Audio jargon

Level: line, instrument, microphone

Microphone level – lowest output level (often -60 dbV), as used in microphones and on microphone inputs on mixers.

Instrument level – output level of guitars, basses etc., and effects pedals.

Line level – loudest output level. Line level can be:

Consumer audio -10 dBv (0 on VU meters)
Professional audio +4 dBu (0 on VU meters). Commonly used in 19" processors, line inputs on mixers and monitors

Outputs on Fractal Audio gear operate at line level. Some are adjustable between +4 dBu and -10 dBv.

"-10 dBV is compatible with instrument levels." source

More information:

Balanced and unbalanced signal

Unbalanced audio signal – Signal carried over a two-conductor cable. The most common cable is a 1/4" guitar cable, where the ground wire wraps around the positive wire. Unbalanced cables are generally good for running signal up to several meters (< 10).

Balanced audio signal – Signal carried over a three-conductor cable which connects a balanced input and balanced output. The two signal wires carry identical copies of the signal, with one of the wires 180 degrees out of phase with the other, creating a differential. At the receiving side the two signals are brought back into phase with one another, and the induced noise will be canceled. These cables usually use XLR or Tip-Ring-Sleeve (TRS) connector end-types. The ground wire wraps around the signal wires and acts as a shield. A balanced connection supports the use of a ground lift switch and noise-free long cable distances.

A DI box transforms unbalanced signal into balanced signal.

More information in this Wicked Wiki article and Wikipedia.

"XLR is only necessary for long cable runs where there is the danger of picking up interference on the cable. Anything less than, say, 10 meters and unbalanced is fine." source

(Axe-Fx II) "The unbalanced and XLR outputs are the same level in the Axe-Fx II. In the original Axe-Fx the XLR outputs were 6 dB hotter. This is not the case with the II." source

(FX8) "For most uses an unbalanced TS cable is fine. The inputs are balanced so that you can get even more hum rejection by using a TS-to-TRS cable from the amp's send." source

Unity gain

What is unity gain? Unity gain means that the input level is equal to the output level.

When does unity gain matter? It is not important when connecting the device to an amplifier or mixing console. It is important in setups where the device is being used as an effects-only processor (e.g. as a pedalboard or in an amp's effects loop) or when using the Four-Cable-Method (4CM) to connect to a guitar amplifier.

How to set up for unity gain? To set up an input/output for unity gain, set the corresponding Output knob to its maximum position.* To test: fill the grid with shunts, and you should get exactly the same signal at the output which you put in.

This does NOT apply to all devices.

"Unity gain mode is a special mode designed for use with the 4CM. When you turn the output levels all the way up whatever you put in you get out (assuming all unity-gain blocks in the chain). If you have an amp block in the chain then you have tons of gain and therefore no longer have unity gain." source

"With the Axe-Fx volume all the way up you would be pushing +20dBu into the amp which could clip the inputs to the amp. Unity gain mode is only desirable for 4-cable-method." source

FX8 and unity gain (from the FX8 Owner's Manual):

Why do I care that the FX8 is designed for unity gain?
A: The FX8 makes it EASY to achieve unity gain. This can be important because amplifier tone, distortion amount, dynamics and noise are level dependent. With unity gain:

The level of the signal from your guitar output can reach your amp input without being altered. Therefore, your guitar-amp interaction sounds and feels the same, offering a transparent playing experience while using the FX8.
The level of your FX SEND can reach your FX return without being altered. The entire system can therefore perform optimally, without unpredictable changes to level, dynamics or noise when you engage True Bypass or bypass all post-effects.

Q: How do I set up the FX8 for unity gain?
A: You don’t need to! Just set up according the basic instructions in Section 3. A default empty preset should sound have the same level as True Bypass Mode.

Q: What might I do to inadvertently upset unity gain?
A: Many SETUP and EFFECT parameters change the gain level. Some of these are intended to change gain levels (how else is a boost supposed to work, after all?) Here is a short list of things to consider:

The LEVEL parameter of every effect increases or decreases the overall level.
Changing MIX on certain effects changes both dry and wet levels. This is to prevent signals from “stacking up” and causing clipping. You can compensate with your ears by turning effects on and off and comparing the level with True Bypass engaged.
If you’re going to change a block’s BYPASS MODE from the default setting of THRU, it is best to check its levels when you engage/disengage the effect BEFORE you switch to something like MUTE FX IN.
The level parameters on the OUTPUT page of the main mode menu increase or decrease overall levels. Incorrect settings on the I/O: AUDIO page can result in gain changes.
The NOISE GATE has a level control.
If your rig is MONO, every BALANCE or PAN control can affect levels.
The Global Graphic EQs affect overall level.
The I/O LEVEL page settings DO NOT affect unity gain. Each setting is compensated internally.

Q: Any last words of advice?
A: Use the TRUE BYPASS switch as a way to make sure your presets and scenes are on track. In general, it is better to be in control of your levels than to be fixated on the “concept” of unity gain. Do what sounds best to you and learn as much as you can about your gear.

Decibels

Cliff's Tech Note about decibels:

"The decibel is a unit of measurement that gives the ratio of the power of one signal relative to another. The formula for the decibel is dB = 10 * log_10(P1 / P2) where P1 and P2 are power measurements. The reason it is called a decibel is because it is 10 bels. One bel would be log_10(P1/P2).
The important thing to understand is that the decibel is a RATIO of powers. A dB is meaningless without a reference power. So if someone says "that signal is 86 dB" that is a meaningless number as it has no reference.
Decibels are convenient because they convert logarithmic perception to a linear scale. Human hearing, for example, is logarithmic. Many other natural phenomena are logarithmic which means that the phenomena exists in the "multiplication domain" as opposed to the "addition domain". For example, human vision is logarithmic. We perceive light such that the light must double for it to appear twice as bright. If we were to plot that we would have an exponential curve of light intensity vs. perceived brightness. If we take the logarithm of the intensity instead we get a straight line. This is why cameras use f-stops which are a base-2 logarithm.
So, back to reference levels. There are many reference levels used in dB: dBm, dBu, dBV, dB re. kPa, etc. dBm refers to the power referenced to one milliwatt. If the measured power is, say, 100 mW then that would be 10 * log10(100/1) = 10 * log10(100) = 20 dBm. dBV is a voltage ratio and not really a true dB but, regardless, is still commonly used. The formula for dBV is 20 * log10(V1/V2) since we need to square the voltage to get the power.
In audio a common unit is dBu. dBu is the power relative to the voltage into a 600 ohm resistor that is dissipating 1 mW. This is roughly 0.77 volts. Back in the early days of telecom 600 ohms was the standard termination impedance, hence the dBu. Most pro audio gear runs at +4 dBu. What does that mean? 0 dBu is 0.77 volts so +4 dBu would be 4 dB greater, or about 1.22 volts. To go from dB to volts the formula is 10^(dB/20).
Consumer audio gear usually runs at -10dBV, or roughly 0.32 volts.
When recording your goal is to get your signal level near the nominal signal level of the equipment being used. This ensures the best S/N ratio. Many recording consoles use VU meters which are calibrated such that "0 dB" is +4 dBu. The goal is to get your signal level around 0 dB.
Well-designed gear has some amount of "headroom". Headroom is the difference between the maximum signal level and the nominal signal level. For example, the Axe-Fx II has a maximum signal level of +18 dBu. If operating at +4 dBu nominal this gives 14 dB of headroom which means that any signal peaks can be over four times higher.
In digital gear we encounter the dBFS, which is dB relative to full-scale. Full-scale is a term that indicates the maximum signal level into or out of an A/D or D/A converter, respectively. With digital converters the best performance is achieved by operating the converter such that the nominal signal level is close to full-scale. The exact voltage is unknown and irrelevant. Most digital gear will have indicators that measure the levels relative to the converter's full-scale value. For example, the input meters on the Axe-Fx indicate the input signal relative to the A/D converter's full-scale value. The "tickle the red" advice aims to operate the A/D converter near its full-scale value as the red LEDs light at 6 dB below full-scale, or -6 dBFS." source

"Decibels are decibels. There is no such thing as "root-power decibels".
By definition a decibel (dB) is a ratio of two powers. The formula is 10 * log10(P1/P2) where P1 and P2 are the power of two signals, respectively.
In electronics, however, we usually manipulate and measure voltage levels. It's convenient to represent the ratio of two voltage levels in dB. To do this you would need to square the voltage to get the power (since P = V^2 / R). We also assume R = 1 for convenience. With a little math you get dB = 20 * log10(V1/V2).
Therefore if we reduce the voltage level of a signal by a factor of 0.1 then the signal is now -20 dB relative to before.
dB is simply an easy-to-read logarithmic-to-linear mapping. Music, human perception, and many other things in nature typically have a logarithmic response. The decay of, for example, a cymbal is logarithmic. If you plot this on a linear axis it's hard to display because of the dynamic range. But if you use a logarithmic axis you "compress" the data into something that's easier to view. Decibels are just a widely accepted mapping. You could use any base for the log; log2, ln, etc but since we have 10 fingers log10 is nice.
The point is that X dB is X dB. If you reduce a signal by 20 dB you've reduced it's voltage to 10% of what it was previously. You also reduced it's power to 1% of what it was previously. These are the same things: 20 * log10(0.1) = 10 * log10(0.01)."

Wikipeda: Decibel

Mono and stereo

Read this: Mono and stereo I/O

I/O hardware specifications

Axe-Fx III

INSTR (front) – 1/4” phone jack, unbalanced, conditioned for guitar use, auto-switching, 1 Megaohm (adjustable), +16dBu instrument level
INPUT 1 (rear) – 1/4” phone jack, unbalanced, conditioned for guitar use, auto-switching, 1 Megaohm (adjustable), +16dBu instrument level
INPUT 2 (rear) – XLR Female and 1/4” combo, L/R, balanced, 1 Megaohm, +20dBu line level
INPUT 3 (rear) – 1/4” phone jack, L/R, balanced, designed for unity gain applications such as 4CM, dual stereo inserts or general purpose, 1 Megaohm, +20dBu line level
INPUT 4 (rear) – 1/4” phone jack, L/R, balanced, designed for unity gain applications such as 4CM, dual stereo inserts or general purpose, 1 Megaohm, +20dBu line level
OUTPUT 1 – XLR, L/R, balanced, ground lift switch, 600 Ohm, +20dBu line level
OUTPUT 1 – 1/4" phone jack, L/R, HumBuster, ground lift switch, 600 Ohm, +20dBu line level
OUTPUT 2 – XLR, L/R, balanced, ground lift switch, 600 Ohm, +20dBu line level
OUTPUT 3 – 1/4" phone jack, L/R, HumBuster, 600 Ohm, +20dBu line level
OUTPUT 4 – 1/4" phone jack, L/R, HumBuster, 600 Ohm, +20dBu line level
A/D and D/A conversion – 48 kHz, 24 bits, 114dB dynamic range, 20 Hz - 20 kHz frequency response (0/-1 dB)
Digital I/O – S/PDIF (RCA Coaxial), AES (XLR), USB Audio 8x8, sample rate 48 kHz
MIDI – IN, OUT, THRU
Headphone output – 1/4" stereo jack, 35 Ohm
Expression pedal ports – 2x 1/4" TRS, 10-100 kOhm, continuous / momentary / latching
FASLINK II port – XLR Female

FM3

IN 1/INSTR – 1/4” phone jack, unbalanced, conditioned for guitar use, 1 Megaohm (adjustable?), instrument level
OUT 1 / MAIN – XLR, L/R, balanced, ground lift switch, line level
IN 2 / FX RETURN – 1/4” combo, L/R, balanced, line level
OUT 2 / FX SEND – 1/4” combo, L/R, Humbuster, line level
A/D and D/A conversion – 48 kHz, 24 bits, 20 Hz - 20 kHz frequency response
Digital I/O – S/PDIF output only (RCA Coaxial), USB Audio 4x4, sample rate 48 kHz
MIDI – IN, OUT/THRU
Headphone output – 1/4" stereo jack
Expression pedal ports – 2x 1/4" TRS
FASLINK II port – XLR Female
USB — USB-A port, USB-B port

Axe-Fx II

IN 1 (INSTRUMENT) – 1/4” phone jack, unbalanced, max +16dBu, conditioned for guitar use, instrument level
IN 1 (rear) – 1/4” phone jack, unbalanced, max +20dBu
IN 2 (FX RTN) – 1/4", L/R, balanced, 1 Megaohm, max. 20dBu
OUT 1 MAIN – XLR, balanced, 600 ohm, max. output +20dBu
OUT 1 MAIN – 1/4” phone jack, unbalanced (hum-canceling)
OUT 2 (FX SEND) – 1/4", L/R, unbalanced, Humbuster, 600 ohm, max. 20dBu
Digital I/O – S/PDIF (RCA Coaxial), AES (XLR), USB Audio

AX8

IN 1 (INSTRUMENT) – 1/4", mono, unbalanced, 1 Megaohm (fixed), max. 16dBu, instrument level
OUT 1 (MAIN) – XLR, L/R, balanced, 600 ohm, max.20dBu
OUT 1 (MAIN) – 1/4", L/R, unbalanced, Humbuster, 600 ohm, max. 20dBu
IN 2 (FX RTN) – 1/4", L/R, balanced, 1 Megaohm, max. 20dBu
OUT 2 (FX SEND) – 1/4", L/R, unbalanced, Humbuster, 600 ohm, max. 20dBu
S/PDIF digital out, 24-bit, 48 kHz (fixed)

FX8

IN [PRE] (INSTR) – 1/4", mono, instrument level, unbalanced, 1 Megaohm (depending on Input Impedance setting), max 16dBu, instrument level
OUT [PRE] – 1/4", L/R, (L/Mono), unbalanced, Humbuster, 600 ohm, max 20dBu
IN [POST] – 1/4", L/R, line level input (+4dBu), 1 Megaohm, balanced, max 20dBu
OUT [POST] – 1/4", L/R, unbalanced, HumBuster, 600 ohm, max 20dBu

Using output ports

Output 1

Output 1 (Main) on the amp modelers is usually used for the direct signal, including cabinet modeling. Most presets, including the factory presets, are set up that way.

The amp modelers (except FM3) provides both XLR and 1/4" ports for Output 1, which can be used simultaneously. They're buffered and protected against phantom power from the console. Output 1 on the FM3 only provides XLR ports.

(Axe-Fx II) "Both outputs should work simultaneously. They are actually buffered so even if you shorted one it shouldn't affect the other." source

(Axe-Fx III) "The outputs are electrically identical. The idea is that one is FOH and the other is your personal monitoring. You can change the volume of your personal monitoring without affecting the FOH." source

Output 2

Output 2 on the amp modelers can be used in multiple ways:

as an auxiliary mono/stereo output
with Input 2: as a mono/stereo effects loop (Output 2 = Effects Send, Input 2 = Effects Return)

When set up as an auxiliary output, you can set the level of Output 1 (usually FOH) separately from that of Output 2 (usually monitors).

Axe-Fx II and AX8 — Output 2 is a set of 1/4" ports. To enable I/O 2 on the layout grid, use the FX Loop block.

Axe-Fx III — Output 2 is a set of XLR ports. If you need to feed an output signal to an external power amp, avoid Output 2 if possible, because it doesn't allow the use of a 1/4" Humbuster cable.

FM3 — Output 2 is a set of 1/4" ports.

I/O 3 and I/O 4

This ports are available on the Axe-Fx III only, as 1/4" ports.

These stereo pairs are designed primarily for inserting outboard gear (rack and pedals), for using the Four Cable Method (4CM), for "guitar level" input devices, for connecting to amplifiers and other purposes. In other words, use these ports:

as an additional "guitar level" instrument input
as an additional output (unity gain)
as an additional Effects Send/Return loop for "guitar level" devices (unity gain)

The input ports support high-impedance sources, such as guitars and basses, besides other gear.

Inputs 3 and 4 do not support the Secret Sauce and do not support variable input impedance.

Configuration:

Choose between Mono or Stereo mode in I/O > Audio.
Adjust Boost/Pad in I/O > Audio, if necessary.
Set the optimal signal-to-noise level in I/O > Input > Input Trim.
Decide whether to use unity gain. To set these loops to unity gain (which is their primary purpose), simply turn the corresponding hardware knobs to their maximum position. Unity gain means that the output level from the unit is the same as the unit's input level. This makes it easy to use these ports for pedals. And as a send to an external amp, because "unity gain" means that you get the same gain as plugging straight into the amp.
Use the corresponding Input and Output blocks on the grid to route the signal. Note that you do NOT need to connect the Output block to the Input block on the grid:
- if you don't connect Output to Input, muting the Input block will mute the signal path from this loop
- if you connect them, bypassing the Input block will not mute the signal. Signal is passed without going through the loop
- if you connect them, with Input engaged, it will output the signal from the effect loop.

"Outputs 3 and 4 are primarily intended for unity gain applications, i.e. fx loops. You can use them as general-purpose outputs as well. When doing this you may need to increase the Output Level of the associated Output block." source

"If you want more signal at those outputs you need to put those Output blocks in the preset and increase the Level in the block. Outputs 3 & 4 are primarily intended to be unity gain outputs for fx loop use." source

"When routing a signal from Input 1 to Output 3 or 4 it's entirely possible for the meters to enter the red zone. This is not an issue." source

"Outputs 3 and 4 are intended to drive "guitar level" devices." source

"Outputs 3 and 4 are "unity gain" and have a different gain constant via the internal number representation than 1 and 2. They are designed for effects loop use, DI sends, etc. If you put 1V into Input 1 and route that to Output 3 and turn its Level knob to maximum you'll get 1V out." source

"Out 3/4 are designed for "unity gain" with respect to an input. If you put 1V into an input, connect that input to Out3 and turn the Level knob on the front panel all the way up you'll get 1V out. This makes it easy to use them as loops for pedals, etc. The effective difference is about 18 dB if Out1/2 are set to +4 dBu, 6 dB if set to -10 dBV. Out 3/4 can also be used as a send to an external amp. Since they are "unity gain" you get the same gain as plugging straight into the amp." source

Creating an effects loop — To establish an effects loop and integrate an external device (like a pedal), use I/O pair 3 or 4. The Output block is the Effects Send, the Input block is the Effects Return. Connect the Output block to the grid to feed the external device a signal. Keep the Output block settings at default. Turn off the noise gate in the Input block. Connect the Input block to the grid to let the signal from the external device enter the grid. The Output and Input blocks do NOT have to be connected to each other. Now turn up the Output knob on the front panel to its maximum postion, this makes sure that the loop is operating at unity gain.

Connecting to a power amp — if you need to feed an output signal to an external power amp, and you have to choose between Output 2, 3 and 4, use 3 or 4. These outputs allow the use of 1/4” Humbuster cables.

Testing an output port

(Axe-Fx) "One way you can test the I/O is to use the synth block. Set the oscillator to pink noise and route it to the various outputs. Be sure to set the filter to 20 kHz (default is 10). Check the spectrum with your analyzer plug-in. It should be flat. You can then route the output of the synth to Output 2 and then jump Output 2 to the Inputs and route the inputs to Output 1 to make sure the inputs are working properly. Route it directly to the Output to test Output 1. Route it only to the FX Loop block to test Output 2. Then run a short cable from Output 2 to the front panel Input to test the input. Run a line of shunts from the input to the output." source

(Axe-Fx III) "It's even easier as you can use the RTA block." source

Setting levels

Main input level

Axe-Fx III – I/O > Input
FM3 – I/O > Input
Axe-Fx II – I/O > Input > Instr In / Input 1 / Input 2
AX8 – I/O > Levels > IN 1 (Instrument) Pad / In 2 (Fx Rtn) Nominal Level
FX8 – I/O > Levels > Input 1 (Pre) Pad

What is Input Level or Input Pad for?
Input Level and Input Pad are NOT GAIN controls. They do NOT affect the overall volume level, they have no effect on output clipping or on amp gain. The control only optimizes the signal-to-noise ratio of the analog-to-digital converters. The adjustment is applied before the A/D converter and is offset by a corresponding but opposite boost at the output of the converter.

Exception: setting Input Level below 5% will have an impact on the signal level. Just don't set it that that low.

How to set Input Level or Input Pad

Make the red Input LED blink occasionally while strumming the strings. This is known as: "tickling the red". Make sure to strum hard, on the loudest pickup! There's a substantial range between orange and red. If you can't make the LED blink red at all, don't worry, it won't have an impact on tone or gain.

The red light turns on BEFORE the instrument input clips. It means "Warning, you're APPROACHING clipping" as opposed to "Warning, you ARE clipping". The red light turns on at -6dB from the point where the signal clips. While the word "clipping" is used here, in reality the input signal never really clips, because of a limiter will kick in before that.

AX8 and FX8: a higher Input Pad value means lower input level ("padding"). Run the pad as low as possible, because padding increases noise floor.

Axe-Fx III: the Instrument input on the Axe-Fx III is more sensitive than the Axe-Fx II's, but it has more headroom / dynamic range. Do not set it below 5%, because at this point gain may be affected. The front panel LED meter bridge provides instant visual status for the inputs.

FM3: to be added.

When using a mono instrument, do not set the Input Mode to Stereo or Sum L+R. Select Left Only (default). Otherwise the level will be attenuated.

Input levels can be also controlled via MIDI CCs.

"For a Strat, near 100% on the input level is not unusual. I run my Strat around there. It has vintage-type pickups." source

"To get the best noise performance it is important that the Instr In trim is set correctly in the I/O->Input menu. Set this as high as possible without clipping the input." source

"You don't HAVE to tickle the reds. Adjust for your hottest guitar and leave it." source

"The AFXII has digitally controlled potentiometers before and after the A/D and D/A converters. Therefore it knows what the input and output gains are. It compensates for these gains in the digital path." source

"Full-scale is a term that indicates the maximum signal level into or out of an A/D or D/A converter, respectively. With digital converters the best performance is achieved by operating the converter such that the nominal signal level is close to full-scale. The exact voltage is unknown and irrelevant. Most digital gear will have indicators that measure the levels relative to the converter's full-scale value. For example, the input meters on the Axe-Fx indicate the input signal relative to the A/D converter's full-scale value. The "tickle the red" advice aims to operate the A/D converter near its full-scale value as the red LEDs light at 6 dB below full-scale, or -6 dBFS." source

(Axe-Fx II) "The Input Trim control in the I/O menu is before the A/D. You can use that to reduce the level into the A/D. If you want 4 dB of gain reduction: A = 10^(-4/20) = 0.63. So you need to reduce your input pad by 37%. The new value is 0.243 * 0.63 = 0.153 => 15.3%" source

(Axe-Fx III) "There is no single optimum setting. If you have hot pickups with thick strings and a heavy hand you may need to set it at 10% or less. If you have vintage single-coils with thin strings and a light touch, 100%. Adjust it to tickle the red when playing hard." source

(Axe-Fx III)"Set the Input Trim so the meters ON THE FRONT PANEL tickle the red when strumming fairly hard." source

Cooper Carter: "The Instrument Level is to make it so the A/D converter hears the best signal it possibly can. So say you have a super low output Strat. You crank up the input level so that that Strat is hitting the converter at a level that is making sure it's well above the (very low) noise floor of the Axe-Fx. The A/D does its work and then brings down the signal it outputs to the processor by the same amount you gained up, so that what is coming in is going out, regardless. Conversely, if you have a super hot guitar, like an EBMM JP15, it's already hitting the A/D way above the noise floor, and you don't want to add unnecessary noise by having the input higher than it needs to be to convert the signal at an optimal level. So you turn the input level down a good bit. The converter then compensates for how much you turned down by bringing up the signal by an equal amount before it outputs to the processor. The signal hitting your grid (i.e. pedals, amps, whatever) is in theory unchanged in level from what is coming out of your guitar. You've just optimized the level at which it's being A/D converted. You "can't" really "clip" the input given that it takes drive pedals and what-have-you just as well as an amp does." source

"Below 5% the gain decreases so, yes, it will be quieter." source

Main output level

The main output levels on the amp modelers are directly controlled with the hardware output level knobs, and also depend on global menu settings and preset settings.

(output knobs on the AX8) "The output doesn't go all the way to zero. This was done due to the plethora of support issues where people would say they weren't getting any sound and it was simply due to the fact that they had the knob turned all the way down. So now you get a little signal and we get less support calls." source

(Axe-Fx II) "We test the output to be flat within +/- 1 dB over the range of the knob. In fact I'd be surprised if there were any measurable variation at all." source

(Axe-Fx II) "The output "pot" is actually a ladder of discrete resistors that is remotely controlled by the knob on the front panel. Other products simply reduce the digital signal going into the D/A converter but this is sub-optimum as you reduce your dynamic range when doing this. The Axe-Fx II strives to keep the signal into the D/A as high as possible for optimum dynamic range and then controls the output level using a programmable output gain. The downside of this approach is that you will hear a small noise when the output switches between the resistors in the ladder." source

(Axe-Fx II) "To place a pot after D/A requires running cables to/from the front panel. These cables can degrade signal quality and pick up noise. The pots on the front panel of the II are remote controls for the digital pots. The signal never passes through them. The digital pots also allow us to boost the level from the D/A and then attenuate it precisely to improve output SNR. The Output X Boost/Pad feature would be impossible without digital pots." source

Axe-Fx II and AX8 — The nominal level of the main outputs defaults to +4dBu line level (adjustable on the AX8). This means that you should connect the Axe-Fx II and AX8 to a LINE level input on the mixing board, when available, because the output signal is too hot for a MIC input. If only MIC inputs are available on the mixing console, try this:

use a pad switch on the mixer to attenuate the signal and prevent clipping
decrease input gain on the channel strip
decrease the output level from the device by turning down the Output knob at the front.

(Axe-Fx II and AX8) "The XLR output is balanced but it's +4dBu nominal. The problem is people connect it to a mic input which is way too sensitive for that level signal. If the board has a mic/line switch you want to set it to line level. Or if it has a pad switch turn that on. Otherwise turn the level knob way down. The thing to remember is that XLR is just a connector. It doesn't imply microphone levels. Most pro stuff like eq's, etc. have line-level XLR's."

(Axe-Fx II) "Optimal gain staging would be with the level knob around noon. Higher than this and you risk clipping the inputs of the downstream device. With the level knob at full the Axe-Fx II will probably incinerate a Soundblaster or other low-cost stuff. The max level out of the Axe-Fx II is +20dBu. Most pro gear can easily handle that but lots of gear cannot and the trend in newer gear is towards lower and lower maximum input levels (due to single-ended designs and low-voltage/low-power constraints). In the old days, +20dBu was routine. Everything could put out and handle +20. Not so much anymore." source

(Axe-Fx II) "The II actually has more output than the I. The II can do about +20 dBu, the I was about +18." source

(Axe-Fx II) "Start with amp volume at noon. Bring up Axe-Fx volume until desired level is reached. If you need more, turn up amp. With the Axe-Fx volume all the way up you would be pushing +20 dBu into the amp which could clip the inputs to the amp." source

Axe-Fx III — The default nominal output level of Outputs 1 and 2 the Axe-Fx III is -10dBV on the Axe-Fx III. It's adjustable: -10 or +4 (Output 3 and 4: n/a). The red LEDs on the front panel come on at -1 dBFS, which differs from previous hardware. A front panel LED meter bridge provides instant visual status for the inputs and outputs. A Meters page in the Home menu and layout grid also shows I/O levels. Finally, the Utility lets you check the performance of all four outputs.

If you need to set the output level of the Axe-Fx III to an exact value (like on the Axe-Fx II), use the Utility > ADC Levels menu.

FM3 — to be added.

"-10 dBV is compatible with instrument levels." source

"The default Output Level for Output 1 and 2 is -10 dBv. This was done to reduce the number of support cases due to people overloading the inputs on consumer-grade interfaces, mixers, etc. (IOW cheap stuff). Most professional gear runs at +4 dBu so if using a pro-grade interface, mixer, etc. you may want to go into the Global menu and change the level to +4 dBu." source

"The Axe-Fx III maximum output level is over 22 dBu (!). The VU meters are calibrated to -12 dBFS. So at the zero line on the VU meters and with the output level knob all the way up you would be putting out around 10 dBu. That's enough to drive any power amp, and then some. The typical power amp has an input sensitivity of 0 dBu for full power so there's more than enough oomph there." source

"+4 dBu is the nominal output. Max output is +20 dBu." source

"The meters on the front panel are the post-fader meters." source

"The meters on the front of the Axe-Fx are relative to full-scale. They are not calibrated in dBu. They are to assist you so that you don't clip the converters." source

"The front panel meters indicate the level INTO the D/A converters. The only place the system can clip is at the converters so the meters let you know when you are in danger of clipping the converters. A signal is generated internally. That signal can be any value from 0 (negative infinity dB) to a thousand dB, in theory. That signal is then multiplied by the "gain" of the output level knob (0 to 1). The knobs have an audio taper but that's irrelevant. If, after applying that gain, the signal exceeds 1.0 the converters will clip and red LEDs on the meters will light. If you think about the meters as level into the converters it all makes sense because that's what it is. To further demonstrate this increase the Boost/Pad setting for Output 3/4 and watch what happens. The output meters are the analog of the input meters. They indicate converter levels. Nothing more, nothing less. And what they do is extremely important and convenient. source

"All Fractal Audio products use floating-point processing. In fact the Axe-Fx III uses 64-bit floating-point in many places. It's impossible to clip internally. The AX-8 and Axe-Fx II use 40-bit in many places and are also impossible to clip internally.
If you are clipping the output, which is the final fixed-point signal to the converters (all audio converters use fixed-point), then your internal signals are far too high. If you use the VU meters and set your output to 0 dB, you are guaranteed 12dB of headroom at the converters with the output level knob all the way up. I've never witnessed a palm mute that was more than a few dB hotter than nominal.
Go into the Layout menu and press the Zoom hotkey. This will display VU meters for the two main outputs. Adjust the level of the Amp block (using the Block Level knob with the Amp block selected) so that the signal hovers around the 0 dB marker. If you do this it's impossible to clip the outputs.
The factory presets are all adjusted for roughly 0 dB on the VU meters. Even with the output level knob all the way up I never get anywhere even close to clipping.
We could've taken a conservative approach and built in a lot of headroom so that clipping the converters was impossible but then you lose dynamic range. The approach taken optimizes the dynamic range of the converters (so you aren't wasting bits) thereby ensuring maximum fidelity and lowest noise. It does require that the user adjust their presets correctly to avoid overflowing the converters but the VU meters make this task trivially easy." source

"It's impossible to clip internally. The output block level meters are intended for use in leveling presets. When routing a signal from Input 1 to Ouput 3 or 4 it's entirely possible for the meters to enter the red zone. This is not an issue. The only place clipping can occur is at the final D/A stage which is indicated by the front panel meter bridge. If the red LEDs light then the D/A is clipping." source

There's NO need to use a DI box to connect the modeler to a mixing board directly.

MIDI CCs can control output levels. To reset them without the help of a MIDI controller, change the assignment to "NONE" in I/O.

The main output level is also affected by the output level of the selected preset, and the Global EQ's Gain control.

Preset level

Read this: Presets

Global menus

Connecting devices and setting levels may require adjusting parameters in the global menus.

More about the global menus:

for all I/O-related parameters: I/O menu
for all MIDI and remote control parameters: MIDI
to configure the FC controllers: FC-6 and FC-12 controllers

Connecting instruments and other devices

Instrument input

Axe-Fx III – INSTRUMENT (front and rear, auto-switching)
FM3 – In 1 / INSTRUMENT (rear)
Axe-Fx II – INSTR (front)
AX8 – IN 1 (INSTRUMENT)
FX8 – IN 1 (PRE)

The Instrument input uses a proprietary circuit and a dedicated A/D converter to lower noise. It's conditioned for guitar through hardware and software ("Secret Sauce"). For best results, use the instrument input for guitar, whether wired or wireless, electric or acoustic, except when running a line level signal.

Make sure that you do NOT use a "balanced" instrument cable between the instrument and input. You can recognize these kind of cables by their "stereo" jacks.

Axe-Fx III — The Axe-Fx III has two instrument inputs: front and rear. The specifications are the same. The rear is meant to be used with racks, wireless units and such. Using the front input, for example with a cable, ALWAYS overrides the rear input. This does not require configuration in Setup.

"The front and rear inputs are identical on the III." source

"The Axe-Fx III input was designed to mimic a typical tube amp input using an average of Marshall and Fender amps for the component values. source

If you think the Instrument input on an Axe-Fx III is faulty, perform this test, courtesy of Fractal Audio (source):

Go to a blank preset.
Create a chain that connects Input 1 to the RTA block. On a separate row create a chain where the Synth block feeds Output 3.
Set the Synth Type = Sine, Tracking = Off, Frequency = 1000 Hz.
Connect an instrument cable from Output 3 (left or right) to the Instrument Input.
Set the Input 1 / Instrument A/D Input Level (Home->Setup->I/O) to 50%.
Turn the Out 3 Level knob all the way up.
The Input 1 yellow LED should be lit. Red should not be lit. Go to the RTA block Config tab. Set Bands to 128, Window to Blackman. Go to the RTA tab. There should be a narrow spike at 1 kHz (three bands). There should be no other bands showing power except maybe a little noise at the highest bands. The spike at 1 kHz should be about 2 1/4 divisions below full-scale.

Secret Sauce on the instrument input

The instrument inputs on the amp modelers feature “Secret Sauce III” on the front instrument input (Input 1). The Axe-Fx III provides "Secret Sauce IV" circuitry on both the front and rear instrument inputs. This feature lowers the noise floor using a proprietary technique along with special analog input circuitry.

(Axe-Fx II XL) "The "Special Sauce III" uses a combination of things to get a lower noise floor. One of these things is new, premium Burr-Brown op-amps in the signal path which have extremely low noise and distortion (and are very expensive). As always I don't design stuff to be cheap, I design it to be good." source

"The spectrum of a guitar is pink(ish). Above 800 Hz or so the energy rolls off dramatically. As luck would have it, humans perceive noise above 800 Hz or so to be most objectionable as it manifests itself as hiss. So the front input pre-emphasizes the high frequencies and then does the inverse in software. This has the net effect of a flat frequency response but pushes the noise floor down by the amount of the pre-emphasis. It's an old trick, used in FM radio and vinyl records. The basic premise is to optimize the data conversion to the information content of the source."

(Axe-Fx II) "You have to set the input selection to match the input you're using. If you're using the front input then you must set the input selection to front and vice-versa. If you plug something into the front and set the input selection to rear it will get MUCH brighter. The front input is optimized for guitar level inputs and has spectral shaping and more gain than the rear input. The front input is optimized for guitar pickups. This is a combination of hardware and software processing. If you set the input source to Analog Rear this turns off the software processing part. If you are plugged into the front it will change the tone since you're still going through the hardware processing. This is why I say you must match the input selection to the input you are using. The rear inputs are standard line-level inputs and can be used with any program material. The front input, as stated above, is optimized for guitar pickups. As such it has more gain and less headroom and may clip if used for non-guitar program material. If you plug a guitar directly into the rear you may find you don't have enough signal level."

Input Impedance

The Axe-Fx II, III and FX8 feature adjustable (variable) input impedance on the instrument input (Axe-Fx III: front and rear).

The AX8 has input impedance fixed at 1 Mohm.

The FM3: to be added.

Read this: Input impedance

Input 2

Input 2 on all devices is a LINE level port. It does not support the "Secret Sauce" and does not support variable input impedance.

Axe-Fx II and AX8 — set of 1/4" ports

Axe-Fx III — set of Combi ports (XLR + 1/4"). These ports support high-impedance sources such as guitars and basses, besides other gear. Because of this, there will be some white noise when Input 2 is connected to an output and nothing is plugged into Input 2 (this does not apply to ports 3 and 4). The signal-to-noise ratio is adjusted through I/O > Input > Input Trim. You can choose between mono or stereo input in I/O > Audio > Input 2 Mode. Use the Input 2 block on the grid to handle the input signal.

"We wanted Input 2 to be able to support both line level sources and instruments. A guitar needs a very high impedance input impedance (1 Mohm). Line level sources typically see input impedances around 10K but work just fine at higher impedances. The self-noise of a resistor is proportional to the resistance. Therefore a 1M resistor will have 100 times the noise power (20 dB!). However the input resistance is shunted by the source resistance so it effectively doesn't contribute to the noise figure. A combi-jack does not have a shorting contact on the 1/4" tip contact like a regular 1/4" jack. The whole reason a regular 1/4" jack has a shorting contact is to short the input to ground when nothing is plugged in. This shorts the noise of the input resistance to ground. Without that shorting contact and nothing plugged in you get the noise of that input resistance and since it's 1M it's significant. Plug something in and the noise will go away. Or simply don't use it with nothing plugged in." source

"As a combo switch, its Input doesn’t get shorted to ground when there’s nothing plugged in, which means it’s left floating, which means it’s susceptible to noise." source

FM3 — to be added.

Input 3 and Input 4

This applies to the Axe-Fx III only.

Read this: I/O 3 and I/O 4

Different guitars

When switching between guitars, there will be differences in level and tone, just like with a traditional amp.

If you want to adjust presets to accomodate these different guitars, several approaches are possible.

Create different presets for different guitars.
Use a separate input port for each guitar, assigning each its own signal chain.
Adjust Level in the Input block. This controls the loudness of the signal entering the grid. It was specifically introduced for this purpose: compensating level differences between guitars. It works per preset, so it needs adjustment per preset, unless the setting is stored as part of a Global Block (Axe-Fx only).
Set up X/Y or Channels in the Amp block for different guitars, using different values for Input Trim for example.
Attach a modifier to Input Trim in the Amp block, connected to a pedal or switch.
Use Input Boost in the Amp block.
Set up a different Amp block (Axe-Fx only) for each guitar.
Use scenes and scene controllers, attached to i.e. Input Trim in the Amp block.
Adjust Amp Gain in the Global menu (Axe-Fx II and AX8 only).
Axe-Fx II only: add a low-CPU block to every preset. Like FILTER or VOL or PEQ (PEQ and FILTER allow additional EQ-ing). Put it at the start of the grid to make it affect the amount of gain in the Amp block. Keep the block neutral and set its Level at i.e. -6 . Make it a Global Block, so you can easily change a setting and have it applied across all presets immediately. Attach its Bypass parameter to an external controller. Engage this block by going into I/O > MIDI and toggling EXT CTRL xx INIT VAL between 0% and 100%. Or assign a general function footswitch to the external controller’s CC and use that for toggling instead (set the switch to Global:Yes in the MFC). It works across all presets.

Wicked Wiki: Settings for Different Guitars

See section 4 of the Axe-Fx III's Owner's Manual for a preset example for a dual output guitar, such as magnetic + piezo.

Multiple instruments simultaneously

Axe-Fx III — Guitar 1 connects to the Instrument input (front of rear). Guitar 2 connects to Input 2, 3 or 4. Even a 3rd and 4th instrument can be connected. Each can have its own signal chain on the grid, and its own output if desired. There are two Amp blocks, so 2 instruments can make use of amp modeling. You don't need an Amp block for an acoustic guitar, piezo, or an electric that should sound like an acoustic. Perhaps neither for bass, with the help of the B7K drive model. You may even get away with a Drive block and a Cab block for clean guitar tones (at the cost of dynamics). See also section 4 of the Owner's manual.

FM3 — to be added.

Axe-Fx II — Set Input 1 to Stereo. Connect one instrument to the front, and the other to Input 1 Right at the rear. Use two rows on the grid. Add a VOL block to each row. Set one to Input left (for the instrument that goes into the front input or rear input 1 Left) and the other to Input Right (for the instrument that goes into rear input 1 Right). Add an Amp block after each VOL block if necessary. You can also leave out the VOL blocks and set Amp 1 to Input Left, and Amp 2 to Input Right; this only works with the Amp blocks in the first column. Continue the rows to the end, adding a CAB and effects to each one if necessary, or merge them if desired. Keep the signals separated by using Balance controls.

AX8 — To use the AX8 with 3 devices: guitar, and two other devices such as piezo or synth, add an Amp and Cab. Put the FX Loop block after the Cab block. This sends the regular guitar sound to Output 2, and lets signals from Input 2 (left and right) enter the grid. Split the signal after the FX Loop block into two rows, and add a Volume block to each one. Set Input Select in one Volume block to Right Only. Set Pan and Balance as desired. Set Input Select in the other Volume block to Left Only. Set Pan and Balance as desired.

More information:

Acoustic instrument

An acoustic instrument with a pickup can be connected to the AX8 or Axe-Fx.

An impulse response (IR) of an acoustic body can add acoustic resonance to the tone. Always use UltraRes IRs when using acoustic IRs when available. There are no acoustic IRs among the factory cabs. You can find some here:

A basic preset for an acoustic guitar suffices: use some compression, some EQ and reverb. An Amp block is not required, though the "Tube Pre" Amp model is often used to warm up the tone.

Audio clip and preset for a piezo-equipped guitar

You can also use Tone Matching with great results:

Bass guitar

The amp modelers provide built-in bass cabs and bass amp models.

The tuner supports bass guitar tuning. The Axe-Fx III provides improved pitch detection for bass guitars.

More information:

External effects

H effects.png

External effects, such as pedals, can be integrated in a Fractal Audio-based rig in several ways.

Between guitar and processor – If you want to connect an effects pedal to an amp modeler, with the processor configured for Amp and Cab modeling, connect it between the guitar and the instrument input on the processor. Remember to check the input impedance (AX8: n/a), and make sure the pedal's output doesn't clip the input of the unit.

(Axe-Fx II) "The input buffer is designed for ~10Vpp max." source

(Axe-Fx III) "The input can handle up to +/- 5V which is greater than any 9V pedal can deliver." source

Effects loop – Alternatively, insert the effect in an effects loop. Make sure to adjust levels where needed (block(s), I/O menu, output level knob on front/top panel. You can include/exclude the effect per scene, or use the effects loop block as an audio switcher. On the Axe-Fx III, FM3 and AX8, make sure to set the OUT knob to its maximum position to achieve unity gain.

Read this: Setting up an effects loop on the Axe-Fx III

(Axe-Fx III) "The outputs have adjustable pads." source

Wireless receiver

A wireless receiver can be connected to the instrument input or to another input.

Axe-Fx III – Connect the wireless to the rear instrument input. If you want or need to use a cable, just plug it into the front input, and it will override the rear one automatically.

(Axe-Fx II) "The front input has a better SNR but if you are using a wireless the better SNR of the front input won't be noticeable since the noise of the wireless will dominate." source

Microphone

A microphone should be connected to a LINE input. That's Input 2 on the Axe-Fx II, AX8 and FM3, and Input 2, 3 or 4 on the Axe-Fx III. The hardware does not contain built-in microphone preamps, so there will be a level mismatch between the line level input and the microphone's output level. The microphone's signal is too low. Increasing its signal on the grid will add noise. It's better to increase the level of the source to get sufficient signal strength into the unit while keeping noise low. This can be accomplished by using an external mic preamp, or a device like Shure's A85F adapter.

"Two awesome options for between a mic and Axe-Fx III are the Summit Audio TBA-221 and the FMR-RNP. A nice inexpensive choice is the Rolls MP13. With one or two of these, you could sell the interface." source

Play sound from computer or other device

The processors handle incoming audio from a computer or audio player in different ways.

- through USB Audio – Audio from the computer can be played through the Axe-Fx II and III and FM3 using USB Audio. The audio is not part of the grid and will be streamed to Output 1. The audio can't be routed to another output on the Axe-Fx II. To route the audio to another output on the Axe-Fx III, see section 3 of the Owner's Manual (Mac), or use ASIO (Windows). To adjust the volume, adjust the volume in the software.

The AX8 and FX8 don't support USB Audio.

The Axe-Fx II does not support USB Audio from an iOS device with an USB connection. The headphones output must be used, see below.

The Axe-Fx III and FM3 do support USB Audio from an iOS device with an USB connection. More information

- through Input ports – External audio can enter the device by connecting a computer or audio player to the inputs.

Axe-Fx III – Input 3 or 4 would be a logical choice
FM3 – Input 2
AX8 and Axe-Fx II – When using Input 1 (rear), set it to stereo, using a preset with shunts only, or with the unit in Bypass mode (Axe-Fx II only)
AX8 and Axe-Fx II – When using Input 2, the FX Loop block must be used and connected to the grid output

How to send signal from Input 2 to Output 2 on the Axe-Fx II

Sustain and feedback

It's as easy to get your guitar to feedback as it is with a regular amp and cabinet (except when using headphones / IEM). If you don't succeed, experiment with the Output Phase parameter in the I/O menu.

Fractal Audio does not provide a dedicated "sustainer" or feedback effect (like Digitech's FreqOut). Forum member Simeon created a feedback simulation preset, mimicking controlled feedback.

Connecting controllers

Read this: Remote control, pedals and switches

Setups

Introduction to amplification

Amplifying a modeler usually requires either a traditional guitar cab with a power amp, or a so-called FRFR monitor or cabinet.

Read this for an introduction to amplification

Full Range Flat Response (FRFR)

See the rig diagrams in Section 4 of the Axe-Fx III Owner's Manual

Why use FRFR monitoring

Listening to a virtual amp with a cabinet model requires an amplification system or listening device that covers a broad frequency spectrum (20Hz up to 20kHz) AND adds as little coloring of its own as possible. Those systems are called FRFR: Full Range Flat Response. Also referred to as: neutral, because what goes in, comes out. Tone shaping is entirely left to the input device, which in this case is a Fractal Audio modeler.

From the Axe-Fx III Owner's Manual:

"A Full-Range Flat Response (“FRFR”) system aims to reproduce the entire audio spectrum without compromise. In comparison, most guitar speakers are narrow range, with no ability to accurately reproduce extended lows and highs. A 1×12 open-back combo is never going to sound like a 4×12 stack. In comparison, full-range flat response studio monitors, high‐quality PA speakers, and FRFR speakers designed specifically for guitar should be able to reproduce anything you play through them."

Many so-called FRFR devices really do not have an entire flat frequency response.

Advantages of FRFR amplification are: portability, no tone coloring, reduced stage volume, consistent tone at all volume levels and in every venue, flexibility of cab modeling, good reproduction of synth and acoustic tones, and the musician hears exactly what the audience hears.

Which systems are FRFR

FRFR systems include:

Studio monitors
Active (powered) FRFR cabs and wedges
Passive FRFR cabs and wedges, powered by a separate neutral amplifier
High-quality headphones
High-quality P.A.

Popular manufacturers of FRFR solutions for stage use include Atomic, RCF, Matrix, Meyer, Friedman, XiTone, Mission Engineering, EAW, QSC and others.

Quality studio monitor brands include Focal, Adam, Genelec.

Close-miking

Miced cab.png

FRFR amplification leaves tone shaping entirely to the modeler. That includes the use of a virtual speaker (cab model). Fractal Audio's cab models are Impulse Responses (IRs). These are sampled sounds of speaker cabinets, with the recording microphone(s) placed within inches of the speaker cap or cone ("near-field"). This is referred to as: close-miking.

Compared to the sound of a traditional amp and guitar speaker (aka "in the room" or "far-field"), close-miked sound has much more bass content, because of the microphone's proximity to the speaker when capturing the IR. Also, there is much more high-frequency content, because the recording mic usually is placed on-axis.

Getting accustomed to then FRFR sound can take some time to getting used to. Instead of listening to a traditional guitar speaker, you're hearing the sound of a close-mic'd speaker cabinet (because of the use of cabinet modeling). The directivity of an FRFR speaker is also different from a traditional guitar speaker.

"You're never going to get a full-range monitor to sound like an amp in the room regardless of the IR used. One reason for this is dispersion. A traditional guitar cabinet has a beam pattern that decreases with increasing frequency. This means less high frequencies when listening off-axis. A full-range monitor will have more highs. Now some will argue that if you capture the traditional cab off-axis in the far field then you'll get the same thing but you won't because the monitor is not interacting with the environment in the same way. The traditional cab will send less frequency content to off-axis which is then reflected off the floor, walls and ceiling. The monitor will send more highs off-axis that are reflected. Our hearing relies a LOT on the spatial cues of reflection and the reflections will not be the same. Compound the above with the fact that 99.9% of IRs are near field captures which sound nothing like the far field. I believe trying to get a monitor to do amp in the room is a lesson in futility. If you really want that sound use a traditional guitar cab." source

"You're not going to hear the same thing through FRFR that you heard from guitar cabs. Your audience will hear something very similar but you won't. What you're hearing through FRFR is a mic'd representation of the cabs. It takes some getting used to. You have to start thinking like a producer/engineer rather than a guitar player. If you start trying to dial out what you call "fizz" and "artifacts" you're going to end up with a tone that doesn't cut. It might sound good to you but it won't fit in the mix. That fizz and sizzle is what makes those classic rock tones work. Listen to some isolated tracks of VH and AC/DC and you'll hear a ton of high-end sizzle. In the mix, however, it's not noticeable. If you remove it then the guitar sounds dead." source

"The sound of an amp in the "far field" is quite different than what you get with close-miking. IR's are made using close-miking and therefore sound nothing like listening to a guitar cab at distance from the cone. Your audience does not hear the far field tone, they hear the close-miked tone as that's what is put through the FOH. It can be quite an adjustment coming from far field amp tone to close-miked tone. Some people just never adjust. Fortunately the Axe-Fx was designed to give you the best of both worlds. You can use the FX Loop and Output 2 to a power amp and conventional guitar cab while routing the fully processed tone with IR to the FOH. See the manual for full details. Rather than using your amp you can use a lightweight solid-state power amp and any of the new, lightweight guitar cabs that use Neodymium speakers. This gives you the classic far field amp tone for yourself in a lightweight package and the polished sound for the FOH direct from Output 1." source

"Close-miked IRs typically have a lot more high frequencies than what you hear at a distance and off-axis from the speaker." source

"All speakers "move air", that's the entire point of their design. Guitar speakers are inherently directional at higher frequencies. So when you stand off to the side you hear less highs. If you have two or four speakers the directivity gets even worse. FRFR speakers have less directivity. This combined with IR technology that almost invariably uses samples of a close-miked speaker and you end up with a different listening experience. To confuse the issue further many combo amps have an open back which changes the frequency response at the listening position even more. Now, if you connect your Axe-Fx to a power amp and traditional 1x12, 2x12, etc. then you will get "amp in the room" but the "moving air" statement has no basis in fact." source

"You can't compare what you are used to hearing "in the room". The close-miked sound ALWAYS has more highs and lows. This is due to the physics of near-field micing. And this is why a highpass and lowpass are frequently employed at mixdown." source

"The classic method is "1W / 1m" which is to apply 1W and measure 1 meter away. When you get the microphone close to the speaker the response is much different and you usually get more highs and lows. This is "close miked" and is the technique normally used in studio recordings. During mixdown the producer/engineer will then often highpass and lowpass the signal to remove these excess highs and lows and to make the guitar "sit in the mix". IRs are almost always made using the same close-miked technique and, hence, will sound like a raw recording. Far-field IRs are possible but very difficult to obtain requiring a large facility and special techniques. Our primary goal is to model an amplifier and speaker as accurately as possible and the latest modeling is astonishingly accurate. We do not purport to be producers or mix engineers and leave the choice of low cut and high cut frequencies up to the user. Furthermore many users rely on the soundman to apply the filtering at the board, just as they would when mic'ing a "real" amp. More importantly the choice of frequencies is highly dependent upon the IR used." source

"IRs are equivalent to close-mic'ing an amp. When you close mic an amp you almost always get more bass and treble than an "amp in the room". The extra bass is due to the proximity effect of the microphone. The extra treble is primarily due to the directivity of the speaker. During mixdown engineers/producers will typically incorporate a low cut and high cut to help the sound "sit in the mix". The thing to take away from all this is that an IR represents the close mic'd sound (unless using far-field IRs which are rare) and the close mic'd sound of an amp is much different than the "amp in the room" sound. As such it is common to use frequency shaping on a close-mic'd amp." source

"The Axe-Fx is extremely accurate in duplicating the sound of a mic'd amp. Your monitoring thus becomes an essential part of the chain and accuracy is paramount. Many "FRFR" monitors are neither FR nor FR." source

"FRFR is just not the same. Traditional head/cab you hear the sound from a bandwidth-restricted speaker at, say, 10 ft. In a typical modeler setup you are hearing what the "mic heard" when the IR was made and that mic was pushed up against the grill cloth. One approach is to use "far field" IRs which are obtained using a measurement mic at a typical listening distance and angle. These are rare. There are a couple stock far-field IRs. They are indicated by (JM) for Jay Mitchell, who created them. Even then it's still not the same because when you are using a traditional setup you move around while playing and the tone changes based on the angle. With a far-field IR the tone doesn't change with angle. When I was gigging I used a power amp and cab behind me and sent the XLR outputs to FOH. More gear to lug but best of both worlds: traditional backline sound, consistent FOH sound." source

"It's not the mic per se'. It's near-field vs. far field. Different mics sample the near-field differently. Mic'ing a speaker is sampling the near-field which sounds dramatically different than the far field. The response pattern of the mic samples the near-field and mics each have their unique pattern. Regardless, it's irrelevant. You'll never get monitors to sound like "cab in the room". If you want that use a SS power amp and cab." source

"FRFR is simply different. It's like mic'ing up the cab in an iso booth and listening from the control room. Therefore it becomes EXTREMELY dependent upon the FRFR speaker. (...) if you have access to some nice studio monitors I'd start there." source

"Apples and oranges. You're comparing FRFR to amp-in-the-room. They will never sound the same. And, IMO, those Matrix FRFR cabs sound like garbage but that's another story. When you use cabinet modeling into an FRFR you're recreating the sound of a close-mic'd amp. It's analogous to being in the control room while listening to your cab in an isolation booth. I.e., how records are made. If you want to compare to a head plugged into a cab you need to run the Axe-Fx into a power amp into the same cab. Get a *good" solid-state or tube power amp and run that into a Marshall cab. A few tweaks and it should sound nearly identical. source

Read AlbertA's explanation of the differences between nearfield and farfield

Fletcher-Munson

The Fletcher-Munson curve is the scientific name for the fact that human ears perceive sound at low volume levels differently than at higher levels. This is VERY important when dialing in tones.

When tweaking tone at low volume levels, a player often turns up treble and bass. This is what the "Loudness" switch on older home stereo systems did.

When the volume is turned up, those high and low frequencies get harsh and boomy. That guitar sound then competes with cymbals, and will lose. Also, the guitar competes with the bass guitar, and will lose.

This is not specific to FRFR systems. But FRFR amplification makes the Fletcher-Munson much more apparent because it amplifies a broad frequency range. In comparison, a traditional guitar speaker operates as a filter, with a quite narrow frequency range.

The "Loudness" controls in old Hi-Fi gear was nothing more than a bass/treble boost. It's a gimmick. It was supposed to compensate for the reduced sensitivity of human hearing at lower volumes. It's not accurate, never was and can never be. There are a myriad of reasons why, the most glaring is that you have no way of knowing what the SPL is (without a meter). Since equal loudness contours are dependent on SPL you can't compensate if you don't know the SPL. The Axe-Fx has no idea of the sensitivity of the amplifier and speakers connected. Therefore it can't possibly know what the SPL is and concomitantly can't know how to compensate. Here's an article on what's wrong with "Loudness" controls" source

More information about Fletcher-Munson:

Fighting extended frequencies and Fletcher-Munson

Above, we've concluded that the FRFR sound has an extended frequency range, which often is undesirable for a guitar sound, and can suffer from the Fletcher-Munson curve when not dialed in correctly.

The solution to these issues is really simple: don't dial in too much top and bottom end. And always dial in your live guitar tones at gig levels (90dB and higher). Do NOT expect excellent "bedroom" or headphones tones to translate well to a rehearsal room or stage, because sound changes as the volume changes. What sounds dull at low volume, may sound fantastic at high volume. And remember that the guitar is a "mid" instrument, so focus on the midrange.

So how do you tweak the sound for FRFR? Here are some guidelines and options.

Use the Low Cut and High Cut parameters in the Cab block to block undesirable top and bottom end. Common values are cutting lows (high-pass) between 80-150 Hz and cutting highs (low-pass) between 5-10 kHz. This may seem to make your guitar sound bad or dull by itself, but it will improve its sound within the entire mix
Put a PEQ at the end of the grid and block the lower and higher frequencies
Use the Global EQ or a GEQ for similar results
Adjust Depth/Bass and Treble/Presence in the Amp block
Boost the mids. For example: put a PEQ at the end of the grid, set a band (use Peaking when using the first or last band) to 770 hz, Q at 0.35, Gain between 2 and 4 dB
Use the Cut switch in the Amp block

More tips in this forum discussion

"Resist the temptation to add bass and treble. The amp designers knew what they were doing (well most of them). If you are applying heavy EQ then you will be disappointed at gig volumes. What sounds midrangey and bland at low volumes will sound great at high volumes. Do some research on Fletcher-Munson to understand this." source

"People often talk about applying low cuts and high cuts. This is because the cabinet models used in modelers are almost always (with a couple exceptions) based on near-field samples of guitar cabinets. IOW, the mic is pushed up against the grill cloth. This just happens to be the way that record producers/engineers mic a cabinet in the studio and the way guitar cabs are mic'd on stage. This is done primarily for isolation reasons. The downside of this approach is that the resulting tone will have a lot more lows and highs than when listening to the amp+cab "in the room". What the mic "hears" when pushed up against the grill cloth is not the same thing that we hear standing 10 feet away. The most common technique to deal with this is to simply cut out the lows and highs using blocking filters, e.g. highpass and lowpass filters. Producers routinely do this when mixing as excessive amounts of lows and highs will cause the guitar tracks to get "lost in the mix". Live sound engineers often do the same thing. The Cabinet block has blocking filters built in for just this very reason. You can also use a couple dedicated filter blocks or a parametric EQ block. For now let's use the Cabinet block. My personal settings are Low Cut around 80 Hz and High Cut around 7500 Hz and Filter Slope set to 12 dB/octave but these are just a starting point. Far-field IRs are available but they are rare due to the difficulty in obtaining them. They require a large facility and special techniques making the process impractical in most cases. So, until an abundant source of far-field IRs are available we need to think like a producer/engineer who is dealing with the mic pushed up against the grill cloth. This means shaping the tone with EQ to remove unwanted frequencies." source

Optimize the Amp block's Output Mode

The Axe-Fx III lets you optimize the Amp block's output for the chosen amplification method. Options are:

FRFR — when using low-to-medium volume FRFR amplification

SS PA + Cab — when using a solid-state power amplifier and a traditional guitar speaker

Firmware Ares 1.16:

"Added Output Mode to Amp block. The default value, FRFR, is the classic mode and designed for use with monitors or recording. The SS PA + Cab mode is intended for use with a solid-state power amp and conventional guitar cab. In this mode speaker compression modeling behaves differently relying on the speaker for compression while still simulating the interaction with the power amp. NOTE: this mode is not intended for use with current drive power amps, i.e. tube power amps, Class-D current feedback amps (Quilter Tone Block), etc. NOTE: this mode CAN be used with FRFR monitors in high volume applications where the monitor’s speakers are compressing thereby achieving a more dynamic response."

FM3: to be added.

FRFR and amp/cab-in-the-room

Use the parameters below to get the sound of FRFR amplification closer to the familiar "amp/cab in the room" sound.

Cab block:

Room Level
Floor Reflection
select a "far-field" IR. The stock ones have "JM" in their name. Or: select a stock cab which has been captured with a neutral mic, such as the Red Wirez ones, and set Proximity to its lowest value for simulate far-field coloring
create the so-called "HAAS" effect by using two IRs in stereo, with a very short delay in the Cab block on one of them
use De-Phase / Smoothing
use Low Cut and High Cut to shave off excessive low and high frequencies and mimic the frequency range of a traditional guitar speaker

Amp block:

use Speaker Compression (AX8: Motor Drive) and Speaker Compliance
set Output Mode to: SS Amp + Cab

In the end, if you crave a real "amp/cab in the room" tone from your modeler, just amplify it through a power amp and a traditional guitar speaker cabinet.

Do not put a microphone in front of a FRFR speaker

When you're using FRFR amplification on stage and you need to provide a signal for FOH, do NOT place a microphone in front of the FRFR monitor. That would make no sense: the source signal already contains the sound of a close-miked guitar cab. Direct-to-FOH is the right way to do it: run a cable from the output(s) to the mixing console. For long distances, use the balanced outputs.

Tweeter squeal from FRFR speakers

Some FRFR speakers can emit very high-pitched loud feedback.

"Tweeter squeal is magnetic feedback from the speaker's tweeter. Move further away from the speakers. This is a phenomenon unique to FRFR solutions." source

"Magnetic feedback is an issue unique to FRFR amplification. The tweeter creates a magnetic feedback loop with the pickups. The closer you get to the speaker the more feedback until the point it squeals. The only solution is to move away from the speaker or turn down the gain/volume." source

"The high-pitched feedback is pickup squeal and is caused by electromagnetic feedback from the speaker to your pickups. FRFR tends to exacerbate this since you have a tweeter feeding back high frequencies. A noise gate can help but the best solution is to move away from the speaker." source

Power amp and guitar speaker

See the rig diagrams in Section 4 of the Axe-Fx III Owner's Manual

Why use a power amp and guitar speaker

If you need amplification, and FRFR (see above) is 'not your thing', you can amplify the amp modeler using a power amp and a guitar speaker.

When choosing this route, there are still choices to be made, as explained in the sections below.

"You'll never get monitors to sound like "cab in the room". If you want that use a SS power amp and cab. No amount of forum discussion is going to change physics." source

Using a tube power amp for guitar (or head or combo)

When using the amp modeler with a tube-powered amp which is designed for guitars (e.g. Mesa, VHT, Fryette) and a traditional speaker cabinet:

switch off Power Amp Modeling. Here's how
disable Cabinet Modeling, because you're using a traditional guitar speaker
set the controls on the power amp as neutral as possible
turn off Speaker Drive, Speaker Compression and Speaker Compliance in the Amp block. More

This also applies when connecting the amp modeler to the Effects Return port of a guitar combo amp or an amp head.

"If you use a tube power amp and don't turn off power amp modeling in the Axe-Fx you will get the impression that the tube power amp sounds "bigger" and "warmer". This is because the tube power amp will have more bass (and highs) than the solid-state power amp since a tube power amp's response follows the speaker impedance. People will ALWAYS find that more bass and treble sounds "better" when listening alone but in a band context that tone will get lost. Speaker designers have been exploiting this fact of human perception for decades. Many "hi-fi" speakers exaggerate the bass and treble because the uneducated customer will think they sound "better". A truly flat speaker will sound dull in comparison to one with exaggerated lows and highs. Over time, however, those exaggerated frequencies lead to fatigue. It's only in comparison that exaggerated bass and treble sound "better". In an isolated context this aspect of human perception is not evident." source

"If you are using a tube power amp you should set any Presence, Depth, Resonance, etc. controls to their minimum positions on that amp (assuming they are conventional controls). On a Mesa power amp, set them to noon. The Presence control on Mesa amps is most neutral around noon. If you turn it up it boosts the highs, if you turn it down it cuts the highs. On most other power amps it only boosts. source

"If using a tube power amp into a traditional cab all should be zero. If using a solid-state amp into a traditional cab I would recommend Speaker Compression and Compliance not be zero." source

Using a neutral tube power amp

When using the amp modeler with a "neutral" tube power amp (e.g. Fryette's Power Station) and a traditional speaker cabinet:

keep Power Amp Modeling turned on
disable Cabinet Modeling, because you're using a traditional guitar speaker
set Low and High Resonance in the Amp block to zero, because the tube amp interacts with the speaker itself. Note: if you use the same Amp block for a separate direct signal (to FOH or FRFR monitor), turning off Low and High Resonance will interfere with this
turn off Speaker Drive, Speaker Compression and Speaker Compliance in the Amp block. More

"If using a tube power amp into a traditional cab all should be zero. If using a solid-state amp into a traditional cab I would recommend Speaker Compression and Compliance not be zero." source

Using a solid-state power amp

When using the amp modeler with a solid-state power amp (no tubes, e.g. Matrix, Seymour Duncan, Crown) and a traditional speaker cabinet:

keep Power Amp Modeling turned on
disable Cabinet Modeling, because you're using a traditional guitar speaker
turn down Speaker Drive in the Amp block. Turning down Speaker Compression and Speaker Compliance is not required. More
set the Amp block's Output Mode to: SS Amp + Cab

"If using a tube power amp into a traditional cab all should be zero. If using a solid-state amp into a traditional cab I would recommend Speaker Compression and Compliance not be zero." source

"Added Output Mode to Amp block. The default value, FRFR, is the classic mode and designed for use with monitors or recording. The SS PA + Cab mode is intended for use with a solid-state power amp and conventional guitar cab. In this mode speaker compression modeling behaves differently relying on the speaker for compression while still simulating the interaction with the power amp. NOTE: this mode is not intended for use with current drive power amps, i.e. tube power amps, Class-D current feedback amps (Quilter Tone Block), etc. NOTE: this mode CAN be used with FRFR monitors in high volume applications where the monitor’s speakers are compressing thereby achieving a more dynamic response."

Finding the resonant frequency with a solid-state amplifier

A solid-state amp doesn't automatically interact with the speaker like a tube amp does. That's why the Amp block provides Resonance parameters. The default Low Frequency Resonance value may not get the best results with the speaker in use. Optimize this by finding the resonant frequency of the cabinet, like this:

put a Filter block after the Amp block
set the type to Peaking, Q to 5 or so and Gain to 10 dB
start with a Frequency of around 50 Hz. Play some chugga-chugga and slowly adjust the Frequency until you hear and feel the cabinet resonate. You need to do this at loud volume level to notice it. Make a note of the frequency
remove the Filter block and set the Amp block's Low Resonance to match.

Alternatively:

add a Synth block (after the Amp block) to the preset and make sure it is connected to the grid output
select Sine wave
turn off Tracking
turn up the volume of your rig
adjust Frequency until you hear and feel the cabinet resonate. You need to do this at loud volume level to notice it. Make a note of the frequency
remove the Synth block and set the Amp block's Low Resonance to match.

Gain-staging a power amp

Make sure not to overload the input of a connected power amp or active monitor.

"The II actually has more output than the I. The II can do about +20 dBu, the I was about +18." source

"Start with amp volume at noon. Bring up Axe-Fx volume until desired level is reached. If you need more, turn up amp. With the Axe-Fx volume all the way up you would be pushing +20 dBu into the amp which could clip the inputs to the amp." source

Note: it's common knowledge that a Matrix power amplifier (GT800FX, GT1000FX) sounds at its best with level at 2 o'clock or higher.

Wet/Dry/Wet rig

Forum discussion

About speaker wire

"The Axe-Fx is designed to recreate the signal at the speaker jack of a tube amp and it does this tremendously well. If I do a Tone Match to the output of the amp vs. the model it's almost always nearly a perfectly flat line. So today I was playing around and did a quick tone match to one of my Plexis and then a Suhr Badger and the results showed a significant mid-scoop (2-3 dB). I was puzzled. Had I messed something up in the new firmware? I repeated the tone match using a DI off the speaker jack and the result was a perfectly flat line. Then I realized that the difference was due to this 30 ft speaker cable I was using because the speaker cab was remote from the amp. Just a bit surprised that that little resistance could have that much effect. Fortunately the new Cab-Lab addresses all this by allowing you to capture reference IRs and we've included reference IRs along with our latest Cab-Pack. To double-check I then captured a reference IR off the speaker and corrected the IR using the new Cab-Lab and viola, perfectly flat." source

"There's a big difference between a long cable between your guitar and amp and a long speaker cord. A long instrument cord loads your guitar's pickups with a reactive load that's mostly capacitive. This changes the resonant frequency of the pickups and rolls off the highs. A long speaker cord increases the resistance between the amp and the speaker which decreases the damping factor. A lower damping factor means the response follows the impedance curve of the speaker more than a high damping factor." source

Combining FRFR and traditional backline

Fractal Audio's amp modelers allow a combination of different output / amplification methods. Many players amplify their modeler on stage in the traditional "backline" way (speaker cabinet with a power amp), with a direct signal going from the modeler to the PA system.

Each of the modelers has a factory preset, designed for this purpose.

(Axe-Fx II and AX8) "Fortunately the Axe-Fx was designed to give you the best of both worlds. You can use the FX Loop and Output 2 to a power amp and conventional guitar cab while routing the fully processed tone with IR to the FOH. See the manual for full details. Rather than using your amp you can use a lightweight solid-state power amp and any of the new, lightweight guitar cabs that use Neodymium speakers. This gives you the classic far field amp tone for yourself in a lightweight package and the polished sound for the FOH direct from Output 1." source

"When I was gigging I used a power amp and cab behind me and sent the XLR outputs to FOH. More gear to lug but best of both worlds: traditional backline sound, consistent FOH sound." source

Axe-Fx II and AX8 – choose between these methods:

enable ECHO OUT2 = OUT1 in I/O. This will duplicate the Output 1 signal to Output 2. The Global EQ on Output 2 lets you tailor the tone, independent of Output 1. The Output 1 knob controls the level of Output 1 and the Output 2 knob does the same with Output 2. This method is great when you want to control the level of your personal monitoring (Out2) separately from the signal that's being sent to FOH (Out1), but DOES NOT WORK with presets that contain a FXL block
insert an FXL block and make it part of the routing but don't connect it to the grid output. The signal before the FXL block will be sent to Output 2. This method is more flexible than the one above, because the position of the FXL block determines which part of the signal is being sent to Output 2. For example, placing FXL before or after a Cabinet block determines whether the Output 2 signal includes cabinet modeling or not. Use this when you want your FOH signal to be "direct" (including cab modeling) and your stage sound to come from a traditional cabinet (without cab modeling). Among the factory presets is a template. The Axe-Fx II lets you put FXL in series or parallel, but the AX8 requires FXL in a parallel row to prevent a feedback loop. Here's a tutorial
split the signal at the end of the grid into a row with a Cab block and a row with a shunt. In the Output Mixer pan those rows 100% left (Cab) and right (shunt). Now OUT1 Left is the signal with cabinet modeling, and OUT1 Right is the signal without cabinet modeling. This method allows you to use the stereo effects loop for other purposes. source

Axe-Fx III – choose between these methods:

enable ECHO OUT2 = OUT1, see above
use multiple Output blocks and signal chains to handle multiple outgoing signals
see the rig diagrams in Section 4 of the Owner's Manual

FM3 – to be added

Four Cable Method (4CM)

The Four Cable Method, or 4CM, is the common term for a rig that lets you run effects before the amp AND after the effects loop.

Axe-Fx III – see the rig diagram in Section 4 of the Owner's Manual. Use I/O 3 or I/O 4, turn the front panel output knob fully open for unity gain, adjust Boost/Pad in the I/O menu for optimal SNR, and adjust levels where needed.

FM3 — to be added.

Axe-Fx II – adjust Boost/Pad and Input Level in the I/O menu to optimize the signal. Also, turn the Output Level knob fully open for unity gain. You can't combine 4CM with cab modeling.

"The very early Axe-Fx II's had more bandwidth than necessary on Output 2. The frequency response extended to hundreds of kHz. When used with certain tube amps this would cause instability in the output drivers. The solution was to limit the bandwidth to a "normal" range of 20 to 20 kHz. We provided the update for free and all units shipped after the first 100 or so had this update included. The Axe-Fx II Mark II, XL and XL+ have a redesigned output circuit that is immune from any of these issues." source

"It is very difficult to minimize the hiss when putting a digital processor in front of a high-gain amp due to the A/D and D/A conversions. The XL is probably one of the quietest processors made but there will still be some residual hiss when using high gain. The Output 2 Boost/Pad feature was specifically intended to minimize hiss in these scenarios by running the D/A converter as "hot" as possible and then reducing the signal level after the converter with an analog pad." source

"The XL+ shares the same amazing low-noise architecture of the FX8. I regularly use my XL+ in 4CM as this is part of the modeling process. It's the quietest device I've ever tried in 4CM." source

"The only product more transparent than the FX-8 is the Axe-Fx III." source

AX8 – the AX8 is not optimized for 4CM, but it will work. The process is the same as with the Axe-Fx II.

Ola Englund's demonstrates 4CM with the AX8

FX8 – see below

Headphones

Axe-Fx II and III, FM3 – use the Output 1 knob to set the headphones level. If you have both monitors and headphones connected and want to listen through headphones only, use one of these methods:

switch off the monitors
feed the studio monitors through another output
use a line level attenuator to turn down the monitors' level without affecting the headphones level.

"The amp on the Mo-Fi's is designed for use with consumer stuff like iPhones and such with really low output drive strength. The headphone amp in the III is a pro-quality amp designed to drive low-impedance pro headphones. It's not designed for use with things like headphones with built-in amps and IEM amps. source

"The volume knob is virtual. It's read by an A/D converter and the data value controls the volume. The data value is quantized so you're hearing the step change." source

"Headphones are hardwired to Output 1." source

AX8 – the AX8 does not have a dedicated headphones output. You can use a Y-cable with the outputs. If you connect your headphones directly to the AX8, you may get less signal level than from a dedicated headphones output on other devices. A popular solution is to use a small headphones amp, e.g. from M-Audio or Rolls.

"It does not have a headphone output but the outputs should be able to drive phones with ease. You'd just need a Y-cable adapter." source

"The output impedance of the 1/4" outputs is 600 ohms IIRC. This may be too high for some headphones. We always use a small output impedance on our designs to help protect the output devices against improper connections, ESD, etc. The outputs were not really designed to drive headphones, they are designed to drive high-impedance inputs (>10K). Headphones will work but it won't be optimum. For optimum results use a dedicated headphone amp." source

If the volume level through your headphones is very low, switch to lower impedance headphones (such as 32 Ohm), use a headphone amp or use headphones with a built-in amplifier (such as Blue Mo-Fi). The Axe-Fx III is better at feeding sufficient signal level to hard-to-drive headphones than the Axe-Fx II.

Article about headphones and impedance

Sound through headphones can be dull:

"Because there's no string and body reinforcement. When you play through speakers the sound couples into the guitar body and strings. With headphones you don't get this so the sound is very sterile and lifeless. Now, if you use speakers during recording and then playback through headphones it will sound fine."

"It's lack of acoustic reinforcement. I did a test a few years ago and I don't remember the actual numbers but having a speaker aimed at the guitar adds many dBs of power to the lower mids coming out of the guitar. IOW, if you measure the spectrum of the signal coming out of a guitar alone and then compare that to the signal coming out with a cab or monitor in proximity at a reasonable volume there are a LOT more lower mids with the speaker present. This results in a "thin" sound without the speaker." source

"The problem with headphones is that there is no acoustic reinforcement of the guitar. There is zero coupling between the speakers (inside the headphones) and the guitar. Without that coupling, which is a type of positive feedback, the sound is lifeless, thin and harsh. When your heroes recorded their guitar parts that weren't using headphones. On "Appetite for Destruction" Slash recorded his guitar parts in the control room. To get even more coupling into the guitar a combo amp was in the control room with him pointed at the guitar. A volume pedal allowed him to adjust the volume of the combo amp so he could control the coupling. With the volume pedal all the way up he could get controlled feedback. I've actually done tests comparing the spectrum out of the guitar when there is no coupling (i.e. monitors turned off) and with typical coupling (monitors loud or using a conventional cab). The boost in the low midrange is significant. I forget the actual numbers but it was at least several dB." source

"I did some studies years ago and having a speaker in proximity to the guitar actually changes the final tone considerably. I compared the frequency response with the amp in isolation to the frequency response with the amp in proximity and measured several dB difference in the lows and mids. It was clearly audible when the recordings were played back." source

Tips for improving sound quality through headphones:

use a far-field Impulse response (IR)
increase Proximity in the Cab block
increase Room Level in the Cab block
always use a stereo input signal
add the Enhancer block to presets, or add Micro Delay in the Cab block
increase De-Phase in the Cab block
use Speaker Compression and Speaker Compliance in the Amp block

More information about headphones:

In Ear Monitoring (IEM)

In-Ear Monitoring (IEM) provides a way for musicians to monitor sound through earbuds, instead of floor wedges etc. While this provides a superior listening environment, it takes getting used to the direct sound into your ears and the absence of surround sounds.

The same tips as for headphones apply.

The I/O architecture of the Axe-Fx III makes it easy to send and receive IEM-specific signals, without the need for an external mixer.

WARNING: always use the built-in limiter of your IEM system to protect your ears against sudden spikes and peaks!

Inserting the Axe-Fx II, AX8, FX8 or FM3 in an Axe-Fx III effects loop

connect Output 3 on the Axe-Fx III to the input on the other device
connect the output of the other device to Input 3 on the Axe-Fx III
turn up Output 3 on the front panel of the Axe-Fx III
in the Axe-Fx III preset, connect IN1 to OUT3 and IN3 to OUT1
when IN3 is bypassed, it just passes the III signal
when IN3 is engaged, you'll hear the other device.

Axe-Fx or AX8 for effects only

POST effects

When using the Axe-Fx II or III, AX8 or FM3 as an effects-only device in an amp's effects loop, you probably want it to send and receive line level signals, at unity gain.

Use Input 2 and Output 2 on the Axe-Fx II, FM3 and AX8, and Input/Output 3 or 4 on the Axe-Fx III
Adjust Input Level for an optimal signal-to-noise ratio
Select the correct input and output settings in I/O
Set the Output 2 knob to its maximum position to enable unity gain
Test the setup by creating a preset with shunts only. The level should be the same as when leaving out the processor. Then start adding effect blocks (no Amp or Cab).

Axe-Fx III: see the rig diagram in Section 4 of the Owner's Manual

"You should NOT use Boost/Pad in this configuration." source

Some amps require inserting a dummy jack into the effects loop's Send to activate the effects loop.

PRE effects

Use Output 2 on the Axe-Fx II, FM3 and AX8, and Input/Output 3 or 4 on the Axe-Fx III
In I/O set output level to -10 dB if possible
Adjust Boost/Pad to make sure the full range of the D/A converter is used and turn up the Output knob all the way for unity gain
Test the setup by creating a preset with shunts only. The level should be the same as when leaving out the processor. Then start adding effect blocks (no Amp or Cab).

Axe-Fx III: see the rig diagram in Section 4 of the Owner's Manual.

Surround sound, quadraphonic sound

Use one stereo output for two front monitors. Use another for two rear monitors. Split the signal on the grid, and send it through 100% wet Reverb to the output feeding the rear monitors. Verify that both outputs are turned up on the front panel.

More information

IR loader

The Axe-Fx, FM3 and AX8 can be used as an IR loader.

Connect the guitar to an amp head
Connect the amp's speaker output to a load box, such as Fractal Audio's X-Load
Connect the loadbox to the AX8, FM3 or Axe-Fx
Use a preset with a Cab block (or the IR Player block on the Axe-Fx III or FM3), and without an Amp block.

Demonstrations:

Digital I/O

Digital I/O: available on which products

Axe-Fx III – USB Audio, SPDIF, AES
FM3 – USB Audio, SPDIF output only
Axe-Fx II – USB Audio, SPDIF, AES
AX8 – SPDIF output only
FX8 – no

To connect the SPDIF port to an optical port, a SPDIF-to-TOSLINK adapter is required.

Why use digital I/O

A digital connection skips the analog/digital conversion stages.

The analog outputs of Fractal Audio's modelers also deliver high-quality audio.

Sample rate fixed at 48kHz

The sample rate of the Axe-Fx series, FM3, FX8 and AX8 is fixed at 48kHz (24-bit).

Digitally connected devices and DAW software always need to be set to the same sample rate. Example

If required, resampling can be handled by software.

"The Axe-FX uses higher sampling rates (oversampling) during the processing stages. This is how it avoids aliasing when non-linearities are applied. But the sampling rate of the audio that is sent to the DAC is the same as the sampling rate coming out of the SPDIF output: 48khz. In other words, it goes from 48khz (ADC) -> higher sampling rate -> 48khz (DAC). So just because these higher sampling rates are used for the processing stages doesn't mean it would be trivial to send a higher rate to the SPDIF output. The 48khz signal would need to be sample rate converted (SRC) at the output stage by a hardware SRC chip and Cliff's whole point is that software SRC's provide better quality than what is available with hardware SRC's."

"IMHO, the ideal sample rate is 64 kHz but that's not a standard. The nice thing about 64 kHz is that you can have a gentle transition band from 20 kHz to Nyquist which results in shorter filters, lower latency, less phase shift, etc. I was very tempted to make the Axe-Fx II run at 64 kHz but people probably would have freaked out." source

"I've long maintained that 64 kHz is the ideal sample rate for audio. But I can't get the industry to change." source

"48 kHz is considered "pro" sampling rate. The reason for 44.1 kHz on CD's is subject to debate. Some maintain that the sample rate was lowered so that Beethoven's 9th would fit on a single CD. Others claim that it was because that rate was compatible with video equipment. IMO 44.1 kHz is insufficient for professional audio. Personally I would prefer 64 kHz. Whilst Nyquist theorem is all well and good most people don't understand the details and simply state "the sample rate must be twice the highest desired frequency". The problem with this is as you approach Nyquist the filter demands become extreme. The more extreme the filter demands the more taps are needed, the more precision is needed, the more latency is incurred, etc. A 64 kHz sample rate would give you a nice, smooth roll-off from 20 kHz to 32 kHz rather than the brick wall you get with 44.1 kHz. There is no hardware advantage to using 48 vs. 44.1. The costs would be the same in either case. Modern converters use over-sampling techniques to implement the necessary anti-aliasing filters thereby reducing off-chip filtering to simple circuits. MP3s have no native sample rate but are typically 44.1 kHz because they are usually derived from CDs. MP3 is a psycho-acoustic compression format that exploits frequency masking to lower the data required to store audio information." source

"If the Axe-Fx were running at 44.1 all the cab IRs would need to be resampled, or there would need to be an SRC chip on the digital I/O. There is no free lunch. The problem isn't the Axe-Fx, the problem is studios stubbornly sticking to 44.1 when 48 is a much better rate." source

"1. 44.1 or 48 KHz is more than adequate for not only guitar processors but ANY audio processor. 88.2 or 96 K makes for nice marketing but, in reality, performance can often decrease when running converters higher than necessary. This is due to activity at the converters digital I/O pins injecting noise into the converters themselves. Personally I wish the industry would adopt a 64 KHz sample rate standard but this is for esoteric reasons.
2. The dynamic range of a guitar, UNDER IDEAL CONDITIONS (i.e. inside a Faraday cage) is not much greater than 100 dB. To capture this you would theoretically need 17 bits (17 bits gives about 102 dB). To allow sufficient "overhead" one should add a couple bits. 20 bits is plenty and yields about 120 dB of dynamic range. Anything greater than 20 bits is marketing. There isn't a converter made that gets much better than 100 - 120 dB dynamic range in the real world. You only need 20 bits for that. AKM has these new 32 bit converters (AKM557x). This is comical as they only have 112 dB of dynamic range so they give 19 bits of data and 13 bits of noise. Once you put a guitar in a real-world EMI environment that dynamic range drops precipitously (60 dB or even less). The ANALOG electronics before and after the converters is far more important. Knowing when to use JFET vs. bipolar op-amps, knowing how to select the right op-amp for the task, etc. far outweigh the sampling rate and advertised bit depth of a converter. Good quality components aren't cheap though.
Internal oversampling determines aliasing performance in nonlinear processing. The higher the oversampling, the lower the aliasing but the more processing power required (= $$$$). Aliasing noise can easily dominate output dynamic range. So, again, sampling rate and bit depth are immaterial in comparison to the things that really matter." source

SPDIF and AES

Axe-Fx II and II — The AES and SPDIF I/O ports can not be used simultaneously. Select the one you want in the I/O menu. The Axe-Fx III lets you set the level of the AES or SPDIF signal can be set in the I/O menu.

How to configure your Axe-Fx II for SPDIF

FM3 — The FM3 supports S/PDIF output only.

AX8 — The AX8 supports SPDIF output only. The strength of the SPDIF signal level depends on the position of the front panel output knob (unlike the Axe-Fx II).

(AX8) "The SPDIF is a digital representation of OUTPUT 1." source

This discussion lists AX8-compatible SPDIF interfaces

forum discussion about AES and Word Clock

USB

Read this: USB

Master or slave

The Axe-Fx II can be either master or slave when using digital I/O.

"The Axe-Fx II can be a slave. Set the Input Source to AES. It will derive its internal clock from the input stream. The input stream must be 48K. Note that SOMETHING must be the master in this case." source source

Word Clock

The clock source for the A/D and D/A converters is either AUTO/INTERNAL or SPDIF/AES.

Axe-Fx III – Word Clock is recovered from the SPDIF/AES input signal.

"Yes via SPDIF/AES in (which actually works better as a word clock than a word clock input)." source

AX8 and FX8 – not supported.

Axe-Fx II — choose between:

Auto – uses the internal clock if the input source is Analog or USB, uses the recovered SPDIF/AES clock if the input is SPDIF/AES.
SPDIF/AES IN – uses the recovered clock for all input sources. A valid 48 kHz data stream must be present at the AES or SPDIF input. If a valid stream is not detected, the unit will fall back to the internal clock and display "NO INPUT CLOCK!". The SPDIF/AES select must be set to the appropriate value, i.e. if the data stream is input to the XLR jack then SPDIF/AES SELECT must be set to AES.

"Set Word Clock to SPDIF/AES In. Connect a cable from the ULN-8 to AES In or SPDIF In. Set SPDIF/AES Select to appropriate input used." source

"The Axe-Fx II will derive its clock from the AES/SPDIF when using Digital In. In Analog In it uses its internal clock." source

Use the modeler for A/D conversion only

To use the device as an analog-to-digital converter:

create a preset with nothing but shunts from input to output.
or: set Input 1 Left Select to to Rear and plug the device into Input 1 Left on the back (Axe-Fx II).
or: connect In and Out and engage Bypass Mode (Axe-Fx II). source

FX8

As a pedalboard (PRE effects)

Guitar goes into IN [PRE] / INSTR. Note: this input only feeds effect blocks designated as PRE.
OUT [PRE] LEFT goes into the amplifier's guitar input. Use a Humbuster cable to prevent noise.
You can use default FX8 settings. Exception: change the output mode in I/O > Audio (see manual for stereo operation)to Mono (see manual for stereo operation).

You can also use this setup to connect an FX8 to the Axe-Fx.

More information in the Owner's Manual, including a description of the cables required.

In amplifier's effects loop (POST effects)

Guitar goes straight into the amplifier.
Amp's effects loop SEND goes into IN [POST] LEFT.
Amp's effects loop RETURN goes into OUT [POST] LEFT. Use a Humbuster cable to prevent noise.
You can use default FX8 settings. Exceptions:
1. Change the output mode in I/O > Audio to Mono (see manual for stereo operation).
2. Change Global Looper Location to OUT POST.
3. Change Global Detector to IN [POST].

The outputs are buffered for long cable runs.

More information in the Owner's Manual, including a description of the cables required.

Four Cable Method (4CM)

The FX8 can be set up to put effects before the amp as well as in the amp's effects loop.

Guitar goes into IN [PRE] / INSTR.
OUT [PRE] LEFT goes into the amp's guitar's input. Use a Humbuster cable to prevent noise.
The amp's effects loop SEND goes into IN [POST] LEFT.
The amp's effects loop RETURN goes into OUT [POST] LEFT. Use a Humbuster cable to prevent noise.
You can use default FX8 settings. Exceptions:
1. Change the output mode in I/O > Audio to Mono (see manual for stereo operation).
2. Change Global Looper Location to OUT POST.

The outputs are buffered for long cable runs.

If you have the FX8 set up for 4CM and want to change this, for example to put the FX8 before a computer, just use a jumper cable to connect OUT PRE L MONO to IN POST L, with OUT POST L going to the computer, amp or whatever. All effects will work and there's no need to change stuff in the configuration.

If you're using the FX8 in a 4CM setup and you're experiencing hiss, try another Post Level value.

More information in the Owner's Manual, including a description of the cables required.

Combine the FX8 with an Axe-Fx or AX8

You can use the FX8 for "pre" effects (plug guitar into FX8) and the Axe-Fx or AX8 for post-effects, including amp and cabinet modeling (plug FX8 into Axe-Fx or AX8). By adding a MIDI connection you can change Axe-Fx and AX8 presets from the FX8.

Relays: switch amp channels and more

CAUTION: Do NOT connect anything to the relays jacks until you've read the warnings in the manual!

What are relays? Relays are electrically operated switches/connectors, which can be used to switch channels on an amplifier and switch other stuff.

How many relays does the FX8 have? The FX8 has two relays.

How can I control these relays? These are controlled through:

Scenes: you can use scenes to switch amp channels through relays. This is configured on the preset's Config page.
Footswitches: you can assign footswitches to the relays per preset, for manual control. Assign the footswitch and configure it on the Footswitch page.

IMPORTANT: a Relay block in the preset will disable the scene's Relay settings.

Do the relays support X/Y switching? The relays support X/Y switching.

What are the possible settings? The relay states are:

Off: nothing connected.
Tip: tip to Sleeve.
Ring: ring to sleeve.
Both: tip AND ring to sleeve.

What are the switch modes of the relays? The switch modes of the relays are:

Latching: the selected RELAY ON state remains connected and the switch LED remains ON as long as the switch is engaged. Nothing is connected when the switch is OFF.
Auto-Off: the selected RELAY ON state remains connected only for a moment when you press the footswitch. The relay then automatically turns OFF, as does the LED.

Which cables can be used? Depending on the amp, you can use TS or TRS cables.

"The FX8 will short tip-to-sleeve, ring-to-sleeve, or both. The circuit is designed to handle 200mA of current. If the current generated by that voltage drop is 200mA or less, then the FX8 will not have a problem." source

"The relays of the FX8 are designed for use ONLY with amplifiers that use “short-to-sleeve” type switching. Do NOT connect the FX8 relays to the switch jacks of an amp that uses voltage differential switching or any other type of switching aside from short-to-sleeve, or serious damage can occur to both units. If you are not 100% sure, contact your amp manufacturer to determine whether your amp is compatible with short-to-sleeve switching. The FX8 relay jacks are compatible with TRS cables, TS cables, or TRS-to-dual-TS split cables. The relays are also fully isolated from the electrical ground of the FX8."

"The FX8 features two TRS (Tip-Ring-Sleeve) relays that can be used to switch the channel or other functions of a connected amplifier or device. If the warning above seems stern, that’s because the last thing we want is for anyone to damage their amp or FX8. In fact, short-to-sleeve relay switched amps are quite common, and your amp may well be perfectly compatible. We need to trust and require you however, to understand how your amp works and make the right choices about connecting it to the FX8 relay jacks. Your amp manufacturer should be able to help if you read them the warning above."

"The FX8 relay outputs employ a "Short-to-Sleeve" connection. Each relay output can short Tip-to-Sleeve, Ring-to-Sleeve, and Both. If the pedal connection uses a voltage drop to power an LED, the relay circuit on the FX8 is rated for a maximum of 200mA." source

I/O connectivity and levels

Contents

Audio jargon

Level: line, instrument, microphone

Balanced and unbalanced signal

Unity gain

Decibels

Mono and stereo

I/O hardware specifications

Axe-Fx III

FM3

Axe-Fx II

AX8

FX8

Using output ports

Output 1

Output 2

I/O 3 and I/O 4

Testing an output port

Setting levels

Main input level

Main output level

Preset level

Global menus

Connecting instruments and other devices

Instrument input

Secret Sauce on the instrument input

Input Impedance

Input 2

Input 3 and Input 4

Different guitars

Multiple instruments simultaneously

Acoustic instrument

Bass guitar

External effects

Wireless receiver

Microphone

Play sound from computer or other device

Sustain and feedback

Connecting controllers

Setups

Introduction to amplification

Full Range Flat Response (FRFR)

Why use FRFR monitoring

Which systems are FRFR

Close-miking

Fletcher-Munson

Fighting extended frequencies and Fletcher-Munson

Optimize the Amp block's Output Mode

FRFR and amp/cab-in-the-room

Do not put a microphone in front of a FRFR speaker

Tweeter squeal from FRFR speakers

Power amp and guitar speaker

Why use a power amp and guitar speaker

Using a tube power amp for guitar (or head or combo)

Using a neutral tube power amp

Using a solid-state power amp

Finding the resonant frequency with a solid-state amplifier

Gain-staging a power amp

Wet/Dry/Wet rig

About speaker wire

Combining FRFR and traditional backline

Four Cable Method (4CM)

Headphones

In Ear Monitoring (IEM)

Inserting the Axe-Fx II, AX8, FX8 or FM3 in an Axe-Fx III effects loop

Axe-Fx or AX8 for effects only

POST effects

PRE effects

Surround sound, quadraphonic sound

IR loader

Digital I/O

Digital I/O: available on which products

Why use digital I/O

Sample rate fixed at 48kHz

SPDIF and AES

USB

Master or slave

Word Clock

Use the modeler for A/D conversion only