I/O connectivity and levels

The information on this page supplements the official manuals.

List of I/O connections

Axe-Fx II

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

AX8

• IN 1 (INSTRUMENT): 1/4", mono, unbalanced, 1 Megaohm (fixed), max. 16 dBu
• OUT 1 (MAIN): XLR, L/R, balanced, 600 ohm, max.20 dBu
• OUT 1 (MAIN): 1/4", L/R, unbalanced, Humbuster, 600 ohm, max. 20 dBu
• IN 2 (FX RTN): 1/4", L/R, balanced, 1 Megaohm, max. 20 dBu
• OUT 2 (FX SEND): 1/4", L/R, unbalanced, Humbuster, 600 ohm, max. 20 dBu
• 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 16 dBu
• OUT [PRE]: 1/4", L/R, (L/Mono), unbalanced, Humbuster, 600 ohm, max 20 dBu
• IN [POST]: 1/4", L/R, line level input (+4 dBu), 1 Megaohm, balanced, max 20 dBu
• OUT [POST]: 1/4", L/R, unbalanced, HumBuster, 600 ohm, max 20 dBu

Simultaneous use of XLR and 1/4" outputs

The XLR outputs and 1/4" jack outputs on the Axe-Fx II and AX8 can be used simultaneously to feed different devices.

Fractal Audio: "Both outputs should work simultaneously. They are actually buffered so even if you shorted one it shouldn't affect the other." source

Instrument input

• 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 ("Secret Sauce"). It's conditioned for guitar ("Secret Sauce") through hardware and software. For best results use the instrument input for guitar, whether wired or wireless, electric or acoustic, except when running a line level signal. It has a soft clip function (IN 1 CLIP).

The AX8, Axe-Fx II XL and XL+ feature “Secret Sauce III”. Fractal Audio: "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

Fractal Audio:
"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."

Fractal Audio:

• (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."
• (Axe-Fx II) "A buffer (in front of the Axe-Fx II) will render the impedance stuff ineffective. It will also add (maybe considerable) noise which may defeat the low-noise advantage of the front input." source
• "The XL+ front input circuit is identical to the AX-8 except the AX-8 doesn't have the variable impedance circuitry." source

Input level

• 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

Input Level and Pad do NOT affect the overall volume level, output clipping or amp gain. They just optimize 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.

The range of the instrument Pad is 0 - 18 dB (default: 12). Set it to make the red input LED blink occasionally while strumming (strum hard, on the loudest pickup). Then adjust it so that it blinks only occasionally ("tickle the red"). A higher parameter value means a lower input (padding)! Run it as low as possible because padding increases noise floor. There's quite a range between orange and red. If you can't make it blink red, don't worry (Axe-Fx II users: make sure that Input 1 Mode is set to Left Only).

The red light turns on BEFORE the front input clips. It means "Warning, you're approaching clipping" as opposed to "Warning, you ARE clipping". Red light turns on at -6dB of the point where the signal is hard limited / clipped.

Fractal Audio:

• (Axe-Fx II) "For a Strat, near 100% on the input level is not unusual. I run my Strat around there. It has vintage-type pickups." source
• (Axe-Fx II) "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
• (Axe-Fx II) "You don't HAVE to tickle the reds. Adjust for your hottest guitar and leave it." source
• (Axe-Fx II) "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
• (Axe-Fx II) "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

Here's a tutorial by AxeFxTutorials.

When connecting a mono signal to Input 2, set Input 2 Mode to Left Only (default). Select Sum L+R or Stereo when connecting a stereo instrument.

Output level

Output levels on the AX8 and Axe-Fx II are controlled with the OUT 1 and OUT 2 knobs on the panel (and with the Global EQ, I/O parameters and blocks on the grid).

Fractal Audio:

• (about the Levels 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)

Fractal Audio (about the 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
• "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
• "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 Ouput X Boost/Pad feature would be impossible without digital pots." source
• "Flutter noise when turning the level knobs is normal. 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

MIDI CCs 11 and 12 control the levels of Output1 and Output2. To reset them without the help of a MIDI controller, change the assignment to "none" in I/O.

Fractal Audio:
"It's inevitable that there will be mismatch in the L/R levels. The output levels are controlled by a stereo potentiometer. The tracking between the two resistors will never be perfect and gets worse as you get near the start of the curve (fully CCW). It's impossible to fix in software as the mismatch will vary from unit to unit. Also, you can't simply put a global balance control because the mismatch varies with the level knob position. The good news is this is one reason why they put faders on mixing boards. I'd hardly consider it a problem or a flaw. Simply compensate as necessary."

I/O parameters (all hardware)

Main Input Source

Hardware: Axe-Fx II.

This lets you set select the input source: Analog (this is Input 1, default), SPDIF/AES or USB. This is useful for reamping and other applications.

Word Clock

Hardware: Axe-Fx II.

Digital audio

SPDIF/AES Select

Hardware: Axe-Fx II.

Digital audio

Input 1 Left Select

Hardware: Axe-Fx II.

Input 1 is split: it appears at the front (mono) as well as on the rear (mono or stereo). This parameter tells the Axe-Fx II if the front input or the rear input is used to connect the instrument. Default: front.

The rear port has the same impedance as the front input (1 Mohm, for guitars) but it operates at LINE level instead of instrument level.

The front input does not disable Input 1 left at the rear. Use the menu to select either the front input or rear input. You can leave everything plugged in.

Fractal Audio:
"Input 1 on the rear is a very high impedance input (1 Mohm). It is compatible with low impedance outputs. Most people don't understand the real meaning of impedance and think you need to connect low impedance to low impedance but that's only with passive devices and is a relic of the old days when transformers were used to get the best power transfer. Nowadays we have active inputs with very high impedance which are compatible with a broad range of source impedances." source

Input Mode

Hardware: Axe-Fx II, AX8, FX8

Axe-Fx II: when connecting a guitar to the front instrument input, set this parameter to Left Only (default). Using Sum L+R can introduce noise (from the disconnected right Input 1) and attenuates the signal level (6 dB). You'd only use Sum L+R or Stereo when connecting a stereo instrument to Input 1 (rear).

Input 2 Mode controls the same for Input 2 on the Axe-Fx II, AX8 and FX8.

IN and OUT Nominal Level, Post Level

Hardware: AX8, FX8

These parameters can be switched between +4 dB (pro line level) and -10 dB (consumer-grade equipment).

IN 2 Nominal Level: use -10 dB (default) when this input connects to the output of a guitar or stomp box. Use +4 dB when it connects to the output of a pro level device.

OUT 2 (Main) Nominal Level: use +4 dB (default) when the output connects to a mixing console or line level input on another device. Use -10 dB when the output connects to a traditional amplifier or pedal.

Post Level

Hardware: FX8.

This switches between +4 dB (pro line level, default) and -10 dB (consumer-grade equipment). Use +4 dB when the unit connects to a line level on another device. Use -10 dB when the unit connects to a guitar amplifier's instrument input or a pedal.

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

NOTE: some users report that the FX8 needs a reboot after changing the value before the changed value takes effect. source

Output Mode

Hardware: Axe-Fx II, AX8, FX8

This determines if the output signal is stereo, or one of two mono modes: Sum L+R or Copy L>R. Out 2 and Out 2 can be set to different modes.

Output Phase

Hardware: Axe-Fx II, AX8.

This lets you change the phase of the signal if necessary.

Output Boost/Pad

Hardware: Axe-Fx II, AX8.

The Boost/Pad parameter optimizes the output level and reduces noise in certain scenarios, such as a 4CM setup or when placing the device in front of a traditional amp. Its range is 0 - 18 dB.

When putting the Axe-Fx II in front of a real amp, maximize Boost/Pad to make sure the full range of the D/A converter is used.

With 4CM: increase Boost/Pad, and prevent clipping. Turn Output 2 on the front panel fully open. Then use Input 2 Level in I/O to finetune the signal level.

Javajunkie: "Boost/pad is not intended for boosting USB output (although it does have that effect). It is for the four cable method (4CM)".

Fractal Audio:
"Usually you will use this when you send the effect loop out before the amp block. The signal will be weak going to the D/A converters. This allows you to boost the signal to the D/A converters w/ maintaining the same output level." source

Output 2 Echo

Hardware: Axe-Fx II, AX8

This determines what signal is sent through Output 2.

• None: no signal.
• Output 1: an exact copy of the Output 1 signal.
• Input 1: an exact copy of the direct input signal (unprocessed).

Output 1 (PRE) Headroom

Hardware: FX8.

This parameter can supply more headroom if the signal through Output 1 PRE is too hot. Downside is that the noise floor is increased. Default is 6 dB, for the least headroom and lowest noise floor. The increased headroom is offset by a corresponding but opposite adjustment internally, so “what you hear” remains the same at all settings. Maximum headroom is 18 dB.

USB/Digi Out Source

Hardware: Axe-Fx II

This lets you determine what is sent out through USB Audio/AES/SPDIF: Output 1, Output 2, or Input (which is the unprocessed DI signal).

USB Buffer Size

Hardware: Axe-Fx II

Defaults at 1024. The buffer can be monitored in Utility > Status.

USB Return Level

Hardware: Axe-Fx II

A level control for the incoming USB Audio signal.

Instruments

Multiple instruments into Axe-Fx II

To set up the Axe-Fx II for use with two instruments:

1. Set Input 1 to Stereo.
2. Connect one instrument to the front, and the other to rear Input 1 Right.
3. Use two rows on the grid.
4. 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).
5. 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.
6. 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.

More information:

• forum thread
• forum thread
• forum thread
• connect 3 instruments to a single Axe-Fx
• Youtube tutorial
• tutorial by AxeFxTutorials

Accommodating different guitars

A preset may require adjustment when changing guitars. Here are methods to deal with this.

• IN/GTE > LEVEL: this parameter controls the loudness of the signal entering the grid. It was specifically introduced for this purpose: compensating output level differences between guitars. It works per preset only, so it needs adjustment per preset, unless the setting is stored as part of a global block. No possibility to attach a modifier.
• AMP X/Y: use different values for Input Trim in X and Y.
• Two AMP blocks.
• SCENES and SCENE CONTROLLERS: attach Input Trim in the Amp block to a Scene Controller. In the Controllers menu set the controller to the desired value for each scene. Switch scenes when switching between guitars.
• GLOBAL AMP GAIN.
• BLOCK: 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.

Acoustic instruments

The front input can be used to connect an acoustic instrument.

The IR of an acoustic body can add acoustic resonance to the tone. There are no acoustic IRs among the stock cabinets in the Axe-Fx II. You can find some here:

• Acoustic guitar body
• Banjo

Use Ultra-Res IRs when using acoustic IRs.

A preset for an acoustic guitar with good pickups can be kept simple: compressor, EQ, reverb. An AMP block is not required but the TUBE PRE model can help to warm up the tone.

This thread contains a nice clip and preset for a piezo-equipped guitar.

Alternatively: use Tone Matching. Tutorials on G66.eu.

Bass guitar

The Axe-Fx II provides stock bass cabs and some bass amp models. The tuner supports bass guitar tuning.

Forums:

• Bass forum
• interesting thread
• discussion at TalkBass
• tutorial by Sonic Drive Studio

Pedals

If you want to use an analog or digital pedal with the Axe-Fx II or AX8 (configured for amp and cab modeling), connect it between the guitar and the device. Remember to check the input impedance on the Axe-Fx II and FX8.

All Fractal Audio devices let you insert an effect device in their effect loops. The effect loops operate at line level.

A pedal can also be placed after the AX8 and FX8 when desired.

Wireless connections

Connect a wireless receiver in the same way as a guitar: to the instrument input. This takes advantage of that input's "secret sauce" for noise reduction. The other inputs operate at line level and are less suited for use with most wireless receivers.

Fractal Audio: "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

Sustain and feedback

It's as easy to get your guitar to feedback with the Axe-Fx II or AX8 as it is with a regular amp and cabinet (with the exception of headphones). If you don't succeed, experiment with the Output Phase parameter in the I/O menu.

Fractal Audio: "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

There's no dedicated "sustainer" or feedback effect in the Axe-Fx II. Forum member Simeon created a feedback simulation preset, mimicking controlled feedback.

Gain staging on the Axe-Fx II

Fractal Audio:

• "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 +20 dBu. 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, +20 dBu was routine. Everything could put out and handle +20. Not so much anymore." source
• "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

About instrument level and line level

Instrument and line level

If you connect a microphone level source to Input 1 or Input 2 at the rear, there will be a level mismatch. You'll need to increase the signal level to line level to get sufficient signal strength, using a DI or microphone preamp.

Both XLR and 1/4" jack cables can be used to connect the Axe-Fx II's outputs to other equipment. These are line level connections.

Fractal Audio: "All outputs are +4 dBu. Please see the manual for full I/O specifications." source

No DI needed on outputs

There's NO NEED to use a DI-box to connect the Axe-Fx II or AX8 to a mixing board directly (with amp and cab modeling).

Connect the device to a Line Level input on the board, when available. The output signal, balanced or unbalanced, is too hot for a MIC input.

Fractal Audio: "The XLR output is balanced but it's +4 dBu 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."

If only MIC inputs are available on the board, do this:

• decrease the output level from the device, f.e. by turning the Global EQ gain down or by turning down the Output knob at the front (9 o'clock or lower).
• use a DI box to change the line level signal to mic level. Use a stereo DI when using left and right outputs.
• use a pad switch on the mixer to attenuate the signal and prevent clipping.

Protection against phantom power

Fractal Audio: "There are blocking capacitors on the XLR outputs (Axe-Fx II) so you should be fine." source

Unity gain

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

To obtain unity gain when using the effects loop on the Axe-Fx II and AX8, set both Output knobs to maximum. If you would then fill the grid with shunts, you'll get exactly the same signal which you put in.

Fractal Audio: "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

Unity gain is useful only when using the Axe-Fx II or AX8 in a 4CM rig, or when using it as an effects-only device.

Fractal Audio: "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. Unity gain mode is only desirable for 4-cable-method." source

About the FX8 and unity gain (FX8 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.

Effects Send/Return (In 2, Out 2, FXL block)

Read this: FXL_block (Effects Send/Return)

Send multiple output signals

The Axe-Fx II and AX8 let you send multiple output signals. For example, Output 1 is stereo and goes directly into a mixing board (with cabinet modeling), while Output 2 is mono and feeds a power amp and a guitar cabinet (without cabinet modeling).

Methods:

1. Echo Out2 = Out1: enabling this setting in I/O will duplicate the Output 1 signal through Output 2. The Output 2 EQ (Global menu) lets you tailor the tone (independent of Output 1). The Output 1 knob at the front controls the level of Output 1, the Output 2 knob does the same with Output 2. This method DOES NOT WORK with presets that contain a FXL block (effects loop).
2. FXL block: insert a FXL block and make it part of the routing (in series or parallel) but don't connect it to the grid output. The signal before the FXL block is 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. For example, placing FXL before or after a Cabinet block determines whether the Output 2 signal includes cabinet modeling or not.
   Among the last presets in Bank C is a template for sending a signal to FOH (with cab simulation) and a signal to an external amp and cabinet (no cab simulation).
3. Left and Right: split the signal at the end of the grid into a row with Cab 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. source

Here's a tutorial by AxeFxTutorials.

Route Input 2 to Output 2

How to send a signal from Input 2 to Output 2.

Set up the FX8

As a virtual pedalboard (PRE)

1. Guitar goes into IN [PRE] / INSTR. Note: this input only feeds effect blocks designated as PRE.
2. OUT [PRE] LEFT goes into the amplifier's guitar input. Use a Humbuster cable to prevent noise.
3. 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 use this setup also to connect an FX8 to the Axe-Fx II.

More information in the manual, including a description of the cables required.

As an effects device in an amplifier's effects loop (POST)

1. Guitar goes straight into the amplifier.
2. Amp's effects loop SEND goes into IN [POST] LEFT.
3. Amp's effects loop RETURN goes into OUT [POST] LEFT. Use a Humbuster cable to prevent noise.
4. 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].

Fractal Audio:

• "Are the outputs buffered for long cable runs? Yes." source

More information in the manual, including a description of the cables required.

4CM (PRE and POST)

The Four Cable Method (4CM). You use 4CM to be able to put effects before the amp as well as in the amp's effects loop.

1. Guitar goes into IN [PRE] / INSTR.
2. OUT [PRE] LEFT goes into the amp's guitar's input. Use a Humbuster cable to prevent noise.
3. The amp's effects loop SEND goes into IN [POST] LEFT.
4. The amp's effects loop RETURN goes into OUT [POST] LEFT. Use a Humbuster cable to prevent noise.
5. 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.

Fractal Audio: "Are the outputs buffered for long cable runs? Yes." source

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 switching to another Post Level value.

More information in the manual, including a description of the cables required.

Connect to an Axe-Fx II or AX8

• You can use the FX8 for PRE effects (plug guitar into FX8) and the Axe-Fx II 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 II and AX8 presets from the FX8.

Relays on FX8

Relays are electrically operated switches/connectors, which can be used to switch channels on an amplifier or switch other stuff. The FX8 relays 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.

The Relay states are:

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

The Relay modes 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.

Depending on the amp, you can use TS or TRS cables.

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

Fractal Audio:

• "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 relays support X/Y switching.

4CM with Axe-Fx and AX8

Unlike the Axe-Fx II and FX8, the AX8 is not optimized for 4CM, but it will work.

When setting up 4CM, adjust Boost/Pad and Input 2 Nominal Level in the I/O menu to optimize the signal. Boost/Pad helps to optimize output levels and reduce noisec in a 4CM setup or when inserting an external device. Increase its level, making sure to prevent clipping. Also, turn Output 2 on the front panel fully open. Then use Input 2 Level in I/O to fine tune the signal level.

Here's a demonstration.

Axe-Fx II and AX8 as effects-only device

In the effects loop of an amp

When using the Axe-Fx II or AX8 as an external effects-only device in the effects loop of an amp, you probably want it to send and receive LINE level signals. So use the input(s) at the rear, not Instrument. When using Input 1 (rear) on the Axe-Fx II, set Input 1 Select to Rear in I/O.

1. Adjust I/O > Input Level for an optimal signal-to-noise ratio.
2. Select the correct output mode in I/O.
3. Use the physical output knob to get the required output level.

Fractal Audio: "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.

As a pedalboard in front of an amp

Set Boost/Pad to 12 dB (to make sure the full range of the D/A converter is used) and turn up Output 1 Level all the way.

Create presets for effects only

To make a signal go through the Axe-Fx II or AX8, connect the grid input to output using shunts. Replace the shunts with the effect blocks you need.Do not add Amp blocks or Cabinet blocks. The TUBE PRE amp type may be an exception to warm up tones. And in some cases an user cab can be useful too, as a container for special IRs (i.e. Echoplex preamp coloring).

Noise and hiss

When nothing is connected to the inputs, there may be some hiss.

4CM can cause because of ground loops. Humbuster cables help to prevent this.

Fractal Audio:

• "As soon as you put a power supply inside a product you are required by law to have a safety ground from the chassis to the third wire of the power cable. This can cause a ground loop. The GSP 1101 uses an internal supply and would be subject to ground loops depending upon the application. In the old days products only used 2-wire power cables and if a conductor shorted to the chassis it wouldn't necessarily blow a fuse and you could get electrocuted. Connecting the chassis to ground ensures that the circuit breaker will trip in the event that one of the power conductors shorts to the chassis. A wall-wart isolates the power supply outside the chassis inside a plastic housing which means that there is no requirement for the chassis to be connected to earth ground. The downsides of wall-warts are well known though. In certain applications (i.e. Hi-Fi) you can use a double-insulated power supply with no chassis safety ground but that is not permitted for "professional installations". With products that use internal power supplies if you encounter a ground loop you then need to either use isolation transformers or "cheater plugs" (which are dangerous). The FX8 eliminates the need for these by using a simple, inexpensive cable. Some products have dedicated ground lift switches but you are required to have at least one switch grounded and if you don't you are inviting disaster (blown output drivers, etc.)." 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 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
• "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