I/O connections and levels

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The information on this page supplements the official manuals.



List of I/O connections

Axe-Fx II

Axe-Fx XL Plus rear.png





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

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:

Input level

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:

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:

Fractal Audio (about the Axe-Fx II):

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.

Read this: Digital audio.


Hardware: Axe-Fx II.

Read this: 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.

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.


Multiple instruments into Axe-Fx II or AX8

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.

To use the AX8 with 3 devices: guitar, and two other devices such as piezo or synth:

  1. Add an Amp and Cab.
  2. Put the FXL 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.
  3. Split the signal after the FXL block into two rows, and add a Volume block to each one.
  4. Set Input Select in one Volume block to Right Only. Set Pan and Balance as desired.
  5. Set Input Select in the other Volume block to Left Only. Set Pan and Balance as desired.

More information:

Accommodating different guitars

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

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:

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.



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.

Alternatively insert the pedal in the effects loop (Effects Send/Return). This requires adding the FXL block on the Axe-Fx II and AX8. Make sure to adjust the levels in the Fractal Audio device where needed. You can include/exclude the pedal per scene, through enabling or bypassing FXL per scene. Or you can turn the effects loop into an audio switcher, by assigning a switch to FXL.

A pedal can also be placed after the AX8 and FX8. Again, make sure to adjust the levels where needed.

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:

About instrument level and line level

Read this: 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:

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):

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 cabinet or monitor on stage, with or without cabinet modeling.


Route Input 2 to Output 2

Read this: FXL block (Effects Send/Return).

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:

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

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:

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

The Relay states are:

The Relay modes are:

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 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 Level in the I/O menu to optimize the signal. Boost/Pad helps to optimize output levels and reduce noise 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.

You can't combine 4CM with (internal) cab modeling.


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:

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