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Input block

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Available on which products

  • Axe-Fx III: 5 blocks (incl. Input USB)
  • FM3: 2 blocks
  • FM9: 4 blocks (incl. Input USB)
  • Axe-Fx II: 1 block
  • AX8: 1 block
  • FX8: no

Channels or X/Y

  • Axe-Fx III and FM9 and FM3: 4 channels
  • Axe-Fx II: no
  • AX8: no

About the Input block

The Input block is the point where the signal enters the grid. Fractal Audio devices provide multiple inputs.

The Input block also provides these functions:

On the Axe-Fx III, FM9 and FM3, an Input block can also operate as Effects Return in an effects loop. See below.

Input block position on the grid

Axe-Fx II and AX8 — The Input block is fixed at the start of the grid

Axe-Fx III, FM9 and FM3 — The Input blocks can be positioned anywhere on the grid

Input sources

The Input blocks are by default linked to the hardware analog input ports.

On the Axe-Fx III, FM9 and FM3, inputs can be linked to digital input. Read this: I/O menu

The Axe-Fx III and FM9 have an extra block: Input USB block. It gets its signal from USB Outputs 7+8 from a computer. It cab be placed anywhere on the grid.

The Multiplexer block can connect directly to a physical input port, without needing an Input block inbetween.

Noise gate in the Input block

Each Input block has its own noise gate. Read this: Noise gate

Create an effects loop

On the Axe-Fx III, FM9 and FM3, an Output block and Input block can operate as Effects Send/Return loop. Read this: I/O connectivity and levels.

Control the level of instruments

The Level parameter in the Input block has a modifier. This makes it possible to adjust presets for the output of a specific guitar using a pedal or switch.

Input impedance


  • Axe-Fx III: variable, front input
  • FM9: variable, front input
  • FM3: fixed (1 Megaohm)
  • Axe-Fx II: variable, front input
  • FX8: variable, instrument input
  • AX8: fixed (1 Megaohm)



The XL+ front input circuit is identical to the AX-8 except the AX-8 doesn't have the variable impedance circuitry.

Auto/Variable impedance

Auto (or: variable) input impedance changes the analog circuitry of the input jack to interact with the guitar pickup. It recreates the way that some classic effects “load down” pickups, causing a change in frequency response.

Input Impedance is also referred to as: Input Z.

In Auto mode, the impedance is set automatically, based on the first non-bypassed effect to follow the input.

The AX8 and FM3 do not support auto input impedance.

Read this: Fuzz, buffers and impedance


Some old stomp boxes, i.e. Univibe, fuzzes, etc. have a low input impedance. This low input impedance will load down a guitar's pickups and change the frequency response of the pickups. You cannot simulate the effect of pickup loading after signal acquisition since the pickup parameters are an unknown quantity. So if you want to exactly model one of these old stomp boxes you need to replicate the impedance loading. The 11R and the Axe-Fx II do this by switching in resistors and capacitors on the inputs that change the input impedance to match the device being modeled. I do not believe that input impedance affects feel in any way. This is simply impossible. I remember people saying they could feel the input "pushing back". Silly. It will, however, definitely affect the sound as the frequency response of the guitar will be altered. The resonant frequency of the pickups will shift downwards and the Q will be altered as well. Now... it is debatable whether this alteration in frequency is actually desirable. If you want to exactly duplicate the sound of a vintage effect then, yes, it is desirable. However, most, if not all, modern effects feature buffered inputs to prevent pickup loading. This is done because typically we don't want to load the pickups. When the pickup designer is designing a pickup he is assuming the pickup will not be loaded. He chooses the winding, magnets, etc. such that the pickups have a desired frequency response into a very high impedance (i.e. 1 Mohm or greater) load. Variable input impedance is a nice feature if you want accuracy but it's not that big of a deal and certainly doesn't make one device better than another. Personally I prefer the sound with it turned off.

[1] The pickups see a load. That load is the impedance of the cable and the impedance of the amp input. The typical input impedance of a tube amp is a resistance plus a small capacitance to ground (120 pF or so). The Axe-Fx input simulates this input impedance. Some effect pedals present a different load. The Axe-Fx replicates this by switching in different load resistors and capacitors when a model of that effect is first in the effects chain.

[2] It's not virtual. It physically switches in a capacitor.

[3] You can simulate the effect of a guitar into a Fuzz-Face using the Input-Z feature.

Duplicating the behavior of a fuzz pedal requires that the pedal be the first non-bypassed block following the Input 1 block and the input block impedance must be set to Auto since fuzz pedals load down the guitar’s pickups.


[4] You can use the Input Impedance control to lower the input impedance and flatten the response. Try 220K, that will be close to your passive DI.

[5] The first block must be engaged for the impedance to change. If it is bypassed the impedance will revert to 1M.

[6] Variable impedance is only used for pedal modeling. Tube amp models are always 1M (no capacitor). The reason it was not included in the FM3 was cost and space. The input impedance of the FM3 and Axe-Fx III without any additional resistance or capacitance modifications is identical and replicates the input impedance of a tube amp. Variable input impedance for our products is used only when modeling certain pedals. I can't speak for other products.

[7] The front input has Auto-Z technology. The rear doesn't. If the preset/scene has a block that lowers the input impedance the inputs will sound different.

[8] The reason it was not included in the FM3 was cost and space.

Forum discussion

Forum member Moke:

You can get around the "1st active block in the chain" thing if needed, by putting any blocks in front of the Drive(s) that you want ignored, one row above. The algorithm only looks at blocks to the right of the 'Input' block in the same row, or below.

The Auto impedance values are:

1MΩ + Capacitor. Use this to simulate a long guitar cable [9]
230 kΩ
230 kΩ + Capacitor
90 kΩ
90 kΩ + Capacitor
70 kΩ
70 kΩ + Capacitor
32 kΩ
32 kΩ + Capacitor
22 kΩ
22 kΩ + Capacitor

Using a buffer before the instrument input disables the auto impedance feature.


[10] A buffer will render the impedance stuff ineffective. It will also add (maybe considerable) noise which may defeat the low-noise advantage of the front input.

[11] A wireless acts as a buffer. Hence changing the input impedance will not be able to load down the pickups.

Guitar volume


[12] When you roll your guitar volume down you increase its output impedance dramatically. It can be upwards of 100K ohms. The self-noise of a 100K ohm resistor is quite high. Now amplify that (a lot) and you'll hear the noise. If you continue to roll the volume down you'll notice the noise goes away because the output impedance decreases.

Prevent the thump

Engaging some effects, like Wah or Phaser, can cause a "thump" in the sound. This may be caused by impedance switching. To solve this, change Input Impedance from "Auto" to "1M".

The above doesn't apply to Fractal Audio devices with fixed (not adjustable) input impedance (AX8, FM3).


The thump isn't the wah, it's the Input Impedance changing. If your preset is set to Auto for the Input Z, when you engage the wah the impedance switches. This causes a slight thump if you are playing since the load on your guitar changes. If you want to avoid this, turn Input Impedance to 1M OHM.

[13] Set the Input-Z on the preset to 1M rather than Auto. The reason the Wah engages abruptly is because it switches the input impedance. Setting Input-Z to 1M overrides the impedance switch (and, frankly, sounds better IMO).

Global blocks: Input 1 only

The Input 1 block can be configured as a Global block on devices which support Global blocks. The other Input blocks do not support this.