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Compressor block

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Compressor block.PNG

Available on which Fractal Audio products

  • Axe-Fx III: 2x
  • Axe-Fx II: 2x
  • FX8: 2x
  • AX8: 1x

X/Y and channels

  • Axe-Fx III: 4 channels
  • Axe-Fx II: yes
  • FX8: yes
  • AX8: yes

There's no MIDI CC available for X/Y switching in the Axe-Fx II XL/XL+.

Compressor block in the Axe-Fx III

  • The old RMS detector has been removed. "RMS" is the same as "Fast RMS" in the AX8 and Axe-Fx II.
  • An "E-L Panel" mode has been added to the optical compressor.
  • There's a single optical type.
  • Input Level has been removed from the Pedal types.
  • The Studio Compressor has been changed; the threshold has been calibrated and its range has been adjusted.
  • The Filter parameter has been replaced with EQ-ing tools.

About the Compressor types

Pedal 1 and 2: "Pedal 2" uses a different algorithm which is smoother and pumps less than "Pedal 1". CPU usage by both types is much less than the "Studio" type.

Dynamics: allows compression or expansion with a single control. When set to negative values, the block compresses the signal. When set to positive values, the block expands the signal.

Optical: based on classic optical tube compressors, famed for their smooth sound. Use it before the Amp block to smooth out your licks and increase sustain, use it after the Amp block for instant “Hit Record” sound.

(Axe-Fx II) "Optical 1 uses a full-wave rectifier as a detector (peak detector). Optical 2 uses a true RMS detector. The LA-2A and many other compressors use rectifiers as detectors because it's easy and simple. Technically true-RMS detectors are "better" but they are difficult to implement in analog hardware. Whether or not true-RMS is better in actual real-world applications is debatable. There are those that claim that true-RMS detectors more closely replicate the natural compression behavior of the human auditory system. Peak detectors respond more rapidly to transients while RMS detectors have a smoother behavior. The only way to know which you like better is to try them." source
"It's not an "LA-2A" but it can sound like one. It's a generic optical compressor that can be adjusted to sound like a variety of compressors. It has adjustable attack, release and compression." source
"The LA-2A, CL1B, et. al. are high-end studio compressors that use optical circuits." source

More information in the Owner's Manual.

Compressor block versus compression in the Amp block

The Amp block also has built-in compression:

  • Dynamics: preamp compression, the same as the Compressor block's Dynamics mode.
  • Output Comp: output compression.

Both types increase CPU usage.

"The Dynamics knob in the Amp block does the same thing as the Dynamics mode of the compressors so you can save a block that way." source

Position of the Compressor block

Placing the Compressor block after the Amp block makes it interfere less with the amp dynamics.

When placing a Pedal or Optical type compressor after the Amp block, set its input to Line instead of Instrument (Axe-Fx III: n/a).


The parameters are explained in the Owner's Manual.

Parameters table

Parameter Axe-Fx III Axe-Fx II AX8, FX8
Type yes yes yes
Threshold yes yes yes
Ratio or Compression, Sustain yes yes yes
Attack Time yes yes yes
Release Time yes yes yes
Output Level yes yes yes
Auto Attack/Release yes yes yes
Light Type yes no no
Auto Makeup yes yes yes
Detector Type yes yes yes
Knee Type yes yes yes
Look-Ahead Time yes yes yes
Dynamics yes yes yes
Sidechain - Sidechain Source yes yes yes
Sidechain - Low Cut, High Cut yes no no
Sidechain - Emphasis yes no  ?
Sidechain - Frequency, Q, Gain or Filter yes yes yes
Input Level no yes yes


"The Threshold is the input level at which the output starts to compress as given by Ratio control. Levels can be and are frequently above 0. Any compressor that has a max level of zero is not calibrated in dBu."


Compressors are used mostly as a 100% wet effect. But there are compressors with Blend or Mix controls. These can be simulated by using the Mix parameter.

Sidechain Select

You can use the signal that enters the grid to feed the compressor, even if the compressor block is placed further upon the grid, through the Sidechain Select parameter (Studio mode only).

Release (Pedal mode)

"Pedal compressors typically have very long release times so the time is scaled by 10 for pedal mode. The time in studio mode is accurate to within a microsecond." source
"The times are the classic "time constants" for analog circuits, i.e. the time it takes for a signal to settle to 37% of it's final value (e^-1)." source


This boosts the high frequencies prior to compression and then lower them after.


Filter is a low-cut filter which can be used to tailor the response of the various compressor types to different material.

In the Axe-Fx III, the Filter parameter has been replaced with EQ-ing tools.


The default Lookahead value is 0.

Fast RMS detector

The “Fast RMS” Detector type in the Studio Comp mimics the fast detectors in classic rack-mount compressors. It uses less CPU than the other detectors.

In the Axe-Fx III "RMS" is the same as "Fast RMS" in the AX8 and Axe-Fx II.

Tips and tricks

Emulating a Boss Compressor/Sustainer

"The PEDAL 1 type in the AX8 is comparable to the CS1/2/3. The COMPression knob is equivalent to the SUSTAIN knob on the Boss pedal. Release is not sustain. Leave it at the default setting. When using this effect, I like to increase level before the compressor. One way to do this is to increase the output level of the noise gate on the input block." source

Using the compressor as a limiter

There's no dedicated brickwall limiter in the unit, but the compressor can be used as such.

Owner's manual:

"Setting the RATIO to “INFINITY” turns the compressor into a “limiter,” reducing any level above the threshold to the threshold, applying a sort of “ceiling” or “brick-wall limiting” above which nothing can rise."

Forum discussion.


Axe-Fx III manual:

"Pumping is an effect that occurs when a sudden strong peak causes a compressor to reduce levels such that the entire signal audibly dips and then returns. The event that triggers pumping might be brief, and can be in any part of the frequency spectrum. To reduce pumping caused by low or high end spikes, you can use sidechain filtering to make the compressor less sensitive to such peaks. Very fast and very slow release times tend to make a compressor seem more resistant to pumping, but can bring issues of their own. Another possibility is to switch to a multiband compressor, which handles lows, mids and highs individually. Now, it cannot go without saying that pumping can also be used as an effect in itself, as when a kick drum is set up as a sidechain source to give electric guitars a bouncing sound common in dance and industrial music."

More information