Always consult the Owners Manuals

Compressor/Dynamics block

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Compressor/Dynamics: supported by which Fractal Audio products

Compressor/Dynamics: X/Y switching or channels

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

Compressor block in the Axe-Fx III

About the compressor types

"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 vs compression in the Amp block

The Amp block also has built-in 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).

Compressor/Dynamics parameters

The parameters are explained in the Owner's Manual.


"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

Use 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."

More information.

More information about compression

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