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Compressor block

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Compressor block.PNG

Available on which products

  • Axe-Fx III: 4 blocks
  • FM3: 2 blocks
  • FM9: 2 blocks
  • Axe-Fx II: 2 blocks
  • FX8: 2 blocks
  • AX8: 1 block

Channels or X/Y switching

  • Axe-Fx III and FM3 and FM9: 4 channels
  • Axe-Fx II: X/Y
  • FX8: X/Y
  • AX8: X/Y

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

Compressor block in firmware Ares and later

  • 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.
  • The Studio Compressor has been changed; the threshold has been calibrated and its range has been adjusted.
  • EQ-ing tools haven been added.
  • Additional compressor types include Tube, DynamiComp, JFET and new Studio Compressor types.
  • Firmware 12.07:
    • "Improved Compressor block. The RMS detector type has been improved resulting in smoother decay. The Auto Attack/Release algorithm has been improved resulting in less “chatter” when using low attack and/or release times. The Tube Compressor and Analog Compressor types now support RMS+Peak detector mode. A new knee shape algorithm in the Studio Compressor has been implemented resulting in a more musical tangential response like that of classic analog compressors."
  • Firmware 12.08:
    • "Decreased minimum Attack Time of Compressor block to 0.1 ms.
    • Added JFET Compressor type.
    • Added three new Studio Compressor types. Studio FF Compressor 2 is similar to Studio FF Compressor 1 (formerly called Studio Compressor) except that it is an “upwards compressor” as opposed to a downwards compressor. Studio FB Compressor 1 and 2 are feedback compressors with downwards and upwards compression respectively. As these are feedback compressors, they yield a different sound than a feedforward compressor commonly referred to as “fat” with smoother dynamics. Note that nature of feedback compressors can result in distortion at extreme control settings (fast attack/release, high ratio, etc.). It is therefore recommended to set Auto Att/Rel to ON in these instances."
  • Firmware 4.01 for the FM3:
    • “Added Compander type to Compressor block. The Transients control adjusts the transient modification. Values less than zero soften the attack, value greater than zero emphasize the attack.“

"A Compander is a compressor followed by an expander. If the time constant of the compressor and expander are the same (and the "channel" is perfect, i.e. linear and lossless) then a Compander is transparent. If you change the time constant of one vs. the other the transients will become distorted. The "Transients" knob controls the time constant mismatch. Negative values smooth the transients, positive values enhance the transients. A Compander with transient enhancement can be used on kick drums, bass, etc. to add punch. Transient reduction can be used to reduce plosives. For guitar you can use the Compander to add or remove pick attack." [1]

"The first step in adding compansion to the delay block was creating a compander algorithm. So I created a Compander type first in the Compressor block. Once that was working I used that algorithm in the Delay block. The Compander allows you to set the Ratio, Time, Transients and Level. Turn Transients down and the attack is reduced, turn it up and the attack is enhanced. You can use it with kick drum to enhance the punch, put it on vocals to reduce plosives, etc." [2]

Compressor types

Firmware Ares and later has these types in the Compressor block:

Pedal Compressor 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" types

Dynamics Processor – 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. This is also available in the Amp block

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

Tube Compressor — based on classic tube compressors like the Altec Lansing 436C. Since this type uses “variable mu” processing it may add subtle, and possibly desirable, distortion to the audio

Analog Compressor — natural soft-knee response, capturing the vibe of the classic compressors of the 70s and 80s

DynamiComp - pedal-style compressor with a faster attack behavior, based on MXR’s classic M-102 Dyna Comp pedal

JFET — Field Effect Transistor-based compression, emulating tube compressors. The most famous FET compressor is the Urei 1176

Compander — The Transients control adjusts the transient modification. Values less than zero soften the attack, value greater than zero emphasize the attack.

Studio Compressor types — Studio FF Compressor 2 is similar to Studio FF Compressor 1 (formerly called Studio Compressor) except that it is an “upwards compressor” as opposed to a downwards compressor. Studio FB Compressor 1 and 2 are feedback compressors with downwards and upwards compression respectively. As these are feedback compressors, they yield a different sound than a feedforward compressor commonly referred to as “fat” with smoother dynamics. Note that nature of feedback compressors can result in distortion at extreme control settings (fast attack/release, high ratio, etc.). It is therefore recommended to set Auto Att/Rel to ON in these instances."

"The Studio FB Compressor 1 might be my new favorite." [3]

(firmware Ares 12.07) "The Studio Compressor is really nice now. I compared it to my favorite hardware unit (dbx 266XL) and the Studio Compressor sounds and measures better (less distortion at low attack/release times)." [4]

(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." [5]

(About the optical compressor) "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." [6]

"The LA-2A, CL1B, et. al. are high-end studio compressors that use optical circuits." [7]

"The Studio FB Compressor 1 might be my new favorite." [8]

More in the Owner's Manuals

Factory presets

The Compressor block is showcased in many factory presets. Search the Factory presets page for "compres*".

Position of the Compressor block on the grid

In traditional guitar rigs the compressor pedal goes between guitar and amp.

Putting the Compressor block after the Amp block makes it interfere less with the amp dynamics. When doing this, set its input to Line instead of Instrument (Axe-Fx III, FM9, FM3: n/a).

Compression in Compressor block versus Amp block

The Amp block has built-in compression tools:

Dynamics – preamp compression, the same as the Compressor block's Dynamics mode

Output Comp – output compression, great to beef up clean tones

"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." [9]

"The Output Compressor is a simplified version of the compressor block." [10]

Both types increase CPU usage.

Some devices provide a dedicated Multiband Compressor block: Axe-Fx II, Axe-Fx III, FM3, FM9.

About ADSR

Attack — The time it takes to go from silence to maximum level

Decay — The time it takes to drop from maximum level to the sustain level or silence

Sustain — The volume of a note while holding it

Release — The time it takes to drop from sustain level to silence after being released


The parameters are explained in the Owner's Manual.

Parameters table

Parameter Axe-Fx III / FM3 / FM9
Type yes
Threshold yes
Ratio yes
Compression yes
Dynamics type: Dynamics yes
Attack Time yes
Release Time yes
Auto Attack/Release yes
Studio type: Auto Makeup yes
Look-Ahead Time yes
Detector Type, Knee Type yes
Light Type (Optical) yes
Sidechain Source yes
Low Cut, High Cut yes
Emphasis yes
Frequency, Q, Gain yes
Filter Type
Mix yes


In firmware Ares and later, a Threshold parameter has been added to all types. In existing presets containing a Compressor type that didn’t have a Threshold yet, the value is set to -60dB. Adjust if necessary.

"The Threshold is the input level at which the output starts to compress as given by Ratio control." [11]

"If defaults to all the way down on old patches so that it doesn't change the sound of them." [12]

"-60 dB is so low as to ensure that the signal will compress even on the most quiet playing -- assuming Compression is higher than 0 or Ratio is higher than 1:1." [13]


"Levels can be and are frequently above 0. Any compressor that has a max level of zero is not calibrated in dBu." [14]

"The output level is irrelevant. It's just a number." [15]

"This is the right way to set the compressor output level:

  1. Bypass everything but the compressor block.
  2. Dial in the desired threshold and ratio.
  3. Alternate between bypassing and engaging the block while strumming hard. Adjust the output level until the volume is the same." [16]


"You shouldn't use a combination of low Attack Time and low Release Time. If one is low the other should be high. Otherwise the detector will not filter enough. This is the nature of compressors. We allow you to use low times but you should be aware that it can cause distortion. Very low attack times will result in detector chatter. This is the nature of detector circuits." [17]

Auto Attack/Release varies with program material as a percentage of the set values.

"I recommend turning Auto Att/Rel on. The Attack and Release controls still work but it uses some fancy math to vary the attack and release times as a function of data statistics. The only downside is that it adds a little CPU hit." [18]


Compressors are used mostly as a 100% wet effect. But there are a few compressors with Blend or Mix controls. Use the Mix parameter in the model.

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.

Sidechain High Cut

"You can use the Compressor block as a "Clarity" control. Set the sidechain high-cut to around 200 Hz. Now low notes will get compressed more than high notes. This will make the lows clearer when digging in. This is like a variable high-pass filter that gets more intense the harder you play. When you play lightly the compressor will do nothing. As you play harder low frequencies will have less gain than high frequencies. For a more intense effect use the filter section of the sidechain. Set the type to lowpass and adjust the frequency to taste." [19]

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." [20]

"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)." [21]


This boosts the high frequencies prior to compression and then lowers 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 firmware Ares and later, the Filter parameter has been replaced with EQ-ing tools.


The default Lookahead value is 0.


"The only thing auto-gain does is attempt to keep the output level the same as the ratio and/or threshold is changed. It has to make assumptions about the incoming level which is just a guess since every guitar is different. The threshold of the Axe-Fx III differs from the Axe-Fx II by about 20 dB so if you are copying presets from the II you need to change your threshold accordingly." [22]

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 firmware Ares and later, there's only "RMS", which is the same as "Fast RMS" in the AX8, FX8 and Axe-Fx II.

Tips and tricks

Pick Attack

To manipulate pick attack using the Compressor:

'"Downward Expander. Set Release Time and Hold Time to minimum. Detector to Peak. Adjust Attack Time, Ratio, etc. to taste." [23]

Latency compensation

If you're using multiple audio routings and need to align them, preventing timing differences (latency), you can use the Compressor block. More information

Pre and Post

For maximum control over dynamics and loudness, use two Compressor blocks. One at the start and one at the end. As demonstrated in some Factory presets, search for "pre- and post-compression".

Emulating the Keeley Compressor

(Admin M@) "I use "Analog Compressor" for Keeley sounds. It works great! Because I play vintage output guitars, I like to boost the level going in by adjusting the output of the input block before it. You can try "Auto Att/Rel", or use the published "classic" Keeley recipes*. Personally, however, I prefer a faster release, in the neighborhood of 75ms or less.

  • Attack Time: typically 15ms for 10dB, 5ms for 20dB, 3ms for 30dB
  • Release Time: 80ms for 10dB, 160ms for 20dB, 240ms for 30dB" [24]

=Emulating the Ross Compressor

I don't hear anything the Dynamicomp model isn't capable of (except for the noise). Turn off Auto Att/Rel. Set the Attack to min, Release somewhat high, Compression as desired and Bob's your uncle.[25]

The key is to use a feedback compressor. So that would be any of the pedal compressors and the Dynamicomp or the Studio FB Compressors. Then either set the threshold all the way down or all the way up if it's an upward compressor (Studio FB Compressor 2 IIRC). Set Attack Time to min, Release Time fairly long. Make sure detector type is peak and Auto Att/Rel is off.[26]

80's clean compression

For that ultra-compressed '80s clean guitar tone, check out factory preset "Clean Pop Rhythm".

Emulating a Boss Compressor/Sustainer

(Admin M@) "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." [27]

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."

"The compressor block is expressly designed to also be used as a limiter. Set the ratio to maximum and detector type to peak." [28]

Using the compressor as a clean boost

A compressor can be used as a clean boost. Keep compression low and set its output level as desired.


From the Owners 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."


"De-essing can be done by adjusting the side-chain EQ in the compressor block." soure

Cascading compressors

An old studio trick is to cascade two compressors in series, especially for slide guitar, reportedly by Lowell George and others.

"Cascading compressors has been used in studios for decades. An 1176 into an LA-2A is a popular combo. The FMR Audio put this into a single package with their RNC (Really Nice Compressor) compressor which has two compressors in series. The "SuperNice" switch engaged a third stage." [29]

Example by Leon Todd

SoloDallas Schaffer Replica

To emulate the compression from the Schaffer Replica, use two compressors, and/or compression in the Amp block, as explained in the threads below. Or check the "Hell's Glockenspiels" factory preset.

More information:

Micro-Delay for stereo enhancing

Use a compressor, Dynamics type, set to a low ratio, adjust the look ahead parameter.

More information about compression