Quick answer: Almost every compressor belongs to one of four families, defined by the part that turns the level down. VCA compressors use a voltage-controlled amplifier and are fast, precise and clean. FET compressors use a field-effect transistor and are very fast and full of attitude, like the 1176. Optical compressors use a light and a light-sensitive cell and are smooth and forgiving, like the LA-2A. Vari-mu compressors use a tube as the gain element itself and are known for soft, cohesive glue on the mix bus. None is better in general: the right one depends on the source and on what you want the compression to do.
If you are still getting comfortable with the controls, start with how to actually use attack and release. This guide is about the step before that: choosing which kind of compressor to open.
The four compressor types at a glance
| Type | What reduces the gain | Character | Classic example | Often used on |
|---|---|---|---|---|
| VCA | Voltage-controlled amplifier | Fast, clean, precise | dbx 160, SSL bus compressor | Drums, mix bus, surgical control |
| FET | Field-effect transistor | Very fast, aggressive, colored | UREI 1176 | Vocals, drums, attitude |
| Optical | Light source and photocell | Smooth, program-dependent, forgiving | LA-2A | Vocals, bass, gentle leveling |
| Vari-mu | The tube itself | Soft knee, thick, glued | Fairchild 670 | Mix bus, mastering, thick vocals |
These are general tendencies, not rules. As John Paterno points out in his variable-mu compression excerpt, even compressors of the same type, and plugins that emulate the same circuitry, often do not sound alike. Use the table to pick where to start, then trust your ears.
VCA compressors: fast, clean and precise
A VCA compressor controls the signal with a voltage-controlled amplifier. The result is a quick, accurate, clean gain change, which makes VCA designs a natural fit when you want to control dynamics without adding much color. The best-known examples are the dbx 160 and the SSL bus compressor.
Two Puremix sessions show where VCA shines:
- Darrell Thorp puts a Shadow Hills Mastering Compressor on the master bus and chooses its Discrete (VCA) mode over the Optical one, because compressing the transients helps tighten a live recording made in a reverberant room. See Mix Bus Compression | Darrell Thorp.
- Fab Dupont uses the SSL bus compressor to "slow the needles down", which means reducing the dynamic range so the mix feels denser, and then shows how to add parallel compression with it. See SSL Bus Compressor | Fab Dupont and our guide to what parallel compression is.
Watch out for: because a VCA is quick and clean, it will not hide a heavy hand. Fast attacks soften punch, and a very fast release can add pumping.
FET compressors: fast, aggressive and full of character
The UREI 1176 is the reference FET compressor. Its field-effect transistor reacts extremely quickly, so it can sound exciting and aggressive. According to the manual for the UA 1176 Classic FET Compressor plugin, the attack runs from 20 to 800 microseconds and the release from 50 to 1100 milliseconds, with four ratios (4:1, 8:1, 12:1 and 20:1). Those times can shift a little with the ratio and the model, and not every 1176-style unit uses the same release range.
Two things trip people up the first time:
- The knobs run backward. On the 1176, the Attack and Release controls work in reverse from most compressors: the slowest setting is at 7 o'clock and the fastest at 5 o'clock.
- The Input knob is the threshold. You push more signal in to get more compression.
In 1176 Settings on Vocals | Greg Wells, Greg Wells uses a hardware 1176 LN while tracking a lead vocal with about 3 dB of gain reduction at most, the slowest attack and the fastest release. He says vocals come out sounding fat and loud rather than choked, and he notes that even at the slowest attack the 1176 always sounds like it is compressing.
Optical compressors: smooth and forgiving
An optical compressor uses light to do the work. In an LA-2A, the sidechain signal feeds a T4 photocell, which turns the signal into light, and the brighter the light gets, the more the unit reduces the level. The response of the cell is smooth and depends on the program material, which is why opto compressors tend to sound gentle and musical.
The LA-2A is simple. It has no attack or release controls, because those are fixed and part of its sound. The Peak Reduction knob works like a threshold and the Gain knob is makeup gain, with no effect on how much compression you get. In LA-2A Compressor: How It Works, How to Set It, and the Biggest Mistake to Avoid, Fab Dupont warns that many people over-compress with it, because a hot input and a high Peak Reduction setting together squash the source and cost it its dynamics.
The least-understood control is Emphasis, which adjusts a filter in the sidechain. F. Reid Shippen shows how moving it changes both the character and the level of a vocal in LA-2A emphasis | F. Reid Shippen.
Not every optical sound comes from an LA-2A. Rich Keller uses the Opto setting of the Empirical Labs Distressor as his typical vocal setting, with a relatively slow attack so most transients pass through. Read Rich Keller's vocal Distressor settings.
Vari-mu compressors: the tube is the compressor
Vari-mu is the odd one out. In VCA, FET and optical designs, a separate element is placed in the signal path to reduce the gain. In a vari-mu compressor, the tube itself does the amplifying, and its amplification factor, the "mu", changes with the signal that goes through it. Nothing is inserted to squash the audio, so the gain reduction happens inside the same device that amplifies it.
That is why vari-mu designs are associated with glue, and why the Fairchild 670 and its descendants are so often placed on the mix bus. They have a soft knee, and their ratio rises with the level of the input, which tends to pull a mix together instead of simply turning it down. Our sister site Process.Audio explains the circuit in plain language in What Is a Vari-Mu Compressor? and the idea of glue in Why Your Mix Doesn't Glue.
On a vocal, John Paterno says he wants a vari-mu because the compressor sounds thicker and still has a relatively fast response. He compares an SPL IRON Mastering Compressor with a UA Fairchild 660 plugin and listens for whether each one is "pinching" the vocal or making it bigger. See John Paterno: Variable-Mu Compression. For plugin options, see the best tube compressor plugins of 2026.
Which compressor type for which job?
Use this as a starting map, then audition two types on the same passage at matched levels.
| Goal | Start with | Why |
|---|---|---|
| Lead vocal, fat and controlled | FET or optical | Greg Wells uses an 1176 for fat, un-choked vocals. Fab Dupont and Rich Keller show optical approaches that keep it smooth. |
| Smooth leveling with little effort | Optical | Fixed attack and release and a program-dependent cell are hard to set badly, as long as you do not over-compress. |
| Tightening transients on a bus | VCA | Thorp chooses the discrete VCA mode on the master bus for exactly this. |
| Gluing a mix together | Vari-mu | Soft knee and a rising ratio make it a classic mix bus choice. |
| Aggressive drums or attitude | FET | Very fast response and added character. |
| Keeping the original dynamics and adding density | Any type in parallel | See Michael Brauer's parallel vocal compressors and parallel compression explained on Campus. |
You can also use more than one. A chain of compressors, each doing a small amount of work, can sound more natural than a single compressor doing all of it, as covered in Vocal Compression Chain: Why 3 Compressors Beat 1.
Plugins are not the hardware
Plugin emulations aim to reproduce these circuits, but individual models differ, and Paterno's comparison shows that two plugins of the same family can lead you to different choices. The practical lesson is to pick a compressor by what it does to your source, not by the name on the label, and to compare at matched levels so loudness does not decide the winner. For a deeper look at hearing what a compressor is doing, watch how to hear compression on vocals.
One option for the vari-mu slot
From Puremix's sister company, Process.Audio: the Rockruepel COMP.TWO is an emulation of an all-tube vari-mu hardware compressor. It offers Input, Output, Mix, Threshold, Attack, Release and a sidechain high-pass filter, plus an optional transformer simulation and Stereo, Dual Mono and Mid/Side modes. A 30-day free trial is available on the product page. Whichever vari-mu you use, the same rules apply: gentle settings, matched levels and a bypass test.
FAQ
What are the four types of compressors?
VCA, FET, optical and vari-mu. They differ in the element that reduces the gain: a voltage-controlled amplifier, a field-effect transistor, a light and photocell, or a tube.
What is the difference between VCA and FET compressors?
Both are fast, but a VCA tends to be cleaner and more precise, while a FET compressor such as the 1176 adds more attitude and character.
What is an optical compressor best for?
Smooth, forgiving leveling, especially on vocals and bass. The LA-2A is the classic example. Avoid pushing the input and the Peak Reduction up together, because it is easy to squash the source.
What is a vari-mu compressor?
A compressor that uses a tube as the gain element, so the compression happens inside the amplifier itself. It is known for soft, cohesive glue and is common on the mix bus and in mastering.
Which compressor type is best for vocals?
There is no single answer. FET, optical and vari-mu compressors are all used on vocals. Try two on the same passage at matched levels and keep the one that serves the song.
Which compressor is best for the mix bus?
VCA and vari-mu are the usual choices. VCA tightens transients and controls dynamics, and vari-mu adds glue and thickness.
Do plugins sound like the hardware?
They are designed to emulate it, but models vary, so compare them by ear on your own material.
Want to hear all four types side by side? The Puremix exclusive Compression topology with Andrew Scheps compares FET, VCA, vari-mu and optical compressors on pianos, vocals, drums and full mixes. For a structured path, take Foundations of Compression on Campus, or see the Puremix plans and watch the free Puremix videos.
Disclosure: Process.Audio, Campus and Puremix are sister brands. COMP.TWO is a Process.Audio product; the other plugins and hardware mentioned are from independent makers. Technical details were checked against Puremix sessions and published manuals in November 2026.
