Mobtown’s Guide To The Different Types Of Compressors

We all know what compressors do: they level out audio as well as act as fancy tone or distortion boxes. (If you need a refresher, check out our primer on what compressors do.) A mentor of mine once said, “Compression is distortion and distortion is compression.” So when you use one, you get the other, whether you like it or not. Sometimes artifacts give you the sound you’re after; sometimes it’s too much. And regardless of how the effect is perceived, compressors always reduce the loud parts of audio.

There are four main types of compressors and each occupies a specific niche in production. Understanding their unique qualities can help you choose what gear will be the best fit for your next project.

Fairchild 670 Compressor

Variable-Mu Compressors

Manley® Variable-Mu Limiter Compressor
Manley® Variable-Mu Limiter Compressor

Vari-mu are the old school compressors. These bad-ass compressors often have a large array of tubes and transformers (not to mention sexy knobs) which may or may not affect the sound. Some engineers call them “space heaters” due to all the heat the tubes generate. Vari-mu employs tubes to control the gain-reduction stage. The input voltage alters the bias of the tubes and this results in a slower, harmonically rich style of compression that shares a number of the same musical characteristics with its optical family member (more on that later).

One of the main characteristics is the “soft-knee” style of compression inherent in all vari-mu designs. The ratio is effectively program dependent, which increases as the input hits the circuit harder. While some of these designs allow for variable attack and release, vari-mu compression will always have a “slow-attack” style of compression thanks to the function of the tube circuits. These circuits allow for generous amounts of compression without unwanted artifacts while adding serious mojo.

What are examples of variable-mu compressors? Fairchild 660 (and its bigger sibling — the holy grail — the stereo 670) which can cost you over $10,000, Federal AM-864/U, Manley Variable-Mu, Altec 436B, and Chandler Limited RS124.

What program audio sounds great with a vari-mu compressor? Mix Buss, Bass, Vocals.

Random fact: the “mu” in variable-mu means “gain” in tube-speak.

Optical Compressors

Teletronix LA-2A
Teletronix LA-2A

Continuing with vibey slow-compression-style circuits, next we have the opto or optical compressor. These compressors use a photocell as the compressor detector and a light bulb or LED to determine the gain reduction. The light will glow depending on the strength of signal passing through it and will reduce the gain accordingly. These compressors are much less sensitive to transients and peaks due to the lag experienced by the photocell. Contrary to what you might expect because of how fast the speed of light is, opto compressors are considered to be slow and smooth. 

Most opto compressors don’t have total control over the attack and release settings, like the Teletronix LA-2A which is arguably the most well known and most highly regarded compressor of all time. The attack time is frequency-dependent which is very likely the main reason these units have such character. Optos are known for their smooth response and great transparency.

What are examples of opto compressors? Universal Audio LA-2A and LA-3A, Tube-Tech CL 1B, and Millennia TCL-2.

What program audio sounds great with an opto compressor? Bass, Vocals.

VCA Compressors

SSL XLogic G-series Compressor
SSL XLogic G-series Compressor

If you walked in to a studio and they had one compressor, more than likely it would be a VCA compressor. VCA stands for voltage-controlled amplifier. Inverse to what you might expect, the amplifier actually attenuates here as opposed to amplifying. Since parts for a VCA compressor can be sourced a lot more cheaply than the other styles of compressors on this list, the majority of low quality/prosumer units use a VCA circuit.

Known for their fully controllable circuits allowing you to really fine tune each setting, VCA compressors are valuable in every aspect of production whether it be tracking, mixing or mastering. They have the ability to be completely transparent while still adding the glue to a mix or drum bus. Drums sound great through these compressors because they excel at transparently taming intense peaks while not destroying the vibe or intensity of the player.

What are examples of VCA compressors? Solid State Logic G-Series Compressor, Smart Research C2, API 2500, and the classic dbx® 160.

What program audio sounds great with a VCA compressor? Mix Buss, Drums, Guitars.

FET Compressors

Urei / Universal Audio 1176
Urei / Universal Audio 1176

FET compressors, which stands for “field effect transistor”, are devices from the family of dynamic audio effects. So how did they come about? As small transistors began to replace large tubes, later compressor devices were based on field effect transistors. They also have transformers which adds a warmth to anything that passes through it. In fact, many great records have been passed through these units with the compressor circuit turned off. Why? To capture the rich sound and subtle harmonic distortion that it adds. So clearly, these compressors add massive amounts of character and color. Think HUGE DRUMS. Usually the slowest attack time available on the FET compressor is faster than the fastest attack time on the variable-mu compressor. For example on the classic Universal Audio or UREI 1176, the attack pot is variable from 20 microseconds to 800 microseconds. Yes, thats 0.02 ms to 0.8 ms. That’s fast!

What are examples of FET Compressors? Universal Audio 1176, Chandler Limited Germanium Compressor, and the Daking FET III.

What program audio sounds great with a FET compressor? Drums, Guitars, Vocals.

So while there are many compressors types, not every one works for every piece of program audio. Sometimes you have to try out different compressors to find which one works best for your instrument and/or workflow. It takes time, but with practice you’ll find all the uses for your different compressors.