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FET, Opto, VCA, Vari-MU…..you’ve probably heard these names to describe types of compressors, you sort of know which type is right for what, but what do they really mean? Each name represents a different type of circuit design that the compressor uses to react to the signal you put through it.
Before I list each type and what they do, know that compressors all serve the same function. You can use them as a leveling device or just for tone or saturation. However, if you choose to use a compressor, know that each style is like a different flavor. It takes time and ear training and a lot of A/B comparisons to distinguish between each.
Here are the 6 most common types of compressors used in music production
FET
Field-effect Transistors, or FETs, use an electric field to control the gain of the unit. The most popular example is the iconic 1176. FET style compression has a super-fast attack and release capabilities, which really allows you to shape the transients. This makes them a favorite on drums for many engineers.
The drawback to their fast speed is there isn’t much range. A fellow engineer once described the release settings to me as being either “fast or faster.” The fastest release setting on an 1176 is 50ms, and its slowest release is 1100ms. The fastest attack time is a blazing fast 20 microseconds, and the slowest is a not-as-blazing-but-still-pretty-damn-fast 800 microseconds. Regardless of the lack of flexibility, it still sounds great on just about anything you put through it. Guitar, bass, vocals, drums, and keys have all found their way through an 1176 with great results.
The 1176 is synonymous with FET compression, but some lesser-known FET compressors include the Daking FET III, Chandler Little Devil, and the popular 1176 Clone by Warm Audio, which I’ve heard great things about.
Opto
These compressors use a photocell as a detector and a light bulb or LED to determine the gain reduction. The light will glow depending on the strength of the signal passing through it and 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 it might seem 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 LA-2A, which is arguably the most well known and highly regarded compressor of all time. The attack time is frequency dependent which is very likely the main reason these units have such character.
Popular optical compressors include the Universal Audio LA-2A and 3A, Avalon AD2044, and Warm Audio WA-2A.
VCA
If you could choose one compressor to use for the rest of your life, your best bet would be a VCA style compressor. They are 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 can be entirely transparent while still adding the glue a mix or drum bus needs. Drums sound great through these compressors because they are very good at transparently taming sharp peaks.
VCA stands for voltage controlled amplifier, but one VCA doesn’t necessarily sound like the next. Since parts can be sourced a lot more cheaply than the other compressors on this list, most low quality/prosumer units use a VCA circuit.
Legendary VCA mix bus compressors include the iconic SSL G Series and the Empirical Labs Distressor.
Variable-Mu
In a Variable “Vari”-Mu design, the gain is controlled by a vacuum tube. The first types of compressors were Vari-Mu and were designed for use in broadcast. The need came from trying to level out inconsistent speech on the radio. Usually, going from a whisper to yelling would require an engineer to ride the fader live, and human reflexes are only so fast. It wasn’t until later that the studio adopted the use of compressors.
The unique quality of Vari-Mu compressors is the ratio of the gain reduction is increased as you hit the unit harder; this is musically pleasing when you lay into it more aggressively. Vintage Variable-MU compressors are also great for saturation, as the tubes and large iron transformers all have unique qualities. Popular vintage Vari-Mu compressors include RCA BA-6A, Altec 436, and UA-175. The most popular modern Vari-Mu is the Manley of the same name, which is used very prominently in mastering studios across the world.
Diode Bridge
Diode bridges have been used in radio for automatic level control for a long time. Diode bridge audio compressors offer fully configurable parameters. Some compressors designed in the ’60s used this basic design, but you don’t see many used in modern compressors. Units that aren’t designed well can be noisy due to the diodes’ low signal level.
Diode bridge circuits allow for the compression curves and the attack and release to be designed independently of the compression element. These compressors are tonally distinctive because of the diode’s harmonic distortion. These designs are flexible and can add a pleasant, colorful character.
Famous examples include the Neve 2264, Neve 33609, and newer examples include the Rupert Neve Shelford Channel Strip compressor.
Fun fact: The 33609 we have at Sabella Studios was incredibly taken from HBO studios when they decided to throw them all out and go digital.
PWM, or Pulse Width Modulation
I admittedly didn’t even know what a PWM compressor was before writing this article. Gregory Scott, a compression guru from Kush Audio, recommended I add a section on Pulse Width Modulator circuits. After a bit of research, I found out why I had never heard of a PWM or used one… because not that many exist. According to Scott, “there are only a few because you truly have to be a rocket scientist to design one, aka Dave Hill or whatever geek at Pye did theirs.”
A Google search turned up an essay written by Dave Hill of Crane Song. In the essay, Dave explains that all compressor circuits have good and bad characteristics. He goes on to say, “in designing a compressor with as few artifacts as possible, the gain control choices are limited. PWM has been used in vintage compressors and also modern devices. If one takes that idea and uses the latest technology, it is possible to build a compressor with very little negative sonic artifacts.”
VCAs let a percentage of energy through depending on the control voltage. The problem is that when the control voltage changes, you start to hear bad sounding artifacts. If there were a switch to control the energy on the output, you’d be able to give a more accurate average control voltage, which would lead to a more sonically pleasing unit. The PWM circuit turns the control voltage into a high-speed variable-width switch that controls the energy that is outputted.
Bonus:
What is the difference between hard and soft knee compression?
Soft knee compression gradually attenuates the signal after it has passed the threshold, while hard knee compression attenuates the signal immediately after it has crossed the threshold. Soft Knee compression is thought to be more musical because it’s not as abrupt and abrasive; hence the name soft.