FET, Opto, VCA, Vari-MU…..you’ve probably heard these names to describe types of compressors, you sort of know which type is good for what, but what do they really mean? Each name describes a different type of circuit design that the compressor uses to react to the signal you put through it.
Before I list each of the types 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 you choose to use a compressor, know that each style is like a different flavor. It takes a time and ear training and a lot of A/B comparison to be able to distinguish between each.
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 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, 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 and the Chandler Little Devil.
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 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.
If you could choose one compressor to use for the rest of your life, your best bet would be a VCA style compressor. 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 being able to add the glue a mix or drum bus needs. Drums sound great through these compressors because they are very good at transparently taming intense 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 styles of compressors on this list, the majority of low quality/prosumer units use a VCA circuit.
Legendary VCA mixbus compressors include the iconic SSL G Series and the Empirical Labs Distressor.
In a Variable “Vari”-Mu design the gain is being 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 radio. Normally 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 actually 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 include RCA BA-6A, Altec 436, 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 bridges have been used in radio for automatic level control for a long time. Diode bridge audio compressors offer fully configurable parameters. A number of compressors designed in the 60’s used this basic design, but you don’t see many used in modern compressors. If the unit isn’t designed well they can be noisy due to the low signal level allowed by the didoes.
Diode bridge circuits allow for the compression curves and the attack and release to be designed independent of the compression element. These compressors are tonally distinctive, because of the diodes harmonic distortion. These designs are flexible and can add a pleasant, colorful character.
Famous examples include the Neve 2264 and Neve 33609.
Fun fact: The 33609 we have at Sabella Studios was actually taken from HBO studios when they decided to throw them all out and go digital.
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 the reason I had never heard of a PWM or used one is because not that many that 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 little 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 the build a compressor with very little negative sonic artifacts.”
VCA’s let a percentage of energy through depending on the control voltage. The problem is that when the control voltage is changed you start to hear bad sounding artifacts. If there was a switch to control the energy on the output you could give a more accurate average control voltage leading to a more sonicly pleasing unit. The PWM circuit turns the control voltage into an incredibly fast variable width switch that controls the energy that is being outputted.
What is the difference between hard and soft knee?
Soft knee compression gradually attenuates the signal after it has passed the threshold while hard knee compression attenuates the signal immediately after it has passed the threshold. Soft Knee compression is thought to be more musical because it’s not as abrupt and abrasive; hence the name soft.