Learning the ins-outs and applications for compression alone can be a stressful task for even the most experienced of engineers out there. For those who have passed the stepping stone and become comfortable with managing the basic parameters of a compression unit (attack, release, threshold, ratio) you may have come across some compression circuit terms on your digital plug-ins and wondered, what is OPTO mode? VCA? FET ALL ABOUT?

Firstly, here are some digital plug-in compressors you may be familiar with.

Softube FET compressor

SSL Comp
Vertigo VSC-2
Renaisance Compressor



An optical compression circuit operates via an optical source and light sensitive resistor which controls the gain reduction of the audio signal. 

This style of compression is very transparent. Slow attack times allow for transients and dynamics to get through relatively unscathed by compression whilst still keeping the tail and body of the signal compressed in the background. This is the key that leads itself to being such a desirable and “warm” sounding unit.

Characteristic overview: Dynamic friendly compressor(Slow attack), warm tone.
Classic, LA-2A


FET (field effect transistors)

A colourful, aggressive and fast-acting GR style of compression circuit. A FET compression circuit lends its hand into really adding some presence to any signal that passes through it especially when pushed.

FET styled compression circuits tends to be my go to when working with elements that really require an up and front presence in a mix. Just be aware the timbre and feel of transient information and dynamics are more likely to be affected by a FET based compression circuit than when using an OPTO or VCA circuit.

Characteristic overview: Edgy tone, good for adding presence and control (fast attack)
Classic, 1176


VCA (Variable control amplifier)

The FET compressor's cousin, VCA. Fast acting and aggressive like a FET circuit but with a more conservative and un-intrusive attitude. 

VCA styled compression I will use 70% of the time throughout my mix. This form of compression is very easy to manipulate and yields results that don't impact drastically on the timbre of the material.

Characteristics Overview: Clean tone, good for controlling elements without affecting the timbre of the source material too drastically.
Classic, dbx 160


So, what's the lesson to take from this all? There's much more to compression units than meets the eye. Next time you're working on a mix and you're selecting a compressor to use, take the time to think which style of circuit will be most appropriate for what you're trying to achieve.