Compressor types: which to use and when (VCA, opto, FET and Vari-Mu)

Ever wondered why there are so many types of compressor and which one to use when? In essence they all do the same thing — reduce the difference between the loud and quiet parts of a sound — but each design does it with a different speed and character. Knowing them saves you hours of trying things blind.

VCA: fast and precise

VCA stands for Voltage Controlled Amplifier. It's the design behind many console compressors and most of the ones that come with DAWs. It stands out for being fast and precise, with very controllable attack and release.

Plugins: the Waves SSL G-Master Buss Compressor, modelled on the master bus compressor of the SSL 4000 G console, is a go-to for the bus. If you work in Cubase, like I do, the stock Compressor or the one in VSTDynamics don't emulate a specific VCA, but they're clean, fast and precise and cover that role.

When I use it: to tame transients and sharp peaks, on drums, on the 808, and as a bus compressor to "glue" a group of tracks. In urban music it's an everyday tool.

Optical (opto): smooth and musical

It works with a light source and a photocell: the louder the signal, the brighter the light and the more it compresses. The photocell's response makes it a slow, smooth compressor, with a release that adapts to the material. The LA-2A is the classic example.

Plugins: Waves CLA-2A or the UAD Teletronix LA-2A Collection, both emulations of the optical LA-2A. Cubase doesn't ship an optical compressor as such.

When I use it: vocals, legato strings, bass and anything that should stay steady and natural without the compression being obvious.

FET: fast and full of character

It uses a field-effect transistor (FET) as the control element. It isn't a variant of the opto: it's a different technology. It can be very fast and adds some harmonic distortion, heard as warmth and aggression.

Plugins: Waves CLA-76 or Arturia Comp FET-76, both based on the classic 1176. In Cubase, VintageCompressor is a vintage-style compressor driven by input gain, with a Punch mode; Steinberg doesn't say which unit it models, but it's the closest stock option for this job.

When I use it: to give presence and attitude to rap vocals, drums, guitars and percussion. It's the compressor that pushes a vocal to the front.

Vari-Mu: the bus all-rounder

It uses valves (tubes) whose gain varies with signal level. Its compression is progressive: the harder the signal comes in, the more it compresses, very gently. That's why it's mostly used on buses and the master, where everything should already be more or less in place and you're only after cohesion.

Plugins: Klanghelm MJUC, which brings together several variable-gain tube compression models, or its free little brother, MJUC jr. In Cubase, Tube Compressor adds tube saturation and smooth compression, although Steinberg doesn't describe it as Vari-Mu.

When I use it: on the mix bus or group buses, with little gain reduction, for unity and a touch of density.

One-knob compressors

Simplified compressors: input, output and an "amount" control. Inside, the settings have already been decided by the manufacturer. They're quick to use and usually light on resources.

Plugin: Klanghelm DC1A, free, with just two knobs (input and output).

When I use them: when I want fast compression on a sustained sound or to try an idea without breaking the workflow. For fine control, I prefer a full compressor.

Before choosing, ask yourself why

The type of compressor matters less than the reason for using it. Do you want to control peaks, add punch, sustain a note or glue a group together? Answer that first and the right design follows. And if a sound doesn't need it, don't compress it: I explain it in less is more.

Bear in mind too that mix compression works best on material that still has dynamics. That's why I deliver my beats with headroom: see the loudness war. And if the frequency balance isn't clear yet, start with EQ.