Compression – Part 4 – Styles Of Compressor.

Find out about the four designs of analogue compressor.

Dom Morley
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27th April 2020

Here’s Part 4 of our series on compression.

It can be overwhelming deciding exactly which compressor to choose when there are so many different types available - both analogue and digital. Fortunately, there are only actually four designs of analogue compressor (VCA, FET, Opto & Vari-Mu), and most plug-ins are emulations of one of these.

So, let’s look at these four designs - what their differences are and what that means to your sound.

VCA

This is the most common type of compressor, and very likely to be design behind your DAW default/stock compressor. There are a couple of reasons for this.

The first is that VCA compressors are quite transparent. They don’t add a ton of colour while they are compressing, so your sound won’t change dramatically just by putting one of these in your signal path.

The second is that they are more controllable than other designs, with relatively fast attack and release controls as well as threshold level – and although they are normally hard ‘knee’ some come with control over that too.

VCA compressors are good for adding punch and work well with transients and percussive sounds. Their full range of controls also make them popular as ‘glue’ compressors for mix busses.

Good real-world examples are the dbx series, and the SSL, API and Neve stereo bus compressors.

FET

These compressors were designed to have an even faster attack than VCA compressors – it’s easy to set the attack too fast on FETs so it can be a good move to start slow and then adjust faster to taste.

They don’t normally come with a threshold control and the amount of compression is generally controlled by the input knob – the more level you throw in, the more compression occurs.

FETs have a lovely warm sound when driven and are great for adding bite to your sounds. They are also very useful for parallel compression as extreme effects are easy to create, which can then be fun to mix in with an uncompressed signal.

The classic FET compressor is the 1176, which you’ll see at least one of in almost every commercial studio in the world.

Opto

The name of these compressors comes from ‘optical’ as the detection circuit uses a light source (such as an LED) and a light-sensitive resistor to control compression.

The speed of the attack and release depend on how hard you hit the input, as that determines how quickly the light source lights up. For this reason, you don’t have control over attack and release times yourself, instead the input knob controls not only how much, but how your signal is compressed.

Equally you have no control over the ratio – that is determined by the particular light/resistor combination in the design you are using.

Opto compressors are great for providing smooth compression on signals that don’t have sharp transients that need fine control. The attack times are slower than VCA and FET designs, so very transient material will either pass straight through (as the transient is over before the attack time is completed) or can push the compressor in to ‘pumping’, which you might not want.

The LA-2A is the best known example of this design. This also has a tube amplification stage which means it’s sometimes confused with a tube compressor, but the compression is all opto.

Which leads us nicely to tube compressors, or Vari-Mu’s…

Variable-Mu

Vari-Mu compressors use vacuum tubes in the compression circuit, which have two important characteristics.

Firstly, their compression is slow and smooth. It’s difficult to get ‘grabby’ effects from these designs, but they do ‘beautiful’ and ‘glue’ very well. They are often found across busses for this reason – they won’t make your snare slam, but they will do a great job of sticking your whole drum bus together.

Secondly, the natural tube distortion they impart sounds warm and creamy, and they can actually handle quite a lot of gain reduction before things get nasty.

Due to the cost of parts, power supplies and so on, these tend to be on the more expensive end of hardware compressors. The classic designs are the Fairchild 670 (stereo) and 660 (mono), of which there are many plug-in emulations (as well as some very good hardware recreations if you have the budget). More modern takes on this circuit are the Manley Vari-Mu and the Thermionic Culture Phoenix.

Hopefully that’s shed a bit of light on the different compressor types you come across and where their strengths and weaknesses lie.

As always though, try them where they are not normally used and see what sort of interesting sounds you can make by breaking the rules!

Check out the other parts of this series:

Compression – Part 1 – Here’s The Basics
Compression – Part 2 – Controls
Compression – Part 3 – Alternative Uses
Compression – Part 4 – Styles Of Compressor
Compression – Part 5 – Getting Personal