Classic Compressor Types

Please research and find out as much as you can about these classic Compressor Circuits, that are emulated in Logic Pro X's Compressor plugin.

What is each one modelled on? Why are they regarded as a classic compressor? What are they good for?

Opto Compressor

Opto compressors make use of photocells in the detection circuit used to trigger compression. In other words when the threshold is reached a light is triggered, which in turn is detected by a photocell, which controls the compression applied to the signal. The characteristics of this type of compressor circuit are a gradual and soft release curve that sound natural and musical. The first 50% of the release time occurs rapidly but the complete release time can take up to several seconds. The photocell also creates a phenomenon called memory, where the release time of the compressor is dependent on the duration and intensity of the light generated. This means that if the signal is loud and the compressor is working harder the release time will be longer. This smoothes out the uneven volume fluctuations caused by other compressor types.

A classic outboard Opto Compressor is the widely used and very highly regarded Teletronix LA-2A

As can be seen in the above image this is a very simple compressor in terms of available controls. There is only a Peak Reduction control, which determines the amount of gain reduction, and a Gain control to put back the volume lost from the compression process (make up gain), with the internal Opto circuit controlling attack, release and ratio dependant on the signal level.

The LA-2A is widely used on vocals, acoustic guitar and bass, as its program dependent nature gives it a very musical sound. It lacks the Attack and Release controls needed for more percussive sounds with aggressive transients.

FET compressor

A FET compressor, or Field Effect Transistor, uses transistors in the compression circuit, which give it very aggressive attack characteristics, making it ideal for drums or very transient material such as picked acoustic guitar.

This circuit type is again very program dependent - this means it responds differently to signals fed in to it.

They offer more control than a classic Opto compressor, adding in input level, Attack and Release controls, plus selectable ratios of 4, 8, 12 and 20 to 1. In addition to this by pressing all four ratio buttons in together a different character of compression can be achieved altogether. The ratio is somewhere between 12 and 20 to 1, the attack and release times change, and the end result is an overdriven tone, with a lag on transients giving a very punchy attack sound. This has become a very famous 'drum sound', widely used in 70's rock, which is a very punchy attack followed by a sucking phenomenon which accentuates reverb and background noise.




VCA Compressor

Here are some classic pieces of outboard to get you started

Teletronix LA-2A

Urei/Universal Audio 1176

SSL G Series Bus Compressor

DBX 160

Focusrite Red 3

Fairchild 670

Manley Vari Mu

Gates STA Level