Last month, I ran you through the function of a compressor and the controls commonly found on one: threshold, attack time, ratio, release time, and output gain. That’s only part of the story, though; over time, a few different types of compressors have emerged as technology has evolved. Many compressors have a distinctive sound colouration or feature set that makes them more suited to a particular application. This month, I thought we’d touch on a few of the main ones.
Tube or Variable-Mu compressors use tubes (valves) in the circuitry and amplification, and have a fairly unique characteristic in that the compression is not linear - as the amplitude of your signal goes beyond the threshold, the ratio of attenuation increases, creating a desirable smoothness. They’re the oldest compressor design, having originated in the ‘50s, and have a distinct ‘vintage’ sound colouration. They are, by modern standards, slow but still useful as a ‘glue’ compressor - applying a sheen and smoothness to a group of sounds. The most famous example is likely the behemoth Fairchild 670, used on many of The Beatles’ recordings.
Optical compression is another old technology that came around in the early ‘60s, but certainly one that left its mark. It consists of a system in which the amplitude of incoming audio affects how brightly a light element lights up - controlling how much a light cell attenuates the audio signal. This made them faster than tube compressors, but with a distinctive ‘soft-knee’ in the attenuation. Many models still used tubes in the output amplification. Perhaps the most famous and common example is the Teletronix/Universal Audio LA-2A. Originally introduced in 1965, they’re a versatile unit, commonly used to bring a nice vintage bite and punch to instruments like drums and a creamy warmth to vocals.
FET (Field Effect Transistor) compression introduced solid state circuits to compression, and with it, incredibly fast attack times. The first and still popular FET compressor was the Universal Audio 1176 which blew minds with its 20 microsecond attack time. FET compressors tend to have a brighter and more focused sound, which makes them less suitable for mix buss or master compressors, and more suitable for aggressive use on drums and rhythm - getting that big punch and ‘whack’.
VCA (Voltage Controlled Amplifier) compressors are your workhorses used more for utility than sound colouration. They are transparently fast and will tame the wild peaks of your track without affecting its overall sound too much. These tend to have the most controls and sound sculpting capabilities, and won’t make themselves known in a mix unless you really want them to. Having said that, there is a vast range of VCA compressors on the market, and there are many like the SSL and API models that have a distinctive sound that many top engineers find desirable.
They’re the main four types of hardware compressors, and generally, software compressors are modelled after one of those systems. Next month, we’ll focus on some different ways to use compressors and introduce some nifty tricks.
Missed Compression 101? Catch up here.