Toolkit: Analogue Compressors
Do you know your FET from your VCA or your Vari-Mu? In the first of our new regular series, we’ll expose all
Like classic analogue synthesizers, the world’s most coveted analogue compressors are capable of fetching eye-watering prices on the second-hand market. Fortunately, the quality of software emulations of analogue compressors has improved dramatically in recent years.
The reason that analogue compressors have such a popular sound isn’t only down to their ability to control dynamic range but owes as much to the non-linearity and distortion which they bring to the signals they treat. However, this is where confusion often starts. You might know the names and model numbers of the most coveted analogue compressors of all time, but did you know that these use different approaches to dynamic range control, resulting in dramatically variable sonic results?
There are four main approaches to controlling dynamics in the analogue domain. The first of these is VCA or ‘Voltage Controlled Amplifier’ compression. VCA compressors use peak detection to control dynamics, meaning that when signals exceed your chosen threshold, they respond, reducing the volume of those peaks to flatten dynamic range. As VCA compressors are capable of extremely quick response times to transient-heavy material, they’re a hugely popular choice for drum sounds in particular.
Perhaps the most famous compressor of all time is the Urei 1176, which is a type of FET compressor. Like their VCA equivalents, FET designs offer quick response times, but rather than providing the clear, uncoloured sound of VCA compressors, FET designs bring warmth and colour in an unapologetic manner; this is a compressor that wants to be heard.
Opto (optical) compressors use light to determine the process of gain reduction, and they’re particularly great at providing a slower, warmer approach to compression. Opto models usually react more slowly to transients, but they sound musical and natural on smoother and less biting program material. As a result, they also make great mix bus compressors or compressors for grouped sounds. The LA-2A, LA-3A and TubeTech’s CL1B are famous examples of Opto compression.
Last but by no means least, let’s look Variable Mu compression. Manley’s Vari-Mu is the most famous example of this compression type, and it employs a vacuum Tube to achieve dynamic range control. Its non-linear sound is partly achieved as Ratio and Gain Reduction increase in tandem and, like Opto compression, Variable Mu compression is particularly popular across grouped instruments or the main output mix bus, owing to its slower, less aggressive response time.