Compressor Topology

Photo: Universal Audio

Feedback and feed‑forward compressors can sound very different. Which type is right for which job?

Most of us have a pretty good idea of what a compressor does, but a less widely understood difference between devices is the choice of a feed‑forward control circuit or a feedback one. To help you understand the pros and cons of each approach, I’ll recap the basic building blocks of analogue compressors before offering some thoughts specifically on feedback and feed‑forward compression.

Compressor Basics

At the heart of any analogue compressor is some type of gain‑control element. You can visualise this as a kind of automated volume knob, though in reality there are no moving parts. (Technically the electrons move but you get my drift!) During the evolution of compressors, we’ve seen various types of gain‑control circuitry, including variable‑mu valves, photocell‑and‑lamp arrangements, diode bridges, FETs (Field Effect Transistors) and solid‑state VCAs (Voltage Controlled Amplifiers). Not all of these offer linear gain control, meaning the output gain may not exactly follow changes in the control signal. (These non‑linearities can contribute in a subjectively positive way but that’s another story.) The most linear of the gain‑control elements mentioned above is the VCA, which is why so many commercial compressors use them; their behaviour is predictable. In the software world it’s possible to create very linear compressors too, or to emulate the behaviour of VCAs or that of less linear hardware. But to avoid having to keep repeating ‘gain control element’, I’ll use the term ‘VCA’ from here on.

The part of the circuitry that instructs the VCA when to bring down the gain is called the side‑chain. This takes an audio signal as its source, and generates a signal that follows the envelope of the audio waveform. Usually, the source of the audio used to feed the side‑chain comes from somewhere within the compressor itself — it’s based on the source you’re processing — but when setting up things like ducking it can be taken from an external source, assuming the compressor has a dedicated external side‑chain input.

Further circuitry is used to set a threshold level above which processing will take place, and also to adjust the compressor’s attack and release times, which determine the speed of its response to changes in the signal level. A ratio controls adjusts the strength of the compression, with higher ratios leading to more compression. For example, a 5:1 compression ratio means that for every 5dB that the input signal level increases above the threshold, the output level increases by just 1dB. There may also be further processing to change the linearity of the processing or to achieve soft‑knee compression, whereby the intensity of compression (the ratio) increases as the signal level increases, rather than a fixed amount of gain reduction being...