compressor types

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#audioenginering🔉 #electronics

What are the differences between various types of compressor?

*Compressors are often described with category names for different circuit designs. Naturally you will wonder "how do they sound", and why would you pick one type over the others.

VCA stands for Voltage Controlled Amplifier. Technically most comp circuits could be described as VCAs in a sense, but in practical usage it means an IC chip that contains transistors that follow your incoming signal level (voltage) to determine how much negative gain to apply. VCAs are typically associated with fast, clean compression with precise controls over attack and release.

FET stands for Field Effect Transistor, and they are essentially a subset of VCAs, with their own particular qualities. FETs are often used as a solid-state emulation of tubes, but the main reason a comp will be advertised as FET-based is so you'll associate it with the famous and coveted UREI 1176.

OTA stands for Operational Transconductance Amplifier, which is a chip like a VCA except its output is variable current, not voltage. It refers to a specific circuit design based around the CA3080 chip (or a modern replacement for it like the LM13700)

Optical comps contain a light source that gets brighter as your signal gets louder, and a light-sensitive resistor that reacts to the brightness of the light by decreasing its resistance. The resistor works in either a feedback loop or a voltage divider to change the level of your signal. In some older designs these were separate components, but in 99% of modern designs the light and the resistor are encased in one small black cylinder.

Optos are generally described as very smooth, slow-releasing, and "organic".
+{}"Tube compressors, almost always, are really optical comps with a simple 12AX7 tube gain stage added on the end. However vari-mu (short for Variable Mu, also a Manley trademark) designs use a vacuum tube in place of a transistor, where the variable voltage input changes a tube's bias instead of a transistor's gain. It has a somewhat different action, generally claimed to be even smoother and more "creamy" or "organic".

Feed-forward means the control circuit will receive ("hear") your signal before the signal goes through the amplifier stage; this is the more modern approach. Feed-back means the controller receives the signal after the amplifier stage, and this is more associated with older designs. "How they sound" is a very subtle shade of distinction, you'd have to try both to have a feel for it. Most of the previously-named circuit designs can be built to feed forward or back.