Depending on the manufactur­er and the type of compressor, the number of controls, and their labelling, will vary across the spectrum, but hopefully the following guide will cover most options.

THRESHOLD/COMPRESSIO­N/ SUSTAIN/INPUT: This sets the level at which the compressio­n is activated. Turning this control up will lower the threshold and the compressor will engage at lower input signal levels.

RATIO: With a 2:1 ratio, a 2dB increase in input level is needed for the output level to increase by 1dB. A 20:1 ratio requires a 20dB increase in input for 1dB of extra output. It’s all about compressio­n strength.

ATTACK & RELEASE: Attack sets the compressor’s response speed when the input exceeds the threshold. Meanwhile, Release determines the rate at which it resets after the input drops below the threshold.

MAKE UP/GAIN/LEVEL/OUTPUT: Heavy compressio­n may reduce overall signal level and this control will compensate for the level drop or even provide a boost.

MIX: Mix allows you to blend the compressed and uncompress­ed signals. This is very rarely seen on studio compressor­s and is more likely to be available on pedals.

HARD/SOFT KNEE: Most commonly seen on studio compressor­s, a Hard Knee setting applies full-strength compressio­n as soon as the input signal crosses the threshold. A Soft Knee setting, on the other hand, begins by adding gentle compressio­n before the input signal reaches threshold level, and compressio­n strength gradually increases as the input signal reaches and then exceeds the threshold.

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