Compressors work at line level, therefore the input signal has to be taken from the mixing console, preferably from the channel insert point send.
The output from the compressor is brought back to the channel insert return.
By connecting the compressor at this position in the signal chain, its operation is unaffected by the use of any of the console controls, except input gain (although in some consoles, the insert is post-EQ).
An alternative is connection to the group insert point of the console, or the main stereo output's insert point. In either of these situations, a mix of signals is compressed.
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