Discrete Compressor

[Eb7+9]

common in the the days of shortwave prior to cheap op-amps ...

http://www.lynx.bc.ca/~jc/sam101compressor.gif

... from a 1968 Sam's book by Robert Brown and Tom Kneitel

[Paul Perry (Frostwave)]

I suppose when the signal level increases, the diodes conduct more, resulting in a lower impedance for small AC signals, and they form the lower end of a voltage divider so less signal goes through. That's a guess, though.