This simple compressor is designed to prevent large sudden changes in the loudness of an audio signal.
D1, D2, D3: HEP154 or SK3311 or NTE116. Diodes having similar characteristics include 1N321, 1N503, 1N504, 1N1008, 1N1028.
Q1: HEP243, SK3512, or NTE128 (Si NPN, max IC 1 A, BVCBO 120, BVCEO 80). Other transistors with similar characteristics include 2SC97, 2SC108, 2N497, 2N498, 2N656, 2N2192, 2N2193, 2N2890, 2N4943.
We haven't tested this circuit; it was passed along by a friend. If you try it, please let us know what results you get.
Other compressor/limiter schematics on the web:
If you go to Google or another good search engine and enter the phrase audio compressor schematic you will find other circuits to study.
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