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.