This circuit with 24 dB per octave attenuation was found in the November 1973 Radio Electronics magazine. The components are for 1 kHz but can be recalculated for other frequencies.
The transistors are 2N918, 2N3464 and other RF equivalents of the time for Q1. For Q2, 2N3399, 2N2894...
This circuit, which can be used as a shield, was found in a 90s Electronic Design. The circuit...
This interesting intercom circuit was obtained from an Argentine publication from the 1970s....