Bass Control (CB21294E)

This circuit with losses of 20 dB at 350 Hz was found in older documentation, but it applies to modern preamp and amplifier circuits.

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Audio controlled switch (CB8435E)
Found in a 90s Popular Electronics, this circuit triggers a load from the audio signal applied to...
Two-way MW radio (CB18318E)
This old receiver (1967) uses germanium transistors from the time and coupling transformers,...
General Purpose Preamplifier (CB10500E)
I found this circuit in a Popular Electronics from 1992. The circuit can be easily implemented with BC549...