Signal generator (CB21057E)

Found in Popular Electronics, this '90s circuit generates harmonic-rich signals for testing audio equipment.

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...