Signal generator (CB21057E)

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

Microphone Micro Amplifier II (CB19625E)
The operational of this amplifier can be the 741 and the resistors R1 and R3 are of 10 k. R2B is...
Tone decoder (CB13936E)
This decoder filter was found in a 1975 documentation. The integrated circuits can be 741. The...
Power supply backup (CB24890E)
This circuit puts the battery in the circuit when the voltage of a source is cut, preventing it...