We found this active crossover in a Radio Electronics magazine from June 1961. Note the different adjustment ranges. The signal must be applied to the amplifier input.
This 4-transistor signal tracer or follower was found in Radio Electronics magazine from May 1960....
This 1967 Raytheon circuit uses an optical coupler of the era and has a range of 800 to 200 Hz....
Assembled with SMD components this circuit can be very small. The range is a few tens of meters. We...