This variable filter circuit was found in July 1975 Radio Electronics magazine. The circuit uses 3 general purpose op-amps like the 741 and must use a symmetrical source.
This circuit can operate with signals up to 60 MHz using the indicated operational. We found it in...
This circuit takes advantage of the noise generated at the reverse biased junction of a germanium...
Equivalent transistors can be used on this 80's circuit. Any general purpose PNP transistor can work...