Found in Exar documentation from the 1980s, this circuit has a 60 dB cutoff at 1800 Hz. The circuit can be adapted to operate at other frequencies.
This circuit was found on a Radio Electronics of July 1984. It consists of a source with the 741...
This receiver for an audio link by modulated light was found in a 90s Elektor. Transistors are common...
We found this circuit in a Radio Electronics from October 1963. The circuit uses an output...