Originally designed for WWV (Standard Time Station) signals, it lets 1 kHz tones pass by rejecting the 440 and 600 Hz tones emitted by the station. The circuit is from a 1964 Electronic Design.
This circuit with transistors was found in an old Japanese publication, translation of an American...
This 8 W amplifier with germanium transistors was found in a Japanese documentation from 1967. The...
This circuit of the 80's makes use of components of the time. The circuit is from an American...