Found in American documentation from 1968, this circuit has an acute cut-off of the frequency to which it is tuned and which depends on RC. Transistors may be modern equivalents of general use.
This circuit was found in Motorola documentation from 1966. The lamps are according to power and...
This circuit, which uses components still common today, was found in a 1997 documentation. It...
This circuit is intended to apply the music signal from a stereo to a telephone line for the...