This circuit for audio applications was found in an English magazine from the 80's. The circuit has a pass band below 3 kHz making it ideal for telecom applications.
This control, from a 1982 documentation, uses an integrated circuit and a Darlington transistor, as shown...
This high-power amplifier with a single operating LM12 (difficult to obtain) is from the Linear...
The coil of this receiver consists of 100 turns of wire 38 to 30 in a ferrite rod and the...