This circuit lets through the 300 Hz to 3kHz range that corresponds to the human voice. We found it in the Radio Electronics magazine of February 1990. The circuit uses both parts of the LM381.
The integrated circuit shown in the figure is nothing more than the LM386 sold by Radio Shack,...
Found in an English publication from the 1980s, this circuit can be built with equivalent...
The transistors of this old circuit from the 60s can be the BC548, in a current version. The...