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.
This circuit for cars with mechanical ignition systems was found in the Electronics Experimenter handbook...
This code practice circuit was found in a 1990s issue of Radio Amaterur Today magazine. It...
The circuit presented was found in the Linear Applications Handbook of the National Semiconductoir of 1994....