The idea of this circuit found in a Radio Electronics from January 1966. The idea of the circuit is to improve the reception of audio signals, mainly telegraphy, working as a filter.
The circuit shown is from an old European publication. In it we have the way to activate a relay...
We found this circuit in a European publication from the 1990s. Transistors are still common types...
This circuit was found in an American documentation and can be added to common amplifiers of any...