We found this circuit in a 1967 American documentation. The diodes are general-purpose and the input frequency can be between 80 and 800 Hz.
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...