We found this circuit in a Radio Electronics from December 1963. The circuit combines valves and transistors and has a high input resistance. The circuit is powered by two batteries.
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...