This commercial earphone receiver was manufactured in the 1960s in Japan. It was powered by a 9 V battery and the transistors were made of germanium.
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...