This circuit, from the 70's, can be assembled with a more modern FET like the BF245. The circuit is powered by a 9 V battery, as its consumption is very low.
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...