This circuit is from a Radio Electronics magazine from May 1986. Observe the voltages applied to the capacitors, taking care that the capacitor under test can withstand them.
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...