We found this circuit for musical instruments in the March 1992 Radio Electronics magazine. The circuit has outputs according to the desired scale from a crystal oscillator at the indicated frequency.
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...