This two-transistor circuit is from a December 1964 Radio Electronics. Modern transistors can be tested. The headphone must be high impedance or crystal with a 2k2 resistor in parallel.
This circuit can be used as a noise canceller for communication receivers or even common ones. The...
We found this circuit in an old American publication from the 1980s. Transistors can be...
This circuit is part of a synthesizer that was published in parts in several issues of Radio...