This two-tube circuit, one being a double, was found in Radio Electronics magazine from July 1958. The distance to the microphone depends on noise and losses. The cable must be shielded.
This two-transistor circuit was found in a 1969 US documentation. Modern equivalent transistors can...
A touch in this circuit turns-on the lamp or the relay. Power supply is according the lamp or...
Using germanium transistors, this amplifier provides a 540mW output power when powered by 9V. We...