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 circuit of various uses on the bench was found in Radio Electronics magazine from January 1960....
This circuit is from a 1957 Japanese magazine, showing the use of OC19 in a high voltage inverter. The...
The integrated circuit used in this power supply is no longer very common in power supplies today....