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.
Found in an American publication from the 90s, this circuit allows the supply of 3, 6 and 9 V...
This is a circuit from the time when some still called the transistor a crystal triode. It...
This circuit for voltages up to about 20 V was found in a 90s Popular Electronics. The LED can be...