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 was found in the ON Semiconductor's Thyristor manual of the 1980s. The high quality...
This simple injector circuit can actually be powered by voltages from 3 to 9 V. The frequency is...
This 10 kHz filter is based on a 741 operational amplifier. The circuit is from Electronics Now...