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.
The circuit shown was found in a 1994 Linear Applications Handbook by National Semiconductor. The...
We found this circuit in documentation from the 1970s; The circuit uses Japanese transistors of...
This traditional crystal oscillator configuration with FET was found in the 1994 Linear...