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 is from an Electronics World of 1967. The circuit slows down the activation of a...
Found in a 90s Popular Electronics, this circuit is intended for amplifying analog video signals....
Found in Popular Electronics, this configuration can be powered by voltages from 6 to 12 V....