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 a 2000 Poptronics. The magazine no longer exists, but the circuit can be...
This 80's circuit has an input resistance of the order of 880 M and gain 10, using operational...
This circuit can be used in a toy organ. The frequencies are determined by the capacitors selected by...