This 1969 circuit uses a 117 V filament tube powered directly by the power grid. The original circuit makes use of a charcoal microphone, a common component at the time.
We found this circuit in a 1979 French publication, but it can still be assembled today with...
This circuit can be used as shield or to drive powerful loads from low current switches such as...
This circuit is from old documentation having a gain of approximately 35 dB at 1 kHz with 2.2 mV...