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 a 2005 Linear Technology manual. The circuit is powered by 3.3 V but can...
This circuit uses two operational amplifiers, which can be of the type 741, in a full wave...
This simple circuit for an audio link uses a lamp, which has a very poor frequency response, but...