This 1967 General Electric circuit makes use of an array of 3 (integrated) transistors for an amplifier stage. The circuit has a 500 mV output.
This circuit was found in the Spanish magazine that no more existis Mundo Electrónico, in a...
This experimental circuit is from a 1975s publication. The coil consists of 80 turns of wire AWG...
We found this circuit in an American 1980s documentation. The transistors can be equivalent as BC548...