This very old circuit, from a publication from the 1970s, uses a germanium transistor and two small transformers to couple the signals from a coal microphone and the amplifier. The power is supplied with a voltage of 22.5 V from a battery that was common at the time, but today it must be obtained from a source. Transistors such as BC558 can be used with possible changes to the 48k resistor, a value that is not common (47k).
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