This simple audio signal tracer or follower can be made with a BC548 instead of the original transistor. The output is made in a small crystal phone. The power can be from 5 to 9V.
This 1950's circuit used one of the first commercial transistors and had two stages for audio...
Using integrated circuits from the time, we found this audio compressor in an English publication from...
This is an experimental crystal radio that uses no power source. The antenna must be very long (at...