In the original publication of the 90s a 556 consisting of a double 555 is suggested. The circuit drives a speaker with good power. The power transistor shall be fitted with a heat sink.
This space mark demodulator for FSK signals was found in an American telecommunication...
This one transistor stage for audio signals was found in an old publication of the year 1958. The...
This audio step operates directly with the 110 V mains voltage rectified and reduced. The output...