The LM3909 tone generator integrated circuit of this application is uncommon nowadays. The circuit can operate on a single battery and is very sensitive.
This is a circuit that we use very often in our projects, and it also appears in many technical...
Found in an American paper from the 90's, this circuit can be adjusted to pass frequencies above 30 to 300...
This preamplifier is indicated for low and high-impedance sources as microphones, transducers,...