This circuit was obtained in a Popular Electronics from the 90's. The gain depends on R4 and R3. Transistors are general purpose types.
This circuit was obtained in an old publication of 1978. The integrated circuit is the critical...
This circuit was obtained in the publication 101 Electronics Projects of 1979. The circuit can be...
This 300 Hz to 3.4 kHz filter was found in the May 1989 Radio Electronics magazine. The circuit...