This filter, which rejects frequencies around 1 kHz, was found in the August 1993 Electronics Now magazine. The fine tuning is done at R3, and the source must be symmetrical.
This circuit using an operational amplifier was found in old American documentation. It can be...
If the circuit oscillates, emitting sound, it is because the transistor is good. This simple...
Like the previous circuit, this is used in the analog part of the telephone lines. The same...