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 is suggested by Motorola, using a 4N26 type coupler and an MC1733 type operational....
Texas Instruments' low power operational amplifiers OPAx196 can be found on Mouser Electronics....
This is the power step of the previous circuit. Output transistors must be mounted on good heat...