Description Regarding this circuit has three change-able notch filters configurations are display . You can take to used in your small pre-amp or amplifier circuit to filter out any HUM for 50 Hz ( European ) or 60Hz . With sustituting the capacitors values in the bridge other frequencies can be apply. About all IC op-amps configuration requires a +/- voltage supply which can easily provided with a voltage divider made of 2 each 100K resistors and a 10 uF filtering capacitor connected from ground to the negative rail . Any op-amps such as the LF353 , TL071, LM308 , They can be used and supply from 9 volt battery or other voltage up to 30 volts can be used Source -http://www.free-circuit.com/2011/02/11/60hz-noise-fillers-adjustable-circuit-and-pcb-parts-assembly/