On this page three adjustable notch filters configurations are shown . They can be used in your small pre-amp or amplifier project to filter out any HUM at 50 Hz ( European ) or 60Hz . By sustituting the capacitors values in the bridge other frequencies can be used . All op-amps configuration requires a +/- voltage supply which can easily provided with a voltage divider made of two 100K resistors and a 10 uF filtering capacitor connected from ground to the negative rail . Any op-amps such as the LF353 , TL071, LM308 can be used and supply from a 9 volt battery or other voltage up to 30 volts can be used .