This filter rejects the hum of 60 Hz from power grid, being placed between the signal source and the input of the amplification or recording circuit. With it, noise due to shielding problems, filtering and sources can be minimized. The filter is adjustable allowing to get the exact frequency of rejection with greater efficiency. Operational amplifiers can be the 741 and must be powered from dual supply. A good filtering of the source is essential and the voltage should be between 9 and 15 V. The input impedance is high and output is low impedance with sufficient signal level to excite most amplifiers. The original circuit is from an old publication, but we made changes to their assembly became feasible with the components available today.
