The function of this circuit is similar with the hum filter in our previous circuit, but it use transistors instead of op-amps. It uses BC109 transistors, but any low noise high gain type will be suitable. This hum filter is adjustable via trimmer pots to fine tune it’s notch depth.   The figure below shows the schematic diagram of the circuit.

transistor hum notch filter circuit schematic

The power supply should be hum free, or the circuit itself will generate hum. The best way is to a battery, but a IC-regulated supply is adequate. [Circuit’s schematic diagram source: elecfree.com]