There are many ways to filter the hum noise from AC line (50 Hz) from audio signal away. This is Hum Filter circuit using electronic coil. A frequency filter usage is a simple ways that include RLC as the electronic coil. In Figure 1 is the specific frequencies filter.

If thoroughly consider to see that if filter frequencies at 50 Hz away, need to use value of Q equal 10 we must to use the inductance is almost 150 uH. Which is difficult and larger. How easy it is to use an electronic coil in Figure 2 works instead.

This circuit operate by two op-amp and R2-R5, C2 and P1 Including as virtual coils. Which the inductance will take place at pin 3 of IC1 to ground. This inductance depends on the value of R2, R3 and C2 (as formula: L = R2 x R3 x C2), and can also adjust photentiometor-P1.

If this circuit is adjusted correctly, The attenuation of frequency 50 Hz, it will be 45 to 50 dB. This is enough to be used for attenuation hum sound for the harmonic distortion meter. Or Hum filter for audio signals from the TV.

**Parts you will need.**

IC1,IC2_______LF356A

C1___________68n or 0.068uF 50V___ceramic capacitor

C2a,C2b_______150n or 0.1uF 50V___ceramic capacitor

C3,C4_________100n or 0.1uF 50V___ceramic capacitor

R1____________4.7K 1/4W Resistors tolerance: 5%

R2,R3_________22K 1/4W Resistors tolerance: 5%

R4,R5_________10K 1/4W Resistors tolerance: 5%

P1____________5K potentiometer

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## Salim Khan

20 Nov 2014Thanks.

## momename

20 Jan 2015Hi Salim Khan,

Thanks for your feedback.

## walter

20 Dec 2018Frohe Weihnachten an alle Bastler

## ElecCircuit.com

22 Dec 2018Hi walter,

Vielen Dank für Ihr Feedback. Frohe Weihnachten und ein glückliches neues Jahr.

## AK DAS

27 Dec 2018I like to buy printed circuit board single/ double layer as per my choice of your various circuits. Kindly let me know the cost of PCB as per sq cm., if is it possible the delivery would be taken by courier.