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

**Figure 1 the simple specific frequencies filter using RLC**

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.

**Figure 2 Hum filter using the electronic coil**

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