I've an audio amplifier that was built from spares from an old kit from 19-something. The power supply for this amplifier is the simplest it could possibly be, consisting of a transformer, bridge rectifier and bulk capacitor.

Even when receiving no signal i can hear a humming coming from the speakers. i have a spare EMI filter circuit that came from some consumer appliance, and my hope is that it will solve the issue, but that's beyond the scope of this question. My question is where on the power supply circuit should I put the EMI filter - on the primary, secondary or the rectified output?


simulate this circuit – Schematic created using CircuitLab

enter image description here

  • 4
    \$\begingroup\$ If you can hear the hum it isn't an EMI concern bit a power quality concern. \$\endgroup\$
    – user16222
    Jul 12, 2021 at 22:47
  • \$\begingroup\$ If you short the inputs of the amplifier, do you still hear the hum? Is your 80mF milli or micro Farads? If uF, that's not enough capacitance for an audio amplifier. \$\endgroup\$
    – qrk
    Jul 13, 2021 at 0:54
  • \$\begingroup\$ It's 4700uF x 80V \$\endgroup\$ Jul 13, 2021 at 4:03
  • \$\begingroup\$ For a one-off, used in your own home, feel free to omit the EMI filter altogether. You still have to tackle the real problem, which is hum, not EMI. \$\endgroup\$
    – user16324
    Jul 13, 2021 at 11:42

2 Answers 2


That filter looks like it's a line filter that goes before the transformer to block conducted emissions from the device back on to mains. If it has a rating of +220V then that is it's intended use. It may block some emissions from mains from getting into your device, but the best to block noise is appropriate filters after the rectifier and also on the voltage regulator of your design.


That's the wrong sort of filter.

If you're getting buzzing generated by the rectifier put small capacitors in parallel with each diode.


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