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Are there any chips or simple circuits available for removing the 50/60Hz mains hum from a regular power supply unit?

So I've got a few bits of audio equipment that have bad PSUs and they have a bit of mains hum, it seems to be a common problem with cheaper audio equipment - so I'm just curious to find out about any cheap or simple solutions to this common problem.

So far I've used basic RC filters and Op Amp based ones too.

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2 Answers

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Are you sure that the noise is coming from your power lines? One other thing to consider is that with cheap sound equipment it tends to be that all of the components and design are cheaper. This can include anything from resistors to opamps, even the PCB used. There also tends to be little shielding on cheaper boards.

If it is the case that your components are cheap with no shielding there could be noise from florescent lights getting into your system. (This is of course if you are in a room with florescent)

I have also noticed hum being introduced to audio systems when cheap dimmers are used for the lighting in the room.

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I'm sure - I have a load of battery operated audio equipment and I'd like to covert them to accept a regular wall wart PSU - I just need a filter to go between the two. They currently don't have a ad/dc input just a 9v battery – Jim Apr 2 '10 at 21:11
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Better filter caps?

It's not always the power supply. Grounding can be a common problem in these kinds of units, ground loops in particular.

If the following circuit is representative of your power supply...

alt text

...C1 and C2 are the filter caps. You can try increasing the values of those to reduce the hum. They are typically electrolytics. If your power supply doesn't have voltage regulators like this circuit does, adding voltage regulators should lower the hum considerably, if it's not a ground loop.

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You're right - that's a big problem too - but it's normally quite cheap to just go over the circuit and make sure it's grounded properly. Is there a particular type of capacitor you would recommend for using with a filter? – Jim Apr 1 '10 at 14:55
@Jim: See my edit. – Robert Harvey Apr 1 '10 at 20:33
Thanks for the schematic - could this be added post ac/dc PSU, on say a regular wall-wart power supply? (they're all plastic moulded & I can't pull them apart) I've been making simple IC based amps that are powered by a 9v battery and I'd like to include the option of plugging them in with a store bought PSU - thanks for the help – Jim Apr 1 '10 at 21:03
@Jim: Yes, some additional filtering caps, and possibly a voltage regulator in-line, should do the trick. Make sure you respect the current capabilities of the wall-wart; if you are pulling current at the limits of the wall-wart's capability, that could be a big reason you're getting hum. – Robert Harvey Apr 1 '10 at 22:51
Cool I'll give the V reg and caps a go Thanks for your help – Jim Apr 2 '10 at 21:13
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