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My Yamaha THR10 amp sounds great in my apartment when running on batteries, but emits a nasty hum when powered by the power outlet in my apartment. I've tried turning off all lights and appliance in the apartment, but the hum is still present.

I'm not sure how to fix this -- are there hardware products that could "clean" the power from the outlet? Or might an electrician might be able to solve this somehow? Or is there some testing or maintenance of the electrical system in my house that I might do?

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    This cannot be accurately answered without knowing your country's (& if appropriate, building's) power specification, & the spec of the power transformer you are using. Many 'wall-warts' are of appalling quality [switch-mode] & additionally are not earthed.
    – Tetsujin
    Commented Nov 2, 2019 at 19:14
  • Dimmer light switches can introduce noise too.
    – Ray Butterworth
    Commented Nov 3, 2019 at 1:27

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Good intuition in your "clean the power" phrase, because the line noise you're experiencing is indeed called "dirty power."

It can result from old wiring or sloppy AC circuits in your house, or interference from other devices plugged in, but yes there are products that can smooth out the AC waveform and maintain a constant voltage called "power conditioners" or "line conditioners."

They're only a little more expensive than normal surge protectors, looking now on Amazon I see some from $50-100.

There are also "online UPS's" (uninterruptible power supplies), which is a battery-backup system whose battery is always engaged (online) between the input AC and the output AC. This requires the electrical power to go through a conversion to DC and then back to AC, which will remove all the noise. But you pay more for that power isolation/reproduction, online UPS's are around $500.

For your scenario I'd give a cheap power conditioner a try, it should do the trick.

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  • … or it might just be caused by a cheap switch-mode wall wart, no earth. Replace with a 'real' transformer, coil wound + earth… if your country specifies a good earth.
    – Tetsujin
    Commented Nov 2, 2019 at 19:12
  • More likely to be a grounding problem than 'dirty power'.
    – Laurence
    Commented Oct 28, 2020 at 15:07
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One thing to check, and it won't take just a moment; Unplug your computer internet modem.

If the noise goes away, that was the culprit.

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