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I'm new to the community and I'm not sure this is the place to be asking, but it seemed reasonable to give it a shot. Also I'm not an electrical engineer so I'm not familiar with the English jargon.

I have a Marshall guitar amp which operates at 127V BUT my house's electric wiring is monofasic, ungrounded, operating at ~220V (ranging from 207 to 230 according to a neighbor) so I have to use a tension converter.

The problem I'm trying to solve is to get rid of a constant humming noise in the back when I turn the amp on which, according to a friend, is caused by the converter. It happens when there's no instrument plugged in and only gets louder as I apply guitar effects or increase drive/gain.

I'm considering buying a noise suppressor, but I wanted to know if there are other equipment designed to handle this kind of setup or get any advice on other options.

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  • If it's a decent Marshall, I'd go the whole hog & get an amp tech to change the transformer - though I can't recall seeing a Marshall that wasn't dual-voltage [not that I've seen anything like them all]... & I just realised you said the house wiring isn't grounded... how is that possible? – Tetsujin Aug 27 '15 at 17:26
  • I read that marshall amps are all 110 by default. Regarding grounding, I'm in Brazil and I'm pretty sure grounding is not mandatory here. – Grasshopper Aug 27 '15 at 17:51
  • That sounds very scary, & could, in itself, be the cause of your issue. Anyway, Marshalls are British; we have 240v here. All the ones I've ever examined have had a dual-tap transformer & a plug or switch on the back that swaps for international voltages... then again, all the ones I've ever used have been old ones, so I've no idea what the newer ones are like, tbh. – Tetsujin Aug 27 '15 at 18:20
  • Which model is the amp? – Michael Hansen Buur Sep 2 '15 at 20:09
  • Marshall MG 15 DFX – Grasshopper Sep 2 '15 at 21:02
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This definately sounds like a grounding issue. How do you rectify this? Get a qualified electrician to install a grounding spike below your electricity meter. This is usually a 3 feet long copper rod around an inch or 2 thick driven into the ground around 2 feet, then an earth wire is connected from the mains to the copper rod.*

WARNING!!! - DO NOT TRY THIS YOURSELF IF YOU ARE NOT A QUALIFIED ELECTRICIAN*

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