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I use the AKG c411 contact mic on a small harp that I play. When I plug into my interface I get a low frequency hum, which goes away when I touch the computer or interface and hold the instrument, which means I need a ground. I can put the interface on the floor and touch it with my foot when I record, which is a pain, but I'm taking this chance to ask about what a good DI box/preamp would be for this mic, which needs phantom power. Anybody got any experience with this mic or similar setups? This would be the live setup too, so I don't want to have to carry more stuff than I need, a combo box would be great. Thanks in advance

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    Sounds like your computer itself is maybe not grounded, or a bad cable. To you have a multimeter to test continuity? – danmcb Nov 11 at 11:37
  • to clarify: 1) is there an amp etc involved or is this hum present from the (presumably) laptop speakers; 2) is it a laptop?; 3) is your laptop powered; 4) is your laptop power supply from the mains a two-pin shielded or 3-pin grounded? – Yorik Nov 12 at 22:11
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It's unusual for a laptop to have any Earth at all. It will also probably have a nasty buzzy, whiny switch-mode PSU.

If you live in a country where Earthing is optional on mains sockets, you may end up having to carry a physical earth strap & find a pipe to fasten it to.

We really need more detail on the interface, the computer & your country's Earthing policy.

I'd suggest a 1:1 isolating transformer but a) they're heavy & b) they're not cheap… also c) they won't kill the nasty buzz of a switch-mode supply.

The other option is to run a laptop on battery power only.

  • I don't think he's reporting a PSU buzz. More a 'mains hum'. But yes, running the computer on battery alone (if it IS a laptop?) will prove the point. – Laurence Payne Nov 11 at 19:58

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