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
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.