can someone explain how i get rid of the ground loop hum created when i plug my preamp into my interface. i assume this is what i'm experiencing. the preamp is powered by a 2 prong wall wort. the interface is usb bus powered. i've read that i have to lift the groung from one end of the chain and not the other but i don't know how to do that. the laptop its going to is plugged into a different outlet - but the hum only occurs when the preamp plugs into the interface (input and headphone gain not affecting the hum at all). thanks for any help.