Define "normal" mic, and the mic cable as well please.
In most cases you want low Z BALANCED cables mic> preamp> converters.
High impedence cables are for guitars, basses, High-Z mics, stuff with HIGH impedences.
If you're using an SM57 or 58 or similar, or a condensor, you want a Low Z cable.
Hum can come from a few different places.
You want your preamp and the computer on the same power circuit. Everything in your rig, both gear and computer, needs to be on ONE circuit.
Turn off your AC, your reefer, any flourescent lights, especially if they're in the room your recording in. All that stuff produces line noise, and turn off any lights that are on dimmers, too. You don't want any gear sharing a circuit with dimmable lighting. Dimmers will produce hum.
You wanna make sure that circuit is properly grounded back to the panel, and that the panel and the service are properly grounded too.
You can buy a recepticle tester at HD for less than $15US.
Once you know your power and grounds are good, then you check the signal chain itself.
IF FOR ANY REASON YOU HAVE TO WORK ON THE POWER CONDUCTORS OR DEVICES, FOR GOD'S SAKE TURN OFF THE POWER AND VERIFY WITH A TESTER THAT IT IS INDEED OFF!!!
Try looking into this stuff first.
If those aren't the issues it could be a dodgy connection on a cable, or a loose ground in the pre, or a few other items as well. Do you have the hum if you run the pre straight into your monitor scheme?
