Does an iMac have phantom power?

no mac supplies phantom power, i'm afraid.

make sure both the mac and the power supply are plugged into the same outlet

There is something called balanced power, but it's a somewhat high end and expensive option. Overkill for what you're doing.

By the way, balanced power is different from a balanced audio signal or cable.

Incidentally, speaking of cables, do you have a decent one for your mic?

I't very unusual that you would get a ground hum problem when using a DC current, AC/DC transformer. You may want to take a look at other sources like lighting or other electrical devices. Sometimes just moving things will get rid of him.

many audio interfases have phantom power such as m-audio. balanced is basicly 3 wires such as a mic cable (XLR) and unbalanced is a guitar cable 2 wires must use balanced for phantom power. about noise you should never run audio cables paralel to electrical cords floresent lights can cause noise and an easy fix sometimes is to use a ground lift plug

FWIW, I was playing around with a friend's interface this past week. It's a newish interface, nothing fancy, but not too cheap either. Anyway, it has both balanced and unbalanced outputs. For some reason, hooking up the unbalanced outputs to the monitors had a weird noise on them, not a typical grounding noise, while the balanced outputs to the monitors was noise-free. Go figure.

So... if there is a choice, balanced is better!

Uh...how are you connecting your mic to the Mac?