I'm encountering what appears to be an induced AC hum in a dynamic microphone. We've tried several makes and models with similar results. The source of the hum seems to be an AC distribution panel about 6 feet from our Voice Over booth, through the wall and across the hall. I've gotten the result with not only our main mixer, but with a battery operated field recorder. I've tried replacing the mic with a 150 ohm load and the hum disappears. I've not tried a low pass filter as the hum seems centered around 60 hz. The hum does not manifest in condenser mics, which seems to confirm the source.
Is there a way to shield the panel from the booth, or is some other solution in order?
You are correct. A dynamic mic has a coil of wire as the voice coil, and this will indeed create an audible hum. Shielding the panel would be difficult, but could be done using Mu-Metal - a type of metal used to shield transformers.
Also consider a dynamic mic with an internal hum bucking coil: Shure SM7B; Shure SM63