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In Reply to: Hum from mics in my recording setup posted by rekordboy on June 29, 2006 at 20:30:57:
Since the hum goes away when you change its position with respect to the equipment it means the mic is picking up RFI/EMI from the equipment. Move it further away. Most high quality mics would not have this problem, so a better mic may also be in order.
Follow Ups:
Thanks for your input Bill. I had my suspicions that I was getting RFI/EMI interference. The mics I was using for this test are drum mics ranging from 50-100 bucks a piece (Audio-Technica). I'll try using our tube vocal mic for the test. Could it also have to do with the quality of mic cable?
I'll go on record saying that cabling could be a problem. Twice I've come across cables with an audio-technica badge that had no business in a sound system whatsoever. No matter what I did, those cables allowed hum into the system. I changed the cables and no more hum.These particular cables were sold as mic cables, but their construction was not a standard shielded twisted pair. Rather, each single conductor had its own shield and the shields were tied together at each end with the conductors as positive and negative.
So, making sure you have decent, properly terminated cabling from microphone to preamp is certainly a good idea.
As an aside, a few weeks ago I had an assistant mic up a guitar cabinet with an SM57. When I opened the channel, there was an obvious hum. He couldn't figure out the problem so I went to the amp and it was dead quiet. The hum was being picked up via inductive coupling between the power tansformer in the amp and the coil in the microphone (EMI). I moved the mic to the other side of the cabinet and hum all fixed.
Hey, Thanks alot.Since my first post I tried using a different mic. I used a Sennheiser dynamic, cardioid vocal mic (I think it was a 431 or something like that, I have a few of them anyway) and the hum was virtually non existant. I could hold the mic anywhere I wanted, even right on top of the console and there was no hum. I tried making my own cables because I had some parts and I found some instructions online and they worked fine with the Sennheiser with no hum as well. That wiring structure you described with each conductor having a separate shield has ground loop written all over it.
Cheers!
Cables will almost never cause noise in a balanced system; junk that does get picked up is eliminated by the opposite phase of the two conductor legs. I suspect your drum mic doesn't have humbucking coils to keep the noise at bay.
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