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Buzz/hum when recording from wired lavalier to AC-powered recorder

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Matthew Ross
Buzz/hum when recording from wired lavalier to AC-powered recorder
on Jan 27, 2012 at 7:52:03 pm

I’m using an AKG CK 99 L lavalier mic wired into a Marantz PMD661 recorder, and if I power the Marantz with AC, I get a buzz in the recordings. In between the mic and recorder, I have an AKG MPA V L phantom power adapter (as recommended by AKG). We don’t have a wireless transmitter/receiver yet, so for now we’ve got to use this wired solution.

Troubleshooting I’ve tried:
- Power the Marantz with batteries instead of AC. This eliminates the buzz.
- Power the Marantz from an AC outlet in a different area of the building. Still has the buzz.
- Power the Marantz from the battery output side of a UPS. Still has the buzz.
- Power the Marantz from a pure sign wave AC inverter (I just happened to have one in my car) coming from the DC power outlet in my car. Still has the buzz when the car is off but in "accessory" mode, but when the car is running, the buzz was either gone or greatly diminished. Hard to tell over the sound of the engine.
- Plug a different condenser mic (Sennheiser MKH 50) into the Marantz. Even with the Marantz powered with AC, there was no buzz.
- Plug the AKG CK 99 L (still through the AKG MPA V L) into a completely different system that supplies phantom power. No buzz.

So the issue has something to do with AC power, and probably a ground problem; I just can’t figure out why it’s an issue with the AKG plugged into the Marantz and not other condenser microphones, and why the AKG doesn’t seem to have the same issue when plugged into another system. The solution for me for now would be to just power the Marantz with batteries, but ultimately I’d like to not have to rely on that.

Any ideas?

Thanks!


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Bill Davis
Re: Buzz/hum when recording from wired lavalier to AC-powered recorder
on Jan 28, 2012 at 6:54:23 pm

Your tests kinda showed that your problem shows in one mode only.

AC powering of the Lav.

That other mics don't exhibit the issue, points to the lav.

My suspicion is that you're using the mic in a situation where there is significant RF interference and the AKG lav has a bad connection and the process (balanced audio) that usually suppresses common mode hum - simply isn't working.

FWIW, doing a "signal chain" check often works best by simply removing or replacing the major system components one at a a time. That's each device - and (and this is sometimes overlooked) each connector cable between them.

Replacing one "system" like an AC powered signal chain - with a battery powered one can provide limited info - like whether the problem is in the replaced sub-chain - but it's usually better to keep the overall system as consistent as possible and simply check out each discrete part of it so there's no confusion as to which discrete part is the problem, rather than which sub-system is bad.

That's a suggestion, however, since everyone here knows that when you're shooting, and you just need the problem to STOP - anything you can do - in whatever order you do it - that gets the result you need - is the correct solution!

Good luck.

"Before speaking out ask yourself whether your words are true, whether they are respectful and whether they are needed in our civil discussions."-Justice O'Connor


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Matthew Ross
Re: Buzz/hum when recording from wired lavalier to AC-powered recorder
on Jan 31, 2012 at 8:35:14 pm

Thanks for the response, Bill.

Since the initial tests, I connected the lav and its phantom power adapter to a Sony EX1, and ran that off battery power, then AC. Sure enough, the AC recording had a slight buzz in it (though not nearly as pronounced as from the Marantz). I believe you are correct that I'm not getting properly balanced audio coming from the mic through this phantom power adapter.

Now I just have to figure out if there is anything I can do about that, or if I'm stuck running the recorder on batteries for now.



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Matthew Ross
Re: Buzz/hum when recording from wired lavalier to AC-powered recorder
on Feb 3, 2012 at 4:12:56 pm

Further tests and help from AKG showed the problem to be just a good ol' fashioned ground loop. The AC adapter for the Marantz has no ground plug, so attaching ground to its chassis made the problem go away. I figure the Sennheiser mic did not have the issue simply because it's shielded differently. Indeed when you hold the AKG lav in your hand for testing rather than on a lapel, the buzz issue was at its worst. And when holding the lav in your hand with the buzz in full effect, if you touch the Marantz chassis, the buzz goes away. That fact was the final troubleshooting step that showed the problem was with ground. And I figure the AKG did not buzz on the other AC-powered system I had initially plugged it into because that system is grounded.



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