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

[Bill Davis]

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

[Matthew Ross]

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.

[Matthew Ross]

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.