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Noise induction on Boom - Audio Puzzler


Derek H

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Hello,

I've got an audio puzzler for you here;

(I've cross-posted this in RAMPS as well)

Been surfing this board the past couple days gathering all kinds of useful tidbits and came across one discussion (regarding XLR wiring; specifically, the pros and cons of connecting pin1 to the XLR shell) that sparked my curiosity about a problem I had earlier this year.

I was bag mixing for a doc and we're setting up for a nice, relaxed walk n' talk shot down North St beach here in Chicago and all of a sudden it seems that my boom pole has turned into a veritable hum magnet.  The hum sounded like a messy version of 60cycle and would vary in amplitude and abrasiveness with movement of the boom in different positions.

Maybe someone with a little more electrical wisdom than I can explain what was happening here.

Factors to consider:

- I was right on the lake shore (lake Michigan)

- It was a cold, wet, drizzly sort of day

- I had clear line of sight to the John Hancock building <1mile south of my position (massive antennae tower)

- The footing was a mix of wet cement and wet earth.

Equipment/Signal Flow in use:

416 T-Power ---> Shorty XLR jumper (pin1 connect to shell both ends)

---> vdb coil cable pole (pin1 NOT connected to shell) ---> Boom to

Mixer XLR jumper (pin1 connect to shell both ends) --->PSC Alphamix

Other factors:

- When the camera op broke to do B-roll and switched on his camera mount shotgun (P48) there was no hum on the recorded sound.

- Even with no cables plugged into the Alphamix I could still hear a bit of the same hum in the noise floor.

- I had wires (lectro 200 series) on the subject and interviewers and vaguely remember having the same problem but to a much lesser extent.

My best guess:

-From what I just read on an old posting it sounds like T-Power mics don't like cables that have the shield tied to the connector shell, something about the signal being present in the shield as well as pin2.  So maybe I inadvertently created an RF antennae and somehow the

416 was amplifying the noise?

-Maybe the coiled cable of the vdb was inducting noise?

-Or maybe (really hoping this isn't the culprit) the PSC Alphamix is just not well designed for RF immunity.

Hope someone has the time and interest to clue me in on this one!  It really made a mess for sound that day and I would love to avoid the

problem in the future.  Thanks in advance!

-Derek Hanson

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Ron,

What do you mean by different grounding scheme in the vdb? 

Jason,

I don't think the monitor return was the issue because I remember hearing the noise when listening to the mixer directly as well.

Thanks,

I've had a lot of great suggestions regarding this problem, and thanks to all who responded, here and also on RAMPS but I'm getting the feeling that the only real way to shed some light on it is to head back out to the same spot with the same gear, as well as backup gear and cables with different soldering configs and just go through the signal flow piece by piece until i find the culprit... not exactly my ideal Sunday afternoon but maybe worth the trouble.

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