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Author Topic: humming mic power supply - is this solution OK?  (Read 2699 times)

bejeeber

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humming mic power supply - is this solution OK?
« on: February 23, 2005, 01:04:56 AM »

I have a new tube mic and unfortunately it's power supply is the only power supply in my studio (or piece of gear period, for that matter) that introduces ground hum.

The reason I feel it's ground hum is that, well it sounds like ground hum in all its 60Hz badness, and if I have a mic cable plugged from its output to my mic pre, whether the power supply is turned on or off, it'll introduce hum. If I pull its power cable, hum gone. If I use one of those 3 prong to 2 prong ground lift adapters on it, hum  gone.

So finally my question: Is there any hazard to using the ground lift adapter in a case like this? Everything in the studio is on the same isolated circuit. This mic will be used for a vocalist who will simultaneously be playing a D/I'd electric guitar, and I'm thinking there's no hazard here, but as you can probably tell by this question I've flunked grounding 101 several times and could use some reassurance.

Thanks.
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Barry Hufker

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Re: humming mic power supply - is this solution OK?
« Reply #1 on: February 23, 2005, 11:21:19 PM »

I am sure you'll get a better answer from someone more knowledgeable than I, but here's my contribution.

It is my thought you are getting a hum because that part of your system is double-grounded.  There is an AC ground and an audio ground.  For safety's sake you should *not* lift the AC ground.  In fact, I believe it to be against the law to do so, but could be wrong.

Instead make an adapter or audio cable such that all three pins of the XLR (pin 2+, pin 3- and pin 1ground) are connected at the end connecting to the power supply.  At the other end, connect only pins 2 and 3.  Leave the wire for pin 1 at its full length but bend it back and secure it (with heatshrink) so that it can't touch anything.  This will give your audio as much protection as possible as it travels but will break the audio ground.

Someone else may have a better idea, but I encourage you to try this.

Barry
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bejeeber

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Re: humming mic power supply - is this solution OK?
« Reply #2 on: February 25, 2005, 02:25:38 AM »

Thanks Barry. So would this indicate that the power supply may be wired incorrectly?

Any other good alternative solutions - like using one of those isolation transformers?
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Barry Hufker

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Re: humming mic power supply - is this solution OK?
« Reply #3 on: February 25, 2005, 08:59:38 AM »

It is hard to say.  Using a volt-ohm meter, use one lead to touch bare metal of the supply case.  With the supply's AC cord unplugged, use the other lead to touch the ground lug on the AC cord.  This will confirm continuity if your meter is set for that or will measure the amount of resistance if your meter is set for that.  In either case there should be continuity and a low resistance.

With that confirmed, plug the AC cord in.  Keep one lead on the bare metal of the case and put the other lead on the bare metal of that case.  Check for continuity and resistance.  Again there should be continuity and relatively low resistance.  If this isn't happening then your problem is in the ground itself.

Often times audio systems are poorly grounded or the grounds are on separate circuits.  You said you are using an isolated circuit.  Is it properly grounded?  Ideally that would mean a thick copper braid connected to the console leading to a large copper stake actually driven into the earth (ground).

It may be that your entire system isn't quite grounded correctly and the power supply is just more sensitive to that.

But if the audio ground lift adapter works, you may get away with only having to do that.  After that, I don't know unless you totally restructure your grounding using a 'star' grounding system  -- too long to explain for now.

Barry
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