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XLR isolation transformer recommendation?

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Bob Cole
XLR isolation transformer recommendation?
on Aug 5, 2009 at 2:56:01 am

Following a shoot last spring where I had a problem with a (ground loop?) hum, I've been meaning to get an isolation transformer, and can dimly recall reading that there are better and worse ones. Something about passive vs. active, the quality of the windings (?)...

Anyway, if someone happens to have the answer to this question, what is the best (reasonable cost) non-battery-powered isolation transformer that will accept XLR cables?

And, does it make a difference that I am not using a mixer, but feeding mics directly into the camera?

Thanks!

Bob C


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Jordan Wolf
Re: XLR isolation transformer recommendation?
on Aug 5, 2009 at 8:32:35 pm

I have used, with much success, the ProCo IT-1. It's passive and accepts a number of pro connector types.

The problem of ground loops (usually) comes with having more than one ground that all your gear is tied to. This can be avoided by making sure any gear you use is plugged into the same circuit and that catering, lighting, etc. gear is NOT anywhere near or in any way connected to your gear.

And, just because it needs to be said: NEVER, EVER lift the AC ground on any of your gear (if it even plugs in). Use the 3-2 prong adapters for troubleshoot and find out where the grounding issue is. From there, use Pin-1 Lifts to deal with the issue.

Wolf
<><


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Bob Cole
Re: XLR isolation transformer recommendation?
on Aug 5, 2009 at 9:15:49 pm

[Jordan Wolf] "And, just because it needs to be said: NEVER, EVER lift the AC ground on any of your gear (if it even plugs in). Use the 3-2 prong adapters for troubleshoot and find out where the grounding issue is. From there, use Pin-1 Lifts to deal with the issue."

Thanks, Jordan.

Just to confirm that I'm understanding this: you say to use the 3-2 prong electrical adaptors to troubleshoot and locate the grounding issue.

So you're saying that you CAN temporarily lift the AC ground on your gear, right?

And then, a Pin-1 lift is the XLR pin 1, right? Which is what the Proco IT-1 and others accomplish.

did I get this right?

B


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Jordan Wolf
Re: XLR isolation transformer recommendation?
on Aug 6, 2009 at 1:42:43 pm

The 3-2 prong adapter should be used for 2 things and that's it:
1. It's original purpose (adapting a 3-prong plug to fit into a 2-prong outlet while bonding the adapter to the outlet via the faceplate screw).

AND

2. As a way to find which outlet(s) are causing hum in your system. It should only be a plug/unplug test, never permanent in any way. Unplug the Device Under Test (DUT) and add the adapter; plug it back in - if the hum goes away or is lessened, you have found a culprit; take the adapter off and fix/workaround the problem (noisy ground) and stop trying to solve the symptom (hum).

(Made generic for everyone, not specific to you, Bob)

I always meter my power before I hook into it - that way I can see if everything is hunky-dory. I agree with Brian - you could get electrocuted if you test the outlet ONLY using the above method. It would make you a responsible and valuable asset to any crew to know what you're doing around power. Be careful and keep on looking for solutions!


Wolf
<><


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Brian Reynolds
Re: XLR isolation transformer recommendation?
on Aug 6, 2009 at 12:21:49 am

Bob you mention in your first post that you are "feeding mics directly into the camera" this can't ever create an earth loop, what you have is induced hum and no transformer will ever fix that.Good cables, connections, and cable placement fixes the problem you experienced.
Earth loops are caused by interconnection with the earthing in the powering of the equiptment ie; mixing console to camera etc. if one or either piece of equiptment is running on batteries the possibility of a loop is broken.
The earth connection on equiptment is there for a reason YOUR SAFETY and must NEVER be broken. (If you try de-earthing for a test and what happens if its faulty....your toast!!! JUST DON'T DO IT) Equiptment with only 2 pins is double insulated to provide the safety factor.




The difference between Knowledge and Wisdom is... Knowledge is the knowing of facts.... Wisdom is the sensible application of good quality knowledge...


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Ty Ford
Re: XLR isolation transformer recommendation? Hum Busting
on Aug 7, 2009 at 12:39:25 pm

Brian's spot on Bob.

Try connecting the same gear at home the same way you did when you were getting the problem (assumedly elsewhere). If it still makes the same sound, try another mic because it could be the mic itself or the mic cable.

I have some "special" cables that are more resistant to noise than bargain brand XLR cables. There are some new Neutrik XLR connectors with built in protection. They are a bit finnicky to wire, but reports have come back that they are working.
http://www.neutrik.com/us/en/audio/204_1603252336/EMC-XLR_Series_productlis...

Was your camera on battery or AC? If on battery, it's not a ground loop.

If on AC, was any sort of other cable attached to the camera? Video cable attached to a video monitor that was plugged in to another AC outlet. That could create a ground loop.

Nearby fixtures, DIMMERS ESPECIALLY, can radiate buzz into your audio, no attachment necessary. Including nearby residential recessed fixtures on wall dimmers and the ubiquitous torchiere floor lamps. Either turn 'em all the way up or turn 'em off. Even residential dimmers in ANOTHER PART OF THE BUILDING can put noise into your audio.

Some "high-tech" industrial, wall-mounted office lighting controls in hotels and convention centers also radiate noise through the air that will get into audio.

The mic cable being parallel to an AC cable can induce hum into the audio.

I THINK your best defense is to have mic cables with really good shields and the shields connected to the shells of the XLR connectors with those new XLR connectors. There are a few applications, like connecting two pieces of SOME powered gear, where these cables may actually cause problems. So mark them so you remember they are your bullet-proof mic cables.

That's my story and I'm stickin' to it!

Regards,

Ty Ford


Want better production audio?: Ty Ford's Audio Bootcamp Field Guide






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Bob Cole
Thanks
on Aug 14, 2009 at 3:52:01 pm

Thanks all. Ty, looks like I'll be looking for someone to wire up some of those Neutrik connectors. If you say they're tricky, I don't want to try.

I haven't had any recurrence of this problem for several months. So I'm inclined to believe that it was a combination of a particular teleprompter rig on one shoot along with my very close-to-camera Kino Flo that created this weird buzz.

Bob C


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