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Author Topic: Not Ground Loop Hum  (Read 5264 times)

Mike Sokol

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Not Ground Loop Hum
« on: May 02, 2016, 11:30:28 am »

So this one threw me for a "loop" on Saturday night. We set up our active JBL PA system and ran XLR cables plus Edison power from a central distro point. But I had this really bad hum on the house left speaker stack, and not the house right stack. I put in a audio isolation transformer on the XRL feed, but then the hum turned into a buzz. Didn't make sense until one of the guys opened up the XLR cable we made in-house and found this mis-wiring condition on a jumper. If you look closely, you can see that the shield is connected from pin-2 female to pin-2 male. Of course, shield is supposed to be on pin-1, since pin-2 is hot. This created a condition where the cable would pass a check using a SWIZZ Army Tester (or any other tester, for that matter) since there was indeed continuity between pins 1-1, 2-2, and 3-3. But now the shield currents were being injected into the pin-2 hot of the speaker input. So the lesson is don't rely on just using a cable tester to check your cables.  Open up the ends and inspect them to make sure that pin-1 is the shield and pins 2/3 are the twisted pair. 
« Last Edit: May 02, 2016, 11:41:07 am by Mike Sokol »
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Chris Hindle

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Re: Not Ground Loop Hum
« Reply #1 on: May 02, 2016, 12:34:16 pm »

At least they were "good" enough to NOT jumper to the shell.
"The gift that keeps on giving" that lovely little swap.
Chris.
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Mike Sokol

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Re: Not Ground Loop Hum
« Reply #2 on: May 02, 2016, 12:47:15 pm »

At least they were "good" enough to NOT jumper to the shell.
"The gift that keeps on giving" that lovely little swap.
Chris.

I've seen a bunch of imported XLR cables as of late with the pin-1/shell jumper. So now that's the first thing I check for when I'm working on a church sound system. Usually just takes a little "nip-tuck" with a pair of cutters to correct.

But I've also seen at least one worship site that bought 2 dozen new mic cables with the shield improperly wired on pin 2. I personally unpacked these cables and checked for miswiring after getting all sorts of hum from the mics. Yes, every single XLR cable was miswired like that from the (Chinese?) factory. I wonder just how common that sort of goof is?
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John Roberts {JR}

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Re: Not Ground Loop Hum
« Reply #3 on: May 02, 2016, 01:12:48 pm »

Hand wiring cables is labor intensive but relatively low skilled work.

It is entirely believable that an entire production run could be wired wrong.

Advise who ever those cables were purchased from, so they can check their inventory.

JR 
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Mike Sokol

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Re: Not Ground Loop Hum
« Reply #4 on: May 02, 2016, 02:08:44 pm »

Advise who ever those cables were purchased from, so they can check their inventory.

Uhmmmmmmm These came from Peavey about 10 years ago. But you asked....  ;D
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John Roberts {JR}

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Re: Not Ground Loop Hum
« Reply #5 on: May 02, 2016, 03:01:25 pm »

Uhmmmmmmm These came from Peavey about 10 years ago. But you asked....  ;D
I quit peavey >15 years ago, so that explains that. I should have told somebody that was important before I left.  8)

Peavey used to make cables at a dedicated operation in Quitman, MS but i think they shut that down even before I left.

I suspect Peavey buys cables from overseas just like everybody else now.

JR
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David Buckley

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Re: Not Ground Loop Hum
« Reply #6 on: May 03, 2016, 12:16:59 am »

A lighting company that servers our area hires out a Tecpro party line intercom system, which the last time I had it, the level of buzz exceeded the voice level.  They had wired their (bright orange!) cables wrong, with the shield connected to whatever pin it is carries the unbalanced audio...
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Ike Zimbel

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Re: Not Ground Loop Hum
« Reply #7 on: May 03, 2016, 09:15:52 am »

I've seen a bunch of imported XLR cables as of late with the pin-1/shell jumper. So now that's the first thing I check for when I'm working on a church sound system. Usually just takes a little "nip-tuck" with a pair of cutters to correct.

But I've also seen at least one worship site that bought 2 dozen new mic cables with the shield improperly wired on pin 2. I personally unpacked these cables and checked for miswiring after getting all sorts of hum from the mics. Yes, every single XLR cable was miswired like that from the (Chinese?) factory. I wonder just how common that sort of goof is?
I encountered this a few years back with a mic cable that was included with an Audio Toolbox I had purchased. I discovered it when the cable became part of my bench test set-up, and yes, it was made in China.
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John Rutirasiri

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Re: Not Ground Loop Hum
« Reply #8 on: May 03, 2016, 09:39:52 am »

Would be nice if someone integrates an LCR meter in an audio cable tester.  That way we can know if shield and hot/cold are swapped or solder connection is poor without taking cable apart.

John R.

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Mike Sokol

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Re: Not Ground Loop Hum
« Reply #9 on: May 03, 2016, 09:54:25 am »

Would be nice if someone integrates an LCR meter in an audio cable tester.  That way we can know if shield and hot/cold are swapped or solder connection is poor without taking cable apart.

John R.

Such a tester would need to check for CMMR on the far end of the cable. So it would need to provide a common-mode test signal (1 kHz?) plus a differential test signal (100 Hz?) and listen for cross-talk, I think.
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Mike Sokol
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Re: Not Ground Loop Hum
« Reply #9 on: May 03, 2016, 09:54:25 am »


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