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Author Topic: Power from multiple sub panels  (Read 10570 times)

Mike Sokol

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Re: Power from multiple sub panels
« Reply #20 on: December 19, 2013, 01:37:05 pm »

I prefer Dick's explanation!
Steve.
I've got to admit, that about sums it up....  ;D
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Mike Sokol
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John Sabine

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Re: Power from multiple sub panels
« Reply #21 on: December 20, 2013, 09:12:46 am »

I have a rack with 3 of these that I've carried around for years and almost never used but they save my bacon from time to time.

http://artproaudio.com/art_products/audio_solutions/splitters_patchbays/product/s8-3way/

I originally purchased them to use as a broadcast distribution for a job I did but they have actually proven quite handy. Once I pulled the rack from my trailer because I was tired of carrying it and never needing it. The next job I had was a 1400 person venue with enormous ground hum issues and a staff electrician who was MIA. Back in the trailer it went.
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Mike Sokol

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Re: Power from multiple sub panels
« Reply #22 on: December 20, 2013, 09:39:33 am »

Back in the early days of mixing consoles, there were transformers on the inputs and outputs. This made it pretty easy to simply lift pin-1 on the XLR and create a shield that was terminated on only one end. This resulted in superior CMRR and zero shield currents. However, transformers are expensive, heavy, and prone to magnetic interference. Once op amps became cheap, virtually all consoles went to electronic balanced circuits for both inputs and outputs. And XLR shields are routinely terminated on both cable ends, which introduced ground loop currents into the mix (literally). You can sometimes get away with pin-1 lifts on electronically balanced inputs, but only up to voltage differentials within the common-mode limits of the input electronics. I've not studied this in too much detail, but I think it's a topic worth considering and discussing.

Have any of you experimented with pin-1 lifting of XLR electronically balanced inputs with various ground-loop differential voltages? I have the gear to do some trials using various voltage offsets, but perhaps this has already been done before. If it's unexplored territory I'll start a new thread and gather some data.   
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Mike Sokol
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Kevin Graf

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Re: Power from multiple sub panels
« Reply #23 on: December 20, 2013, 10:32:53 am »

But back in the early day's,  the XLR jack pin-1 was incorrectly wired to the audio circuit ground rather than being connected to the chassis/shield creating all kinds of problems.  It has taken 20 years (and counting) to get everyone on-board.
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Mike Sokol

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Re: Power from multiple sub panels
« Reply #24 on: December 20, 2013, 10:53:37 am »

But back in the early day's,  the XLR jack pin-1 was incorrectly wired to the audio circuit ground rather than being connected to the chassis/shield creating all kinds of problems.  It has taken 20 years (and counting) to get everyone on-board.

And still counting. There's plenty of late model active floor wedges with the pin-1 problem. Also a number of studio monitor speakers I've run into that will hum with low ground-loop currents.

But the real question is this... just how much ground-loop current are you willing to accept in your XLR shields? Even if it doesn't create hum in your particular situation, I really don't like the idea of several amps of current flowing through shield paths. While XLR pins are rated for something like 7 amps of current, I've found AV system wiring with 5 amps of ground loop current which seems like a bad idea.
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Mike Sokol
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TJ (Tom) Cornish

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Re: Power from multiple sub panels
« Reply #25 on: December 20, 2013, 11:02:28 am »

And still counting. There's plenty of late model active floor wedges with the pin-1 problem. Also a number of studio monitor speakers I've run into that will hum with low ground-loop currents.

But the real question is this... just how much ground-loop current are you willing to accept in your XLR shields? Even if it doesn't create hum in your particular situation, I really don't like the idea of several amps of current flowing through shield paths. While XLR pins are rated for something like 7 amps of current, I've found AV system wiring with 5 amps of ground loop current which seems like a bad idea.
Particularly when that 7 amp XLR pin is connected to a 28-ga snake wire.
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Mike Sokol

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Re: Power from multiple sub panels
« Reply #26 on: December 20, 2013, 12:11:17 pm »

Last year I did a graduation gig that required I send program audio to a video trailer about 300 ft from the FOH console. And yes, the video trailer pulled its AC power from a separate building on campus with it's own sub panel. There were complaints about all kinds of hum in the video feed the previous years, so I did my simple DI press mult fix. While I have a rather fancy active press mult, it wasn't with me at the time. So I just plugged in a Whirlwind IMP2 from the TRS aux out of the console, did the ground lift, then ran a XLR cable over to the video truck. You can even daisy-chain a number of these passive DIs together to feed several video cameras during a shoot. (See diagram). Since the input impedance of a passive DI is quite high (perhaps 50K or so) you connect a dozen or more of these together without any impedance loading issues.

While a press mult is a nice thing to have, if you're doing small corporate or local political work often you just need to get audio to one or two video cameras. Don't be tempted to just run them a direct XLR feed from the console even for a pro camera since many of them don't play nicely with line-level XLR (even if they claim to). And many pro camera grounds have terrible hash noise from all their on-board switching power supplies. So send them something that looks like a mic-level signal, isolate the shield from your sound system ground via a DI box, and things will be beautiful.
« Last Edit: December 20, 2013, 01:37:13 pm by Mike Sokol »
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Mike Sokol
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Frank DeWitt

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Re: Power from multiple sub panels
« Reply #27 on: December 20, 2013, 01:05:43 pm »

So I just plugged in a Whirlwind IMP2 from the TRS aux out of the console, did the ground lift, then ran a XLR cable over to the video truck. You can even daisy-chain a number of these passive DIs together to feed several video cameras during a shoot. (See diagram). Since the input impedance of a passive DI is quite high (perhaps 50K or so) you cab connect a dozen or more of these together without any impedance loading issues.

This is one of those ideas that is so crazy simple it is wonderful.  I will keep it.

BTW  A Jensen transformer equipped DI box will be about 140 K

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