Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums > AC Power and Grounding

Power from multiple sub panels

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TJ (Tom) Cornish:

--- Quote from: Mike Sokol on December 20, 2013, 10:53:37 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.

--- End quote ---
Particularly when that 7 amp XLR pin is connected to a 28-ga snake wire.

Mike Sokol:
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.

Frank DeWitt:

--- Quote from: Mike Sokol on December 20, 2013, 12:11:17 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.

--- End quote ---

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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