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Old 17th June 2013, 10:24 AM   #1
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Default Ground lift switches

Anybody got a nice link to standards on how to wire XLR connectors in cables for balanced lines for microphones and between equipment, and how to wire a ground lift switch on a patchbay, etc.?
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Old 17th June 2013, 10:34 AM   #2
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i dont recall ever seeing a patch bay with a ground lift switch ....

ground lift switches normally exist behind amplifiers but still these are not necessary to anyone that knows how to install audio equipment properly .

If you focus and manage to understand what causes hum in PA installations you may only use a ground lift switch only in a case of emergency .

Also i think that if anyone installs/adds a ground lift switch in a patch bay is wrong practice ....
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Old 17th June 2013, 01:03 PM   #3
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Patch bay may be a poor choice of words. I don't want to explain this now, so nevermind, I'll find what I need myself. This is more like an intermediate node in balanced lines, and will usually connect the third pin thru, but sometimes I might not want to (which is a wonderful feature of balanced differential lines).
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Old 17th June 2013, 01:30 PM   #4
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I had a patchfield with ground lifts, once, while touring a big keyboard rig. Eighteen keyboards, some mono, some stereo, and four stereo subgroups. The house mix and the monitor mixer could choose individual outs, grouped outs or the entire mix, which also went to the local monitor (but had effects mixed with it, that the FOH guys didn't always want.

Some of the keyboards had three connector mains leads, some two, but all had transformer balanced outputs. Every ground lift switch had a neon round it; if this lit up I knew there were serious problems. Outputs were on XLRs and multipole connectors, separately isolated. Mic preamps (for grand piano, leslie and pipe organ) were brought up balanced in the patch. Sometimes it took a while of flicking switches to achieve optimum performance (and you couldn't start until the lights were rigged and radiating their nasties), but if I ever toured an equivalent system I'd build another. Independent DI boxes might do the job quite adequately, but it's so nice having everything grouped together (isn't that why we invented patchfields in the first place?)
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Old 17th June 2013, 04:00 PM   #5
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I think I found some of what I wanted to know. If you lift (don't connect) the ground pin on one side, the plug and jack shells will still connect. Which is why you're not supposed to connect the shell (or panel or jack) to the ground pin on the female. Does this sound right? I've been out of the business for far too long to remember... This has other implications I need to remember...and I need to check some binding screws on some connectors. Somebody must have the guidelines somewhere...I need to search more.
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Old 17th June 2013, 05:19 PM   #6
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If you're going to do a ground lift on an RTS jack (or bantam jack), you have to have the sleeve isolated from chassis. Most professional patch fields give you this automatically, and let you bus your grounds. With an XLR patch, you generally don't connect the cable screen to the plug shell anyway, so a pin 1 lift works fine.

With a proper balanced signal, there should be nothing passing by the ground, anyway; it's just a reference, irrelevant (except on phantom powered circuits). So you can put a smallish resistor, or paralleled head to tail diodes from chassis to pin one, so if any voltage is generated between apparatuses it's got an escape route.

Is this for fixed installation/studio or a touring rig? All my patch leads have always had ground continuity, and the patch has always been the central star ground (however little this is supposed to count in balanced systems, and in forty years of studio I've never had a ground current problem when the signals were truly balanced, or floating. I can not say as much for outside gigs, live recording or PA.
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Old 17th June 2013, 08:33 PM   #7
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Like I said, this is all XLR. The first thing I'm making (finishing tonight) is a rack panel with a bunch of sections. Each section has one XLR female input and two XLR male outputs and a DPDT switch that reverses pins 2 & 3 of the input female for a "polarity" (or 180 relative phase) forward/reverse switch. This is just for convenience and speed for ad-hoc setup of mixed equipment and snakes to various active crossover inputs and outputs for various mixes, channels, and feeds (plus an organized visual display of the state); and the dual paralled outputs are just instead of "Y" cords, usually to drive both channels of stereo amps with the same program material. Might occasionally use some sections for getting stage mics and di boxes into correct relative phase with each other etc.

Diagrams I've seen show XLR cables (like mic cables) wired with the metal cover shell over the female connector not connected to pin1, and warning that ground lifts are not really SAE standard at all. So if you typically buy mic cables, are the shells (metal cases over the plug or jack on a cable end) connected to anything at all (besides each other when plugged into each other), or is pin 1 only connected to the "shell" on the males?

I'm starting to think I'm just going to leave the gournd lift switches out, as I won't need them immediately at all. I was just kind of thinking ahead and used to having them available on snakes if I ever needed them.

Last edited by cyclecamper; 17th June 2013 at 08:38 PM.
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Old 17th June 2013, 11:26 PM   #8
Enzo is offline Enzo  United States
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Try this. Wire your patches up complete. Then make a couple short (6" or 12", whatever) XLR cords with no ground between ends. Now any time you need a ground lift, insert your short cable in series with the connection. No switches, no special jacks. When I was touring, I would never go out without a few of those in my problem solvers case.

Other items in there are a TRS plug with simply tip to ring jumpered inside. Bypasses insert jack normals. And a shorted plug to stuff in a footswitch hole if needed. And a bunch of adaptors.
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Old 18th June 2013, 09:41 PM   #9
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Yes, I have boxes of that stuff from the 1970's. In-line XLR w/ built-IN T-pad or H-pad, various adapters.

Looks like I had that backwards, they more often connect the female shells to ground or don't connect either shell to ground. And apparently the US and Europe have opposite standards for the absolute phase of pins 2 & 3.
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Old 18th June 2013, 11:36 PM   #10
Enzo is offline Enzo  United States
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I wire my cords pin 2 to pin 2 and pin 3 to pin 3. An so I don't care at all which one the equipment decides is +. At least not as far as my cords.
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