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#1
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Connecting XLR shields
Hi!
Just out of curiosity - does anyone know why XLR cables almost never have the shield connected? How would it afftect things if it were connected? Harry |
#2
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"Harry Houdini" wrote ...
Just out of curiosity - does anyone know why XLR cables almost never have the shield connected? How would it afftect things if it were connected? Presumably you mean the connector shell and not the cable shield (screen)? Seems highly doubtful that you could get away with leaving the shield floating. OTOH, making a connection to the shell (at either end) appears to be nowhere near as critical. I suspect that people who operate in high RF areas can't be as casual about that as the rest of us. |
#3
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"Richard Crowley" wrote in message ... "Harry Houdini" wrote ... Just out of curiosity - does anyone know why XLR cables almost never have the shield connected? How would it afftect things if it were connected? Presumably you mean the connector shell and not the cable shield (screen)? Seems highly doubtful that you could get away with leaving the shield floating. OTOH, making a connection to the shell (at either end) appears to be nowhere near as critical. I suspect that people who operate in high RF areas can't be as casual about that as the rest of us. Not casual - deliberate. If you are going through a stage ar wall box, with multiple connectors, you will instantly have lots of potential ground loops which are very susceptible to rf interference. Even just a single cable connected to a grounded metal wall box will likely cause problems. geoff |
#4
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The shell of any panel-mounted *metal* flange XLR connector, say on a
facilities panel or wallbox, is usually grounded by its corner or radial screws to the metalwork, assuming that the panel *is* already grounded - for Safety reasons. When a flying lead *metal* case XLR connector (male or female, 3 -pole or more) is inserted there, its shell gets automatically grounded. Therefore it should not be linked to pin 1 which would get grounded twice!. If using lightweight *plastic* XLRs, Caution - there is not always a 4th pole embedded in the connector. XLR pin 1 is supposed to be longer than the other two - and so mates first and breaks last, allegedly. But the *shell connection tag* of the wandering end metal-cased 3-pole female XLR cable socket should be linked to pin 1 to make the microphone (or whatever source) safe and immune from RF. I always connected studio mic cables' and mic extenders' XLR females from pin 1 to shell. Even if the signal-pair part of the cable in a technical area is interruptible by normalling jacks and patching, the screen should be continuous to ground. No matter what series length and how many add-on extenders are used, pin 1 should be traceable to the active electronics chassis elsewhere, which good practice dictates is always at robust ground potential. I believe the same philosophy does not always go for *shell connection tag* of studio/gig wander ends with metal-cased 3-pole MALE XLR plugs used for taking feeds, foldback, etc. ANOMALY... If o/p cable metal connector shells are always linked to their pin1, should the metalwork for an i/p cable accidentally touch the metalwork for an o/p cable on stage or in a studio, there could be a subtle ground loop. |
#5
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As I re-call connecting the shield to the shell creates a ground loop
finder. Handy if you need ground loops. Harry Houdini wrote: Hi! Just out of curiosity - does anyone know why XLR cables almost never have the shield connected? How would it afftect things if it were connected? Harry -- Hope this helps Bill Mayhew Mayhew & Company, Rental, Sales, Service http://www.mayhewco.com |
#6
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Richard,
Thank you so much for your response. Yes, I did mean the shell. On Sun, 20 Mar 2005 12:02:35 -0800, "Richard Crowley" wrote: "Harry Houdini" wrote ... Just out of curiosity - does anyone know why XLR cables almost never have the shield connected? How would it afftect things if it were connected? Presumably you mean the connector shell and not the cable shield (screen)? Seems highly doubtful that you could get away with leaving the shield floating. OTOH, making a connection to the shell (at either end) appears to be nowhere near as critical. I suspect that people who operate in high RF areas can't be as casual about that as the rest of us. |
#7
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Geoff,
Your input is appreciated! Thank you. I am, in fact, setting up two junction panels with lots of Mic interconnets. Do you suggest connecting to the chasis of the metal face-plates? On Mon, 21 Mar 2005 08:29:54 +1200, "Geoff Wood" wrote: "Richard Crowley" wrote in message ... "Harry Houdini" wrote ... Just out of curiosity - does anyone know why XLR cables almost never have the shield connected? How would it afftect things if it were connected? Presumably you mean the connector shell and not the cable shield (screen)? Seems highly doubtful that you could get away with leaving the shield floating. OTOH, making a connection to the shell (at either end) appears to be nowhere near as critical. I suspect that people who operate in high RF areas can't be as casual about that as the rest of us. Not casual - deliberate. If you are going through a stage ar wall box, with multiple connectors, you will instantly have lots of potential ground loops which are very susceptible to rf interference. Even just a single cable connected to a grounded metal wall box will likely cause problems. geoff |
#8
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Jim,
Thank you for your thoughtful and generous response. Questions: So if I understand you correctly, I should ground the panel chassis and then solder the XLR interconnect shells to the chassis? Then, I should solder the shells of any XLR cables going into/out of the panels to pin 1. Right? Can I connect the chassis plates to the ground on a nearby AC line or do I have to run a separate line? Can I connect both junction faces to the same ground? Kind regards, Harry On Sun, 20 Mar 2005 21:08:18 GMT, "Jim Gregory" wrote: The shell of any panel-mounted *metal* flange XLR connector, say on a facilities panel or wallbox, is usually grounded by its corner or radial screws to the metalwork, assuming that the panel *is* already grounded - for Safety reasons. When a flying lead *metal* case XLR connector (male or female, 3 -pole or more) is inserted there, its shell gets automatically grounded. Therefore it should not be linked to pin 1 which would get grounded twice!. If using lightweight *plastic* XLRs, Caution - there is not always a 4th pole embedded in the connector. XLR pin 1 is supposed to be longer than the other two - and so mates first and breaks last, allegedly. But the *shell connection tag* of the wandering end metal-cased 3-pole female XLR cable socket should be linked to pin 1 to make the microphone (or whatever source) safe and immune from RF. I always connected studio mic cables' and mic extenders' XLR females from pin 1 to shell. Even if the signal-pair part of the cable in a technical area is interruptible by normalling jacks and patching, the screen should be continuous to ground. No matter what series length and how many add-on extenders are used, pin 1 should be traceable to the active electronics chassis elsewhere, which good practice dictates is always at robust ground potential. I believe the same philosophy does not always go for *shell connection tag* of studio/gig wander ends with metal-cased 3-pole MALE XLR plugs used for taking feeds, foldback, etc. ANOMALY... If o/p cable metal connector shells are always linked to their pin1, should the metalwork for an i/p cable accidentally touch the metalwork for an o/p cable on stage or in a studio, there could be a subtle ground loop. |
#9
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"Harry Houdini" wrote in message ... Geoff, Your input is appreciated! Thank you. I am, in fact, setting up two junction panels with lots of Mic interconnets. Do you suggest connecting to the chasis of the metal face-plates? That is a little difficult to avoid in all but all-plastic sockets ! geoff |
#10
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Thank you for your thoughtful and generous response.
Questions: So if I understand you correctly, I should ground the panel chassis and then solder the XLR interconnect shells to the chassis? Then, I should solder the shells of any XLR cables going into/out of the panels to pin 1. Right? Long Answers: Yes to grounding/earthing the panel/metal enclosure, but No, and No only to the outgoings from panel into studio. Just ground the metal panel securely using a shakeproof washer/shakeproof solder tag with decent, at least 15Amp, solid/stranded wire run to a permanent, reliable earth/ground point. Nothing else needs to be done this end. The earth/ground point ought to be part of the building's tested and approved wiring and should not be interruptible by plugging to obtain it!! The add-on should be labelled "Safety Ground! DO NOT Remove." [I may sound like I am teaching my grandmother to suck eggs - but I don't know the relevant bye-laws in your country.] Explanations Any shells of metal XLRs plugged into the panel metal-flanged receptacles will now make to chassis-ground by default. Simple! If the panel connectors have insulated barrels and wells, on the types I've used, they still had a "shoe" that makes with an inserted shell. All Pins 1 will go their own route as screens to/from technical equipment which is hopefully and advisedly grounded during use and again hopefully and advisedly during inactivity (just switch off at wall outlets but without unplugging relevant mains cords). NOTE Pin 1 at XLR *female* end (outgoing from mic) of a studio wander cable would be linked to shell tag as it is often handheld or manhandled. BUT regarding incoming feeds from control room equipment on roving male XLRs at the studio floor end, it remains optional for the user to link /isolate the shell to /from its pin 1, dependent on *no-hum results* with mains-operated folback/heaphone/comms amps, etc. Remember, all exposed metalwork on mains appliances should be grounded/earthed by their cords (well, that's the regulation over here, at least). Q Can I connect the chassis plates to the ground on a nearby AC line or do I have to run a separate line? Answer: As I've said, run it separate, not from plug-in cords - so making it permanent at each end. Of course, these are only safety routes to neutralise potential danger to manhandled hardware, no current is involved [till a possible live mains calamity is prevented by shorting to ground]. Q Can I connect both junction faces to the same ground? Answer: Yes, several even, if the captive screw is long enough to accommodate the new wires securely between disc washers and a locknut (or via brass ground bus connector terminals). This paralleling is normally called a star connection. Hint. Choose the shortest grounding/earthing route possible to good and proper electrical infrastructure. Ideally anchor it at a mains consumer unit, or it's acceptable at a ring-main outlet node, but avoid grounding at a spur outlet box where resistance is slightly higher. |
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