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Harry Houdini
 
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Default 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
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Richard Crowley
 
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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.
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Geoff Wood
 
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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


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Jim Gregory
 
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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.


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bill
 
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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




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Harry Houdini
 
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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   Report Post  
Harry Houdini
 
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Default

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


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Harry Houdini
 
Posts: n/a
Default

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   Report Post  
Geoff Wood
 
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Default


"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   Report Post  
Jim Gregory
 
Posts: n/a
Default

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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