Audio Connector Wiring

[Steve Foy]

Does anyone have a definitive answer for whether or not to connect the cable 'Shield' to the connector 'Case'

Standard wiring for 'Balanced' Input/Output 3 Pin XLR is:

Pin 1 Shield

Pin 2 +

Pin 3 -

But should you jumper Pin 1 'Shield' to the connector 'Case'

It seems that every time I ask someone I get a different answer ranging from "Absolutely yes" to "Definitely not" and even "Yes for location audio but no for studio audio" or "Only at one end" or "No way because that 'Case' lug is for attaching the string in the cable to provide strain relief".

Comments from the crowd

[Johnny Karlsson]

I researched this recently and came to the conclusion that it's better to connect the jumper because it helps protect against RF.

[chris_bollard]

If you do connect to shell then connect only to one end

[Jack Norflus]

At one point I connected the ground to the shell. And I would occasionally have issues with it. But currently I have no grounds connected to any shells - and for 20 plus years no issues.

[Jay Rose]

FWIW...

I was asked to straighten out a puzzle for an a/v company. They had one venue where certain mics could plug into some of the house XLR inputs and work perfectly, but plug into others and hum like crazy. Weirder, it always the jacks on the wall that hummed, and the ones set into the floor that didn't. Even weirderer, some of their mic extension cords would fix the problem and others wouldn't.

You, of course, know exactly what the problem was, and why the wall jacks were more apt to have it than the floor ones.

[chrisnewton]

Don't leave us hanging, what was the solution?

[studiomprd]

" Does anyone have a definitive answer for whether or not to connect the cable 'Shield' to the connector 'Case' "

a definitive answer: "NO"

but, whether or not, yes: "it depends!"

[Jay Rose]

spoiler alert, but since Chris asked:

Some of the mics had their XLR shell tied to shield. The wall jacks at the venue were in steel boxes, mounted to steel studs. When you used those boxes, you tied the audio ground through the XLR shell and wall plate to all sorts of dirt in the venue's AC grounding system, thanks to the studs' conducting from all the properly grounded power boxes. The floor jacks were set in concrete and so were relatively isolated.

Some of the extension cables lifted the shield from shell, isolating the problem and cleaning up the mics again. Others didn't.

Moral: at least in my world, keep the shells isolated.

[Hartley]

A good guide for all sorts of connections http://www.rane.com/pdf/ranenotes/Sound_System_Interconnection.pdf

[Steve Foy]

Thanks everyone for your valuable input, it seems there is still a variety of ideas on this which is not surprising as it has been hotly debated for over 60 years.

The Rane guide is excellent, thanks Hartley. According to them, a standard audio cable (XLR Male to XLR Female) does not have or does not need Pin 1 connected to the 'Case', however it is interesting to note that on their diagram it shows that at the Female end: Pin 1 is 'Shield', Pin 2 is '+' and Pin 3 is '-' as you would expect but at the other (Male) end: Pin 1 is N/C, Pin 2 is '+' and Pin 3 is '-'.

I can see the logic of this, however out on 'Location' there are times when you might for example need to add a length of cable to another cable to extend the length and then you would effectively have no shielding on the added cable/s. or what ever else was at the end of the cable run, so I can also see some shortfalls with this method.

Given all of the above, I think I will go for Pin 1 'Shield', Pin 2 '+' and Pin 3 '-' at both ends with no jumpers to the connector 'Case'.

[Jay Rose]

it is interesting to note that on their diagram it shows that at the Female end: Pin 1 is 'Shield', Pin 2 is '+' and Pin 3 is '-' as you would expect but at the other (Male) end: Pin 1 is N/C, Pin 2 is '+' and Pin 3 is '-'.

Pin 1 n/c? How would that cable pass phantom power?

[studiomprd]

" Pin 1 n/c? "

that is a special adapter we call ground lift, often needed particularly when there is a "pin 1 - shield" issue elsewhere in the system; it does not pass industry standard "Phantom Power".

[RandyHall]

" Pin 1 n/c? "

that is a special adapter we call ground lift, often needed particularly when there is a "pin 1 - shield" issue elsewhere in the system; it does not pass industry standard "Phantom Power".

I was just thinking that this is why people sell or make their own ground lifters. A good phantom power supply will isolate the ground anyway because hey, you're using it for power, duh.

[Jay Rose]

A good phantom power supply will isolate the ground anyway

Up to the supply's designer. I've seen plenty with ground continuity; the isolation happens on pin 2 and 3 with capacitors or a transformer. And the majority of phantom supplies I've seen have shield continuity, which was the original topic.

--

Senator, sure you can use a ground lifter when needed... but the quote was about an extension cable, which - I'd assume - is something you want to grab when you need to move a signal over a distance, not a tool for stopping ground loops.

[studiomprd]

" the quote was about... "

...a really long adapter, too long.

(that's what my quote was about )

[Steve Foy]

I hadn't even considered the 'Phantom Power' aspect, but regardless having Pin 1 N/C at the 'Male' end of the cable didn't make any sense for our typical cable applications. However as I previously said, I could see the logic in doing this, but I should probably have qualified that I was referring to seeing the logic of doing this in a strict 'Equipment A to Equipment B' style hook up, as in two components in a studio rack for example. After thinking about it a bit more, I realised that the 'Rane Guide' whilst full of very good information, was more specifically referencing exactly that, permanent or semi-permanent component type connections, in fact that particular 'Guide' is even titled 'Sound System Interconnection'. This would make perfect sense as 'Rane' make components like 'Crossovers', 'Amps', 'Equalisers' etc.

If you go and have a look at the "Rane Guide" here:

http://www.rane.com/...rconnection.pdf

You will see that the diagram I was referring to at the top of page 7 which was their standard XLR interconnect cable and not a 'Ground Lift' cable which they also refer to as a 'Test' cable which appears on page 4.

Thanks again everyone for all your valuable feedback, at the end of the day I will be making sure all my cables are wired as I was originally shown which was Pin 1 'Shield', Pin 2 '+' and Pin 3 '-' at BOTH ends with NO jumpers to the connector case.