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Why does XLR connectors use 3 pins instead of just 2. The wiring is

Pin Function
--------------------------------
1   Chassis ground(cable shield)
2   hot
3   cold

(http://en.wikipedia.org/wiki/XLR_connector#Three_pin_-_audio)

Why do they use an extra pin instead of the connector shell to connect the cable shield?

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The cable connector body must contact the chassis connector body, which is generally metal, conductive, and bolted to a conductive metal case. Therefore the only choice for the connector body is a connection to the equipment case.

This (the equipment case) is obviously grounded - to safety earth. There is no choice about this.

However it is common for safety earth connections to form ground loops or be connected to a noisy earth point shared with, e.g. multi-kilowatt triac dimmed lighting systems (i.e. spiky 50Hz waveforms.)

So audio equipment design must separate the two functions of an earth connection: safety, and noise reduction.

Connect external metalwork to a safety ground, without worrying about the noise on it.

Connect signal ground to a separate low noise ground, without worrying about whether it is fault-current rated.

And this dictates the need for a separate pin for signal ground connections.

When I worked in broadcast audio, "pin 1 is ground" was one of those rules we didn't even have to think about.

Consider many pieces of equipment connected to an audio mixer by XLR cable : there may be many signal ground connections - yet we must eliminate ground loops. There are options on some of these boxes to "float" the signal ground - isolate it from safety ground - using the signal cable to provide a signal ground reference from the mixer.

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  • \$\begingroup\$ Would the shield of a cable therefore be connected to the XLR connector itself, and grounded that way? \$\endgroup\$ – rthur Apr 22 '16 at 2:51
  • \$\begingroup\$ Generally no, see Passerby's answer. The cable shield is about noise reduction, not safety isolation (line level audio isn't dangerous!) so connect it to audio ground (pin 1) not safety ground (connector body) \$\endgroup\$ – user_1818839 Apr 22 '16 at 11:02
  • \$\begingroup\$ Thanks! Does that not have the potential of introducing additional noise (picked up by the shield) into signal ground? \$\endgroup\$ – rthur Apr 22 '16 at 16:45
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Wiki article you linked has an explanation:

AES standard [AES14-1992] recommend that shells of cable-mounted connectors should never be connected to pin 1 or the shield, because inadvertent contact of the shell with another grounded surface while in use can create unwanted current paths for fault current, potentially causing hum and other noise.

Since the shell should not be used as ground, because touching it will cause ground issues, and added noise, which is unwanted in audio setups.

Since AES and EIA standards are behind pay walls, it's hard to provide better justification for it.

A better reason is that not all XLR connectors have metal casings in the first place. Plastic cases are just as common.

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You mean, why aren't XLR female plugs designed more like DC barrel plugs, with a metallic casing that forms one of the contacts, and then just two holes?

Electrically, this would work just as well. There is no "deep, electronic" reason.

However, note that such a plug still had better be asymmetric, otherwise it could easily be inserted backwards, resulting in a phase reversal.

The advantage of a two-conductor plug to use a round casing for one of the contacts is that it can be inserted at any rotation whatsoever. You never have to fidget with an AC adapter's barrel plug's alignment with the jack! Nor with a tip-ring or tip-ring-sleeve connector.

As soon as you have two pins that have to line up with holes, this advantage goes away. The plug has to be at a prescribed angle in order to go in. If the plug were surrounded by a metal casing, this would be a waste of metal because only specific spot on that metal casing would touch the mating contact point in the jack.

Of course, the plug could use a casing with multiple sections, like tip-ring-sleeve plugs. And, guess what? TRS plugs/jacks are sometimes used for balanced audio!

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The XLR 3-pin connector is used for balanced audio connections therefore it needs two signal pins and one ground pin. The ground pin I refer to is signal ground and not the metal outer shell of the connector. The metal outer shell will (or can inevitably) be connected to power grounds in the system and these are not recommended for small-signal audio grounds such as from microphones.

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    \$\begingroup\$ But that's what OP is asking. WHY use 3 pins, instead of two signal pins and use the connector shell as ground. \$\endgroup\$ – Passerby Apr 17 '13 at 7:40
  • \$\begingroup\$ The metal of the frame could be connected to power ground and this is not the cleanest way to provide audio connections. I'll alter the answer to clearly state this - thanks @Passerby \$\endgroup\$ – Andy aka Apr 17 '13 at 7:43
  • \$\begingroup\$ There is no such thing the as 'signal ground' in a balanced connection. It's a contradiction in terms. \$\endgroup\$ – user207421 Apr 17 '13 at 19:58
  • \$\begingroup\$ @EJP - I'm making the distinction between "power ground" and the "earth" required on the screen that is needed with proper balanced-twisted-pair cables. On a slightly different note it is used as "ground" for phantom powering condensor microphones. \$\endgroup\$ – Andy aka Apr 17 '13 at 20:10
  • \$\begingroup\$ But why make any distinction? It doesn't matter how noisy the screen is. It doesn't even matter how noisy the balanced pair is, as long as it's the same noise. \$\endgroup\$ – user207421 Apr 17 '13 at 21:10
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The shells of many connectors are not intended to provide a reliable electrical connection so using a connector pin for ground is required to ensure that the ground connection is carried through the cable.

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Very good question. These is no such thing as 'signal ground' in a balanced audio connection, which is what XLR connectors are used for, so it isn't that, and to be effective the shield ground should contact the chassis at the earliest possible opportunity at both ends of the cable. There is a very good discussion in the latest Linear Audio (5) in which it is argued that it is a design mistake.

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Many XLRs do provide a terminal for termination to the shell as well, which can be used to maintain an additional ground connection, usually 'chassis ground' which is often connected to the mains 'earth' (for non-double insulated devices).

More recently the termination is used to maintain the EMC performance of the cable over the last few mm in electrically noisy environments (e.g. see the Neutrik EMC series connectors).

In practice, equipment is most likely to work with no connection to the connector chassis, even though it may be technically superior in some situations to use the connection. A large quantity of XLR connections are used in situations requiring regular reconfiguration (e.g. live music and other audio applications), so the 'most likely to work' option is also the most popular!

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