Electronic – Why does an XLR connector use 3 pins instead of two


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


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

Best Answer

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.