Mostly XLR (X-series with Latch and resilient Polypropylene, later known as Xternal, Line, Return) are used where longer cables are required, f.e. microphones, etc. or where a minimum of noise and hum is required.
Due to the rigid construction they are also used to connect loudspeakers, etc. today.
How can it be that an ingenious, symmetrical connector like this is used the way we do and "killing" some advantages: ground is not in the (symmetrical) middle, but at pin 1. The output is always a male connector, so a shortcut is possible quite easy, especially where has to be worked under pressure. Third issue is that in the USA for long time both signals (reversed 180º) are connected the other way around.......
In todays equipment US follows the rest of the world for once.
Unlike RCA connectors though, ground is made before any signal line is connected !
Pin1=(signal)GND, Pin2=Live (in phase), Pin3=Return (same signal, 180º out of phase), body= equipment GND.
Outputs are MALE, inputs FEMALE.
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