Right you have your hot +, cold - and shield (I think it's gnd). The three XLR pins of any standard XLR plug. But all plugs have a fourth pin to the actual metal housing of the XLR plug.
I have seen on other peoples work where the shield is also soldered to the fourth pin. Perhaps this is something I should not have done on any particular cable?
Any ideas lads?
As I understand it, the metal casing is connected to ground so as to shield that last inch of stripped wire within the plug. I solder one to the other as a matter of course, but to be quite frank, it must make very little difference...
Not this old chestnut.
I was very flummuxed by this at one point in my sorry wee ground looped hell of a life,
and Sound on Sound even printed this whole saga in their letters page! Gosh!
HERE'S THE COMPLETE AND UTTER DEFINITIVE ANSWER:
Generally, ewe do *NOT* want to also join the XLR plug shell to the ground pin.
The XLR plug shell will generally, when ewe plug it in, be in electrical contact with the "chassis" (casing) of your audio equipment. This means it is connected to "mains earth" via the audio equipment's mains lead.
The ground pin carries the "audio ground" (the return current path for the audio signal) and in a balanced system, this audio ground is generally separated off from the mains earth.
EWE DO NOT WANT THOSE TWO EARTHS TO GET MIXED TOGETHER AGAIN - cos otherwise, the return current of the audio signal will get "coupled" to any return currents in the chassis, and ewe will have the dreaded MAINS HUM (a ground loop) in your audio.
This arrangement also maintains shielding for the audio signal, as the cable shielding is in place along the length of the cable, and then the plug (which is acting as an extension of the equipment's chassis) shileds the last wee bit.
THE ONLY TIME THIS WON'T WORK is if you want to make a really long cable run, and join two XLR cables end to end.
In *that* situation, you WOULD need to solder the cable screen /earth pin onto the XLR plug shell, otherwise at the bit where the two cables joined, ewe'd have the two shell's not earthed to anything, but floating.
Thus there'd be no screening for about 2 inches, and the audio could pick up interference.
OR worse still, if you were running AES-EBU digital signals down the cable, it would blast out as a transmitter at the cable join.
However, you could avoid that whole scenario by just having a balanced XLR cable that was extrememly long, and thus not needing to join two together (i mean they can go for 100's of meteres)