In a typical situation you can not allocate a particular XLR cable to just one mike, and cables move tasks freely in a studio or live rig.
Please confirm that the XLR cable in question was connecting to a microphone, not between two other components.And, I repeat: an open shield on even one side can lead to hash/noise in the mic.
It was indeed a cable going to a microphone! (So: connected to nothing else.)At the moment pin 1 of the XLR on the microphone input was connected to the shell of the connector (=mains ground), a clear mains hum was audible. (I suppose this was due to a design fault in the mixer. This happened on every channel.)
... chassis ground connected to audio ground is the norm in most vintage supplies.
What's the best procedure for an XLR cable connecting a tube mic power supply to a mixer? The cable is not directly connecting to a microphone. Is it more akin to a cable between two powered units, even though it is not carrying line level?
It's a bit more than that, you must first determine if chassis ground (AC ground) is internally connected to power ground (audio ground return path). If so it can be isolated via a 10 ohm resistor between them. A .1 uf cap across the resistor will provide an AC connection above 500 hz to remove buzz and any rf. I do this on rack mount mic preamps to avoid any issues when rack mounted.Otherwise it's a crap shoot every time you interconnect various AC powered audio pieces.
Kai,Your last sentence is not clear to me: as I mentioned, even Neumann does not separate audio ground from AC/chassis ground.Please explain in simple words what you mean. I also recommend to not stray too far from the original subject. Maybe rackmount design goes a bit too far into the woods for this forum's users.