R/E/P > Klaus Heyne's Mic Lab

XLR shell connections to ground in a microphone cable

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RuudNL:
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.)

Jim Williams:

--- Quote from: klaus on April 03, 2021, 12:31:42 PM ---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.

--- End quote ---

Again, the locking tab connects the shell to the ground connection, there is no need for an extra internal wire from the ground tab to pin 1. Use a continuity meter to confirm this.

Kai:

--- Quote from: RuudNL on April 04, 2021, 12:09:26 PM ---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.)

--- End quote ---
The cable’s internal connection of Pin 1 to Shell bypassed and deactivated the mixer’s ground (lift) configuration, by interconnecting the formerly separated Audio Ground with Chassis Ground.

Specially bigger analog audio mixers have a hard time avoiding internal ground loops, usually there is one single central Audio Ground - Chassis link, or a really thick copper rail as ground reference.

DarinK:
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?

klaus:
My first job is to never see a mic again after I work on it, due to something I did or did not attend to while the mic was in my care.

I always terminate the internal wiring inside the power supply between multi-pin mic connector and audio XLR exactly the same way I described: Ground and shield and connector to ground, both sides of the cable (by the way, chassis ground connected to audio ground is the norm in most vintage supplies. (See as example the grounding scheme in the NG).

If there were ever a problem in all the years I used that scheme, I'd think at least ONE person would have notified me that there was something wrong where there was nothing wrong before. I am still waiting for that call.

I will address the critics of the Neumann termination in a few days, and show more examples of factory-terminated mic cables.

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