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

XLR shell connections to ground in a microphone cable

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Jim Williams:
Since this subject was blocked I thought I'd re-open the subject since it needs further discussion. In a microphone body the ground is connected to the body as a screen. Every mic I've seen has an internal connection from pin 1 to the mic case. That in turn connects to the XLR connector shell via the locking tab on the cable. One can use a continuity meter to confirm this. There is no need to solder an extra wire from the ground tab to pin 1 inside the cable.

On the other end it's a bit more complicated. Mic preamp designs vary. In many cases there are two grounds at work, one is power ground, the return path for the audio electronics, the other is chassis ground, that which connects to the ground tab on the AC power connector = shield.

In many designs those are kept separate for good reason. In those cases pin 1 (power ground) is not directly connected to chassis ground. It's isolated via a cap, a resistor or a combination. 10 ohms isolation is common. The reason is ground loops. You do not want the pin 1 power ground connected directly to chassis ground as that creates a loop via the AC chassis ground flowing back through a rack case into the next piece of gear. The 10 ohm isolation resistor prevents that loop, the cap across it allows for 500 hz and up to be AC connected to ground to avoid buzzes. 120 and 60 hz won't pass through the small cap and won't loop.

Again, the XLR cable case does connect to chassis ground via the locking tab, that provides all the extra shielding you need, connect that to power ground (pin 1) and issues arise.

Bill Whitlock is the reference on this subject, he's an international expert on audio grounding, has written dozens of articles for the AES and the trades on the subject and was the President of Jensen Transformers. I'd love for him to weigh in here.

The point I was trying to make, well put. 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. And in that situation, grounded shells cause problems.


--- Quote from: RadarDoug2 on April 02, 2021, 01:31:19 PM --- 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.

--- End quote ---
There is never any excuse for slop.
If you dedicate and label some cables exclusively for mic use, and terminate them correctly as mentioned, you avoid any misapplication of using these cables in line-level connections, especially those line level connections between two AC-powered units.

Keep cables for these two functions separate, and you will not have issues with ground loops.

Back to the correct grounding/shielding scheme for mic cables:
If there were a ground loop problem, I would be the first to hear about it from my clients. No one in all these years has complained about added noise or hum, after following Neumann's recommendation, which I came to follow, strictly from my own experience: eliminate RF/cable hash by terminating shield/ground in mic connectors on both ends.

In the past I had a different problem: increased mains hum when pin 1 of the XLR connector was connected to the shell of the connector going to the input of a mixer..
At the moment I disconnected the shell, the mains hum was gone!
It seemed there was a potential difference between the audio ground (pin 1) and the case (safety) ground of the mixer.
So you never know...
(This says probably a lot about the quality of the mixer too!)

Please confirm that the XLR cable in question was connecting to a microphone, not between two separately powered components.
And, I repeat: an open shield on even one of the two connectors of a mic cable can lead to hash/noise in the mic.


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