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

XLR shell connections to ground in a microphone cable

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David Satz:
(to DarinK's question) Yes, the latter. The microphone cable can be connected as Klaus suggests (and as Neumann does it) only because a microphone isn't normally connected to any other external "ground" reference. In nearly all other cases it's a better bet to let the connector shells and cable shields connect only to the equipment enclosures, and to connect the cable shield to the ground pin only on one side of the cable.

Unfortunately this wasn't well understood when the conventions for XLR cabling were established, and now there's a lot of resistance (no pun intended) to realizing that the best arrangement for microphone cables isn't also the best for making connections between other types of equipment. People understandably want XLR cables to just be XLR cables; they want to use them for all purposes interchangeably. But that isn't the best actual choice in most cases.

I'm hedging my statements, though, from hard experience. Sometimes there isn't time to investigate the causes of a particular problem, and you gotta do whatever works to get rid of hum or other noise from interference. I'm a strong supporter of standards such as AES48--but to my knowledge, no one approach can instantly and reliably fix all hum and RFI problems. I think that all advice on this subject has to be hedged with "of course if there's still a problem, try something else, even if it doesn't seem to make theoretical sense at the time."

Things that you wouldn't necessarily think of can cause problems--for example, Jim W. mentioned that shield and ground are generally connected within microphones. But sometimes that's by means of a screw that holds the housing onto the body of the mike, and that screw can become loose or dirty, or it might be painted or corroded so that it doesn't make good contact. (I've had that happen.) Or sometimes the connection is by way of a wire that isn't short enough to be fully effective at the higher and higher radio frequencies that are increasingly prevalent, or the wire can break or come loose, or ... or ...

Decades ago, microphones weren't designed to protect against problems that didn't exist yet. At best they were built to work in broadcasting studios where a conventional AM, FM or TV transmitter was in the same building or nearby. That's a challenging enough problem--but what we have today is categorically more severe sometimes, e.g. boom operators for film and video sound who connect their microphones directly to digital wireless transmitters. Microphone manufacturers are having to keep up with rapidly changing conditions.


--- Quote from: klaus on April 07, 2021, 02:07:21 PM ---... chassis ground connected to audio ground is the norm in most vintage supplies.
--- End quote ---
Itís a user selectable option on a lot of studio equipment... vintage or contemporary.


--- Quote from: DarinK on April 07, 2021, 01:55:19 PM ---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?

--- End quote ---
I have added two pictures of tube mic connections.

The first one shows male and female factory-terminated Neumann 67 cable connectors.
Note that the cable is terminated as per my recommendation (as if could take credit): ground and shield connected to ground pin and connector housings, on both ends.

The second picture shows the factory-terminated female chassis-mounted XLR connector of the current-edition Neumann U67 power supply.
Note that the connector is terminated as per my recommendation: ground and shield connected to ground pin and connector housing (which is also connected to chassis and circuit ground).

I have received a few ugly responses to my initial recommendation how to terminate microphone ground and shield connections. I have removed these, but kept the posts with disagreements on the subject.
What this whole discussion tells me is two things:

1. Solid empirical data trump theoretical evaluations, no matter how logically sounding. I have terminated god knows how many microphone cable and connector ends in my life; not once have I received a report that what I regard as proper terminations then turned a previously quiet mic into one with a ground hum. Not once.

2. The ground and shield terminations I recommend have in almost 100% of cases eliminated RF hash, radio station interference, or other forms of external noise induced into the mic due to faulty shield/ground terminations.


IME, ground loops are caused by current flowing in the shield of the interconnect cable, caused by the gear having different chassis potentials with referenced to ground.

Yes, this may be cured by lifting the pin 1 connection but there is usually an underlying problem such as a leaky power transformer or AC bypass cap and that problem really should be corrected first.

My first step in addressing ground loops is to voltmeter the chassis-to-ground potential of the interconnected units.

And ground loops are a completely different problem then RFI/EMI.

Jim Williams:
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.


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