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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.

[RadarDoug2]

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.

[klaus]

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.

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.

[RuudNL]

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!)

[klaus]

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.

[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]

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.

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]

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.)

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.

[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.

[Kai]

... chassis ground connected to audio ground is the norm in most vintage supplies.

It’s a user selectable option on a lot of studio equipment... vintage or contemporary.

[klaus]

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?

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.



 

[boz6906]

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.

[klaus]

Since my last post, I received three more requests to troubleshoot RF in condenser mics - two solid state, one tube. In all three cases the clients were again able to solve the issue by following Neumann's (and my) grounding/shielding scheme.

P.S.: Not sure whether I had previously mentioned it, but in the current Neumann U67 power supplies AC, chassis and audio ground are one.

[Kai]

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.

ABSOLUTE NO-NO!
Don‘t anybody mess with the connection of mains (safety-) ground to chassis!
This is deadly dangerous.
If a device is designed that way, then this connection is essential for safe operation of the unit.

It’s possible to interrupt CIRCUIT ground from chassis / safety ground IF NO CONDUCTIVE PARTS OF THE CIRCUIT STICKS OUT AND CAN BE TOUCHED.

Few devices are built like that, and usually doing so takes some effort, as multiple connections need to be isolated. None of this, I suggest, is a layman's job.

In the case mentioned, I suggest to use Humfrees for rackmount.

[Jim Williams]

I did not suggest that. AC chassis ground connections must be maintained unless it's a non-grounded AC feed like many common dbx compressors. The audio power ground is isolated in many designs. These devices use isolated TRS jacks and XLR connectors from the chassis. If audio power ground is connected to chassis ground via multiple connectors, isolation is not possible.

Yes, you need to know about this subject before you start probing. It's intended for designers, not end users.

[Kai]

I was mislead by your terminology.

What you call “AC ground” is in fact “SAFETY ground”, “Schutzerde” in German (yellow-green wire in most countries).
The term audio power ground was unclear to me. I think you mean the PSU’s secondary DC ground.
A PSU has a multiple of those, one for each voltage, mostly, but not always, combined into -> (audio) circuit ground, often clearly visible as a starpoint on the PC-board.


On current devices, to avoid RFI problems, the audio circuit ground is connected to chassis on multiple positions, isolation is almost impossible and kills RFI immunity.

On devices with safety ground, doing so could be a safety problem too, i.e. imagine a short in the power transformer or other loss of isolation between primary and secondary.
I’ve seen this, not only on decades-old devices where isolation materials are far beyond their intended lifetime.
Even current devices can lose isolation, caused by over-voltage from mains or nearby lightning strike.


Humfrees are the better, safer and more user friendly solution.
Or even simpler: all my studio racks have wooden mounting rails, where isolation comes for free.

[Jim Williams]

Here I use standard metal rack rails for support. Some devices like the Bricasti M7 have a lot of rear weight pulling on the rack screws, metal is far more secure than wood. The sag from behind can cause the device cases to touch even though there is isolation on the rack rails. I do use isolation washers, those with the ability to float the screw away from the rack rail.
I use a continuity meter to test isolation after an install. A strip of isolating plastic across the rear of the device will prevent sag connections.

However, that will not prevent possible ground loops simply because in some devices the audio supply power ground still connects directly to AC safety ground internally. Patch those devices together and you create dual ground return paths, one through the DC audio power ground via connectors and another through the AC connected safety ground. This is where the 10 ohm isolation resistor has its benefit.

I recall doing extensive testing on outboard pieces with the Audio Precision back in the 1990's. What I found was a crapshoot. It became apparent that audio gear designers failed at interfacing and testing their products in a real world app, isolated on the test bench they all tested ok. When interconnected, the issues arose.

[Kai]

Don’t underestimate wood.

I do use wooden rack rails for ages and never any screw came loose.
Even on the deep and heavy 1U Lexicon PCM 70, and I’m lazy and do use two screws only even for this one.

You just have to use the right sized screws: 5 x 30 mm SPAX half-round head, with plastic washer underneath.
The wood itself is nothing special, simple spruce, 20 x 50 mm, predrilled 2 mm holes at the mounting positions to avoid splitting.

Try to pull out a screw like this by human force, you will find that‘s not possible.
A rack device uses four...

Back support can be done the same way with units that need it.



To get rid of hum loops you have to keep the loop area as small as possible.
This means laying all lines, audio and power (!) in one big strand.
When using quality properly shielded cables, no stray hum goes from power to audio lines in my experience, even with my country’s  230 V mains voltage.
I do use double shielded multicores with heavy gauge shield.

Opposed to that microphone cables should be kept distant from power lines, specially from the high current ones for stage lighting!

[klaus]

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.

[Kai]

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.

I edited it, hope it’s clear right now.
I do not advocate separating grounds beyond what the device designer intended.
By no means I suggest to change internals of devices.

Hum loops and the like are a common problem in studios and closely connected to the topic I think, so hopefully I did not digress too far.

Wooden rack rails is my invention (no Pat. pending) and hereby I share it with the world