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Capacitor between XLR shell and ground pin

Dilettante

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The topic isn't new, but still seems to be contested.
Here and here it is advised to add 100pF capacitor in a way that both pin #1 and shell are connected.

qa_1120_01-sI1R.cYcWBsxyyEeUVHq1M7H2PrysXeZ.jpg


Is this what's normally recommended or connection between the cable and pin should be severed?

Untitled.png


Also, what is the optimal capacitor type and capacitance?
I've seen values ranging from 0.1 (zero-point-one) to 10000uF. Quite a range!

Specifically, I would like to quote a message from another forum:

To minimise problems with ground loops through the mains safety earthing of separate equipment (in poorly designed products) many designers therefore contrived ways to separate the audio and chassis grounds (at least at audio frequencies).

In this kind of equipment, linking pin 1 to the shell of XLRs will obviously bridge the two grounds and potentially open a whole world or ground loop worms.

So if you suspect you might be using less than perfectly designed equipment that might be susceptible to ground loop issues, then the best advice is NOT to link pin 1 and the XLR shell.

Full screening will still be in place because the XLR shell is grounded to the chassis once plugged into the socket anyway.

Where the screening breaks down is when joining two XLR cables. In this situation, the short bit of exposed wiring inside the connectors (but outside the cable screen) is potentially open to interference becuase the metal XLR shells are floating (not grounded to anything).

In practice, this is unlikely to be a major problem and I've certainly never had problems with it... but you can get the best of both worlds by using a small capacitor to link between pin 1 and the XLR shell. Something like a 0.1uF ceramic disc capacitor will provide a very low-impedance path to ground for RF interference, while maintainig a very high-impedance path at audio frequencies -- so the XLR shell acts as a proper RF screen but won't bridge the audio and chassis grounds at audio frequencies, and so there is no risk of ground loops.
 

Blumlein 88

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I don't think you mean 10,000 uF. Maybe 10,000 pF which is .01 uF. Wouldn't fit in the shell anyway.

I'll let others chime in otherwise. I can see situations where the cap is a good idea and those where it isn't. I don't think I have ever seen an XLR with the cap in it though I've read about them. I don't sever the connection in my XLRs.
 

somebodyelse

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I'll just throw in Jensen Transformers' application note AN007 section 3.6 for some more explanation, but the rest of it provides some good context. If it's been the subject of argument in a standards committee for multiple years there probably isn't a clearly 'correct' answer. If I've understood what's going on correctly (and it's a big IF) you're looking at workarounds for specific problems usually caused by manufacturers not following AES48, and the lack of a 'correct' answer is because the workaround that's right for one problem is wrong for another.
 

HarmonicTHD

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The topic isn't new, but still seems to be contested.
Here and here it is advised to add 100pF capacitor in a way that both pin #1 and shell are connected.

qa_1120_01-sI1R.cYcWBsxyyEeUVHq1M7H2PrysXeZ.jpg


Is this what's normally recommended or connection between the cable and pin should be severed?

View attachment 212451

Also, what is the optimal capacitor type and capacitance?
I've seen values ranging from 0.1 (zero-point-one) to 10000uF. Quite a range!

Specifically, I would like to quote a message from another forum:
See here plus the datasheet linked on the site.

 

Speedskater

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It's not about a cap from pin #1 to the shell. It's about a cap from the shield conductor to pin #1 at the receive end.
The AES has a standard about not connecting pin @1 to the shell. The worry in pro audio situations is that, the shell of portable cables might come in contact with metal objects that are at different potentials.

With dedicated cables, connecting pin #1 to the shell is a good idea.
In phantom mic cables, the shield needs to be hardwired to pin #1.
 
OP
Dilettante

Dilettante

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@Speedskater , what do you think about the scheme suggested in the document linked by @somebodyelse ?

If this is inadequate, or the frequency is lower (such as AM radio), a low-pass (high-frequency reject) RFI filter may be necessary in the signal line.

The schematics below show sample filters for unbalanced and balanced line-level applications.

For best performance and highest audio quality, use NP0 or C0G type ceramic capacitors with leads kept extremely short (well under ¼” preferred). The 680 uH inductors are small ferrite core types — current rating and DC resistance are not critical.

For the balanced filter, the inductors and capacitors should be ±5% tolerance or better

1655134005088.png
 
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somebodyelse

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That's another specific solution to a specific problem, not something for general use.
 
OP
Dilettante

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You make me wonder if there is a free space in DBX body to add that scheme...
 

somebodyelse

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I just wonder if it can be a solution for 50-150Hz range.
No - if your problem is 50-150Hz you don't have the specific problem it's a solution to. Have a look at sections 3.3 (Pin 1 Problems and The Hummer) and 3.4 (Finding the Problem Interface) as that's more likely what your problem is. Is this a rehash of your USB Bus Noise thread, but without the pertinent information about the hardware configuration?
 

eliash

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The topic isn't new, but still seems to be contested.
Here and here it is advised to add 100pF capacitor in a way that both pin #1 and shell are connected.

qa_1120_01-sI1R.cYcWBsxyyEeUVHq1M7H2PrysXeZ.jpg


Is this what's normally recommended or connection between the cable and pin should be severed?

View attachment 212451

Also, what is the optimal capacitor type and capacitance?
I've seen values ranging from 0.1 (zero-point-one) to 10000uF. Quite a range!

Specifically, I would like to quote a message from another forum:
In case you need to decouple the sending XLR cable ground from the receiving end ground, due to ground loop hum, I would recommend a 1nF cap to short any RF pick-up EMI between both points.
In my case the problem was a connection from a mains grounded preamp output (RCA) to a STAX electrostatic headphone amp, which was also grounded via another mains cable. Unfortunately the Stax amp´s mains filter and transformer generate a strong ground current, which, because of the mains cable ground lead resistance, generates the hum voltage. Solution in my case was to use the Stax differential XLR input instead of the RCA input to connect the preamp's RCA signal and ground to its hot pins 2 and 3. Since Pin 1 of the XLR input is also connected to the Stax mains ground, it made sense to block any EMI between the RCA ground and the XLR ground with a cap. Dimensioning is as follows: As big as possible to set to lowest possible EMI pickup frequency and small enough, not to generate any noticeable hum voltage on the RCA cable's ground lead resistance. Here 1nF ceramic was an uncritical value, which serves both mentioned purposes and fits into the XLR plug.
 
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Speedskater

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Ignore the links in the original post. Not that they are wrong, it's just that they are confusing and much of those threads are written by posters who don't understand balanced interconnects and noise & interference.
 

DonH56

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I have not read this whole thread.

The reason I have seen for adding a small capacitor is to provide an RF "sink" or short from signal ground to outer shield. Whether that is needed, and whether it helps or hurts, is very system-dependent. I have never seen it used to "break" a ground loop; it would have to be a very large capacitor, and that would allow noise from the shield to couple to the signal ground (return) path so seems like a bad idea to me. It also increases the shock hazard; the shield is meant to help isolate the signal return from the user, so in general you don't short it to pin 1 (but many do...) The shield is sometimes connected at just one end to help break a loop. A truly differential system could also float pin1 (signal ground) at one end (or use a direct box or similar approach to break a ground loop). Floating pin 1 is an issue if you are driving from a balanced (XLR) source to a single-ended (RCA) input since pin 1 is the signal return in that case.

If you have an RFI problem, then 100 pF to 1 nF (1000 pF) are what have been used IME to provide a RF path from signal return to ground. 1 nF is about 8k-ohms at 20 kHz so a rather low value; 100 pF is what I usually used. IIRC, 1 nF was used on some PA systems that were used for voice only (announcements, not music), again IME. In most (practically all) installations I have done it is not needed since the shield isolates the signal return anyway. There have been a few special cases I can recall in the primordial past that benefited from the RFI cap due to either excessively noisy environment (e.g. at or near a radio station, which back then was usually some CB'er and his "linear" amplifier that was a distortion generator) or poor design (e.g. some mixers had no RF filtering).

FWIWFM - Don
 
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Dilettante

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Is this a rehash of your USB Bus Noise thread, but without the pertinent information about the hardware configuration?

The hardware configuration is on the 1st page: click me. That being said, it isn't a rehash of the topic you have linked, but an extension of this particular post. I do admit, however, that it gets rather dense quickly. I am trying to keep information short and to the point.

What problem are we trying to solve here?

Was trying to, this.

Have a look at sections 3.3 (Pin 1 Problems and The Hummer) and 3.4 (Finding the Problem Interface) as that's more likely what your problem is.

Did that, lifted the ground on XLR end - no difference. Please note the the issue was narrowed-down to a particular combination of factors.
 
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