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Old 14th February 2008, 05:40 PM   #1
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Default XLR Pin 1 question - chassis vs circuit ground

I've tried searching but I can't find a definite answer to this question.

What do I do with Pin 1 of an XLR connector?

In my audio chain, my DAC, Preamp, and amp (Aleph 30) are all balanced out/in, but I'm never quite sure about how to connect pin 1 of the xlr in/out.

1. Does circuit ground connect to XLR pin 1? I've read people say both yes and no to this question. As far as I know, XLR pin 1 is shield which is NOT circuit ground. If pin 1 IS supposed to connect to circuit ground, then does it connect at both input AND output (eg., preamp and amp)?

If I understand correctly, XLR shield will no longer be effective as a shield if it's connected to circuit ground because the shield will pick up noise and inject it into the audio signal but if it's connected only to chassis, then the chassis and XLR shield act together to protect from RFI/EMI..


2. Do I connect XLR pin 1 to chassis ground? If so, does it connect at both input AND output? In the Balanced Zen Line Stage (Bosoz), XLR inputs and outputs are paralleled with RCA jacks and RCA ground should isolated from the chassis but if XLR pin 1 is connected to chassis and also connected to RCA ground, then isn't RCA ground no longer isolated from the chassis, and thus also connected to circuit ground?

3. Does chassis ground connect to IEC ground (earth)?



Hope my questions make sense..
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Old 14th February 2008, 08:43 PM   #2
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Pin 1 is supposed to connect to chassis ground.

It's all in here; but there is also an AES standards paper on XLR / balanced connections you may want to look for.


http://www.rane.com/note110.html

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Old 14th February 2008, 09:12 PM   #3
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Thanks. THis article was very informative but I still don't understand what to do where the RCA in/out is paralleled with the balanced in/out.

It seems clear that pin 1 of the xlr is connected to chassis ground and isolated from signal ground.

This is a bit confusing to me: (taken from Bosoz article)

"In parallel with the XLR connectors
are 2 RCA connectors, one for each polarity, with XLR pin 2 connected to the "live" of the positive RCA and pin and XLR pin 3 connected to the "live" of the negative RCA. XLR pin 1 is attached to the grounds of both RCA connectors. These grounds are isolated from chassis ground. The case of the XLR connector, if metal, and shield are attached to chassis ground. "

Here, it says XLR pin 1 is attached to ground of RCA connectors and these are isolated from chassis ground. Seems to be different from the AES standard.

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Old 14th February 2008, 09:26 PM   #4
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Quote:
Originally posted by HeadSh0T
Thanks. THis article was very informative but I still don't understand what to do where the RCA in/out is paralleled with the balanced in/out.

It seems clear that pin 1 of the xlr is connected to chassis ground and isolated from signal ground.

This is a bit confusing to me: (taken from Bosoz article)

"In parallel with the XLR connectors
are 2 RCA connectors, one for each polarity, with XLR pin 2 connected to the "live" of the positive RCA and pin and XLR pin 3 connected to the "live" of the negative RCA. XLR pin 1 is attached to the grounds of both RCA connectors. These grounds are isolated from chassis ground. The case of the XLR connector, if metal, and shield are attached to chassis ground. "

Here, it says XLR pin 1 is attached to ground of RCA connectors and these are isolated from chassis ground. Seems to be different from the AES standard.


Yes it is not conform the standard. The RCA ground pin should of course be connected to signal ground. Why they connect XLR pin 1 there, I don't know. It may have to do with the way they 'make' the balanced output, I am not familiar with the Bosoz. Maybe ask Nelson Pass?

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Old 14th February 2008, 10:28 PM   #5
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If the XLR pin 1 is connected to chassis ground at both ends you
will have huge ground loops formed by the power cord grounds
and the XLR cables. It will generate a lot of hum. I spent last
weekend trying various combinations of ground configs between
a DIY Aleph P 1.7 and my "Xono Clono". I will post a paper
with a grounding scheme that I found to work best.

I understand what you may mean about having noise induced
into the signal ground if the XLR pin 1 is connected to it, but it
was by far the least worst solution that I found - and in fact
with the XLR1 connected to chassis, the system was unusable
as the hum was 20 to 40 dB louder than the "signal" (I'm
guessing the numerical value because I can't measure it).
Meaning - the induced RF noise affects the circuit orders of
magnitude less than the relatively large voltages induced by
the ground loops.
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Old 14th February 2008, 10:32 PM   #6
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It is not possible to make a piece of equipment compliant to AES48 when XLR1 connects directly to the signal ground, as this would be the case with a RCA GND connected to XLR1. AES48 is only a recommendation, though. AES48 reads as: Connect XLR1 to chassis only, and get some clean galvanic connection (often a R//C//D) to signal GND elsewhere, to provide a reasonable common mode potential for the balanced inputs/outputs. This implies that gear with safety-grounded metal chassis cannot have their signal ground floating (unless one strictly uses xformer coupled I/O, and even then this is questionable). And if metal-housed gear is not constructed conforming to class-II isolation (double isolation) the chassis must be safety-grounded (as per German/EU eletrical codes, US/Cananidian codes I don't know in detail).

In the end, it's a tradeoff between potential RFI/ground-current threats and the best possible common mode hookup.

See also:
http://www.rane.com/note151.html

- Klaus
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Old 14th February 2008, 10:45 PM   #7
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Quote:
Originally posted by wayne325
If the XLR pin 1 is connected to chassis ground at both ends you
will have huge ground loops formed by the power cord grounds
and the XLR cables. It will generate a lot of hum.
This problem is commonly encountered, but the root cause isn't always clear. In gear with no "pin 1 problem" the only possible cause is ground currents so high that they induce hum with non-perfect cables (then termed "Shield-Current Induced Noise", papers to be found here). High ground currents are absolutely unlikely in domestic situation, especially when all gear runs off one mains outlet. So, the "pin 1 problem" inside the gear is usually the cause, assuming that the common mode rejection in the receiver end is up to the task. One can work around that with so called hybrid shield connection (realized in the cable): one end -- the receiver end -- of the cable shield is connected to XLR1 and XLR shell via a R//C, to stop huge DC/LF shield currents from flowing and still keep the shield connection for RF.

- Klaus
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Old 14th February 2008, 11:15 PM   #8
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Not being a member of AES, I simply do what works for me. If I
get complaints from customers, I'll do something else.


Later:

Cyclotronguy refers me to

http://www.aes.org/standards/b_comme...aes48-xxxx.cfm

particularly the comments by Kim Roshoej and Sven Aage
Christenson and Bruce Olson's response.
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Old 14th February 2008, 11:45 PM   #9
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I have been making XLR cables for over 20 years, and have never connected the XLR chassis to ground, (and neither has any other tech I know, or have I found any cables wired as such). Pin one is connected to the shield, and pins 2 and 3 to the twisted pair. That's it.
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Old 15th February 2008, 12:15 AM   #10
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PM, probably a semantics problem. With "chassis" you mean the cable connector's shell? I was referring to the device's (ie amp's) metal enclosure.

"Officially", the device's receptacle shall be responsible to apply a shield potential to the connector's shell, but this has proven to be an unreliable contact because of painted/anodized shells etc, so one should connect the cable connector shell also. As an example, my active nearfield monitors don't connect the shell at all (clearly a bug), so I have to use a cable with connected shell otherwise I get some buzz from the CRT computer screen located between them.

@Nelson:
This double shielding variety is often used in instrumentation, with good results.
AES48 is controversly discussed, as doesn't allow for combo inputs/outputs, where a TRS 1/4"-jack is used with a unbalanced (mono) plug, shorting cold phase and shield, expecially in the XLR/1/4"-combo input jacks.

- Klaus (with 24yrs of XLR'ing on my back )
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