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[John Neiberger]

I've got a video conferencing system that has been suffering from weird audio problems for a while. We think that there's a lot of RF in the area and that the system has some grounding issues. We've done quite a bit of rewiring and I think we've made quite a bit of progress. In fact, there's a video conference going on right now and it sounds great. I didn't hear any noise in the system at all when I was up there before the conference began.

I'm still worried about one connection in particular, though. I have a video conferencing unit (Polycom ViewStation 128) that has low impedance line-level inputs and outputs. That unit is out in the board room connected to its own electrical outlet.

The audio connections go to a Gentner AP800 audio mixer in a closet that has balanced three-wire phoenix connectors. I'm worried about how this is wired. It seems to be working at the moment but I still don't know if it's wired 100% correctly.

At the moment there is a three-wire cable running between the systems (hot, cold, shield/ground). The video unit uses unbalanced RCA connections so the hot and cold pins are wired but the ground/shield is snipped and taped off. At the mixer side, the hot wire is wired to positive, while the cold and shield are twisted together and wired to ground.

I've been hearing differing opinions about this. I was told that it was up to me whether to leave the ground connected or not. Last night I read a pretty thorough white paper discussing the differences between unbalanced and balanced grounding problems as well as the differences between signal ground and chassis ground. That author gave the impression that if you're not using the ground then it shouldn't be connected on either side because it can act as an antenna. If it picks up noise, it will pollute the reference voltage and cause noise in the audio system.

So, what do you guys think? What is the proper way to wire a two-wire low impedance system to a balanced three-wire system? I'm beginning to think that hot and cold should be wired on the RCA side, then it should be hot-to-positive and cold-to-ground on the mixer side.

Any thoughts?

Thanks!
John

[John Ivan]

If you're not hearing a noise now, I would leave it alone . Having said that. I would wire RCA tip to Pin-2 hot.RCA sleeve{the second conductor, not the shield} to pin 3 cold and jumper pin 3 to ground. I would leave the shield {braid/foil/drain, which ever it is} Lifted and I would star ground the units chassis's to the same earth ground. Make sure all audio unit's are on the same outlet or set of outlets on the same circuit.

JI..................................................

P.S. Make sure you can jump pin 3 cold to ground without hurting anything.. It sounds like you already have it wired this way.

[John Neiberger]

Right now, I think we have RCA hot wired to the hot post on the mixer, but the cold and shield are twisted together and wired to ground on the mixer. Is that correct if I'm going from an unbalanced two-wire connection to a balanced three-wire connection?

I guess I can disconnect the shield at the mixer side so that it isn't wired to ground, but then I wonder about connecting cold to ground. It's working now so that must be at least partially correct.

Thanks!
John

EDIT: I also meant to mention that these outlets are not on the same set of outlets and may not even be on the same circuit. We're going to run a new line so that all of them can be on UPS-protected clean power on the same circuit.

[danlavry]

John Neiberger wrote on Fri, 21 April 2006 19:26

Right now, I think we have RCA hot wired to the hot post on the mixer, but the cold and shield are twisted together and wired to ground on the mixer. Is that correct if I'm going from an unbalanced two-wire connection to a balanced three-wire connection? I guess I can disconnect the shield at the mixer side so that it isn't wired to ground, but then I wonder about connecting cold to ground. It's working now so that must be at least partially correct. Thanks! John EDIT: I also meant to mention that these outlets are not on the same set of outlets and may not even be on the same circuit. We're going to run a new line so that all of them can be on UPS-protected clean power on the same circuit.

As a rule, one should wire the shield at the driver side, and leave it open ended at the reciver side. That is assuming that all other connections are done correctly.

Regards
Dan Lavry

[Gerry]

Good info on wiring, gives you some templates.

http://rane.com/note110.html

[danlavry]

Gerry wrote on Thu, 27 April 2006 20:02

Good info on wiring, gives you some templates. http://rane.com/note110.html

I am not sure I agree. I would start by connecting the shield (XLR Pin 1) ONLY at the driver side, not at the destination. This is the preferred way. See "Grounding and Shielding Techniques in Instrumentation" by Ralph Morrison (Wiley publication). This book may or may not be out of print, but it is certainly an excellent publication and highly regarded. The shield comment is well explained!

There are times when connecting the shield at both driver and receiver end does improve noise. But such cases indicate that something else is not wired optimally.

Of course, if the system works well, I would not muck with it, and I am not against trying various schemes. I am suggesting that the STARTING POINT should be with a shield connected only at one end - the driver end.

Note, the above remark does not apply to microphones.

Regards
Dan Lavry
http://www.lavryengineering.com  

[Mark Donahue]

Dear All,
It seems to me that this is actually a pretty straight forward question, it doesn't have to do with shielding specifically, but rather the proper way to connect single ended systems to balanced systems.
Like some others have already said, the shield (Not audio ground) should normally only be connected at the source.
I am making the assumption that the wire is all twisted pair with an overall shield.

When connecting a single ended output to a balanced input, wire as follows.
1) Connect the hot lead (RCA center pin)to pin 2 of the XLR or the Hot pin on the terminal block.
2) Connect the RCA Ground to pins 1 and 3 of the XLR or cold and ground.
3) Connect the shield of the wire to the ground of the RCA.

However, the wiring of balanced outputs to unbalanced inputs is a little more complex. There are 2 different balanced output schemes. Transformer coupled/Cross coupled and electronically balanced.
For electronically balanced systems, connect as follows.
1) Connect XLR Pin 2 or the hot source to the RCA center pin
2) connect the XLR pin 1 or Audio ground to the RCA ground.
3) If available, connect the shield to the chassis of the XLR. If this is connected to audio ground inside the box(Use an Ohm meter to check for DC continuity), connect the shield to pin 1 (Ground)

For transformer/cross coupled  outputs, You do as above except you need to tie XLR pin 3(Audio cold)to ground. Not doing so will result in a loss in level as well as grungy and distorted audio.
All the best,
-mark

[danlavry]

Mark Donahue wrote on Tue, 02 May 2006 19:16

Dear All, It seems to me that this is actually a pretty straight forward question, it doesn't have to do with shielding specifically, but rather the proper way to connect single ended systems to balanced systems. Like some others have already said, the shield (Not audio ground) should normally only be connected at the source. I am making the assumption that the wire is all twisted pair with an overall shield. When connecting a single ended output to a balanced input, wire as follows. 1) Connect the hot lead (RCA center pin)to pin 2 of the XLR or the Hot pin on the terminal block. 2) Connect the RCA Ground to pins 1 and 3 of the XLR or cold and ground. 3) Connect the shield of the wire to the ground of the RCA. However, the wiring of balanced outputs to unbalanced inputs is a little more complex. There are 2 different balanced output schemes. Transformer coupled/Cross coupled and electronically balanced. For electronically balanced systems, connect as follows. 1) Connect XLR Pin 2 or the hot source to the RCA center pin 2) connect the XLR pin 1 or Audio ground to the RCA ground. 3) If available, connect the shield to the chassis of the XLR. If this is connected to audio ground inside the box(Use an Ohm meter to check for DC continuity), connect the shield to pin 1 (Ground) For transformer/cross coupled  outputs, You do as above except you need to tie XLR pin 3(Audio cold)to ground. Not doing so will result in a loss in level as well as grungy and distorted audio. All the best, -mark

I am not too thrilled with adding a transformer in series. I am even less thrilled about the "electronically balanced" circuitry. So in my gear I have a third way to go. I provide jumpers to change from balance to unbalance pin 2 hot or unbalanced pin 3 hot. One can also do it with the proper adaptors or by making or modifying their cables.

Regards
Dan Lavry

[prozak]

Hi Dan,

Quote:

So in my gear I have a third way to go. I provide jumpers to change from balance to unbalance pin 2 hot or unbalanced pin 3 hot

Does this mean that the balanced output effectively becomes an unbalanced output?

Best Regards

Tim

[danlavry]

prozak wrote on Tue, 01 August 2006 19:39

Hi Dan, Quote:  So in my gear I have a third way to go. I provide jumpers to change from balance to unbalance pin 2 hot or unbalanced pin 3 hot Does this mean that the balanced output effectively becomes an unbalanced output? Best Regards Tim

Yes. You can have it balanced,
or unbalanced pin 2 Hot (pin 3 grounded)
or unbalanced pin 3 Hot (Pin 2 grounded)

Regards
Dan lavry
http://www,lavryengineering.com

[JGreenslade]

An important note to bear in mind with relation to "electronically balanced" outputs (or where you have separate amplification stages for each hot / cold leg) is that you really do not want to common the cold leg of the output to ground at any point (as you may do a cross-coupled stage). If the inverted side is connected to ground, you're asking the inverting amplifier to drive - what is in effect - a short...

The only answer can be to know what type of output stage every piece of gear in your chain has accurately and wire accordingly - in this scenario, there is no "one cable for all".

Justin

[Sin x/x]

There seems to be a lot of confusion about balanced vs unbalanced cabling.

Properly balanced equipment has the shield connected to chassis ground. This way earth currents and RF interference can't influence the audio signal. And this is the best way to connect equipment.

Unbalanced cables have the shield connected to signal ground.
Connecting these together gives reasonable sound quality because there can be Rf interference and earth currents influence the audio signal. But with few equipment and short cable's its ok for consumers.

The problem arises when people want to connect properly balanced equipment with unbalanced equipment.
Signal earth and chassis earth are now connected
When you do this you will notice a very strong buzz.

There are 2 way's to solve this buzzing problem:
- With an active DI box or with a trafo.
This is the only proper way to do this.
- By  connecting the shield of the balanced equipment to signal ground.
Now both shields are connected to signal ground. And the strong buzz is much much weaker, but not gone.
RF interference and ground currents still influence the audio signal.

Most manufacturers connect the shield to signal ground, so that unbalanced cables can be used without much buzz.

If you want to connect improper balanced equipment (shield to signal earth) together, you hear the o so familiar buzz.
Again there are 2 way,s of solving this problem.
- Disconnect one end of the shield.
Now your equipment is connected unbalanced and all the manufactures efforts to eliminate noise is mostly down the drain.
- Get a soldering iron and connect all the shields to chassis earth. Now you have created a decent balanced system, and RF interference and ground currents can't influence the audio signal.
I think this is the (only) proper way to do things.



I hope this helps.

[Andy Peters]

Sin x/x wrote on Wed, 02 August 2006 07:47

Properly balanced equipment has the shield connected to chassis ground. This way earth currents and RF interference can't influence the audio signal. And this is the best way to connect equipment.

Exactly ... always remember and never forget that the shield is an extension of the enclosure.

"Broken" balanced gear has the shield connected directly to the circuit ground, which is bad because shield noise is dumped right into your circuit reference (the absolute worst place for it to be).  This is the "Pin 1 Problem."

Of course the circuit ground has to connect to the chassis ground but that must be done only at one point, generally at the mains safety ground connection.

-a

[Tom C]

Sin x/x wrote on Wed, 02 August 2006 16:47

Properly balanced equipment has the shield connected to chassis ground. This way earth currents and RF interference can't influence the audio signal. And this is the best way to connect equipment.

Just out of curiosity, in most parts of Europe we have an electrical system with 3 connectors: hot, signal ground and earth (ground).
That makes proper cabling very easy (at least if no dumb-ass has connected signal ground and earth somewhere in the house and you have to find out where. In my sisters house it took me half a day to find out and re-cable properly...).

Do you guys on the other side of the big pond have the same?

[Bob Olhsson]

It's not a very good idea to generalize about this. Different factors are important for different designs.

It would be nice if there were interface standards but there aren't and even where there are, many popular manufacturers have chosen to "cheap out" rather than provide a high quality, bullet-proof interface.

Dan's approach is just about the wisest I can think of.