Studio interconnect cable

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scodu

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Hi,
I am in the process of making interconnect cables for my studio and I have a couple of questions about the cable.

I am using Gepco cable with 12 multi-channel pair. Each pair is individually foil shielded with a drain wire. Then the main cable jacket has a foil shield around the 12 pair with a braided drain wire also. My question is what do I do with this main drain wire? Is there a reason for this extra wire? Or should I just cut this off?

Thanks,
Scott
 
Good question...!

I'm using this stuff but I'm not at the stage of wiring it yet.... I'll have the same question when I get there too!!!!!!!!!
 
Pin one gets the drain wire of the individual pair, pin two gets the + wire (red), and pin three gets the - (black). Personally, I'd fly the drain on the wrap that goes over the whole thing or maybe only connect it to a ground at one end only.
 
Track Rat said:
Personally, I'd fly the drain on the wrap that goes over the whole thing or maybe only connect it to a ground at one end only.
I understand the individual channel wiring, but what would be the point of floating the multicore foil overwrap like that?
 
Trying to keep any RF induced on the shield either isolated from the pair gounds or shunted off to ground at one end. Also trying not to encourage ground looping.
 
Hey Blue Bear!
I have that same cable in the 26 pair configuration. I'm going to terminate it in 90 pin Elco connectors to connect my desk to the two rack towers I have on each side of the desk. One of the towers is a complete tracking station that I can roll around my apartment and use in the different rooms, I do lots of self tracking. I haven't started on the multipin connectors yet and I think those are going to prove to be a very big challenge.

Track Rat, I don't understand what you mean by "I'd fly the drain on the wrap...". Cut it off?

I live in the city of Chicago right next door to a hospital and 2 blocks from another hospital. The RF in this area is horrible! I can't use my cell phone in my apartment and barely out on the street. So I want to do everything I can to get rid of the RF. I'm not very knowledgable about grounding but if I use your second recomendation about grounding one end where would be a good place to connect that ground too?

Thanks for all the help,
Scott
 
Leave the overall shield drain wire disconnected at one and, and connect it to a good chassis (NOT signal!) ground reference at the other end. I'd initially ground it at the end that drives the most signals into the cable: this is called "telescoping the shield", and effectively extends the chassis of the driver right up to as close as possible to the chassis of the receiver, without contacting it.

Don't chop the free end off right at first, though. You may find it useful to try grounding the other end instead, if you find that you still have RFI/EMI problems. There is really no hard and fast rule for telescoping shields: you do whichever one gives the best noise immunity. What you probably _don't_ want to do is to ground both ends,: connecting chassis grounds together is very likely to create a ground loop, and make problems worse. But be prepared to try it, just to see.

For this sort of overall shield, you want to use chassis ground instad of signal ground so that you can dump as much of the EMI/RFI hash as possible into ground as far away from the sensitive circuitry you are trying to protect: stick it into the chassis rather than Pin 1 of an XLR. Lots of gear is designed with questionable signal/chassis ground relationships, where noise on Pin 1 of an input gets coupled right into the signal path. If you have an overall shield, use it to your advantage to dump the hash far, far away from your preamps and signal receiving circuitry...

This is called the "Pin 1 problem", by the way- there are some really good sites with discussions of the problem, and whole special issue of the AES Journal on it: the June 1995 issue, which was the special edition on shielding and grounding. It's a must have, and is available from the AES for $15 (or $10- if you're a member). get it: http://www.aes.org. You can see a lot of the material from that issue from Bill Whitlock at Jensen Transformers at this link: http://www.jeffrowland.com/tectalk6.htm to decide if you want to snag the whole issue...

Hope that helps, anyway.
 
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