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Old 16th September 2011, 03:07 AM   #31
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No. That will possibly create a ground loop, which would actually conduct RF into an amp. Sheild should be connected at only one end. Beldfoil supplies 100% sheilding around the inner conductors. What little RF might get in would show up as common to both inner conductors and should cancel capacitively between the conductors and sheild.
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Old 30th September 2011, 03:02 AM   #32
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I gave in. I couldn't sleep at night with the horrors of running unshielded 24AWG right past the mains transformers and all that junk...and then the same stuff for wiring up the potentiometers...it was plauging my soul...so...

I wound up buying 10ft of Belden 8451. A few feat left over for some other stuff down the road maybe.

And then I bought 4ft of Belden 7787a..which is 3 wire coax supposed to be used for component video (I believe), however, I'm going to use it for my potentiometer wiring and just leave the braided sleeve as "shielding". $3 a foot...better be good. Any ideas on how to optimize the use of this would be appreciated.

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Last edited by TheLaw117; 30th September 2011 at 03:06 AM.
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Old 30th September 2011, 03:04 AM   #33
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I'm in a wooden case I might add. So the Beldfoil I just solder to the source side only? A little confused...
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Old 30th September 2011, 03:29 AM   #34
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Originally Posted by TheLaw117 View Post
Any ideas on how to optimize the use of this would be appreciated.
I noticed on the Belden 7787a spec sheet that the minimum bend radius is 4.5", so you probably want to get rid of the outer sheath as a start (to try to get a bit more flexibility).

Also, be careful when soldering the centre conductor, as some of the foam plastic insulators between the conductor & shield are not too heat resistant. A lot of the coax nowadays is intended for crimp fittings, not soldering.

A bit late to mention it now, but....
A few times, I've 'made up' a shielded cable by using copper braid shield stripped from cable and threading the (twisted) conductor bundle into the sheath. With a bit of care it's possible to make up a 'wiring harness' with conductors exiting the shield at different points. Use heat shrink to tidy up the ends.

The % coverage isn't enough for critical RF stuff, I suppose, but it works OK for most audio projects where I tried it. I do have the advantage of a few hundred feet of RF coax lying in the junk box.

To connect to the shield(s) the neatest method is to solder a solid conductor to the twisted braid shield, and use that to make your connection.

An older ARRL Radio Amateur handbook is a good source on dealing with coax.
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Old 30th September 2011, 03:54 AM   #35
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Originally Posted by TheLaw117 View Post
I'm in a wooden case I might add. So the Beldfoil I just solder to the source side only? A little confused...
Yes, only connect one end of sheild. Traditionally the source end, but things being equal either will do. Best answer is sheild should be connected to end which offers best groundpath for RF.

The 8451 will actually offer better (100%) sheilding than your fancy new multi coax, Whups, I take it back. I see the coax has a foil wrap.
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Last edited by thaumaturge; 30th September 2011 at 03:57 AM.
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Old 30th September 2011, 03:58 AM   #36
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internally, I think it looks awkward with those thick stiff cables
and risk of breaking a solder joint is really high
yeah, did it that way myself, once
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Old 30th September 2011, 04:01 AM   #37
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Yes, only connect one end of sheild. Traditionally the source end, but things being equal either will do. Best answer is sheild should be connected to end which offers best groundpath for RF.
ahh, also called 'pseudo balanced'

I would suggest to try all shields connected to preamp

yeah, if all wires have own shield, split the cable, as suggested
and you can disregard the foil
just cut it so it doesn't short anything
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Old 30th September 2011, 04:10 AM   #38
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Sorry, beg to differ tinitus. Sheild should be connected toone end. Letting it float it becomes an antenna that may capacitively couple RF to inner leads. Best to hook one end to local ground.
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Old 30th September 2011, 04:11 AM   #39
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ideally I think it should be two signal wires, with one common shield
which basicly is a 'stereo' wire
or maybe more correct, a 'balanced' wire
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Old 30th September 2011, 04:16 AM   #40
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Agreed. I'm just referencing standard practice in broadcast industry. I've used it on three ground up studios, two radio and three TV stations.
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