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Old 17th August 2005, 07:11 PM   #1
Bryan is offline Bryan  United States
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Default Shielded Input Wire

Hey Guys,

I was wondering if some more experienced builders may share some advice.

I am working on a preamp, and I need to run the input wires a small distance, and therefore would like to use shielded cable.

I have some very nice solid core teflon coated silver which is used for surgical applications, and I was planning on using this as the input wire from the unput jacks to the source selector switch, and then to the stepped attenuator, and then to the tubes....

Is it possible to fashon a shield and use this silver wire?

Or, does anyone have any suggestions as to HIGH quality shielded cable which will work well in this application?

Also, what is the best way to hook up the shield to ground...?

Thanks,

Bryan
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Old 17th August 2005, 07:27 PM   #2
rdf is offline rdf  Canada
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If the wire is to your liking have you considered using twisted signal/ground pairs? The circuit crosstalk will in all probability swamp that of the internal wiring.
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Old 17th August 2005, 07:50 PM   #3
PRR is offline PRR  United States
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> preamp, and I need to run the input wires a small distance, and therefore would like to use shielded cable.

No tubes nor teflon, still this is a well-regarded box and they just stuff the wires:

Click the image to open in full size.

Some concessions to cross-talk: the phono inputs run on the other end of the chassis from all this line-level stuff (but still end up at the switches seen here), and the tape-out line goes straight back rather than bunched with the others.

Basically, if the case is a good shield, the wires inside may not need obsessive shielding. The approach shown here can have trouble if the FM tuner is left on a LOUD station while you try to use the phono, though in practice it isn't a problem.
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Old 17th August 2005, 07:54 PM   #4
lgreen is offline lgreen  United States
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Default braid

You could get some braided cable and put your teflon cable inside the braid. Here is an example:

tubular braid

It comes in different diameters, you might also need an insulating braid to go over the tubular conducting braid depending on your installation.
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Old 17th August 2005, 08:02 PM   #5
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I sheilded the fine silver/teflon wire of my phono cable leads as follows: I got a roll of desoldering braid at Radio Shack and compressed it along its length (imagine trying to escape from the "Chinese handcuffs" toy) which makes it easier to find the hollow center of the braid. I did this by putting it on a smooth countertop and pushing a short length of it together at a time.

Then I folded over a piece of stiff wire into a "U" so that it had a smooth leading end and it wouldn't get snagged, and I slid it up through the center of the braid. When the "U" emerged from the far end, I bent it onto an "o" with a tail pointing back away from the braid, again so it wouldn't get snagged. Then I attached the signal cable to the stiff wire and pulled it back through.

None of these steps went smoothly! But eventually I had a shielded cable.

For the application you describe, what I would do is twist the signal cable with another solid-core (so it's fairly stiff and will keep its shape) wire. Ground the other conductor at the input socket, and leave the end near the destination taped off (no connection). This will give enough sheilding for your application, I'd bet, and be 10,000 times easier to do.

-j
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Old 17th August 2005, 08:26 PM   #6
zobsky is offline zobsky  India
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wouldn't shielded 3 cnductor (2 wires and 1 copper braided sheath) microphone cable do, .. not more than a few cents per foot ?
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