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In Reply to: 89259 IC with copper foil posted by MylesJ on November 06, 2001 at 13:59:27:
Since the shield for the twisted pair is not carrying any signal current, and is grounded at one end only, it would not have any advantages over a braid, and would be stiff and could lose coverage if it "split open" when bent. It is also hard to make a good seam in foil that is not a problem at RF. The various cable companies have patented various means to seal the seam properly.The most important thing about shielding a twisted pair is to make sure that the shuield is spaced away from the twisted pair, especially the crease between the cores.
Jon Risch
Follow Ups:
What if he didn't ground one end of the foil shield (properly folded)to the rca jack and instead hooked it to the system's earth ground?I've got these high purity 26 gauge silver solid cores in a teflon tube that I plan on twisting and terminating. I have some extra 1.5" wide 3M copper foil and I am thinking of wrapping the twisted pair with the copper and grounding it to the ground lug on the isolation transformer that's feeding the AC to the equipment that's producing the signal. The copper foil shield will be slipped into a teflon tube.
Mike Bates
This brings up the old question: is the AC ground worth a hoot?In many cases it is not, and "grounding" the telescoped shield to it will only tend to raise the possiblity of injecting RFI or other interference into the cable conductors. Certainly, the capacitive coupling between the shield and the hot and signal ground wires will be affected, perhaps over all capacitance loading will be increased.
As is often the case, try it very carefully, rasing the volume slowly, and cautiously, and see how it works.
I have heard of dedicated audio grounds that connect ONLY to cable shields, using litz wire run to the chemically treated ground rod that is NOT AC connected anywhere. If done properly, these can be quite effective.
Jon Risch
Hmmm.. I suspected that the foil will act like an antenna and may contaminate the AC ground.. I have some extra copper ground rods maybe I'll try a seperate ground! This makes sense.Have you ever experimented with active sheilding of IC's and speaker cables?
Mike Bates
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