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In Reply to: actually... posted by Dave Cigna on April 9, 2007 at 14:10:14:
>>"It is not necessary to have one side grounded in a twisted pair to get benefit from twisting the wires, but it improves the situation."
Hum rejection in twisted pairs generally depends on both wires having equal impedance to ground (i.e. 'balanced.')<<you are making the case for best use of twisted pair wires, not why you twist the wires in general. I was trying to explain why we would want to twist them, and it helps in almost all cases.
Try an unbalanced approach to twisted versus non-twisted: the leads of an oscilloscope, for example. Make a big loop from the center conductor probe to the outer conductor shield, and you have a loop antenna picking up hum. Then twist them together while keeping the input shorted, and the hum nulls from the scope trace.
You could also, if it weren't for the static electricity problem, take the single-ended phono output (not differential) and feed it to the single-ended primary of a step-up transformer, all floating in the air, and not get the kind of hum you'd get if you ran two wires untwisted. In practice, you need to bleed off the static electricity from this floating arrangement because that causes other problems with noise.
But for best hum and noise rejection, you provided the best case scenario for twisted pairs, almost. For even better performance, add a grounded shield around the twisted pair. After all, in your circuit suggestion you have the established ground potential available, so why waste it? Just a little extra cost, perhaps.
Again, what was the original question? :-)
Kurt
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Follow Ups
- Still... - kurt s 14:57:12 04/09/07 (2)
- Re: Still... - Dave Cigna 15:49:03 04/09/07 (1)
- Re: Still... - kurt s 08:03:02 04/10/07 (0)
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