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Not simple addition

Posted by Jon Risch (B) on December 3, 2005 at 08:20:02

In Reply to: Re: milli, micro, nano, pico..nt posted by Mudcat on December 2, 2005 at 18:44:41:

You can not just add up the individual cable capacitance of twisted pairs when they are bundled OR braided.

The close proximity of other twisted pairs contributes to the overall capacitance of the total cable aggregate, increasing the total sum capacitance over that of the individual pairs.

Just as a star quad wired cable has more than twice the capacitance of two pairs of wires (actually very close to about 3 times the C), so do the multiple wire pairs interact with each other.

Unfortunately, there is no ready multiplier that can be used, as it depends on if they are merely bundled together (as in still in the jacket) or braided, and how tightly they were braided.

On average, for an adequate guesstimate, I would say that the multiplier would be somewhere between 1.25 and 1.5 over the capacitance of the number of pairs just added together.

BTW, 10 pairs is an odd number if you are working from sets of 4 pairs in a jacket, so most folks would end up with some multiple of 4 pairs for a single run, as this is the most convenient number to work with.
If braiding, then it becomes easier to deal with multiples of 3 and or 2, allowing one to 'three-braid' or double up via twisting, or some combination of both.

ChrisVH recommends a full 27 pairs for full range use, and 9 pairs for HF bi-wire, while I have recommended no more than 16 or 18 pairs for a full range, or for each of a bi-wire. Either way, that's a lot more capacitance than you have been talking about, thus the practical limitations of about 10-12-15 feet of braided/bundled CAT5 (depending on how it is configured precisely), due to the sheer capacitance it represents.

Unfortunately, a large number of power amps are NOT unconditionally stable, and will become marginally stable, and prone to oscillation with SOME value of capacitance hung off the output, it is just a matter of how much it takes. Some NAIM amps will be prone to problems with ordinary zip cords, while other amps can have 1 F hung off the output.

Jon Risch

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