Since the 15 pair cable was fairly easy to make, and very good sounding, the next step was obviously a 21 pair version. During braiding, instead of using 5 single braids (hereafter called 'strands') where each 'braid' operation crosses 2 strands, use 7 strands, crossing 3 strands for each operation. The shrinkage is the same, or very slightly larger (1.4).
As for the 15 pair cable, it is important for best possible sound that all the pairs have the same twist rate. Make a template to ease the checking of correct twist rate. Simply fasten a piece of reference wire to a stick, and check by holding the wire beeing twisted up over the template.
If you don't have a drill to twist the pairs extra with, you can ensure even twist rate by using only one colour in one cable. You may use only green wires for one cable, and only blue wires for the other cable. Of course to do this you will need twice as much cat-5 cable, but on the other hand, you can make two sets. Perhaps a project for a couple of friends?
Braiding 7 strands is a little different from braiding 5 strands. The braid stays too open, so it has to be closed by squeezing the sides every so often. This part can be hard on your fingers, so start easy and don't keep it up for too long in the beginning. You will be sorry.
Some data on this cable:
A finished speaker cable 3 m long, consisting of 21 pairs, will have a square area of 4.12 mm², and a round trip resistance of 0.035 Ω.
Terminating the 21 pair cable is actually easier than terminating the 15 pair cable. This is because 21 wires can be grouped in 3 groups of 7 wires each, and 7 wires is a perfect number for twisting.
It is smart to make sure that all the plus and minus wires in each 7 wire group (white and coloured) come from the same 7 pairs. This way the cable can be used for bi-wiring, by using one group for the treble, and two groups for the base. Since the base drivers use most of the energy, it makes sense to allocate 2/3 of the cable to the base, and 1/3 to the treble.
This pair sounded every bit as good as the 15 pair version, even at once, and given the tendency of these cable to improve with use, they will be my permanent cables for a long time.
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