Mwiliams; Is it not possible to put the monos next to the speakers? - that seems to be the layout of choice most of the time. Mine are about 2 feet from the speakers while the interconnects are about 7' away in either direction without any issues. Just a thought.
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There is no hard and fast rule about maximum interconnect length, because it is highly dependent on both the output impedance of the preamp, and the capacitance of the interconnects. The lower the better in both cases.
If you choose interconnects with capacitance of say 20 or 30 or 40 picofarads per foot (pf/ft) or less, and if you have a preamp with an output impedance of say 100 or 200 ohms or less, you'll be fine. If both parameters are significantly higher than that, upper treble rolloff may result.
Also, the resistance of the path through the shield from one end to the other needs to be low, or 60Hz hum may result. Most quality cables should be adequate in that regard, but the only way to know for sure is to try it.
Thanks for the detailed info. It was very helpful.
One more question
I have a Bryston intergrated amp with Wilson Sofia's and using 35ft. Tranparent Ultra's. I was thinking of trying some tube mono's and using the Bryston as a pre. Would it be better to put the amps with the pre and keep the speaker cables and shorter interconnects or the longer interconnects and shorter speaker cables
Assuming the interconnect capacitance and preamp output impedance satisfy the constraints I described above, and the speaker cables are of adequate gauge to support the run length (the Transparent Cables site doesn't seem to specify the gauge of the Ultras, though), then there is no unequivocal answer based on technical grounds. And the long speaker cable/short interconnect vs. short speaker cable/long interconnect debate is an age-old one, with divergent opinions (as Stan alluded to).
My instinct in this case, though, would be to go with short speaker cables. Your speakers have an impedance which is quite low at mid-bass frequencies, and which varies widely across the rest of the frequency range. The tube amps will most likely have higher output impedance than your Bryston, and you don't want to have the speaker cables adding to that higher impedance any more than necessary.
If the sum of amplifier output impedance and cable impedance (resistance and inductance) becomes significant relative to the speaker impedance, two things will happen. First, bass damping will suffer. Second, in a situation like this where the speaker impedance fluctuates widely as a function of frequency, then an uneven frequency response will result. That is because the voltage losses across the cable impedance and the amplifier's output impedance will vary as a function of the speaker's impedance vs. frequency characteristic.
Audiofeil, what I thought would be obvious to anyone, and I was clearly wrong, was that keeping the wires short would be a benefit that would compensate to some degree for any loss in the longer cable run. I suppose I shall have to join Galileo in praising those of limited understanding who forced him to explain what he thought everyone already knew.
Let me expound even further on the subject. We apparently agree that in cables short is better than long. Let us further assume that if one is longer then the other can be shorter. We should then expect a degradation on the longer cable and an improvement on the shorter one, the exact proportion of each will vary with each case. This was the whole and sum of my statement about compensation.