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Cable Asylum Interconnects, speaker wire, power cords. Ask the Cable Guys. |
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Cable Asylum Interconnects, speaker wire, power cords. Ask the Cable Guys. |
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In Reply to: Re: Cable Suggestions posted by SpecialK on July 28, 1999 at 17:46:34:
A passive preamp usually only has potential problems with the cable coming out of it to go to the power amp, this is the critical one.Depending on what value of volume control pot is used, it can have a high source impedance to the power amp. The problem is, if you make the pot too low in value, it loads the source adversely. The lowest practical value that can be used is a 10 k ohm pot, this loads the source components with a 10 k ohm load, which is as low as some SS power amps get. Most all sources can safely and successfully drive a 10 k ohm load, but it is possible that some might be stressed by even this amount, a more common value of pot is either 20, 25 or 50 k ohms, to avoid this type of potential problem.
With a 10 k ohm pot, the worst case output impedance occurs when the volume is at -6 dB, or the electrical one half way point. The output impedance is then approx. one quarter that of the pot (one end is connected to a low Z source, and the other to ground, with the wiper in the middle, hence, the two 5 k ohm sections are in parallel).
If we set the amplitude loss criteria to be less than a 0.1 dB loss at 20 kHz due to the cable run from the pot, then the -3 dB point needs to be approx. 200 kHz. To acheive this requires that the total cable capacitance be less than 318 pF. For a typical interconnect with about 45 pF per foot, this is a 7 foot run, or a 2 M cable. Lower capacitance cables, such as stock Belden 89259, which run about 17 pF per foot, can be as long as 18 feet. Some high end cables that are high capacitance include the Kimbers, and other woven type cables, as well as some of those with multiple conductors and an intimate geometry.
For a 50 k ohm pot, worst case output Z of about 13 k ohms, the cable length for a 0.1 dB criteria is limited to 61 pF, and a typical cable would have to be less than 1/2 M in order for it to avoid roll-off.
For a less stringent amount of 20 kHz roll-off, say 1 dB of HF loss, the figures from above can be multiplied times four, so that with a 10 k ohm pot, the total amount of capacitance could now be 1272 pF total, which would be a long cable using low C cable. The 50 k ohm pot would now be usable with up to a 5 foot cable, or almost 2M. A 1 dB loss at 20 kHz will sound a tad soft and will take the edge off of transients, but not in an unpleasant way if any of the components tend toward brightness or harshness.
The moral is, you should keep the output cable of a passive preamp as short as possible, ANY run over 2M should be reconsidered, and even low capacitance cables will not allow runs of much over 10-12 feet with 50 k ohm pots if you want to preserve transients and HF air.
Jon Risch
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Follow Ups
- Re: Cable Suggestions - Jon Risch 07:03:09 07/31/99 (0)
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