Originally Posted by Dan in St. Louis
You would really have to do the math. If the output impedance of the preamp is 600 or 1000 ohms, a capacitive reactance of 20k ohms at 20kHz would lose just 5% of the signal, or less than 0.5dB. Clearly for subwoofer use you are many octaves away from that so the loss would be negligible.
But let's say you need a 50' cable for some reason. The capacitance of RG-59 has specs spread all over the place. Let's just decide it is 2500pF for 50'. The reactance is now around 3k at 20kHz so the loss is definitely measurable even if not audible to an aging audiophile like me.
If I messed up the arithmetic above just blame it on Easter dinner. The real math requires the "j" operator and I am not ready for that today.
I don't have a convenient tool for complex arithmetic. I find, though, that Excel does it, in a somewhat kludgy manner. I've never tried to calculate the effects of a shunt capacitance before.
If I did it correctly, a 15m (50') cable at 69 pF/m would have a capacitance of around 1000 pF. Assuming a 1k source resistance and a 28k input resistance on the amp, the loss (compared to zero capacitance) would be .07 dB at 20 kHz. Increasing the cable's capacitance to 167 pF/m to give about 2500pF total would give -0.38 dB.
Incidentally, a 1.83m (6') long cable at 69 pF/m gives 0.0010 dB of loss due to capacitance, with the other parameters the same. Raising the capacitance to 167 pF/m increases that to 0.0060 dB.
I'm not confident in these numbers, but they don't suggest to me that I need to buy low-capacitance cables to make a 6' connection between my AVR and my amp. Audiophiles, who are sometimes unimpressed by numbers, may disagree.