Q: “What’s the advantage of using low-capacitance cable? How long of a cable can I run before high-frequency loss becomes apparent?”
A: With cables that are paired, or coaxial, the two conductors with insulation in between form a capacitor, which holds an electrical charge. The capacitance is small (measured in picofarads). However, it is additive – usually displayed per foot or per meter, it must be multiplied by the actual cable length to get the total capacitance. Capacitance affects the signal level and is frequency-dependant. The higher the frequency, the greater the reactance caused by the capacitance and the greater the signal loss.
Lower cable capacitance allows more of the natural “brightness,” “presence,” or “bite” of an instrument to reach the amp, which in turn allows the treble controls to be run lower, reducing hiss and other unwanted noise. High-frequency loss from the cable gradually becomes audible and objectionable depending on the source, the amplification and other circumstances. There is no point at which high-frequency loss suddenly appears or disappears. Raising the source impedance or increasing the length of the cable increases the loss. Guitars typically have much higher source impedances at higher frequencies because of the inductive nature of their pickups, which aggravates the effect of cable capacitance. A guitar will often sound noticeably “muddier” when run through a 40-foot cable, while keyboard instruments, samplers, mixers and other line-level devices with low source impedances can usually drive cable runs of hundreds of feet without problems.