Written by 5:00 am Cables

It’s Not All That Simple

Roger Skoff agrees that it’s all R, C, L and ZO … except that…

For as long as I can remember, there’s been ongoing controversy about cables for high-end audio: Those who are willing to accept the evidence of their ears say that which brand or model of cables you use can make a significant difference — either positive or negative — in the sound of your system. Another highly vocal faction of orthodox others shouts the magic words “placebo effect,” “measurement” and “double-blind testing” and, dragging out their textbooks or encyclopedias as proof, declares that listening by any standard other than their own is invalid [sic] and insists that the only things that matter about cables are their resistance (R), capacitance (C) and inductance (L).

cable_hammer.jpgInstead of making this article just one more brick in the wall between the two sides (thanks, Pink Floyd), let’s do something entirely different: Let’s assume that the naysayers are correct and that R, C and L are truly the only things that affect the performance of an audio cable. (Please remember, also, that the subject of this article is cables for AUDIO and not for frequencies in and above the RF range.) And let’s actually design an audio interconnect — actually a pair of interconnects, to serve the two channels of a stereo system, but the second cable will be identical to the first, so there’s no need to describe it.

First off, in order to complete a circuit and provide the R component, we’re going to need to have at least two conductors: a “going” lead, to carry the signal to the next component in the chain (from a preamp, for example, to a power amplifier), and a “coming” lead, to bring the signal back to its component of origin (back, in this case, from the power amp to the preamp).

Each of those conductors could be made of one or a multiplicity of wires, so how many wires per conductor should we actually use? Well, one per conductor is the minimum, so let’s use that. What should the wires be made of? The two most common choices are copper and silver, so let’s use copper, which is the more popular.

How big (what AWG gauge) should each wire be? Remember that the smaller they are, the more resistance they will offer to large current flows, but that, on the other hand, if they are too big, there may be a problem regarding skin effect, even at audio frequencies. Remember, too, that if they’re not stranded, they could (if very large) be too stiff to use, but that if they are stranded, although they will be more flexible, because very thin layers of copper oxide act like diodes, and because virtually all copper that has ever, no matter how briefly, been exposed to the atmosphere, is covered with a thin layer of copper oxide, the individual strands making up each wire will likely act like a massive series of tiny diodes, and rectify-out low-level signal information. So which should we use?

Since our cable is going to be an interconnect, and will therefore likely carry only very low levels of current, let’s go with non-stranded (“solid core”) wires and, to avoid skin effect in the upper frequencies, let’s use a relatively small gauge, like 26AWG. Now that we know the material and the gauge of our wires, we can simply look up the value of R for the lengths of wire that we will be using, but that will still leave us with the problems of C and L.

Fortunately, inductance and capacitance have a “see-saw” relationship, where more of the one automatically results in less of the other and vice-versa, and both are determined (for the bare wires we are talking about at this point), solely by how far our coming and going wires are apart from each other. That ought to make it simple. So how far apart should they be? Closer together will result in more capacitance and farther apart will result in more inductance, so let’s pick a distance that will allow those two factors to be in whatever we choose to be a reasonable balance. Twenty thousandths of an inch (0.020″) is a convenient number that might give the desired result, so let’s use it. Now, how can we get and hold that distance between our two conductors?

One easy way to set the distance between the conductors and to protect against accidental “shorts,” too, is to use insulated wire. If the insulation thickness we apply is 10 thousandths of an inch (0.010″) per wire, the minimum distance possible between our two wires will be 20 thousandths: the total combined insulation thickness of the two wires at their points of contact. If the two wires are laid parallel and touching, that will guarantee our desired minimum distance.

But what about the maximum distance? How can we control that? With both the maximum and minimum desired distances being the same, the easiest way to maintain it will be to just twist the two insulated wires together. That way, the thickness of the insulation will keep the wires from getting too close and being wrapped around each other will keep them from getting too far apart.

The problem, of course, with using insulated wire is that the insulation will, itself, act as the dielectric of a capacitor. It will affect the C of the cable, and may, just for that reason, require that we recalculate the desired distance between our two wires. Not only that, but as the cable’s insulation discharges stored energy (remember that that’s what capacitors do — they store and release energy), the released energy will result in signal cancellation near the “zero line” (the point on an oscilloscope at which a sinusoidal signal changes polarity) and/or the addition of actual out-of-phase capacitive discharge artifacts which may be created and added to the carried signal as a whole new signal source that will affect the cable’s sound.

What I’ve written here just barely touches on some of the concerns regarding resistance, capacitance and inductance that are involved in designing a high-performance audio cable. I hope that it has shown you that how they are derived — meaning the materials and construction of the cable embodying them — is fully as important to the design process as the R, C and L values, themselves. There are still other things that affect the performance of cables in specific applications — characteristic impedance (Z0), for example, for very high frequencies or very long runs and, in every application, the kind, materials and even mass of the connectors to be used. All of those are important aspects of the design process and, whether recognized and considered or not, will still have their effect on the cable’s performance.

The foregoing wasn’t intended to be (and shouldn’t be taken as) a treatise on cable design. Its only purpose was to help you understand that cables are far more than — as others might lead you to believe — just simple pieces of wire affected only by R, C and L.

It’s not all that simple.

(Visited 137 times, 1 visits today)