Low inductance speaker cables occur because of some magnetic field hocus-pocus. But in order to understand what is occurring, it will be necessary to delve into where the inductance comes from in the first place.

If we look at a single wire carrying current in a space free from any other influences, we can then ascertain the origin of the wires inductance. We must first make the distinction between the flow of a steady direct current, and one that alternates it's polarity, like an audio signal. A direct current flow is a steady unchanging current flow, the magnetic field due to the current flow is steady and unchanging. There is virtually no effect on the current flow distribution due to the magnetic fields presence.

However, once we start looking at the AC situation, representing music signals, now the magnetic field has an influence. In order to see why, lets examine the magnetic fields as we alternate the current. If we start from 0 current flow and then proceed to the beginning of positive current flow, we go from a condition of zero magnetic field, to an expanding magnetic field. As the current flow increases, the magnetic field increases and expands, the field coming into existence within the conductor, and expanding within it, and finally creating magnetic field lines of force outside the conductor. One could imagine these magnetic lines of force as expanding from the center of the conductor, and increasing until they expand out into the space surrounding the conductor. At the peak of the AC waveform, the current flow will be at a maximum (for a resistive load, lets not get into reactance quite yet), and the magnetic field will have reached it's furthest extension from the conductor.

Now, as the current flow decreases due to the AC waveform now heading toward zero level again, the magnetic field due to the current is decreasing proportionately to the decrease in current flow, that is, the magnetic field is collapsing from it's peak extension point, until as the waveform goes to zero, the current goes to zero, and the magnetic field collapses to zero. The swing of the signal into the negative region is just the same as the above description, except the polarities are reversed,

As with many things, there always seems to be a reaction to an action, much of physics revolves around this. As the magnetic field is expanding through the conductor, it will tend to generate a current flow that is the opposite of the original current flow. Any time lines of magnetic flux cut through a conductor, they generate a current flow. Fortunately for us, this current that is generated is not as strong as the original current flow itself, or there would be no output signal at all, the self-inductance of the wire would cancel out the original current. The amount of current generated by magnetic flux lines cutting through the conductor is proportional to the rate of flux change, hence the faster the currents alternate (the higher the frequency), the more of this self-induced back current is generated.

Thus, as the frequency goes up, the wire tends to impede the flow of current more and more, as the induced back current gets greater and greater. This is called the self-inductance of the wire, and does not require another wire to interact at all for the self-inductance to manifest.

So how do loudspeaker cable geometry's affect this self-inductance, and reduce the inductance of the overall cable?

Well, when you place another wire nearby carrying current in the opposite direction, it also has a magnetic field that expands into the space around the wire, only it has the reverse polarity of the wire next to it. If the two wires are very close, the effects of the self-inductance are nearly canceled out by the mutual inductance of the two wires. The closer the second wire gets to the first wire, the closer it comes to completely canceling the self-inductance of the first wire with the reverse polarity mutual inductance between the two conductors. Of course, the reverse holds true for the second wire with respect to the first wire, it too cancels out the self-inductance of the second wire via the mutual inductance of the two wires.

BTW, for the inductance aspect, it makes no difference if the wire is solid, or is stranded, the magnetic field still expands from the center of the stranded bundle, and expands outward through all the stranded wires. Individually insulated wires in a bundle, act just like a solid wire, except for any loss of cross-sectional area due to the insulation. In the real world, a bundle of insulated wires can not be easily be kept completely in the same location with respect to the whole bundle, and then it will react slightly differently, see below.

So the more intimate the current carrying conductors, the lower the inductance. For a simple construction, the ultimate expression of this is two wide flat strips of conductor, with just a minute amount of insulation separating them from each other. You can also see that if you had a multiconductor ribbon cable, and alternated the hot and ground currents in the wires that constitute the ribbon, that this would cause the mutual inductance to couple from both sides, (except for the end wires), and would further reduce the self-inductance of all the individual wires. The first mentioned two wide flat conductors could become a multilayer sandwich, which would gain the same benefit that the multiple conductor ribbon cable has and further still reduce the total inductance of the speaker cable.

There are other ways to address the total inductance of a speaker cable. One way is to attack the self-inductance of the wire itself, instead of depending on the mutual inductance to reduce it. By running many insulated runs of wires, the inductance of all the wires is effectively in parallel, so that the inductance can be reduced by the amount of individual wires present for each polarity. In order for this to be truly effective, the wires must not be close to one another, or the mutual inductance, being of the same polarity, will begin to add to the self-inductance, and actually make things worse.

Then there is the Litz construction, which has been touted as being effective against skin-effect, which is a different beast than self inductance (I will cover that some other time), but it also helps reduce self-inductance. This is accomplished by having the individual wires take turns being on the inside and on the outside at any given point along the length of the wire. Despite the fact that the wires are in close proximity, and that they are being affected by a similar magnetic field as a solid wire, since they trade-off along the length of the conductor bundle, no single strand is always on the inside being exposed to the maximum amount of magnetic field.

So even though a Litz construction is designed for the skin-effect, it will help with self-inductance some as well.

Of course, you can combine these techniques and use more than one at a time. When you braid your CAT5 cable pairs, the twisted pairs achieve a fairly intimate spacing, and then, these pairs are braided together, weaving the pairs in and out, in psuedo-litz construction.

You could take this mixing of techniques to extreme levels, by using the twisted pairs, then braiding them, then taking the braided bundles and twisting them together, and then braiding the twisted pair of bundles together again, until the whole assembly was too large and stiff to do anything to, or your fingers were raw and bleeding, which ever comes first!

Jon Risch

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