Skin Effect in Solid/Stranded/Litzendraht Wire -Guy Macon

This is just thinking aloud - it might be nonsense...

So to minimise the skin effect (not that it is significant for home audio systems in the first place), what you really want is a cable made from individually insulated strands, with the signal on half the strands and the return on the other half, with the strands intermixed. That way each strand is surrounded by strands generating an equal and opposite flux, and thus the flux at any point inside (or outside) the cable will be tiny. This would give you minimal inductance, minimal skin effect, and maximal rejection of common mode interference - in effect, you have an exaggerated twisted pair with many pairs twisted together. I suspect you'd have a high capacitance, however.

So, go do some research on RF transmission lines. Capacitance per se is not bad; it is an integral part of what makes the line have a particular impedance. Series inductance and shunt capacitance, primarily, determine the impedance at RF frequencies. At audio, the series resistance has a profound effect on the impedance, but it also doesn't make much sense to worry about it as a transmission line unless it's pretty long. A line from one end to the other of any house I've ever seen probably wouldn't qualify as "long" at 20kHz, where the wavelength in a typical cable is roughly five miles (assuming a velocity factor of 0.5).

I recall reading some reasonably objective evaluations of various speaker wires. One of the best (with respect to measured transmission of transients, and instrumented frequency response of cable-output versus cable-input, over the audio range) was many-conductor ribbon cable, with alternate strands conducting opposite directions. That's very easy to do using the typical insulation-displacement ribbon cable connector, as alternate strands connect to opposite sides of the two- row connector. But there's a lot of question in my mind just how much difference this all makes anyway, given the high resistance of the wire in the drivers in the speaker system, at least for runs of modest distance. Also, as Guy notes, the transducers aren't inherently "flat" enough to warrant putting a lot of effort into it. If you really think it makes a difference, why not just put the amplifier at the speaker, and run low-level or fiber to the amp?

For sure, Litz wire is useful for certain applications. I've been playing a lot with RF filters in the 1MHz region lately, and get far higher (like, a few times higher, not just tens of percent) Qu in coils wound with the proper Litz wire, as compared with coils the same size wound with single-strand copper wire. But I was somewhat astounded when I priced Litz wire at how expensive it is! That was from commercial wire houses, not from audio-phool sources. Good thing I found some assorted sizes kicking around here.

Skin depth in copper at room temperature is about 2.6 mils at 1MHz, and goes as 1/sqrt(freq); at 10kHz it's 26 mils, so unless you're working with pretty big conductors, there's not a lot to be gained from Litz at audio. In coils, I believe things are rather different because of the addition of proximity effects.

Cheers, Tom

That all fits in with what I understood from before (but it's always nice to get confirmation).

I don't think it makes a *significant* difference, but some people certainly do! I'm just trying to think about how to improve the flat frequency response of the cable (over audio frequencies) from merely insignificant skin effect down to virtually non-existent skin effect.

The best idea I heard of for speaker cable is heavy extension cables (the kind used for electric lawnmowers) - you have thick cables and therefore low resistance, and excellent value for money. Apparently Quad (who make very expensive speakers, among other things) used them during a trade show.

As for ribbon cables, it certainly sounds like a convenient way to make the cables I thought of. Perhaps using the 80-pin IDE cables would be even easier, and some even come rolled up in advance. The would not be long enough off the shelf, but I'm sure rolls are easily available.

I'm aware that the skin effect is somewhere between negligible and non-existent for audio frequencies and reasonable sized speaker cables, but since snake oil salesmen get hefty mark-ups on their cables by reducing the skin effect, I was wondering about ways to get even less.

mvh.,

David

Speaker wire ..phhhftpp :P Fiber optic link to active speakers. Now that's a soaker.. It'll be so expensive I'll make "special wire" look like a deal..

D from BC

Nope. You use thick Teflon insulated SPC wire, not mag wire strands. This keeps the individual strands far enough from each other so as to not create to high a capacitive effect over then span of the wire, and they are not individual twisted pairs, but interwoven braid. It's like macrame with no knots, of course, however.

You end up with an equivalent of like #10 gauge wire. Kick ass speaker cables that are flexible, mostly flat (physically and electrically), and quite durable.

This is actually the way the true audiophiles do it now.

The amp is right next to the driver. Thumbs up there. There can actually be an amp for each driver in a cabinet as well. Another thumbs up. The optic link can be analog in nature, so no crying allowed by the "valve tards".

Yep. That is the wave of the future. Next thing will, of course, be to BlueTooth it over to the driver cabinets, and do away with the hard links altogether.

I'll bet the valve tard boys couldn't even tell the difference in a blindfolded side by side comparison test if it were engineered correctly.

Not one iota!

Ahhh... the wonders of QAM 256!

Only at 70k ft in a low level pressurized cabin.

Is that up there around the area of 99.99% oxygen free Monster speaker wire? :)

Just as a thought experiment, one could, in theory, reduce the capacitance to zero by wrapping each strand inside a driven shield. Not at all practical, but cheaper than some of the stuff I have seen for the Golden Ears audio market... :)

The audio snake oil sellers cause real problems in AC power. An non-power engineer who knows that skin effect is too small to make a difference in a speaker cable at 20-20K Hz. will sometimes assume that skin effect is too small to make a difference in a high-current bussbar at 60Hz.

There are other advantages to keeping the return close to the supply as well. A few months back we had a customer who was running two 1000A+ siggle phase 400Hz lines through two holes in a shipboard bulkhead. He had made an error and routed both hots though one hole in the bulkhead and both neutrals through the other. His efficiency took a hit and the bulkhead got hot...

Why would one WANT to reduce the effective capacitance to zero? What's wrong with making an 8 ohm transmission line, assuming you want to drive an 8 ohm load? (That is, what's wrong with the concept; in practice at audio it's pretty tough if you don't use superconducting wires.) I put the particulars of a 24 conductor, 26AWG ribbon cable into a transmission line calculator; assumed Er for the dielectric of

About cables through bulkheads: the US National Electric Code forbids running unpaired AC conductors through holes in steel (e.g. through conduits) for the reason you mention -- it can also be a safety hazard for people working on the wiring, too, since it's very hard to know where the circuit goes if the wires aren't paired (or bundled in the case of multi-phase).

Yes, at 60Hz, skin effect is significant even in long distance power transmission lines, whose conductors may be a couple inches in diameter. In copper at 20C, skin depth is only about a third of an inch. If the conductor is a bundle of seven strands (each of which may comprise a multitude of smaller strands, of course), it seems reasonable to make the center strand steel, for strength, since it's not conducting much current. Is that done? Or is the corrosion from dissimilar metals reason to not do it?

Cheers, Tom

In fact, they often use steel cored aluminum and copper wires. Excellent strength. Supposedly, I've heard of running transmission through the center as well (what kind of signals, electronic or fiber optic, I don't know), which would work, but seems awfully hard to get at.

Tim

-- Deep Fryer: A very philosophical monk. Website @

I have a one-foot-long cross-section of an ac 115kV HV transmission cable, from when the 234MW power plant across the street was installed. It's 4.2" thick with a 2" dia copper interior. Although the very center of the conductor carries no current, it's still made of copper, like the rest. BTW, this is an underground HV cable. They installed three sets of four cables for the entire plant output, IIRC, one set being a spare. The cable has a 1/8" outer plastic or rubber insulation followed by a 1/6" thick inner lead sheath. I was told the lead sheath was grounded, to act as a short-inducing element in the event of a cable breach. The cable was manufactured by Pirelli, the tire people. It's marked 115kV / 138kV.

The lead also stops rodents and insects. I had to add plasticized lead sheathing to my pool control lines... otherwise replace every six months :-(

...Jim Thompson

Hi Win,

Ah, back when copper was cheap... ;-)

Of course, that's an entirely different application than the one I was thinking of, where the wire is strung between towers and generally is not insulated -- and where strength matters. Tim W. says that yes, cores are commonly steel. I tried a web search just now but didn't turn up anything useful. Maybe I'll drop a note to our local PUD or the like and see what they say. I suppose with the current price of copper, Pirelli would consider making the core from some relatively inexpensive and much lighter material. I wonder if the construction of long distance transmission lines was covered in that old power engineering book you got from me about three years ago. Might be interesting to compare "then" and "now."

Cheers, Tom

Except that they get utilized in PARALLEL, not series.

You mean you don't KNOW how it is done?

HIGH TENSION refers to the steel carrier center "strand" cable!

The lines are steel cables shrouded in aluminum wires!

"Commonly" Try ALWAYS! Long spans between tie points ALWAYS require a steel carrier strand.

Even long cable TV coax runs need a steel carrier strand to keep them from damaging the coax at the tie points. Not talking about hard line here, as that gets bundled to a STEEL carrier strand as well!

Fascinating! How many strands, and how thick is each one?

Any remote chance that this was a high voltage DC line?

I am curious about the four cables. Three phase is usually configured as delta (no neutral), not wye at those voltage and power levels. HVDC tends to use two condutors, and even the fairly rare six phase and twelve phase systems don't use four conductors. Any idea what was going on there?

References:

...

Perhaps each of the 3 sets included one spare conductor? Between "cost of digging trench" and "Cost of being down" a high degree of redundancy might well make sense .vs. saving money on cables at installation time.