I recalled earlier discussions on 'skin effect' here, for instance later down this thread (showing Owen's fine room) -

darklantern.proboards.com/thread/1270/work-progress

It reminded me that I had been unconvinced at audio frequencies, so went looking and found a couple of calculators.

www.daycounter.com/Calculators/SkinEffect/Skin-Effect-Calculator.phtml

shows that at 20kHz, skin depth is 20.1 mil (approx 0.51 mm, see www.convertunits.com/from/mils/to/mm ) and that wire gauge such that Rac=Rdc is 18 AWG.

Put in 48kHz and it's 12.9 mils and 22 AWG,

I'll get to the second part in a moment, but along the way I also found this (more attuned to RF, which is where it does become significant)

voltage-disturbance.com/engineering-calculators/skin-effect-calculator/

and a quite technical discussion, the math is way over my head ~

Some Skin Effect Notes
fermi.la.asu.edu/w9cf/skin/index.html

Anyway, the quite prosaic thing that started me wondering about this was, looking through my old stash of wiring, speaker cables, and discovered 3 + 3 + 7m lengths of old DSE (Dick Smith Electronics) cables that I even think I remembered reading about getting oxidised within the plastic, a bad batch perhaps, and wondering if or how they might be compromised.

Copper oxide being a semiconductor, and yet only a thin film, and easily scraped off.

These cables were terrible to try to split apart, even with a sharp craft knife or by starting a split and pulling apart.

Easily veered to expose copper on one side instead of splitting evenly down the middle. Not "TrueRip" !

Yet quite chunky, I estimate about 1.8mm overall conductor thickness diameter.

I've renewed some ends, merely flattening and scraping while rotating to get back to copper - this pic in low light, adjusted.

You can see the green under the insulation, especially on the left.

My question: Do you think the oxide on the cable strands surfaces internally (despite cleaned terminating ends) will affect the audio signal?

I wish to give these to a broke neighbour whose early NZ made Roberts amplifier (70's?) I've replaced one channel output transistors in, and who scored a pair of Kef Carlton III for free on a renovation job (not wanted by new owners, looked too dull and boring for makeover premises!) - and likely will anyway, but just wondered as above - will a very thin surface film of green copper oxide (reaction with the plastic sheath?) internal in the cable conceivably cause any audible anomalies?

Thanks for indulging me! ;=})

More to the point: will those cables fit into the Roberts speaker terminals as iirc they are not overly large by modern standards ?

Hey thanks for the reply;- good point.

However my neigbour has a couple of thinner lengths which we may also need as it's quite a long room, lounge area and open plan dining kitchen.
Else I have some terminators that accept cable at one end then go to a small square cross section rod, clumsy description.

You know of the Roberts amplifiers?
All research suggests that the company has now ceased to exist, which is sad, although the original designer was still working there some years ago.
It's a pity because there are a couple (or couple of pairs) of trim-pots on the board, I know not what they do, I guess AC & DC balance perhaps.

It's a Model STA-50 - bought from new apparently, but out of use for ages.
Would be nice to get some data.

The oxide layer is so thin I won't worry about it - more important to get some sounds er music happening for him.
It'll be a huge change from laptop into small Logitech maybe computer sat sub speakers!
The room is like a long shoe box, the Roberts appear to drive the Kefs to a reasonably modest volume, an apartment surrounded by others so likely enough.

Yes I know the Roberts amplifier range well. The STA50 mark 3 was very well regarded back in the day as it was one of the few amps that could drive Linn saras. I have been looking for one for donkeys ages. I did have one back in the day but foolishly "upgraded". I also has a 100S (55w) and a 100 mk2 (85W). I also had a later R05 for a short time.

Roberts amplifiers grew out of Panther electronics which were originally upstairs in a small workshop off Fitzgerald ave (?) The proprietors were Phil Roberts and Chris Cotter (relative of the well known cotter electrical store in High St). Chris died in mysterious circumstances in Queensland I believe. Phil continued on but got into cashflow issues supplying Smiths City Market (which had a reasonable HiFi and record dept in those days). This is hearsay so please dont take it as gospel.

Phil then bought Dann & Wise the electrical service agents, and most recently I understand has been on the staff of Soundline doing repairs. Which is more than ironic bearing in mind that Frank never had a good word to say about him back in the day.

He is probably still around somewhere, was living out west Melton way last I heard, but if you get stuck drop me a line as I think I have his contact details somewhere.

Supper nice guy BTW.

My reference on the question of 'skin effect' in audio cabling, has always been Prof. Malcom Hawksfords Essex Echo series, originally published in HI-FI NEWS & RECORD REVIEW in the early-90s.

Re-published by STEREOPHILE in 1995 - www.stereophile.com/reference/1095cable/

This has been my reference over the years, not because i understand the electromagnetic & wave theory, but because it has been borne out in my own humble experiments with DIY cables.

Specifically this conclusion...

" At a diameter of around 0.8mm, the conductor becomes closer to a low-valued ideal resistor at audio frequencies. "

DarkLantern blog - darklanternforowen.wordpress.com

RDM,

How did your mate with the Roberts amp get on ? BTW did you know they also made speakers ?

Skin effect on audio cable is interesting a subject I find fascinating and often misunderstood.
OK BW 20KHz but rise and full time of a clash is what? well when measure it can be as fast as 1uS ok this relates to 1MHz.
Now imagine lots of crash bashing whistles of good complex music, each sound rising and on there leading and lagging edges at different speeds and at differing starting and stopping points.
So do I use slow 30Amp solid power cord of Litz and not covered in copper oxide but lets say a good dielectric PTFE.
Which cable will mix and phase screw the music?
Oh and the other fly in the wine, is you have a horrid inductive load that changes in operational mode that need to be damped so it backing EMF does not cancel the small signal going to it.
The subject is extremely complex, and I have spent many a good day on it, from the 1970 designs of High Power Sonar to massive stepper motor controls. And high speed multiplexed digital signals.

So read a bit about

www.antenna-theory.com/tutorial/electromagnetics/capacitance.php

patents.google.com/patent/GB2527777B/en

en.wikipedia.org/wiki/Michael_Faraday

en.wikipedia.org/wiki/Maxwell%27s_equations

en.wikipedia.org/wiki/Litz_wire



Have fun best Col

That is very complex Col, I will do well to update my knowledge!

AMR-iFi R&D