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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 8th Jun 2018, 4:09 pm   #101
John M0GLN
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Default Re: Speaker Cable of Yesteryear

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They don't give a c.s.a. in their catalogue, but it measured about 0.85 mm diameter on my digital micrometer (difficult to be exact, since any pressure from micrometer jaws will distort profile - but at least in the right direction for "safety"). On my calculation that gives a c.s.a. of 1.8sq mm.
Is that right? The area of a circle with diameter D is πD²/4, so 0.85² x π/4 = 0.567mm² not 1.8mm²,

from the copper wire tables the nearest to that would be 20SWG which is 0.914mm dia and has a CSA of 0.6567mm².

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Old 8th Jun 2018, 4:57 pm   #102
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Default Re: Speaker Cable of Yesteryear

Correct, but I used PI R SQUARED. Of course some pies are round.

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Old 8th Jun 2018, 5:04 pm   #103
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Bell wire would have been ideal with screw terminals, it would be just the right size for the screws.
If the bell wire was solid core, there would be no worries of loose strands of wire touching the other terminal.

Seems bell wire was ideal. It is 20 gauge, and according to the chart on previous page, you can run a length up to six metres. In a domestic environment, that is more than enough.
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Old 8th Jun 2018, 5:12 pm   #104
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@ John Mogln & Station X

Sorry, my (very,very rusty O’level) maths is abysmal - formulae and order of operators always did get me stuffed!
But using πr2 I get 3.146 x 0.85 0.85 = 2.27 sq mm. What am I doing wrong?

I'm only a simple philosopher, and formulae have always bothered me. It’s remedial maths for me!
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Old 8th Jun 2018, 5:18 pm   #105
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What am I doing wrong?
Using the diameter instead of the radius?
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Old 8th Jun 2018, 5:24 pm   #106
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Ditto.

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Old 8th Jun 2018, 5:35 pm   #107
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"Using the diameter instead of the radius?"

Indeed, thanks. I had somehow got it firmly into my head since measuring and first posting that the diameter was 1.7mm, and thus the radius 0.85. Put it down to age.. So, 0.57 sq mm it is then - about 3A - still probably adequate up to 50W. And at that price...
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Old 8th Jun 2018, 9:03 pm   #108
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Shows just how much rubbish is said about speaker cables. Bell wire works fine up to 6 metres. Yet the irony is the larger the gauge past 16awg the more you get an increase in capacitance. Yet I was always told the larger the cable, and the more expensive the better . . . .

I think if I went in to a HiFi store and told the sales person " I use bell wire for speaker cables " they would break out in a cold sweat, and then they would throw me out the store . . . .!
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Old 8th Jun 2018, 9:18 pm   #109
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Default Re: Speaker Cable of Yesteryear

My goodness this Thread is really running on as I suspected, but to get this, er, Nerdy is a whole new World to me....
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Old 8th Jun 2018, 10:03 pm   #110
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Default Re: Speaker Cable of Yesteryear

Surely this must be the most excitement that bell wire has ever induced (geddit!!?) in its long and rather humble history!
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Old 8th Jun 2018, 10:44 pm   #111
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I think if I went in to a HiFi store and told the sales person " I use bell wire for speaker cables " they would break out in a cold sweat, and then they would throw me out the store . . . .![/QUOTE]

Just adding a bit of humour to the thread . . . .

Nothing more to add really . . . .
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Old 8th Jun 2018, 10:48 pm   #112
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Default Re: Speaker Cable of Yesteryear

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Yet the irony is the larger the gauge past 16awg the more you get an increase in capacitance.
Some decades ago someone read some textbooks on transmission line theory and learned that the ratio between inductance per unit length and capacitance per unit length set a characteristic impedance of a line. They then thought that well, speakers are 8 Ohms, so let's make some cables that act a lot closer to 8 Ohms line than the usual stuff does.

They must have missed the small technicality that the speaker may look like 8 Ohms (at least at some frequencies), but the amplifier tries to look like a lot less (by a factor called the damping factor) so which one should they try to match?

They also overlooked the other technicality that the length of speaker cables is nowmally a very very very small fraction of a wavelength at audio frequencies (15km at 20kHz) and so it all doesn't matter at all.

But they went ahead and made speaker cables of much lower impedance by increasing the capacitance between the two conductors as much as they could by using a lot of very fine enamelled strands all interwoven - with both conductors of the cable divided up and interwoven together. The net result should have been vanishingly small in all cases, but it wasn't. A number of amplifiers were not stable with so much capacitance so close to their outputs. Certain rather expensive models of amplifiers destroyed themselves. That result certainly was audible. Silence, and maybe just a little bit of smoke, replaced music

Bit of an Ohm goal, I thought. A length of Woolies best bell wire, as the non-cognoscenti used would have saved them the price of a big name amplifier as well as the cost of some silly cable. And it would have worked quite well.

The designers of some amplifiers had followed a trend which said that the full "Zobel networks" found on the tail end of most sensible amplifiers, had a series network (L bridged by R) which the non-mathematically inclined didn't understand what it did, and it looked to them like it 'got in the way of the sound'. Did they learn from the ensuing wreckage? Of course not. The esoteric cable industry got into full swing and brought out different cables developed by trial and error and lots of listening by the goldenest of ears. There was a real audible difference this time, the amps stopped blowing up. The capacitance was a lot less, but they explained it all in reams and reams of hoodoo.

All because of a design problem that had been solved many years previously. But it had been done so successfully that few people understood what four little components did, and few people even knew that there had been a problem.

It's like people refusing immunisation because they know certain diseases aren't around any longer and not understanding WHY they aren't around much any longer.

David
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Old 8th Jun 2018, 11:04 pm   #113
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Interesting you mention woven cable, because one cable manufacturer (won't mention the name in case my information is not accurate) specialise in woven cables.
Inductance and capacitance is supposed to cancel each other out due to the woven design, and cancel out rfi. I don't know if this is true.
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Old 8th Jun 2018, 11:14 pm   #114
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Default Re: Speaker Cable of Yesteryear

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I think you'd find it rather hard to produce speaker-cable (or any other cable) that was entirely UK-sourced....
Try telling that to the thousands that worked at the copper works in Cheadle in the next town to me (until recently). Ore in at one end, wire out of the other.
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Old 8th Jun 2018, 11:22 pm   #115
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... Inductance and capacitance is supposed to cancel each other out due to the woven design, and cancel out rfi. I don't know if this is true.
It's not. The truth about inductance and capacitance is that as you reduce one of them by changing the cable's shape, size and geometry you increase the other. You can also influence the capacitance to an extent (not a very big extent) by your choice of insulator. But that's it. James Clerk Maxwell has spoken https://en.wikipedia.org/wiki/Maxwell%27s_equations. And he is not wrong.

Cheers,

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Old 8th Jun 2018, 11:33 pm   #116
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Just what I thought, snake oil . . . .

Thanks for the link grimjosef.
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Old 9th Jun 2018, 12:21 am   #117
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Default Re: Speaker Cable of Yesteryear

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Interesting you mention woven cable, because one cable manufacturer (won't mention the name in case my information is not accurate) specialise in woven cables.
Inductance and capacitance is supposed to cancel each other out due to the woven design, and cancel out rfi. I don't know if this is true.
It's about as close to the truth as a reasonable simplification can get except it isn't something exclusive to weaving the wires, and it only works under a special circumstance.

If we take a length of cable - a there-and-back connection. It doesn't have to be for loudspeakers. It's a rather elegant trick that's done to connect lots of laboratory test equipment, and lots of radios to their antennae. It's how the coax cable to your TV works. There are also parallel-wire transmission lines.

If I make a transmission line of a uniform design along its length, it will have a certain amount of inductance per unit length and a different amount of capacitance per unit length. By changing the shape and the proximity of these condutors I CAN change the ratio of inductance to capacitance per unit length. There is some interaction with changes affecting both L and C per unit length, but there is enough freedom to be able to change their ratio to an extent. I can also add dielectric material (like plastic insulation) and increase the capacitance without affecting the inductance. I could also add magnetic material to increase the inductance, though this isn't much done nowadays, phone lines used to have inductors added every so many miles to make their inductance and therefore impedance more favourable for equal delay across their frequency range.

Once I've made this cable I can play around with the impedance of my source and load. Impedance means the ratio of voltage to current that they either present or absorb.

If I pick a source and load impedance which match (IE equal) the characteristic impedance of my cable, waves can travel along the system without creating echoes at the ends. The source feeds both voltage and current waves into the cable, they flow along it, and the matched load gobbles them both up at the end. Everything works nicely. There will be some loss in the cable due to the resistance of its conductors and possibly any leakage in its insulation. But waves of different frequencies will get fairly similar losses and fairly similar delays.

But if my source and/or load aren't the right impedance, there will be a mismatch at one end or at both. At these points the waves will be in the wrong ratio of voltage to current for the next thing to accept all of them. There will be a reflection, an echo will go back along the line. With a mismatch at both ends, there will be reflections at both ends and diminishing waves bouncing back and forth like a hall of mirrors. It sounds chaotic? It is. Signals get garbled and different frequencies get different effects due to the time delay along the cable creating different phase shifts at different frequencies. It is a mess. This is what the people who tried to match speaker cables to speakers were trying to avoid.

What they lacked was a sense of scale. Waves travel along wires at speeds approaching that of light... about 60% of the speed of light in typical plastic-insulated cables. nearly 100% in air.

So the time taken for echoes to bounce backward and forward is a small fraction of a microsecond for typical sparker cable lengths and the echoes have all died down before anything can be noticed.

Once you get to higher radio frequencies, these transmission line techniques become important. At audio you need to think in terms of cables many kilometres long before they become necessary.

Long, long ago when the first trans-Atlantic telegraph cable had been laid, they were trying to send Morse code along it, but at the receiving end the electromagnetic sounder just sort of clacked erratically. The powers that be thought it was a weedy signal and wanted to get the line driven with higher and higher voltages. That would sort it, they were sure.

A young guy, William Thomson was there and he was of a more mathematical bent. He was a genuine smart cookie, but was then young and in a junior position. He wasn't the first to have worked out the maths of transmission lines, but he deduced that the line was of the wrong impedance for what was driving and loading it. The big names of the day weren't listening. They got the voltage wound up to beyond the capabilities of the undersea cable's insulation and they burned the thing out. What was happening was that different frequency components of the Morse signal were getting different delays and there were echoes, and the signal was getting garbled by the line.

Bit of an expensive mistake!

William Thomson invented lots of things, did lots of good science and was an all-round good guy. He's remembered as Lord Kelvin.

So transmission lines are important things and do some good work for us, but not in loudspeaker cables. They're too short and work at too low frequencies for transmission line effects to have a chance of being noticeable.... unless you've decided that there are no limits to the discrimination of your hearing, then you can't allow yourself to dismiss anything as inconsequential.

Of course, there's profiteering and snobbery added in and that's where science takes a back seat. Those things don't take kindly to facts.

David
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Old 9th Jun 2018, 9:37 am   #118
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Thanks David, I enjoyed that. Hunt in Chap1 of 'Electroacoustics' has a great historical summary of what it took to build up to 1950s audio, including a lot of interesting stuff on early telegraphy (and the invention of PCM in 1790s!).

It struck me while reading through that it would be interesting to stick a small pot in series with my (8 ohm nom.) HiFi speakers and see how much R I need to add to bring in a noticeable drop in volume. Hearing theory suggests a 1dB change is about the limit in a lab, and up to 3dB in the field (imagine that - assuming you've run out of things to horn load, you double the power of your touring PA system - twice as much lorry space - and the audience scratch their heads and say 'hmm, maybe it got a little louder but I'm not sure'

Fletcher and Munson (curves of equal loudness - Phons) suggest that when it gets quieter, the spectral balance will appear to change - less HF and LF. So swapping a *really* lossy cable for a non-lossy one - in an instant, and comparing one after the other with no opportunity to forget what one just heard - might well produce an 'oh, that sounds so much clearer' type response. It would be fun to find out just how catastrophically lossy the 'bad' cable needs to be. Never thought of doing it this way when I had students to do it for me
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Old 9th Jun 2018, 10:35 am   #119
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I enjoyed reading that David. Although some of it went over my head !
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Old 9th Jun 2018, 10:35 am   #120
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You're making me think of the Minions films, Mark...

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