UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 14th Jan 2021, 8:24 pm   #1
samjmann
Hexode
 
samjmann's Avatar
 
Join Date: Feb 2014
Location: Nottingham, Notts. UK.
Posts: 384
Default Exotic speaker cable - problems.

There's been much discussion here and many other forums about the so called 'exotic' speaker cables and the effects on output stages in certain amplifiers. From what I understand some amplifiers can go unstable at high volume when connected to these cables with resultant s/c output transistors etc.

Is there a problem with the design of the Zobel network in the amplifier, insomuch that the low resistance and possible capacitive effects of the 'exotic' cable
cause it to 'take-off'?

I read about this on and off over the years, and some high-end amps seem quite prone to damage in this way. But few explanations really go into any detail about exactly what's happening.

Anyone fancy a write up or a link to good article somewhere?


Regards, SJM.
__________________
It's never been right since we've had it...
samjmann is offline  
Old 14th Jan 2021, 9:08 pm   #2
kevinaston1
Hexode
 
Join Date: Aug 2014
Location: Featherstone, West Yorkshire, UK.
Posts: 297
Default Re: Exotic speaker cable - problems.

See

https://www.passdiy.com/project/articles/speaker-cables
kevinaston1 is offline  
Old 14th Jan 2021, 11:01 pm   #3
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 17,139
Default Re: Exotic speaker cable - problems.

Where do I begin.........

Once upon a time, amplifiers were designed by engineers, using maths and normal scientific principles. At first there were plenty of things they didn't know, but as problems came to light they worked to analyse and circumvent them.

Speakers were relatively tame beasts and people traditionally wired them up with bell wire from woolies.

Time passed. People wanted louder music, and the ohmic drop in Woolworth's finest looked like it was worth reducing with some thicker copper. Not terribly thick, just a good bit thicker than bell wire,

Meanwhile great advances were being made in marketing techniques. Some companies overdid the copper size increase thing and made claims that it would sound even better. here was an outbreak in gigantism in the connectors as well. This trend has continued to the point where cables are the size you could jump-start a tractor with and the connectors are finely machined engineering jewellery. It's almost as if people aren't aware of the differences of electrical and mechanical energy and the differences in ways of coupling them. Again there are plenty of claims of audible superiority.

Human perception is famously susceptible to immediately prior experience and other things around for contrast. Stare at a blue wall for a few minutes, and your colour perception will be skewed for several minutes. This is a characteristic of our eyesight and of how our brains handle it. No big deal. The sense of smell is the same. Go work in a cow shed for a day and you'll stop noticing it, but maybe you shouldn't trust yourself to check perfumes for a while.

For some reason, in the hifi world, the comparable psychological effects on the perception of sound must not be admitted. You mark yourself out as inadequate if you admit your perception is not infinite and absolute. Yeah, right!

No-one dare admit "Well, I can't hear any difference." So the stage is set for a showing of the Emperor's New Clothes.

So far, I'm just considering increases in the cross sectional area of copper.

But that wasn't esoteric enough for the marketeers. Something more exclusive could be more profitable. But they wanted some scientific background to give it some legitimacy.

Someone had heard of transmission lines, commonly used for radio frequency connections, say between receivers or transmitters with their antennae. These transmission lines are real and serve a necessary purpose when their length is anything more than a small fraction of a wavelength at the frequency concerned.

I've used them professionally to frequencies of tens of billions of Hertz. Things get more difficult up there, conversely, things get a lot easier as you go down in frequency, and 20 kiloHertz is a long way down from millimetre waves. Audio wavelengths in cables are about 9km at 20kHz.... now, how big is your lounge? But the audiophile world has evolved rules and the number one consequence of believing that there is no limit to your hearing's capability is that there is no imperfection which can be considered so small that it does not matter.

So your speaker cables need to be designed as transmission lines. Transmission lines have a natural 'Characteristic impedance' usually abbreviated to Zo. Oooh, so what Zo should be chosen? Well, the loudspeakers are 8 Ohms... it says that on them right by the terminals. Let's make some 8 Ohm transmission lines! the impedance Zo is set chiefly by the ratios of the spacing of the conductors to their diameters, and the dielectric (capacitive) constant of the plastic/air/whatever insulation between them.

8 Ohms is a problem. The spacing would have to be tiny, In fact you have to take a different approach of interleaving the two conductors within each other. You divide each conductor of your pair up into a lot of thin insulated strands in parallel and you plait them in and amongst those of the return conductor.

This looks GREAT! Do the strands in different colours. Have fancy braiding schemes. Dress them up with some fancy woven jackets, with elegant end pieces to stop them fraying and lastly, most importantly rev up the prices to the limit. You'll need some slick marketing words to go with them, and above all a slick, slightly clever name. Plenty of people will swear they hear a difference. You'd think, the way this tale is going, that the difference is psychosomatic....

Well, it wasn't always imaginary. Sometimes the listeners really did hear a difference.

The best valve amplifiers from the end of the valve's reign relied a lot on feedback, but the amount they could use had to be limited because stray effects in their output transformers would build up unwanted phase delays well above the audible band, but before all the amplifier's gain had rolled off. This risked instability. The designers simply had to limit how much gain/feedback they could use. The losses in the output transformer helped by buffering just what the valves could see at much higher frequencies. The dragon slumbered.

Transistor characteristics, like valve characteristics are curved. But the transistors' are more sharply curved and there is the issue of switching devices on and off round the cycle for class B. So transistors need a lot more feedback than we ever did with valves. This is OK, because with transistors we can easily do away with the output transformer. We can have lots of feedback if we want it.

We can have a wideband amplifier and tons of loop gain, and lots of feedback. Distortion figures are pushed right down, the response is flat over a very wide band, and the output impedance is very small scoring points for 'damping factor'

There is a problem.

Such feedback amplifiers don't like working into capacitive loads at high frequencies. The capacitance load creates an extra pole (mathematical term for a point at which a roll-off and a phase lag come into play) in the response, and this acts within the feedback loop. OOps, the stability margin just flew out the window. The amplifier will now oscillate at rather high frequencies, far above audibility even for bats. Things get hot, bothered and go up in smoke (or just pop internally with no external visibility)

So, weren't the designers aware of this?

Indeed they were. What's called an 8 ohm loudspeaker is a joke. At some frequencies they drop to a few ohms (nearer 2 for Isobariks) and at others they can reach over a hundred. But these instabilities we're talking about are far above audio. Your tweeter has turned to a piece of cheese and all the parts in your crossover are acting mostly like their strays. Your 8 ohm cable is not seeing an 8 ohm load, so neither does your amplifier.

If you have a woven (Litz wire) speaker cable, then they do tend to look strongly capacitive at radio frequencies. Straight-forward cables look lossier, and aren't so dangerous but still could excite oscillation.

So the designers added a little network between the amplifier proper and its output terminals. It's job is to disconnect the amp from the speaker cables in the frequency region at risk, and also to bring in some damping to help out. It has an L-R parallel network from amp to terminal, and a series R-C network from amp to ground.

People tend to call this a Zobel network. Otto Zobel was a very smart cookie at Bell Labs. He worked extensively in network theory and filter synthesis, doing original research. THere is no one Zobel network.... there is a book full of different ones.

So, all was OK, until a boutique amplifier manufacturer decided he didn't like having that network there. It offended his sensibilities. It got between his beautiful brainchild and the speakers. It must be doing no good. Most speakers and cables were OK without it, so he sold amplifiers with no output isolation networks.

Some other manufacturers picked up this trend, some scaled back the networks they still had.

The fates chose this time for the esoteric speaker cables to enter the scene.

The rest is history

Who says the fates don't have a sense of humour?

But only few people understood what had gone wrong and why. It survives as rumours and legends. A few people remember that Naim amplifiers and various esoteric cables didn't go together, but there is plenty more vulnerable stuff.

If you use moderately thick speaker cables and don't need a motorbike to get around your lounge, and you can really really hear differences in speaker cables, then one setup or the other has something going very badly wrong.

David (Both typing fingers now sore!)
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 15th Jan 2021, 12:06 am   #4
Julesomega
Octode
 
Julesomega's Avatar
 
Join Date: Dec 2012
Location: Stockport, Greater Manchester, UK.
Posts: 1,242
Default Re: Exotic speaker cable - problems.

um, shouldn't the cable Zo match the amplifier output impedance?

Talk me out of that one, David
__________________
- Julian

It's good here
Julesomega is offline  
Old 15th Jan 2021, 12:48 am   #5
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 17,139
Default Re: Exotic speaker cable - problems.

The moving coil loudspeaker is both a motor and a generator. If you apply a fixed voltage to it, current will flow, applying force to the moving mass which will accelerate. As the coil gains velocity, in moving through the magnetic field, it will generate voltage opposing the current (Lenz' law and Faraday) eventually the coil will reach speed where the 'back EMF' opposes the applied voltage and decreases the current. The force decreases and the cone velocity is at a limit set by the applied voltage.

At constant voltage, the cone wants to move at a constant (and proportional) velocity.

A cone moving at constant velocity tries to shift a constant volume of air per second

A constant flow rate (litres/second) acts as a constant pressure.

So a loudspeaker tries to be a voltage-to-pressure transducer. Our ears are essentially pressure sensitive, so this all seems to fit.

This is why we try to drive speakers from controlled-voltage sources, and why we don't get too upset by speaker impedances wandering around rather a lot.

We want the voltage supplied to the speaker to be rather stiff... to not vary much if the current taken changes. This has a nice side effect of trying to damp and control any resonances in the speaker as well. So the ratio of speaker impedance to amplifier impedance is called 'Damping Factor' It isn't the be-all and end-all, but you want a reasonable amount.

Um, just a minute... the bigger the damping factor, the bigger the difference in the speaker input Z and the amplifier output Z..... so the match is worse!

Right! Who said they needed to be matched? We have a situation of what we want is an efficient, unidirectional flow of power. AND the distance is trivially small at the frequencies concerned. An Amplifier output impedance of damn all is perfectly fine, and an 8 Ohm speaker is OK.

Zero Ohm transmission line is tending towards infinite diameter conductors spaced zero mm apart. So the cable shamen chose to match the speaker's nominal value and ignore the amplifier Z. Actually this is OK if we really had a match at the speaker end. But speaker impedance bounces all over the shop, and at the hundreds of kHz and above region where the oscillations start up, then all bets are off. So the matched at either end business is a big joke. Just thank your chosen deity that the cable is far shorter than a wavelength, and so it doesn't really matter at all. Except to people with infinite aural capability.

When cables are long compared to a wavelength, say a radio transmitter to an antenna. It is usual to match the cable to the antenna (minimising any reflection when the signal reaches that end of the line, and to not care about matching the transmitter to the line - it doesn't matter if there is no backwards flow of power to be reflected.)

Many people Think the transmitter and line are matched, but they aren't. A transmitter is designed to feed say a 50 Ohm line, but that doesn't mean it looks like a 50 Ohm source. It just means that the voltages and currents in a transmitter working into that load are comfortable, and nothing is about to burn up. The transmitter suits a 50 Ohm load (and vice versa) but this isn't the same as matching in the mathematical sense.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 15th Jan 2021, 1:10 am   #6
joebog1
Octode
 
joebog1's Avatar
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 1,293
Default Re: Exotic speaker cable - problems.

As always, BRILLIANTLY explained!!!. I wish you had been my lecturer at uni. I might actually know something. Thank you David

Joe
joebog1 is online now  
Old 15th Jan 2021, 9:18 am   #7
Stockden
Heptode
 
Join Date: Mar 2008
Location: Forest of Dean, Gloucestershire, UK.
Posts: 526
Default Re: Exotic speaker cable - problems.

Quote:
Originally Posted by joebog1 View Post
As always, BRILLIANTLY explained!!!. I wish you had been my lecturer at uni. I might actually know something.
Yes, me too!

I’m now struggling against the temptation to send this to my audiophool inclined friend. He’s a clinical psychologist so he won’t understand the engineering but he might understand David’s “human perception” paragraph.

On the other hand it might spoil his enjoyment of his sound system....

Hugh
Stockden is offline  
Old 15th Jan 2021, 10:21 am   #8
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 17,139
Default Re: Exotic speaker cable - problems.

There is another psychological factor kicking around as well. It's that there can be satisfaction in having something you think is nice, even if it has no objective, functional justification.

We all have our collections of vintage electronics, with no rational basis for the pleasure they give us. But the pleasure is real nevertheless.

I ride a horse for heaven's sake. A rational person might question what's wrong with my range rover? It's just my preference and I don't have to justify it.

We all have our interests, but we should be careful not to get sucked into some of the pitfalls of trying to justify them.

'High end' audio becomes truly risible once they wheel out pseudoscientific justifications. I don't know whether to laugh or cry. If they'd just got the sense to not try, the fancy gear becomes jewellery and can be simply enjoyed for what it is. For some reason the human race is addicted to pecking orders and the urge to list things in order of some sort of goodness (I cite almost the entire internet as evidence!) Why bother? the goodness is spread across many parameters and comparisons at best are indirect and often misleading. It all depends on an individual's weighting of the different aspects.

I find it odd that much of the public is prepared to disbelieve a bona-fide scientist and to immediately believe a bona-fide conspiracy theorist.

So, your audiophool friend is perfectly within his rights to have his fancy equipment, and I'll defend his right to do so, but I'd recommend he doesn't try to trot out any of the tomfoolery justifications. I'd only laugh.

Maybe we can wean him off. Once you stop worrying about the gear, you start to notice the music.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 15th Jan 2021, 11:24 am   #9
samjmann
Hexode
 
samjmann's Avatar
 
Join Date: Feb 2014
Location: Nottingham, Notts. UK.
Posts: 384
Default Re: Exotic speaker cable - problems.

Thank you RW for such a comprehensive and easy to read reply. I hope your typing fingers are in 'recovery'

What had prompted me to post was that I've just finished a repair on the original Creek CAS4040. This one was the very first one before the o/p stage was redesigned. This amp was the subject of a long thread on here and David had posted about some compromises in amps design. In particular stability problems with exotic speaker cable. In this case all that was wrong was the volume control and not the output stage thank goodness!!

Thanks also kevinaston1 see post#2 the article is well worth the time taken for a proper read.

Take care out there everyone. Regards, SJM.
__________________
It's never been right since we've had it...
samjmann is offline  
Old 15th Jan 2021, 6:57 pm   #10
Vintage Engr
Hexode
 
Join Date: Jan 2008
Location: Shrewsbury, Shropshire, UK.
Posts: 274
Default Re: Exotic speaker cable - problems.

Thank you RW/David. All totally agreed.

I've travelled a long way in my various careers, until retirement a few years ago. I've worked in both domestic electronics, and later in broadcast TV & audio.
Twenty years ago, when I moved up to Shropshire, I visited a local Hi-fi emporium. Mainly just out of interest, but partly thinking that I might do some part-time work.
I was being shown round the elegant sales area, and a locked cabinet was pointed out, which contained..... phono leads & speaker cables.
I was not expecting to see 2m phono leads priced at 99.00, or indeed speaker cables at similar prices.
Unfortunately, I was unable to control my my immediate response, much laughter, and comments such as 'you must be joking'. I do especially like the gold-plated IEC mains leads...
I was immediately escorted off the premises, in case I upset the customers!
I didn't go back.

David.
Vintage Engr is offline  
Old 15th Jan 2021, 7:32 pm   #11
knobtwiddler
Pentode
 
Join Date: Mar 2016
Location: London, UK.
Posts: 233
Default Re: Exotic speaker cable - problems.

Removing Zobels from commercial amps is an audiophool pastime. A client was charged handsomely for the service. He kept wondering why his tweeters kept blowing... He brought said amp over to me... Square response was shocking, although not as much as the smoke emanating from said amp....

Harold Black was one of the greatest innovators in audio. His discovery was NFB, which audiophools rail against (am not equating low global FB with lack of stability btw - too much FB can cause instability in some circuits). Same audiophools rail against Mr Zobel's invention.

One day I shall market a 'high feedback amp' - with a Zobel!
knobtwiddler is offline  
Old 15th Jan 2021, 9:12 pm   #12
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 17,139
Default Re: Exotic speaker cable - problems.

That's quite standard, and comes under the general heading of 'A little knowledge is a dangerous thing'

What that little network on the output of an amplifier does is very important. The lack of one can drain your bank account. HOW that little network does it is quite subtle and to see how it has to be designed in order to work needs knowledge in the Bode and Laplace areas of engineering maths. Most batchelor's graduates ran away from the control theory modules of their courses, so framed paper on the wall is no surety of competence or understanding. Amusingly, it isn't actually hard, it just involves some weirdly alien concepts that can be hard to assimilate.

However, pontificating on how things can be done is not held back by any lack of technical knowledge. The results might just be affected, though.

Railing against feedback?

Well, if you don't understand something, you are scared of it, you do not feel in control of it. If you do understand it, it becomes a tool in your toolbox, to be used when appropriate. From here on, it gets funny, verrry funny.

So some pundits declared that feedback is evil, and that a non-evil amplifier must not have any. So amplifiers were 'designed' without any overall feedback loop and they all felt much happier and set to imagining thrusting new ways of describing the audible improvements they convinced themselves they must hear.

They'd removed the overt, obvious feedback path. They'd missed all the local feedback systems like stage degeneration.

An emitter follower is a feedback amplifier. The feedback path is one and the same with the signal path. through the transistor. Same with cathode followers and undecoupled cathode resistors.

So far, the anti-feedback people aren't even aware that they've missed most of the feedback in an amplifier. Even less are they aware that we've spotted their omission and are quietly having a jolly good laugh.

The next joke is that emitter followers, through the internal capacitances of the transistor and the inductances of their legs and circuit connections can go unstable at RF. No overt feedback path need apply. Nearly all transistor amps end in emitter followers as their final stages. One of the conditions for the oscillating follower problem is good RF capacitance to ground via low inductance on their output.... Ah, those damnable esoteric cables again. They work their magic even on pundit-certified feedback-free (well, the obvious feedback that is) amplifiers. The amplifier hoots its little head off, and the heat from cross-current sends up a smoke-based distress signal.

Serves them right?

Mostly.

There have been innocent errors in this area. Peter Walker is almost sainted because of his Quad II, and Peter Baxendall is up there in the pantheon as well. They made a mistake that wasn't a mistake at the time. Those triplets of transistors in the output of the Quad 303 are naturally unstable unless the TO3 transistor is dramatically slower than the two smaller devices driving it. Their mistake wasn't an error at the time, their circuit worked, but they failed to see that the devices they used had open ended specs and were being improved all the time. The TO3 transistors got improved more than the TO5 and TO18 drivers. The gap between the transition frequencies closed up. So nowadays it is a major problem to buy a replacement TO3 power transistor BAD enough to work in that circuit.

I stress that what they did was fine with the devices of the period and no-one was predicting quite the improvement that was coming.

But fire up a Quad 303, apply an 8 ohm load, maybe with a bit of ordinary cable capacitance, drive the amp somewhat, not even close to clipping, and look with a scope. You will likely see a short burst of RF oscillation (around 10MHz) hanging around one place on each cycle of the audio sine you're testing with. It looks like a smudge on the scope trace. Not all 303s do this, but a lot with replacement transistors do.

There seems to be a reciprocal law at work. The more boutique-y an amplifier is, the less of a clue the designer seems to have had, when you do a hard analysis.

It's as if the structural engineering for a bridge or large building was done on religious principles. What could possibly go wrong?

Anyway, the speaker cable problems are inextricably entwined with output stability networks and they are entwined with feedback design and your toe bone's connected to your.....

This thread has been about sensible, engineering based, answers to a perfectly real issue that samjmann raised. It's an area that I felt wasn't well enough understood in the world at large and has had tons of oofle dust dumped on it by a few decades' worth of shamen and mystics.

The way samjamm phased his question made me feel he wasn't just another cultist with a mind already made-up. It was a serious issue, asked carefully, and deserved the best serious answer I could give.

For the really silly bits of audiophoolery, there is the appropriately named "The audiophoolery thread" not very far away. It's where you'll find happy stories of 1800 mains cables, smart rocks and unicorn hide. It's where you can have a damned good laugh.

So let's keep this thread about high-profit cables and amplifier stability, please. This way we'll have a nice record for the next person to realise something funny is going on when fancy cables cause smoke, to be pointed towards. (And I don't have to type it all over again!)

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 15th Jan 2021, 10:22 pm   #13
Cobaltblue
Moderator
 
Cobaltblue's Avatar
 
Join Date: Dec 2010
Location: Exeter and Poole, UK.
Posts: 4,636
Default Re: Exotic speaker cable - problems.

Thanks to Davids Hard work (and sore fingers) there is a lasting record of these issues.
It's time to close the thread unless new and pertinent information comes to light.

Cheers

Mike T
__________________
Don't care if it was a bargain why's it in my kitchen
Mike T BVWS member.
www.cossor.co.uk
Cobaltblue is offline  
Closed Thread

Thread Tools



All times are GMT. The time now is 11:43 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Copyright ©2002 - 2021, Paul Stenning.