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Old 13th Oct 2017, 10:30 am   #121
Craig Sawyers
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

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Originally Posted by Radio Wrangler View Post
We don't often disagree, Craig, but I think you've missed something.

The full Zobel network on the output of an amplifier consists of two sections. One is a series R-C pair which shunt the output of the amplifier. Together these provide a nice, resistive load to the amplifier at frequencies above audio, without burdening it significantly in the audio range. This is nice but isn't terribly important with well designed amplifiers.

The second part of the full Zobel network is a parallel indictor-resistor in series with the output (after the Zobel shunt). The job of this part is to prevent capacitive loads (exactly like those woven speaker cables) from creating excess phase shift in the feedback loop by the load capacitance and the output impedance of the output transistors creating an additional pole in the loop. The amplifier is isolated at RF from the cable/crossover capacitance and it sees the series resistor instead. This takes the cable/crossover C out of the loop.

Some high faluting amps of the nineteen eighties eschewed the series path Zobel network (presumably on religious grounds) and were renowned for taking off at RF when confronted with high-C interwoven cables. Smoke and large bills followed.

So high-C speaker cable isn't an alternative to the full Zobel, it is something which needs the full Zobel.

The business of running feedback amplifiers and feedback controlled voltage regulators into capacitive loads is a classic problem in design. The maths of loop stability scare a lot of people, but the problem is real.
We still agree. But the amp Zobel and series inductor/resistor are rarely mounted hard to the output connections (they ought to be of course). The exposed wiring between the terminals and the output network itself is an RF problem - a few centimetres of exposed cabling is an antenna at mobile phone frequency. So OK - twist the wires tightly on the way to the output terminals....

A cylindrical inductor is also a problem, since it is an RF antenna. At least one audio amp designer uses an air cored toroid for the inductor to overcome, at least to first order, this issue.

And yes - loop stability is a big issue. Get it even mildly wrong and you can put an HF pole high Q at high hundreds of kHz to low Mhz that is lurking there ready to oscillate with the wrong cable and destroy the output devices.

Lack of Zobels etc is alive and well - it was not just a feature of the 80's. Denis Morecroft's power amps have no output parts and no protection circuits (and are mounted in plastic boxes). And you have to use his speaker cables - two widely spaced wires - because a capacitive load is anathema to his designs http://www.dnm.co.uk/intro.html

Craig

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Old 13th Oct 2017, 10:31 am   #122
G8HQP Dave
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

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My only audiofoolery is the speaker cables - for which I use Kimber. This is a woven construction, with very low inductance and very high capacitance - so it inherently acts like an RF filter - although another way of skinning the RF cat is just to use a Zobel at the speaker end.
Woven construction is likely to be very poor for RF pickup i.e. very good at picking up RF. Certainly not an RF filter. I suspect the popularity of woven (and other unsuitable audio cables) is precisely because they do pick up RF, and the resultant background noise is misperceived as extra 'detail' or 'sparkle' around the musical instruments. A Zobel at the speaker end may help at those frequencies where the cable length is resonant, or if the amp cannot cope with a capacitive load. Anyway, you admit that this is audiophoolery.

I once saw someone try to justify woven speaker cables by saying that earthing straps are woven, so therefore woven is good for RF.

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I believe that when listeners report different sound quality between cables, they are mainly hearing the difference in RF characteristics when linked up to their particular system, in their particular location.
Yes, I suspect the same. As a general rule, the more expensive or more 'exotic' the cable then the more it picks up RF. The best cables are ordinary competent (i.e. quite cheap!) screened for interconnects, and ordinary (but fairly thick) for speakers.
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Old 13th Oct 2017, 10:36 am   #123
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

Anyone on this forum listen to music

Lawrence.

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Old 13th Oct 2017, 10:38 am   #124
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

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Anyone listen to music on this forum
Ugh? I thought it was normal to put a scope on the end of your speaker wires?
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Old 13th Oct 2017, 10:45 am   #125
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

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Speaker cables of such construction are a double edged sword, because some amplifiers object to an almost perfectly capacitive loading. Typically 500pF/m but only 100nH/m inductance..
That looks like a misguided attempt to make a cable with a characteristic impedance which is in the region of typical speaker impedances. 500pF/m and 100nH/m have an RF characteristic impedance of 14 ohms. Misguided because:
1. speaker impedances vary quite a lot with frequency
2. domestic speaker cables are electrically short at audio frequencies so no need to consider transmission line issues
3. at audio frequencies the characteristic impedance is nothing like the RF impedance

For a low loss cable the characteristic impedance is sqrt[(R + jwL)/(G + jwC)]. At RF R and G are negligible (apart from causing some loss) so the characteristic impedance is approximated by sqrt(L/C). Unfortunately people see this and assume it applies at all frequencies. For most audio frequencies the correct approximation is sqrt(R/jwC): this is higher than the RF impedance, not resistive, and varies with frequency. Hence the idea of 'matching' an audio cable to a speaker is misguided. The only benefit of attempting it is that, perhaps by accident, the cable often turns out to have low resistance and that is what is actually needed.
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Old 13th Oct 2017, 10:58 am   #126
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

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Woven construction is likely to be very poor for RF pickup i.e. very good at picking up RF. Certainly not an RF filter. I suspect the popularity of woven (and other unsuitable audio cables) is precisely because they do pick up RF, and the resultant background noise is misperceived as extra 'detail' or 'sparkle' around the musical instruments. A Zobel at the speaker end may help at those frequencies where the cable length is resonant, or if the amp cannot cope with a capacitive load. Anyway, you admit that this is audiophoolery.

I once saw someone try to justify woven speaker cables by saying that earthing straps are woven, so therefore woven is good for RF.
The audiofoolery comment was actually rather tongue in cheek.

As part of another aspect of cable development I had a set of measurements taken for woven cables for differential and common mode transmission up to 1GHZ and RF pick up also to 1GHZ, at 3C test http://www.3ctest.co.uk/ . All as compared to the regular "kettle" lead.

Since this was all under contract, I alas cannot share the detailed results. But you alas are wrong on this. Basically RF is picked up by the inductive component of a cable - so minimising inductance is a good goal to go for, and is proven by measurement. And the 14 ohm characteristic impedance is quite correct, and is an accident of the construction, not a design goal.

Craig
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Old 13th Oct 2017, 11:00 am   #127
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

Oh - just to say we're away over the weekend from about now - so alas cannot reply to the inevitable firestorm I've created
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Old 13th Oct 2017, 11:34 am   #128
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

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Speaker cables of such construction are a double edged sword, because some amplifiers object to an almost perfectly capacitive loading. Typically 500pF/m but only 100nH/m inductance ...
That suggests a dielectric constant of well over 4 for the insulator, which is pretty high. Do they say what the insulator is ? As to whether it's 'perfectly capacitive', well the reactance of even 10m (5nF) of this cable only becomes comparable with the impedance of a typical speaker up in the megahertz region. It might tend to neutralise the inherent inductance of most speakers I suppose, but amplifers which depend on that inductance for their stability are perhaps best avoided in any case.

Cheers,

GJ
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Old 13th Oct 2017, 11:37 am   #129
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

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Basically RF is picked up by the inductive component of a cable
Not true. A coax cable has inductance but near zero pickup. A twisted pair has inductance but near zero pickup. A woven cable, or other poor designs, will pick up RF and for these cables the amount of pickup may vary with inductance.

To avoid RF pickup what you need to aim for is symmetry and/or screening, not low inductance. Coax works because of the screen provided by the outer, and the fact that the outer and inner are concentric. Twisted pair works because of the frequent polarity reversals. Now it may be possible to design a woven cable which is almost as good as twisted pair, but why not just use twisted pair? Most people use untwisted pair quite successfully for speaker cables, but of course there is only little money to be made from untwisted pair as this is a commodity item.
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Old 13th Oct 2017, 11:56 am   #130
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

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Not true. A coax cable has inductance but near zero pickup ...
I can confirm this. In a previous life I looked after a large pulsed power source which accelerated an electron beam to drive a gas laser. When the switching was well synchronised the electrical part of the machine could raise the e-beam current from zero to 600kA in about 15ns. That's quite a dI/dt and also quite an effective source of pulsed RF.

We screened it as well as we could, but the laser light had to get out somehow, and the beams were approaching 40cm in diameter. So the wiring for our diagnostics had to be, um, insensitive to RF pickup. We could get away with good quality braided-outer co-ax over short runs relatively far from the machine. But cables which had to go any distance or run anywhere near the loud end of things had to be solid-outer. We liked Heliax a lot http://www.commscope.com/catalog/cab...s.aspx?id=1342. I've yet to see any used for audio. But maybe it's just a matter of time ...

Cheers,

GJ
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Old 13th Oct 2017, 1:14 pm   #131
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

Stevehertz, thanks for the comment, it's nice to be appreciated & I hope you enjoyed the website. I liked your analogy of ABX tests being garlic to a vampire.
Audiophoolery is one of those hipster trends which generally causes little harm - except to the 'phool's bank account - & provides a perverse source of amusement to pragmatic engineering types like me.
However a particularly egregious outgrowth of this is the aquisition of vintage gear - particularly valve amplifiers - for the purpose of aquiring transformers & original valves. The 'phool considers a shagged out, low emission Mullard ECC83 wrenched from a hapless elderly PA amp to be a greater prize than a new high spec part because it's a 'black plate, yellow print' version presumably blessed with the appropriate amount of magic pixie dust & virgin's tears. As Lucien Nunes expressed his dismay at this practice earlier in this thread.
Woe betide the valve amp collector, invariably outbid on the auction sites by 'phools desperate to liberate the aformentioned glass trophies & wound components from their metallic prisons. Hey! They're PA amps, guys! Not HiFi. You can't make a monoblock out of it. If I hear the word monoblock again, I think I'll scream.

I think I'll take my medicine & have a little lie down now.
Mark.
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Old 13th Oct 2017, 2:27 pm   #132
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

Mark, all understood. However, just to throw a spanner into the works, in my other life I am a long serving guitarist - acoustic and electric. As you may well imagine, I have bought and used many different guitar amplifiers including expensive vintage models. For many years, I didn't believe that the valves in these amps made much, if any difference. One day, unhappy with the sound of a Mesa Boogie pre amp, I decided to swap the Chinese ECC83s for old (but tested good) Mullard ones. Now, you have to understand that what I am about to say refers to the sound when the pre amp is driven into distortion - as is normally the case with such a high gain guitar amplifier. With the stock Chinese valves in place the distortion characteristics were gritty, harsh, odd harmonics, thin - just generally nasty sounding. When I put the Mullards in, I could not believe the difference! It was, honestly, chalk and cheese. Now, the sound was sweet, 'choral', rich and meaty. So, just to be reiterate, I'm not commenting on how valves compare when run within their 'hifi capabilities', but certainly, when used in valve guitar amps in distortion mode, the difference is vast. And in that respect, I have over the years conducted many, many tests and comparisons and it has to be said, original Mullard ECC83s sound better than any other ECC83 valve. It's a similar story with other valves such as EL84s and EL34s. I don't know why, but there's definitely something about the construction of the original Mullards that makes them sweet sounding when driven into distortion in guitar valve amp.

This article may throw some light on it, and in any case it is interesting reading re valve construction: http://www.effectrode.com/signal-tub...al-comparison/
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Old 13th Oct 2017, 2:33 pm   #133
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

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what kind of specs should one be looking out for; what are the essential features (and figures) to look for?
Easy, low resistance, less then a few percent of the 'speakers rated one. Bog standard two core cable, twin and earth too (ignore the earth). I have used Maplins 'speaker cable only because it is flexible and I won't confuse it with other ones.
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Old 13th Oct 2017, 2:42 pm   #134
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

Well yes, that's the easy, and with respect, obvious part. Craig seemed to hint at a much more 'high tech' version, here's what he said:

"Also mains cables need careful consideration to manage the junk that comes out our mains, particularly in these days of ethernet over mains. Once inside a chassis, that can radiate via anything that looks like an antenna. Mains filters a-la Shaffner are next to useless since they are only specced to operate up to 30MHz (or occasionally to 60MHz), and can often resonantly amplify interference at higher frequencies."

There's also his follow up answer:

"I specifically did not mention mains cable types; I have to declare an interest, since a range of EMC attenuating cables of my design is on the market on a royalty basis for prices that would make your eyes water.

"I hesitate to mention a good alternative that sounds great, but is dirt cheap. I did on another forum and was excoriated so completely and repeatedly on the basis of safety that I quit the forum. So I remain stum on that - sorry."
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Old 13th Oct 2017, 2:52 pm   #135
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

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... So, put another way, in looking out for such a unit, what kind of specs should one be looking out for; what are the essential features (and figures) to look for?
Peter Walker, I think, once said that cables do matter. In particular they need to be long enough to reach between the two pieces of kit they're supposed to be connecting. If they don't meet this spec then the sound quality can be seriously degraded .

Cheers,

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Old 13th Oct 2017, 3:01 pm   #136
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

Brilliant!

What worries me is that whoever thinks up this nonsense is busy thinking up the sequel (or should that be prequel these days)
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Old 13th Oct 2017, 3:07 pm   #137
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

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Peter Walker, I think, once said that cables do matter. In particular they need to be long enough to reach between the two pieces of kit they're supposed to be connecting. If they don't meet this spec then the sound quality can be seriously degraded
Well, that's one for the audiophools, not me, I'm not in any way, shape or form a 'cable freak'! On the contrary I have track record of boo-hooing high priced, audiophile cables for years. No, Craig suggested that there are ways to stop mobile phone hash getting into equipment via mains leads, that's my interest as he raises a good point if you read his post on the subject. Otherwise wet string (or if it's serious, bell wire) does for most of my hookups
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Old 13th Oct 2017, 3:23 pm   #138
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Default Re: Audiophoolery. 'Cable Break In' - I never knew that!

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... No, Craig suggested that there are ways to stop mobile phone hash getting into equipment via mains leads, that's my interest as he raises a good point if you read his post on the subject ...
I'd be interested in that too. He mentioned 'EMC attenuating cables'. I imagine that those act as in-line filters, attenuating EMI which is propagating along the cables, rather than simply as shielded (if that's the right word) cables which attenuate EMI as it tries to leak into them. Shielding would be of limited use given that most of the length of any mains cable is made up of twin-and-earth hidden in the wall plaster or running in conduit or above/below a suspended ceiling/floor.

Cheers,

GJ
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Old 13th Oct 2017, 3:36 pm   #139
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Default Re: 'Cable Break In' - I never knew that!

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The Quad VA-One is available with “audiophile-grade” valves treated by freezing at -300F for 48 hours.
This one is actually true. Spending 48 hours at such a low temperature is ‘character building’ for the valves (similar to DofE or one of those American military schools). They then go on to be well-rounded/superior sounding members of society/your hi-fi system.

Non-cryo-treated valves lack perspective and never realise how lucky they are, whereas those which have been to the North Pole are always thankful for the warmth of their filaments.

Very interesting thread by the way.

Liam
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Old 13th Oct 2017, 4:04 pm   #140
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Liam, you don't give a single, science or physics based reason why a cry-treated valve is better than an untreated one. I have to say, the use of phrases and words like 'character building', 'well rounded/superior sounding', and 'perspective' are exactly the reason why non-audiophools respond to such claims with phrases such as smoke and mirrors, voodoo and snake oil. I'd love to hear, from you, in your own science based words (not a link to someone else's similar 'beliefs') exactly why cry-treated valves are better? I don't believe they are until I see the proof, although you say quite clearly and knowingly, "This one is actually true". Prove it. And for sure, 'proof' isn't an audiophool telling me that they can hear a difference. I can't speak for everyone (clearly) but in general we're a pretty science based bunch on here, we grew up on electronics and radio theory, and we use that theory every day to repair, set up and even modify audio equipment old and new. Notional views are only of use to those who want to believe them and for whatever reason choose not to look for objective based answers. Then again... you could be joking..
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