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Old 17th Mar 2020, 5:40 pm   #1301
avocollector
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Default Re: The Audiophoolery Thread.

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Originally Posted by Craig Sawyers View Post
Another odd positive side effect of valve rectifiers is the lack of reverse recovery. They therefore do not excite the leakage inductance/interwinding capacitance resonance of the mains transformer.

Regular solid state rectifiers generate reverse recovery transients like a champ, so every time each rectifier diode turns off, a burst of oscillation occurs via the above effect usually in the range 200kHz to 1MHz with a Q of 5-10.

Of course you can use more sophisticated soft recovery diodes, and/or use a C-RC snubber across the secondary winding of the transformer, which kills the oscillatory transient dead if you get the RC in particular correct.

There is a small circuit on DIY audio that enables these values to be determined by direct measurement https://www.diyaudio.com/forums/powe...-test-jig.html

Craig
So does that explain the weird signal display I got when I once fed the ac into a diode and put a scope on the other side - instead of the smooth series of humps like the textbooks show it was completely crazy/all over the show. When I put a load resistor in and the scope probes across that I got the expected series of humps - but I wondered about the other just why.
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Old 22nd Mar 2020, 2:01 pm   #1302
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Default Re: The Audiophoolery Thread.

I thought "Audiophoolery" was the term used to desribe the selling technique used when selling speakers and audio equipment from the back of white vans.
I have a set of very lovely looking Symphony speakers in my garage that a friend of mine purchased from a white van for AUS $300.
The sound is absolutely aweful and tinny for a large wooden cabinet.
He believed they were some awesome home theatre speakers made in England. Until you peel the product of GB sticker off to see made in Taiwan underneath.
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Old 22nd Mar 2020, 2:05 pm   #1303
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Default Re: The Audiophoolery Thread.

That's the opposite, Audioconnery!
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Old 22nd Mar 2020, 2:18 pm   #1304
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Default Re: The Audiophoolery Thread.

Audioconnery

Is he Seans half brother
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Old 22nd Mar 2020, 7:40 pm   #1305
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Default Re: The Audiophoolery Thread.

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I thought "Audiophoolery" was the term used to desribe the selling technique used when selling speakers and audio equipment from the back of white vans.
I have a set of very lovely looking Symphony speakers in my garage that a friend of mine purchased from a white van for AUS $300.
The sound is absolutely aweful and tinny for a large wooden cabinet.
He believed they were some awesome home theatre speakers made in England. Until you peel the product of GB sticker off to see made in Taiwan underneath.
There was a similar scam in the UK during the 90s, but without the nice cabinet. I got exposed to it when a relative fell for it, buying a gigantic set of speakers - which weighed about 2 kilos! They barely functioned at all. Cabinet was akin to cardboard or balsa wood, with a laminated shell. Massive mylar cones - but with magnets the size of speakers seen in kids toys.

I was once in a record shop, when white -van men pulled up and started offering similar speakers to the owner. Without handling them, judging them on looks, he opened the till and started counting cash... There was no way I was going to articulate verbally that he was being truly ripped off, as the van men looked rough, and I would've been looking over my shoulder the whole way home. Instead, I stood there, glaring at him, gnashing my jaw. Eventually, having enacted something out of a Python sketch, he twigged and didn't buy them.
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Old 22nd Mar 2020, 11:50 pm   #1306
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Default Re: The Audiophoolery Thread.

Ah yes, the infamous "white van speakers". I was with some colleagues in London when they rolled up and tried the spiel - supposedly studio monitors from a dealers bankruptcy auction or some such tale. But we were all professional sound engineers, and we had a bit of fun and gave them a right winding up...

I was working in Vienna in the mid 90's and exactly the same thing was happening there (even down to a similar wind-up by a couple of Tonmeisters who were approached). The marketing method was obviously an international effort.

They were also sold in classified ads, Exchange & Mart, Loot etc (long before online auctions) normally under the brand name "Acoustic" and always with the term "liquid cooling" in the descripton - presumably ferrofluid tweeters were involved.
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Old 23rd Mar 2020, 12:33 am   #1307
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Default Re: The Audiophoolery Thread.

Your memory is better than mine! The speakers my relative had were those exact ones! A '3-way' design, with impressive 'cooling' credentials! I dare say the coils would've needed cooling, if you connected an amp larger than about half a Watt!
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Old 23rd Mar 2020, 2:06 am   #1308
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Default Re: The Audiophoolery Thread.

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Originally Posted by avocollector View Post
Quote:
Originally Posted by Craig Sawyers View Post
Another odd positive side effect of valve rectifiers is the lack of reverse recovery. They therefore do not excite the leakage inductance/interwinding capacitance resonance of the mains transformer.

Regular solid state rectifiers generate reverse recovery transients like a champ, so every time each rectifier diode turns off, a burst of oscillation occurs via the above effect usually in the range 200kHz to 1MHz with a Q of 5-10.

Of course you can use more sophisticated soft recovery diodes, and/or use a C-RC snubber across the secondary winding of the transformer, which kills the oscillatory transient dead if you get the RC in particular correct.

There is a small circuit on DIY audio that enables these values to be determined by direct measurement https://www.diyaudio.com/forums/powe...-test-jig.html

Craig
So does that explain the weird signal display I got when I once fed the ac into a diode and put a scope on the other side - instead of the smooth series of humps like the textbooks show it was completely crazy/all over the show. When I put a load resistor in and the scope probes across that I got the expected series of humps - but I wondered about the other just why.
Surely Schottky diodes would not suffer from this reverse recovery effect either?

Steve.
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Old 23rd Mar 2020, 8:22 am   #1309
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Default Re: The Audiophoolery Thread.

Yes, Schottkys are good in that regard - but even so a properly calculated or measured CRC snubber is good practice.
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Old 23rd Mar 2020, 10:11 am   #1310
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Default Re: The Audiophoolery Thread.

Schottky diodes are somewhat limited in voltage rating, so you can use them if you want for the sort of supply rails that transistor amplifiers use, but you'll have trouble looking for any for use as HT rectifiers in valve equipment, similarly for mains rectifiers in switch-mode power supplies. This is where the ultrafast version of the ordinary diode (P-N junction) come in. Ultra-fast rectifiers reduce but don't entirely eliminate charge storage effects.

Another benefit of sophisticated modern rectifier diodes is that they are usually designed to have a controlled breakdown characteristic, and therefore don't get killed quite so easily by the transients found on mains supplies.

Schottkys are more efficient with their lower forwards voltage drop as well.

Fourier analysis of even a perfectly fast turn-off/turn on at a zero potential difference across a diode has some transient energy, so good diodes don't eliminate the need for filtering and snubbing, but they greatly reduce what you have to do.

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Old 23rd Mar 2020, 2:35 pm   #1311
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Default Re: The Audiophoolery Thread.

EI transformers are much worse than toroidal mains transformers for ringing.

With EI there is a large leakage inductance and small winding capacitance (where the primary and secondary are wound separately in two winding chambers). So for a given resonant frequency, the Q w0L/R (or w0CR) can be ~10-15, so there is a tendency to ring.

With toroidal, the capacitance is high (~10nF) but the leakage inductance is a few uH. So the Q is much lower (typically 2 or 3), and with less tendency to ring. However you can still stimulate the ring using the little circuit in DIYAudio (it is called Quasimodo - the bell ringer!), and very quickly find the optimum snubbing values.

Of course you need to take any extended wiring into account, both from inductance and capacitance reasons.

This is getting to be a decent engineering discussion - and definitely not audiophoolery!
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Old 25th Mar 2020, 7:07 pm   #1312
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Default Re: The Audiophoolery Thread.

Just looking at a period hifi advert for sale on Ebay, above the actual ad that is for sale is a piece of editorial about Monster cables, cutting attached. I'll let you read it in full, but it says it "eliminates phase inaccuracies, as well as aligning high and low frequencies to travel at the same speed for dramatically improved frequency response and three-dimensional imaging". Wow! all that?! And that's not their best cable!

I kinda know my stuff as a qualified electronics engineer and a hifi buff for over 45 years, but seriously, there are guys on here who design hifi and complex electronic equipment and I'd love to hear the views of these true experts please? Maybe they could they pick those statements apart and advise whether there is any substance in them at all, and secondly if there is, is the amount of 'substance' in listening terms effectively equal to the square root of sod all?

Er, I do have my own views on this but I'll keep you guessing..
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Old 25th Mar 2020, 7:14 pm   #1313
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Default Re: The Audiophoolery Thread.

30 foot of cable, at 66% velocity factor (far too slow for twin feeder but it makes the arithmetic easier) the one wavelength frequency is 20MHz, top audio 20kHz. I think it is much less than the square root of sod all.
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Old 25th Mar 2020, 8:06 pm   #1314
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Default Re: The Audiophoolery Thread.

Transmission lines made out of real-world materials exhibit 'dispersion' where the speed of propagation is not exactly equal at all frequencies they can carry.

Consequently, if you follow Monsieur Fourier and consider a complex waveform to be a collection of components at different frequencies, then take into account the differential delays of the components, then do the inverse Fourier transform to get the voltage/time waveform, you'll see the shape has indeed changed or you could just stab it with a scope probe and look at the picture on the screen.

The effect is real, but you have to get up to decently high frequencies, many megahertz for it to become significant on a short length of cable. At audio and the size of a normal house, it's a bit of a joke.

But the main principle of audiophilia is a solid belief that nothing can ever be too small for the most golden ears to be able to detect.

They goofed with the first transatlantic telegraph cable, and made it with the wrong ratio of inductance per metre to capacitance per metre and it distorted the Morse being sent so badly it was garbled out of recognition. The stuffed shirts of the day elected to use more volts, lots more volts, and broke down the cable's insulation. A younger guy, William Thomson worked out what was really wrong and the idea of loading coils was born. A Frenchman suggested a cable with conductors spiralling round an iron core, but the idea of just having lumped inductors every so many miles was found as good, and was a lot cheaper. Land-based talky phones also got loading coils every so many miles to flatten their response. When long distance networks were converted to PCM, the coils came out.... I think they were 88mH toroid lumps and radio amateurs scrounged lots to make audio filters for improving CW reception. The PCM networks exploited the existing lines, but chose to make appropriate impedances to load the wires, rather than mucking about with the wires themselves.

People with no sense of proportion will just latch onto anything that seems to be pointing their way.

Speakers aren't nice tame 8 Ohms, they wander all over the shop, so trying to make a cable to match what they do is a right laugh. Some people tried to make low impedance cables by plaiting lots of fine wires together to make more capacitance per metre. Yeah, fine... but did it make any difference? You bet it did! Certain rather expensive British amplifiers did not like the high-frequency capacitance they now saw loading their terminals and went unstable and went up in smoke. Their designer didn't believe in the value of 'Zobel' networks, whose job is to stave off just this conflagration.

The whole field is filled with comedy.... just as long as you aren't the one paying for the cult cables and cult amplifiers, then you may see it as more tragedy.

David

Mr Thomson's name got changed to Lord Kelvin in his later years. He was a genuinely smart cookie.
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Old 25th Mar 2020, 8:50 pm   #1315
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The whole field is filled with comedy...
To quote a "man of the land" Eee may I larf, I pee mye draws. The American land telegraph used iron wires, did this do anything to make the dispersion less? Market opportunity for iron 'speaker wires I wonder?
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Old 25th Mar 2020, 9:52 pm   #1316
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Default Re: The Audiophoolery Thread.

When I was invoved in the design of the predecessor to Ptarmigan, the filters that were necessary for minimising pulse distortion to ensure reliable transmission of data had to have a linear phase characteristic to provide uniform group delay. The amplitude frequency characteristic was far from uniform. Not that I suppose this is really relevant to audio, where a linear amplitude response is normally desirable.
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Old 25th Mar 2020, 10:37 pm   #1317
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Default Re: The Audiophoolery Thread.

Yes, linear phase types are needed for data transmissions. The obvious ones are Gaussian and Bessel characteristics, but in more recent equipment, the root-raised-cosine filtar has come to the fore. The big idea is to have one root raised cosine filter in the transmitter, shaping the occupied channel so as to not annoy the neighbours, and to have a second one in the receiver acting as the channel filter. Together, their characteristics multiply together, giving a simple rasied cosine shape. This filter, if the bandwidth is chosen appropriately for the symbol rate produces nulls in the time domain waveform right in the centre of the eye diagram for all the other symbols. This maximises eye opening and reduces error rate for the signal to noise ratio (Eb/No)

There once was a politically-incorrect computer programme called the jive translator into which one could feed some ascii text and something came out that was supposed to be funny. Maybe we need an audiophile translator to feed text like I've just written, above, through? It'll just come out with some synonym of 'It's digital so it sounds crap" I suppose.

David
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Old 26th Mar 2020, 2:12 pm   #1318
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Default Re: The Audiophoolery Thread.

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When I was involved in the design of the predecessor to Ptarmigan,
Mallard?

There were bits of it all over the Plessey site in West Leigh looked like it got to B model.

Cheers

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Old 26th Mar 2020, 5:15 pm   #1319
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Default Re: The Audiophoolery Thread.

Yes, Mallard, at Plessey, Vicarage Lane. Interesting days designing compact solid state UHF broadband amplifiers using lumped components instead of the more bulky microstrip designs. Killed off by the defence cuts and the rhree-day weeks of the oil shortage era. Nuff said, o.t.
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Old 30th Mar 2020, 9:46 am   #1320
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Default Re: The Audiophoolery Thread.

I nearly sprayed tea over my monitor when I saw the price of some cassette decks on ebay £2000 plus
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