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Tubes are objectively best! Feedback is bad!, University of New Hampshire MSEE thesis
Axon
post Jun 21 2005, 06:22
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In the course of an excruciatingly long Audio Critic thread on Head-Fi, somebody dug up this.

In it, the author makes some very powerful statements about distortion audibility, and after a review of the literature and quite a few measurements, comes up with a theory to explain why tube amps (single ended triodes in particular) sound so much better than solid state amps to some people, as well as a figure of merit to objectively rate amplifier quality based on harmonic measurements. Based on said figure of merit, the author's hand-built SET amp (using 80-year old tubes shock1.gif) scores very very well, while a generic bipolar class A amp is dragged out and shot.

I must admit the analysis is extremely persuasive. In a nutshell, the author proposes: that extremely high order harmonics are far more audible than previously thought; that existing THD/IMD measurements cannot distinguish between relatively inaudible low-order distortion and high-order distortion, and so are mostly useless; that negative feedback inherently trades off a small amount of low order distortion for lots of high order distortion; and that existing tubes (triodes in particular) generally offer less open-loop distortion than existing solid state designs. Therefore, SETs win. It's really worth reading in detail.

This sort of thing ought to be highly testable - if it's not completely bunk.

However I do take issue with a lot of points of the paper:
  • He is clearly not arguing from an objective position. The first chapter of the paper states that his SET amp is universally considered as subjectively better than a SOTA solid state amp, even though it measures uniformly worse. The rest of the thesis continues off the conclusion that the existing measurements are crap. To put it mildly, He Is Not On Our Side.
  • He is unusually selective with his choice of amplifiers. SETs are arguably the cream of the crop of tube amps (or at least the most expensive), yet to demonstrate his figure of merit he pairs it up against a bipolar amp he doesn't even name by brand or model. In regards to the high-end discrete amp he first listens to, the author declares that the proper way to measure the amp is by castrating its feedback stage and then computing a figure of merit based on the open-loop distortion spectrum. Gee, what a surprise, it's really bad! He devotes most of his thesis on how feedback is objectively bad, yet he doesn't even run the discrete amp through his own test. Inexcusable in my opinion.
Nevertheless, this paper excites me a lot, and I think the harmonic measurements are within the resolving range of a 1212m or even my RME PAD. Anybody else interested in testing this out?

This post has been edited by Axon: Jun 21 2005, 06:28
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jimboelrod
post Jun 21 2005, 06:37
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I play guitar, unfortunately not very well. I remember reading a comparison of guitar amplifiers when I was a kid.

As I remember, the magazine basically gave those amps with the most tubes the highest ratings.

Someone wrote a letter the next month, he worked for an amp manufacturer, and completely tore up the magazines rating system.

I don't remember much of what he said, basically he stated how the amount of tubes was not relevant. He gave an explanation about how the tubes interface with the amp and how this affects the sound.

He was objective, not arguing for any specific company but explaining how the magazine did everything wrong.

This is really interesting now. Tommorow I'm going to read the article you mention.

thanks


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Defsac
post Jun 21 2005, 06:52
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There's no such thing as "objectively best". It could be objectively proven to be more transparent but that does not mean it sounds better.
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Axon
post Jun 21 2005, 07:03
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Mr. Cheever is postulating a psychoacoustic effect that makes low harmonics universally more pleasing, and high harmonics much less pleasing, and gives testable and objective criteria for each. To some extent, he is trying to redefine the objective notion of transparency, or at least neutrality.

Oh yeah, one more major peeve: his figure of merit, TAD, is maximized (highest distortion) for the case of no distortion. Or so it seems...
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RockFan
post Jun 21 2005, 18:45
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QUOTE (Axon @ Jun 20 2005, 09:22 PM)
........... Based on said figure of merit, the author's hand-built SET amp (using 80-year old tubes shock1.gif) scores very very well, while a generic bipolar class A amp is dragged out and shot.


*


laugh.gif Ah, that is funny!

But seriously, I'm glad you're taking the idea seriously.

'Weighting' of the importance of various analogue-domain artifacts is undoubtedly important, and quite possibly does need rethinking.

If one went on measurements alone, then almost all 'properly' designed SS ampilfiers should sound identical (THD figures down in the 2nd and 3rd decimal places etc), but they certainly do not. And of course it would be the valve amps which would be put out of their misery (>5% THD being the norm at rated outputs).

Technics made an extremely powerful and notionally 'perfect' 'high-end' transistor design back in the 70's, and it univerally underwhelmed everyone who listended to it, by all accounts.

R.

edit - reading the PDF right now, and thus far it's rather intriguing.

I can just see all the major audio-electronics manufacturers of the world getting behind a new measurement protocol which rates most of their products as 'poor' in absolute terms. Not.

This post has been edited by RockFan: Jun 21 2005, 19:00
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Woodinville
post Jun 21 2005, 19:44
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You know, I was at a tutorial given by JJ (James D. Johnston), then from AT&T Bell Labs Research. His strongly made point about SNR, ThD, and most any other analytic number was that all of them were "mostly harmless", unless one actually attempted to use those numbers to argue for accuracy. In the hall afterward, he argued rather at length (I was there) with a couple of people who insisted that ThD was a very relevant measure.

He seemed to regard the issue as closed, as he can provide examples of perceptually coded material at 20dB SNR that listeners can not distinguish, under good conditions, from the original, as well as provide test signals that are indistinguishable from the original at 6dB SNR, or other test signals that are distinguishable at 70 or 80dB SNR. In fact, this distinction is related both to the kind of material (noise vs. tones) and the frequency content of the noise vs. that of the signal. If you're interested, it would be better to ask him first hand. I will not take the material he's created.

I would think that anyone who has looked at a masking curve would notice that low-order harmonics are harder to hear than high-order harmonics. After all, the slopes that Manfred Schroeder used in his pioneering work (with Joe Hall and Bishnu Atal) in perceptual coding of speech were 15dB per critical band for upward spread of masking. That puts the masking threshold for a 2nd order distortion mechanism somewhere around the -45dB spot. It also puts the 10th order distortion mechanism at threshold of hearing, without other maskers.

On the other hand, loudness "enhancement" is a well-known effect of low-order distortions, and is one well-known reason why guitar amplifiers use tubes for their soft(er) clipping behavior. I wonder, sometimes, if the same kind of loudness enhancement that provides an excess of percieved dynamic range over actual dynamic range in an LP also provides a percieved (I won't say "false", since this is a question of perception and preference, not absolute measure) increase in "dynamics" for a tube amplifier.

Maybe, just maybe, "sounds flat" is a synonym for less distortion at high levels. At some level, this is speculation, because I don't have any work that I can mention that demonstrates this, but I do believe that it's worth an examination.


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Woodinville
post Jun 21 2005, 19:47
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QUOTE (Axon @ Jun 20 2005, 09:22 PM)
In regards to the high-end discrete amp he first listens to, the author declares that the proper way to measure the amp is by castrating its feedback stage and then computing a figure of merit based on the open-loop distortion spectrum.


Oh. crying.gif

Do I understand this properly? Are you saying that this wonderful gentleman argues that one must first break an amplifier before measuring its performance?
rolleyes.gif rolleyes.gif mad.gif yeahright.gif gun2.gif


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WmAx
post Jun 21 2005, 20:09
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QUOTE (Axon @ Jun 21 2005, 01:22 AM)
I must admit the analysis is extremely persuasive.


I read this paper(a couple of years ago -- I'm not going to tread through the bull again), and it is not persuasive of anything, after you tread through the the issue from all perspectives. Looking at this paper form the most basic elements: using some very old graphs of isolated sine wave signal audiblity for harmonic distortion(s), Cheever attempts coorelate these(inapplicable) graphs with music audiblity, assuming that it's the same. He offers no credible controlled listening tests to verify audibility. This paper is not persuasive to me, but it does appear to be an impressive attempt to distract from reality and reinforce the egos of many audiophiles. Approximately 2% THD, as produced by typical solid state amplification, is the threshold for known audibility, according to the latest efforts of such listening tests(which was over 2 decades ago). Please refer to a real expert on THD audiblity, such as James Moire. He promoted much the same thing as Cheever, back in the 50's and 60's. However, by the late 70's, his conclusion was different, after he had done proper coorelation and listening tests. Refer to this standard reference:

Just Detectable Distortion Levels
James Moire, F.I.E.E.
Wireless World, Feb. 1981, Pages 32-34 and 38

I'll e-mail you scans of the article upon request.

If you are interested in hearing from someone that has conducted many real-world listening tests, under blinded protocols, please refer to and contact Richard Clark at caraudioforum.com. He has conducted many blinded listening tests on audiophiles, music engineers and others; comparing vastly different grades(prices and topologies) of amplifiers. No one has yet demonstrated, conclusively, that they could tell a difference when the frequency response was the same and when no audible noise(s) were present. You can contact Clark directly from that forum. Tom Nousaine(you can find him on the rec.audio.high-end newsgroup) has also conducted blinded tests on amplifiers, using various audiophiles as subjects.

-Chris

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RockFan
post Jun 21 2005, 20:23
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QUOTE (Axon @ Jun 20 2005, 09:22 PM)
[list]
[*]He is clearly not arguing from an objective position. The first chapter of the paper states that his SET amp is universally considered as subjectively better than a SOTA solid state amp, even though it measures uniformly worse. The rest of the thesis continues off the conclusion that the existing measurements are crap. To put it mildly, He Is Not On Our Side.

*


I'm not sure I agree with you here.

Surely he's proposing a measurement protocol - which if implemented is then defacto, and therefore 'objective'?

It could be argued that this protocol is arbitrary, but then is there one that isn't?

Agreed standards in engineering are just that - agreed upon, and essentially (and often actually) arbitrary, but they nonetheless then provide base-line or data to work with.

And also his proposal seems intrinsically more sophisticated, in that it factors rather more into the tests which result in the 'TAD' number, than the THD calculations which we all know and love (?).

Some of his descriptions of the ear's 'masking' throught it's own intrinsic (mechanical and electrical) generation of harmonics are fascinating, and this isn't even in the arcane area of 'psycho-acoustics' - it's a set of effects which appear prior to 'processing' by the brain itself.

I've never really needed convincing that SET's do amplify a music signal 'better' subjectively (!) than other types, despite, as Cheever observes, the poor measurements they turn in using current methodology.

I agree that he's produced a very interesting and worthwhile analysis/synthesis (although I don't think his methodology/protocol, or anything like it, is going to be adopted broadly anytime soon, somehow),


R.
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WmAx
post Jun 21 2005, 20:26
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QUOTE (RockFan @ Jun 21 2005, 01:45 PM)
If one went on measurements alone, then almost all 'properly' designed SS ampilfiers should sound identical (THD figures down in the 2nd and 3rd decimal places etc), but they certainly do not. And of course it would be the valve amps which would be put out of their misery (>5% THD being the norm at rated outputs).


TOS #8 violation? Where are the audiblity studies proving that a 'properly' designed SS amplifier has an audible difference?

-Chris
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seanyseansean
post Jun 21 2005, 20:31
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QUOTE (WmAx @ Jun 21 2005, 08:26 PM)
QUOTE (RockFan @ Jun 21 2005, 01:45 PM)

If one went on measurements alone, then almost all 'properly' designed SS ampilfiers should sound identical (THD figures down in the 2nd and 3rd decimal places etc), but they certainly do not. And of course it would be the valve amps which would be put out of their misery (>5% THD being the norm at rated outputs).


TOS #8 violation? Where are the audiblity studies proving that a 'properly' designed SS amplifier has an audible difference?

-Chris
*



He stated 'on measurements alone'.

You're just another in a long line of idiots shouting TOS violations when you have nothing better to add yourself.
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WmAx
post Jun 21 2005, 20:39
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QUOTE (seanyseansean @ Jun 21 2005, 03:31 PM)
QUOTE (WmAx @ Jun 21 2005, 08:26 PM)
QUOTE (RockFan @ Jun 21 2005, 01:45 PM)

If one went on measurements alone, then almost all 'properly' designed SS ampilfiers should sound identical (THD figures down in the 2nd and 3rd decimal places etc), but they certainly do not. And of course it would be the valve amps which would be put out of their misery (>5% THD being the norm at rated outputs).


TOS #8 violation? Where are the audiblity studies proving that a 'properly' designed SS amplifier has an audible difference?

-Chris
*



He stated 'on measurements alone'.

You're just another in a long line of idiots shouting TOS violations when you have nothing better to add yourself.
*



Actually, I added quite a bit in a prior post. But, you did not read the quote concerning TOS violation properly. Your accusation is unfounded.

As for the TOS, if you don't like it, don't participate. It's one of the primary elements that keeps this forum a rational place to discuss issues.

-Chris

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RockFan
post Jun 21 2005, 20:40
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QUOTE (WmAx @ Jun 21 2005, 11:26 AM)
QUOTE (RockFan @ Jun 21 2005, 01:45 PM)

If one went on measurements alone, then almost all 'properly' designed SS ampilfiers should sound identical (THD figures down in the 2nd and 3rd decimal places etc), but they certainly do not. And of course it would be the valve amps which would be put out of their misery (>5% THD being the norm at rated outputs).


TOS #8 violation? Where are the audiblity studies proving that a 'properly' designed SS amplifier has an audible difference?

-Chris
*



I fully retract the statement - they do all sound the same. Happy?

R.
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seanyseansean
post Jun 21 2005, 20:44
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QUOTE (WmAx @ Jun 21 2005, 08:39 PM)
Actually, I added quite a bit in a prior post. But, you did not read the quote concerning TOS violation properly. Your accusation is unfounded.

As for the TOS, if you don't like it, don't participate. It's one of the primary elements that keeps this forum a rational place to discuss issues.

-Chris
*


Do you not see how 'TOS Violation' just kills a thread? How about asking someone for more information?

You're right I didn't read your response properly, for which I apologise. I don't apologise for getting narky with people that have nothing to add other than stamping 'TOS Violation' on a post.
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RockFan
post Jun 21 2005, 21:14
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QUOTE (RockFan @ Jun 21 2005, 11:40 AM)
QUOTE (WmAx @ Jun 21 2005, 11:26 AM)
QUOTE (RockFan @ Jun 21 2005, 01:45 PM)

If one went on measurements alone, then almost all 'properly' designed SS ampilfiers should sound identical (THD figures down in the 2nd and 3rd decimal places etc), but they certainly do not. And of course it would be the valve amps which would be put out of their misery (>5% THD being the norm at rated outputs).


TOS #8 violation? Where are the audiblity studies proving that a 'properly' designed SS amplifier has an audible difference?

-Chris
*



I fully retract the statement - they do all sound the same. Happy?

R.
*



That is of course fecatious, I can't think of a study which 'proves' it either way.

So I assume you're saying that any statement or remark to the effect that amplifiers which 'measure' the same either do or don't sound the same is a TOS violation in and of itself?

My own personal experience (probably another TOS) is that they do, and it's not difficult to find that one will 'prefer'' one amp to another.

The Creek 4330, for example, achieved it's fame purely on the basis of it's sound, not the numbers in it's specifications or test results, and was unanimously regarded as sounding 'better' than any other amp in it's price-range/class.

I would seriously recommend that if you do intend spending a substantial amount of money on audio hardware that you listen to the item you're thinking of buying, if you get the chance, as well as looking at the numbers or reviews.

R.
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WmAx
post Jun 21 2005, 21:43
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QUOTE (RockFan @ Jun 21 2005, 04:14 PM)
So I assume you're saying that any statement or remark to the effect that amplifiers which 'measure' the same either do or don't sound the same is a TOS violation in and of itself?


Yes. So far as 'sameness', one could only say[accurately] that no credible perceptual research demonstrates[or otherwise gives good reason to suspect] that an unknown variable exists which causes a difference in sound of amplifiers.

QUOTE
My own personal experience (probably another TOS) is that they do, and it's not difficult to find that one will 'prefer'' one amp to another.


I suspect such evaluation(s) were conducted under non-controlled, sighted(or poorly produced blinded) tests, of which no coorelation/verification of measurements were subsequently conducted to ensure proper operation within known human thresholds[of known measurable parameters and perceptual research in relation to said measurements].

QUOTE
I would seriously recommend that if you do intend spending a substantial amount of money on audio hardware that you listen to the item you're thinking of buying, if you get the chance, as well as looking at the numbers or reviews.


Thank you, but I am not currently in need of any advice so far as my audio equipment is concerned.

-Chris
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rjamorim
post Jun 21 2005, 22:40
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QUOTE (seanyseansean @ Jun 21 2005, 04:31 PM)
You're just another in a long line of idiots shouting TOS violations when you have nothing better to add yourself.
*


We have several of those here.


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boojum
post Jun 21 2005, 22:57
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Gas on the fire! OK, this senseless bleating about "Tubes are good; transistors are bad" gets me. Let me re-tell the Bob Carver story. Bob is a savvy audio engineer and amp designer. He brought one of his amps to the idiots at Stereophile and was able to make it sound exactly like any tube amp they could produce. The guys at Stereophile were unable to distinguish between Bob's transistor amp and their revered tube amps. End of story. It is hokum. I suppose that it is just a concidence that tube amps are all but forgotten and surrounded in myth and snake oil. Same with speaker cable, inter-connect cable, speaker stands, and little bits of wood to stick on the wall. The list is endless, as are the suckers to buy the stuff. cool.gif

/rant off.


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RockFan
post Jun 21 2005, 23:40
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QUOTE (boojum @ Jun 21 2005, 01:57 PM)
Gas on the fire!  OK, this senseless bleating about "Tubes are good; transistors are bad" gets me.  Let me re-tell the Bob Carver story.  Bob is a savvy audio engineer and amp designer.  He brought one of his amps to the idiots at Stereophile and was able to make it sound exactly like any tube amp they could produce.  The guys at Stereophile were unable to distinguish between Bob's transistor amp and their revered tube amps.  End of story.  It is hokum.  I suppose that it is just a concidence that tube amps are all but forgotten and surrounded in myth and snake oil.  Same with speaker cable, inter-connect cable, speaker stands, and little bits of wood to stick on the wall.  The list is endless, as are the suckers to buy the stuff.    cool.gif

/rant off.
*


So, what do you listen to music on?

And how much, in your wisdom, do you think someone should spend on an amplifier before any further expense is a waste of money and marks one out as a sucker?

For example, was the Creek 4330 I mentioned too expensive, and an obvious rip-off (500)?

Just so we can nail this down to specifics, rather than idiotic pejoratives.

R.
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KikeG
post Jun 21 2005, 23:48
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If two amplifiers measure the same under same load (speakers), I can't think of any reason why they should sound different. One could debate if measurements accuracy is high enough, which I think it is, by far. In the case we agreed it was, what is left, that could cause a difference... magic?

Edit:

BTW, the cost of an amp is not totally indicative of its quality. I'm quite convinced that many stereo amps around 300 are, for moderate listening levels and non-exotic loads, as good as anything you can get.

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Woodinville
post Jun 21 2005, 23:54
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QUOTE (KikeG @ Jun 21 2005, 02:48 PM)
If two amplifiers measure the same under same load (speakers), I can't think of any reason why they should sound different. One could debate if measurements accuracy is high enough, which I think it is, by far. In the case we agreed it was, what is left, that could cause a difference... magic?
*



Hear, Hear.

If they read the same to -100dB, i.e. if their difference signals are that far down, it's very hard to imagine any kind of difference in audibility that could possibly exist.

One might contrive one, perhaps, but not with the kinds of effects I've ever seen in any decent amplifier.


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Woodinville
post Jun 22 2005, 00:09
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QUOTE (KikeG @ Jun 21 2005, 02:48 PM)
If two amplifiers measure the same under same load (speakers), I can't think of any reason why they should sound different. One could debate if measurements accuracy is high enough, which I think it is, by far. In the case we agreed it was, what is left, that could cause a difference... magic?


I do think that it depends very strongly on what the measurement is. If the measurement is 60dB (which it can be with some amplifiers) it is possible that two different amplifiers with 60dB SNR, which I cheerfully grant you is pretty awful by present standards, it is possible that they sound different, if one amp has only second harmonic distortion and the other very high-order distortion.

In such cases, I have met an amplifier that had very bad crossover distortion, in which you could hear a 1.5 kHz (think ear-canal resonance range) "tweedling" when a low-frequency sine wave was the input. This amp was close to -60dB in SNR, although I don't recall the number well at this point. Note, also, that I am talking about an amplifier that I wouldn't refer to as "good".

For upwards of 90dB SNR, I can't see anything happening, unless, of course, the noise floor is audible due to bad gain structure or something like that, which is a different kind of pathology.


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KikeG
post Jun 22 2005, 00:31
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Well, of course, I'm talking about measurements taken with decent equipment, capable of measuring well below -100 dB. Not a difficult thing even with prosumer equipment today.
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RockFan
post Jun 22 2005, 00:36
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QUOTE (KikeG @ Jun 21 2005, 02:48 PM)
If two amplifiers measure the same under same load (speakers), I can't think of any reason why they should sound different. One could debate if measurements accuracy is high enough, which I think it is, by far. In the case we agreed it was, what is left, that could cause a difference... magic?

*


What exactly is being measured in the first place - and how meaningful these measurements are, was the subject of the paper Axon provided a link to at the top of the thread.

To reiterate, many valve amplifiers create 'THD' measuments which, if produced by transistor amps in their characteristic fashion, would render them completely (and I mean completely) unusable for music reproduction.

Whatever one's views on vacuum-tube amplifiers and the whole-digit THD figures they produce, they are certainly not 'unusable', and this alone should give one pause for thought.

R.
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Jebus
post Jun 22 2005, 00:42
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Slashdot style (didn't RTFA) response:

Tube amps kick ass for playing feedback! Especially with a good ol' '62 Jazzmaster... They distort real good at high gain levels. Solid-state amps just can't pull that off at all. No, I have no idea how to abx that, and yes, I know this is probably not even what the thread is about smile.gif

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