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Topic: Tubes are objectively best! Feedback is bad! (Read 31618 times) previous topic - next topic
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Tubes are objectively best! Feedback is bad!

Reply #50 –
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I'll look to see if any existing music might work tomorrow.
[a href="index.php?act=findpost&pid=308047"][{POST_SNAPBACK}][/a]


Try the intro to most of the Cowboy Junkies pieces on Trinity sessions, where just the bass is playing.
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J. D. (jj) Johnston

Tubes are objectively best! Feedback is bad!

Reply #51 –
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It would seem pretty clear that they don't, so looking for 'threshholds' of audiblity of THD seems futile.
[a href="index.php?act=findpost&pid=308063"][{POST_SNAPBACK}][/a]



Of course not. If you can show that all of the distortion components are below the zero loudness curve, that's very relevant, and not futile at all.  That is, however, harder than what some folks may have expected.

Btw, the crossover distortion example appears to be -71dB down, not -90 something as somebody said. Perhaps each individual component is down -90 something dB.
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J. D. (jj) Johnston

Tubes are objectively best! Feedback is bad!

Reply #52 –
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Btw, the crossover distortion example appears to be -71dB down, not -90 something as somebody said. Perhaps each individual component is down -90 something dB.
[a href="index.php?act=findpost&pid=308140"][{POST_SNAPBACK}][/a]

Correct, each harmonic is -90db. That was a little misleading on my part.

Tubes are objectively best! Feedback is bad!

Reply #53 –
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Btw, the crossover distortion example appears to be -71dB down, not -90 something as somebody said. Perhaps each individual component is down -90 something dB.
[a href="index.php?act=findpost&pid=308140"][{POST_SNAPBACK}][/a]

Correct, each harmonic is -90db. That was a little misleading on my part.
[a href="index.php?act=findpost&pid=308143"][{POST_SNAPBACK}][/a]



What does the total energy in the range of, say, 1400 to 1700 Hz, amount to in terms of total signal energy?  That's the ear-canal resonance range, and the place you're likely to be most sensitive.

btw, you probably know this, but the transform of your chosen signal looks just like a sine wave plus a pulse train at the period of the sine wave, hence explaining the particular frequency content.
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J. D. (jj) Johnston

Tubes are objectively best! Feedback is bad!

Reply #54 –
If you need musical samples with pure low frequency tones, I can provide some :

Sines

Transwave - Helium - Hypnorhythm
Pete Namlook and Ritchie Hawtin - From Within 2 - Future Surfacing (What lies ahead), at 32'10"

Noise :

Lustmord - The Place Where the Black Stars Hang - Sol Om On

...or some other tracks that have the ability to make the right channel of my Senheiser HD600 rattle.

Tubes are objectively best! Feedback is bad!

Reply #55 –
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What does the total energy in the range of, say, 1400 to 1700 Hz, amount to in terms of total signal energy?  That's the ear-canal resonance range, and the place you're likely to be most sensitive.

The energy? Well, assuming a critical band of the ear of about 2khz (2khz-4khz - not really sure where you get 1.4-1.7), the ratio of critical band energy to sine wave energy is about 2.8 * 10^-7.

Quote
btw, you probably know this, but the transform of your chosen signal looks just like a sine wave plus a pulse train at the period of the sine wave, hence explaining the particular frequency content.

Right. When I FFT it, the pulse train turns into a constant harmonic coefficient (just like the fourier transform of a delta function).

Tubes are objectively best! Feedback is bad!

Reply #56 –
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I'd gladly take that, although I'm not sure I'd call this an expectation. More of a guess  Would you take my samples in return?


I'll get ripped in a day or so, and e-mail to you. Yes, please send me samples. If you prefer, I can give you a temp. ftp account to do these file transfers. Contact me via e-mail.

-Chris

Tubes are objectively best! Feedback is bad!

Reply #57 –
Can I just suggest my favourite 'test signal'?

Brass, or often even better, muted brass (because it's often relatively close-miked) -  trombone for lower freq. stuff, trumpet/cornet otherwise.

Just get a load of the rows of harmonics. Cornet apprantly produces significant pressure levels right up in to the 50KHz region.

(Yeah, i know we either can't hear them, or they're band-limited out of the recording, but I would argue they don't cease to exist by virtue of this).

I often put on the CD of Trilok Gurtu's 'The glimpse' as a test recording, but obviously there's no shortage of very well engineered jazz and classical.

R.

Tubes are objectively best! Feedback is bad!

Reply #58 –
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The energy? Well, assuming a critical band of the ear of about 2khz (2khz-4khz - not really sure where you get 1.4-1.7), the ratio of critical band energy to sine wave energy is about 2.8 * 10^-7.
[a href="index.php?act=findpost&pid=308207"][{POST_SNAPBACK}][/a]


Now hold on.  That's an egregious mistake.  Critical bandwidths above 700Hz are very close to 1/3 octave. You've listed an entire octave.


1400 to 1700 is a pretty clean estimate, according to Fletcher, and the subsequent work by Scharf.

In any case, the energy is down about 70dB, or a bit less. That's audible. But a critical band is not an octave, it's more like 1/3 octave. Where did you get this number?
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J. D. (jj) Johnston

Tubes are objectively best! Feedback is bad!

Reply #59 –
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Quote
The energy? Well, assuming a critical band of the ear of about 2khz (2khz-4khz - not really sure where you get 1.4-1.7), the ratio of critical band energy to sine wave energy is about 2.8 * 10^-7.
[a href="index.php?act=findpost&pid=308207"][{POST_SNAPBACK}][/a]


Now hold on.  That's an egregious mistake.  Critical bandwidths above 700Hz are very close to 1/3 octave. You've listed an entire octave.


1400 to 1700 is a pretty clean estimate, according to Fletcher, and the subsequent work by Scharf.

In any case, the energy is down about 70dB, or a bit less. That's audible. But a critical band is not an octave, it's more like 1/3 octave. Where did you get this number?
[a href="index.php?act=findpost&pid=308236"][{POST_SNAPBACK}][/a]

Sorry, I had no idea there was a technical definition to "critical band". I meant it notionally  as the hump on the equal-loudness curve where the ear is most sensitive.

Tubes are objectively best! Feedback is bad!

Reply #60 –
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Quote
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The energy? Well, assuming a critical band of the ear of about 2khz (2khz-4khz - not really sure where you get 1.4-1.7), the ratio of critical band energy to sine wave energy is about 2.8 * 10^-7.
[a href="index.php?act=findpost&pid=308207"][{POST_SNAPBACK}][/a]


Now hold on.  That's an egregious mistake.  Critical bandwidths above 700Hz are very close to 1/3 octave. You've listed an entire octave.


1400 to 1700 is a pretty clean estimate, according to Fletcher, and the subsequent work by Scharf.

In any case, the energy is down about 70dB, or a bit less. That's audible. But a critical band is not an octave, it's more like 1/3 octave. Where did you get this number?
[a href="index.php?act=findpost&pid=308236"][{POST_SNAPBACK}][/a]

Sorry, I had no idea there was a technical definition to "critical band". I meant it notionally  as the hump on the equal-loudness curve where the ear is most sensitive.
[a href="index.php?act=findpost&pid=308244"][{POST_SNAPBACK}][/a]


A critical bandwidth is an approximation to the filter bandwidth at a given spot on the basilar membrane in the organ of Corti.  Much more discussion is possible on this subject.
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J. D. (jj) Johnston

Tubes are objectively best! Feedback is bad!

Reply #61 –
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Can I just suggest my favourite 'test signal'?

Brass, or often even better, muted brass (because it's often relatively close-miked) -  trombone for lower freq. stuff, trumpet/cornet otherwise.

Point taken. However, extremely wideband brass is exactly what I'm trying to avoid - any THD would pile up against the already existing harmonics, which would dominate. Better to have something that is recognizably detailed yet involves few harmonics.

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A critical bandwidth is an approximation to the filter bandwidth at a given spot on the basilar membrane in the organ of Corti. Much more discussion is possible on this subject. smile.gif

Yeah, um... This would be a good time to point out that I have no formal training nor reference material on psychoacoustics whatsoever. Suggestions welcome.

Tubes are objectively best! Feedback is bad!

Reply #62 –
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Point taken. However, extremely wideband brass is exactly what I'm trying to avoid - any THD would pile up against the already existing harmonics, which would dominate. Better to have something that is recognizably detailed yet involves few harmonics.

Which is why I suggest one of the Cowboy Junkies tracks taht starts with string bass and nothing else, off of "Trinity Sessions".
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Yeah, um... This would be a good time to point out that I have no formal training nor reference material on psychoacoustics whatsoever. Suggestions welcome.
[a href="index.php?act=findpost&pid=308288"][{POST_SNAPBACK}][/a]


Brian C. J. Moore, something like "The psychology of hearing". It's not in my office or I'd be more specific. It discusses the phenominology.

William Yost - something like the "Physiology of Hearing".  Ditto, but it discusses the actual mechanisms, although I have some qualms with some of the details.
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J. D. (jj) Johnston

Tubes are objectively best! Feedback is bad!

Reply #63 –
Guys,
When you see hot arguments about tubes vs. solid state amps, don't omit noticing that there is alot of miscommunication. For example, and thats very important, all amps are measured for their specifications into *dummy* load, which is purely resistive. Similar specs into such load has *nothing* to do with how different amps behave when faced with "difficult" real loads - complex reactive impedance of loudspeakers.

Whenever you get into "sounds best" arguments, you really ought to look into how the amp-speaker interface affects the operation of the amp. And that drags you away from static THD measurements as relevant but not sufficient, and to something that quantifies response to musical transients. Afaik there is no formal methodology to do that, and we are forced back to comparing apples to avocados.
There are some logical arguments in favour of no-feedback amps, but its far from being clear-cut to warrant even a hint of such topic name.
It really really did sound different. Not in a placebo way.

Tubes are objectively best! Feedback is bad!

Reply #64 –
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If exactly the same signal reaches a loudspeaker from two different amplifiers (as best as we can measure), they'll sound the same.
Wrong. The amps damping factor has direct influence on the speakers behaviour at resonance frequency. Dont underestimate the direct corellation speaker <--> amp in this discussion. I found that tube amps will sound 'better' in many cases because they have a lower damping factor, leading to more bass  ...

Christian

Tubes are objectively best! Feedback is bad!

Reply #65 –
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I found that tube amps will sound 'better' in many cases because they have a lower damping factor, leading to more bass  ...

Christian
[a href="index.php?act=findpost&pid=311267"][{POST_SNAPBACK}][/a]


You are wrong.
With low damping factor you obtain a non linear frequency response; this is not high fidelity, this is only coloration added to the sound.
High Fidelity is to reproduce the original music signal as faithfully as this is possible; and tubes amps are poor in this, because they add a lot of harmonic distortion, and they have (as already I mentioned) low damping factor that it produce a non linear frequency response in the output.

In contrast, a well designed transistor amplifier have very low harmonic distortion and high damping factor, and, a high damping factor produces a linear frequency response; this is in reality the concept of high fidelity: a linear frequency response with very low harmonic distortion.

Many stupid audiophiles, that lack of any knowledge of basic audio physics and as this works; they say frequently that tubes amps sounds "supposedly" better, when in fact they make clear their preference by a sound loaded of coloration that it is very far from being a High Fidelity Sound.

BTW, the author of this thesis is a complete lunatic. The negative feedback has been used by engineers by decades, and it is a very useful design tool that reduces dramatically the harmonic distortion and improves (increases) the stability and damping factor of any amplifier; in other words, if you want design a high fidelity audio amp you obligatorily needs to use the negative feedback law of the electronics.

For me is very clear that the author of this thesis wants to disinform to the people, I don't know the reason of this; but particularly, for me is very strange that nobody has realized it.

Tubes are objectively best! Feedback is bad!

Reply #66 –
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If exactly the same signal reaches a loudspeaker from two different amplifiers (as best as we can measure), they'll sound the same.
Wrong. The amps damping factor has direct influence on the speakers behaviour at resonance frequency. Dont underestimate the direct corellation speaker <--> amp in this discussion.[a href="index.php?act=findpost&pid=311267"][{POST_SNAPBACK}][/a]


I don't. In the hypothetical comparison you describe, the signal at the loudspeaker terminals is measurably different for the two different amps.

I said "If exactly the same signal reaches a loudspeaker from two different amplifiers (as best as we can measure), they'll sound the same" - in your example, the signal is not the same - it will be measurably different, therefore it should come as no surprise that it can sound different.

I am, of course, stating the obvious. But many audiophiles would implicitly argue against the point one way or another.

Cheers,
David.

Tubes are objectively best! Feedback is bad!

Reply #67 –
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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 ) 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?
[a href="index.php?act=findpost&pid=307719"][{POST_SNAPBACK}][/a]


Interesting article - unfortunately it's littered with subjective opinions, half-truths and glaring errors. But hey, we all know that if you paid $15k for an amp it would HAVE to be good, don't we? :-P

To say that a triode is the most linear device is questionable at best, but to state that output transformers are perfectly linear is blatant hooey. As for negative feedback being a Bad Thing, does he imagine someone might not have noticed this before if it were true?

Seems to me that the main thrust of the article is not that our current measurement systems are wrong (which might be plausible) but that his SET amplifier is wonderful and all those years of progress are wasted...entertaining idea that one!

Tubes are objectively best! Feedback is bad!

Reply #68 –
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If exactly the same signal reaches a loudspeaker from two different amplifiers (as best as we can measure), they'll sound the same.
Wrong. The amps damping factor has direct influence on the speakers behaviour at resonance frequency. Dont underestimate the direct corellation speaker <--> amp in this discussion. I found that tube amps will sound 'better' in many cases because they have a lower damping factor, leading to more bass  ...

Christian
[a href="index.php?act=findpost&pid=311267"][{POST_SNAPBACK}][/a]


Uuunnnhhh. No.

If amp is controlling the speakers differently, YOU WILL SEE A DIFFERENT SIGNAL AT THE LOUDSPEAKER INPUT and AMPLIFIER OUTPUT.

Now, certainly changing damping factor can (often not as much as people imagine) change the speaker's performance, but one certainly can, and shall, if one tries, see this at the amplifier output in terms of the signal there being different.
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J. D. (jj) Johnston

Tubes are objectively best! Feedback is bad!

Reply #69 –
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As for negative feedback being a Bad Thing, does he imagine someone might not have noticed this before if it were true?[a href="index.php?act=findpost&pid=311404"][{POST_SNAPBACK}][/a]


One might as well point out that one of the reasons that triodes are fairly linear is that they have a lot of internal, built-in feedback, via the actual field equations that govern their operation.

It is true, if you actually look at how a triode works, that they have built-in feedback. This, alone, I think, puts a serious crimp in the whole work.
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J. D. (jj) Johnston

Tubes are objectively best! Feedback is bad!

Reply #70 –
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As for negative feedback being a Bad Thing, does he imagine someone might not have noticed this before if it were true?[a href="index.php?act=findpost&pid=311404"][{POST_SNAPBACK}][/a]


One might as well point out that one of the reasons that triodes are fairly linear is that they have a lot of internal, built-in feedback, via the actual field equations that govern their operation.

It is true, if you actually look at how a triode works, that they have built-in feedback. This, alone, I think, puts a serious crimp in the whole work.
[a href="index.php?act=findpost&pid=311453"][{POST_SNAPBACK}][/a]


In the overall scheme of things, that's one of the more minor inaccuracies. His SET amp has frequency-dependent feedback via the cathode leg anyway.

Tubes are objectively best! Feedback is bad!

Reply #71 –
Richard Pierce DAMPING FACTOR
It really really did sound different. Not in a placebo way.

Tubes are objectively best! Feedback is bad!

Reply #72 –
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In the overall scheme of things, that's one of the more minor inaccuracies. His SET amp has frequency-dependent feedback via the cathode leg anyway.
[a href="index.php?act=findpost&pid=311603"][{POST_SNAPBACK}][/a]


It's got a cathode resistor and he's complaining about feedback!?

Oh well.
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J. D. (jj) Johnston

Tubes are objectively best! Feedback is bad!

Reply #73 –
That makes me wonder what a tube amp without any feedback would sound or evel look like. I guess you'd have to use a pentode instead of a triode? And it would behave more like a single-ended transistor, with a highly quadratic response?

Tubes are objectively best! Feedback is bad!

Reply #74 –
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That makes me wonder what a tube amp without any feedback would sound or evel look like. I guess you'd have to use a pentode instead of a triode? And it would behave more like a single-ended transistor, with a highly quadratic response?
[a href="index.php?act=findpost&pid=311669"][{POST_SNAPBACK}][/a]


In order to have any hope at voltage linearity at all, you'd have to use a triode.

Pentodes are basically current devices, not voltage devices. The "screen grid" gets rid of most of the internal feedback that is imposed by the physics of a triode, and raises the output impedence by a great lot.

Now, solid state devices have an exponential response, at least in terms of base-emitter-voltage to current at the collector.
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J. D. (jj) Johnston