How close to the real thing?

Irvrobinson, in a nutshell, frequency response variation is not why tubes sound different! I've heard that idea expressed before, but its hard to find real world examples so I have to chalk it up to mythology. This is easily proven by using a speaker with a flat impedance curve.

The fact of the matter is that the ear interprets non-clipping harmonic distortion as tonality. With tubes, quite often we see a great deal of lower ordered harmonics, which the ear hears as warmth or 'bloom', IOW because the lower orders are seen by the ear as musical, humans are more tolerant of their presence although such will mask detail.

In the case of transistor amplifier audible distortions we are indeed talking about -70, -80 db phenomena. General Electric proved how sensitive humans are to odd orders back in the mid 60s- its not like this is rocket science, but OTOH if you don't know about this quality of human hearing there may be nothing I can say.

So I think quoting research references just clouds your argument.

So when making a point of fact, one should never point to basic long-standing research?? Don't confuse the situation with facts??

However, it is readily proven, and here is how it is done. Take any amplifier and speaker, you will also need a VU meter and a sine/square wave generator. This is very simple test equipment. Set the sine wave to 1KHz 0VU into the speaker.

Now cover the meter and turn the signal all the way down. Set to square wave (odd ordered harmonics). Turn up the volume until you perceive the same sound pressure. Uncover the meter. You will find that it is reading between 20 and 30 db less.

The human ear is very sensitive to odd orders because it uses them to tell how loud sounds are. To claim that we cannot hear something that is 70 db down is beside the point- we are not talking about something that is being masked. I think you must be thinking that these harmonic distortions are somehow going to always be buried and they are not. This is one of the most basic rules of human hearing.

Now negative feedback is well-known to inject odd ordered harmonics into the output of the amplifier although at low level. Its easy to hear too- but best done on an amplifier that is functional operating open-loop (transistor amplifiers that meet that requirement are rare but they do exist). Norman Crowhurst pointed out in the mid-50s that the addition of negative feedback injects harmonics up to the 81st into the output of the amplifier- this stuff is not imagined by any stretch. Take a look at Nelson Pass' article on distortion:

Audio, Distortion and Feedback at
http://www.passdiy.com/projects.htm

if you have trouble believing that I do my homework. Nelson Pass is one of the leading designers alive today.

Chaos Theory says that an amplifier that has loop feedback is an example of a chaotic system that exhibits several stable states. Interestingly and not by coincidence, the formulae you see to express feedback in an amplifier are *identical* to the classic formulae for basic chaotic systems.

Now Chaos uses the term 'bifurcation' to refer to distortion and what it says confirms what Norman Crowhurst pointed out decades earlier, that the addition of feedback will destabilize the amplifier and inject low level harmonic and inharmonic noise (the inharmonic noise is the result of intermodulations at the feedback node).

The way this happens is that the amplifier has a time period, called propagation delay, which is a finite time in which it takes the input signal to propagate to the output. It is nowhere near the speed of light! In fact it is so slow that at high frequencies by the time the feedback gets back to the input of the amp to do its work, the input signal will be seen to have changed. For this reason the negative feedback is always lagging behind and so is unable to correct the signal it was supposed to. As frequency goes up, the phenomena becomes more pronounced.

With steady-state signals the damage is not too severe, but with a constantly-changing waveform (music) the resulting distortion is much higher than -70 db.

Now most transistor amplifiers are push-pull and so they have even-ordered harmonic cancellation at the output. If the amplifier is balanced throughout (and many of them are) then this even ordered cancellation will occur throughout the amplifier (we do this in our amplifiers for the same reason). So really, the main distortion components of a transistor amplifier are going to be almost entirely odd orders!

Now you may think I am a big tube proponent, but if that is the case you may not have read some of my earlier comments. It is *easier* to make tubes sound more like music because it is easier to build tube amps that work without feedback. But go back and look at what I said about transistor amps without feedback.

12-17-10, Irvrobinson: Your explanation above is such nonsense that I can't help but reply.... And then concluding that all of this mumbo-jumbo you're discussing indeed describes why solid-state amps sound harsh and lack low-level detail is just so much malarkey. Solid-state amps have neither of these characteristics you claim they do just for promoting your tube products.

Irv, I would suggest that it would not be too difficult to find more appropriate targets for this kind of insulting language than someone who has devoted a career to studying these matters, and who for many years has designed and manufactured some of the world's finest and most respected amplifiers.

Disclaimer: I have no affiliation with Atmasphere or his products, not even as a user.

Regards,
-- Al

Agree and not just about Irv's response to Atmasphere , but for everyone in general no need to make it personal when in disagreement.

That aside It must be said Ralph did peg SS a bit in his response before pulling out the encyclopedia and totally forgot to mention Tube fog and distortion ..... :) :) :)

I think the most interesting aspect to the original poster's question is the fact that it forms the logical basis to objectively improve our art - that is, "Does it sound real?" is actually a falsifiable question. The answer is definitely highly dependent on an unfathomably large number of personal differences in perceptional acuity . . . but one can always ask oneself this question and come up with a relevant answer. So the main thing we *should* be striving for in the technical studies of audio (i.e. measurement and controlled listening tests) is repeatability.

For reading on some of the logical basis of falsifiability, I'd recommend starting with the work of Karl Popper, or at least his Wikipedia page:
http://en.wikipedia.org/wiki/Karl_Popper
http://en.wikipedia.org/wiki/Falsifiability

Under the logic of falsifiability, it takes just ONE instance of having experienced reproduced sound as being real . . . to validate the conditional plausibility of all the technical and methodological basis for its reproduction. Not that the reverse isn't true as well, but if "reproduction" as an abstract is the goal, then at least for me its occurrences are much more noteworthy than its absences.

Here's the problem I have with setting up camp in any of audio's little belief-systems (i.e. tubes/transistors, cones/panels, feedback/no-feedback, analog/digital, etc.). If I'm truly honest with myself, I have at times been dumbfounded by amazing experiences from really crappy stuff, and also underwhelmed by my experiences with some really beautiful machinery.

And since audio is really a trivial pursuit (no lives at stake, no big "origins of the universe" questions), I view the the craft of audio thusly (my quote):

To examine a wide variety of data from both measurement equipment and listening experiences, and establish correlations that logically fit as much of the data as possible . . . then assume causality based on well-established principles of physical science.

This is why I think audio is so interesting in general -- we get to explore a truly genuine frontier, consisting of abstract and profound questions of the limits of human perception in sound and music. We also get to explore many aspects of human psychology in the listening process, and sociology in the marketplace and hobbyist communities. And really, the assemblages metal, wood, plastic, paper, and whatnot that we fashion to this pursuit are not really so profound . . . after all, it's all going to be in a landfill in a couple hundered years at the very most.

Atmasphere, here's the main reason why I'm so routinely perplexed by your dogged determination to proselytize the whole anti-feedback thing . . . What do you do when you have an amazing experience listening to music through a system, and THEN learn that it's a solid-state amplifier using a pile of global NFB? Because it's happened for me often enough that I can't imagine it hasn't happened at least a few times to anybody that's made a career in audio.

Well, the whole negative feedback issue has been beaten to death pretty thoroughly already in other threads.

My conclusion is that it is just one of many design and execution factors that affect results depending on how well it is executed.

Having heard a lot of gear over the years, my ears tell me that good engineers working for companies making good products have a much better handle on all such things that matter these days than they did 30 years ago.