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In Reply to: RE: Listen for yourself posted by Tre' on July 07, 2017 at 11:55:13
"I believe it's not the (relatively) high amounts of 2nd HD that make them "sound so good", it's the lack of 5th, 7th and 9th.
I believe (but can not prove) that the smallest amount (maybe even an amount that can't be measured) of 7th and it's game over. The electronics sound like electronics instead of music."
You may be right that it is a lack of higher odd-harmonic distortions that is responsible. (Is this actually a property of an SET, by the way? I can see why it would tend to have more even-harmonic distortion than a PP amplifier, but is there a reason why it would have less higher odd-harmonic distortion? I'm not doubting, just asking.)
However, what about the suggestion that the higher odd-harmonics below the threshold of measurability, could be responsible? A modern state-of-the-art spectrum analyser is amazingly sensitive. Is there really any reason to suspect that amounts below the threshold could have audible effects? Or is this an example of the very human desire (in some humans at least) to believe that there are things that lie beyond our understanding? If it could be demonstrated in convincing experiments that such effects were occurring, I would be only too happy to accept them. But I wonder what evidence there might be?
As a matter of practicality, I would have thought that any real-world amplifier in existence today would have 5, 7 and 9th order distortions that are actually within the range of measurability by present-day spectrum analysers, and so speculation about the possibility of such distortions below the measurable threshold would be, at this stage at least, academic. (Again, if I am wrong about this, I would be happy to be corrected.)
Edits: 07/07/17Follow Ups:
It has been noted that the use of negative feedback can reduce second harmonic distortion, however Dr. Earl Geddes (among others) has noted that added second harmonic distortion is relatively difficult to hear compared to other types of distortion. The second harmonic is an octave above the fundamental and harmonizes with the fundamental which makes it difficult to separate in our perception from the fundamental. While negative feedback reduces the second harmonic, it can actually increase higher odd order harmonic distortion. As the second harmonic is the biggest number, reducing it makes the total harmonic distortion number look better at the expense of increasing higher order odd harmonic content which has been shown to be perceived as unmusical. But you don't have to believe me on this, Jean Hiraga wrote several articles examining this subject going back into the late 70's and 80's which we didn't get here in the US unfortunately until the oughties when they were reprinted in AudioXpress. See the link here for the Amplifier Musicality article which was followed by rebuttals from two experts, and which was followed by a rebuttal by Hiraga with no rebuttal to Hiraga's rebuttal.
I'm pressed for time right now but a DHT is very linear. If loaded and driven correctly, and without NFB, there is not much of a mechanism for producing upper ordered HD.
"Norman Crowhurst wrote a fascinating analysis of feedback multiplying the order of harmonics, which has been reprinted in Glass Audio, Vol 7-6, pp. 20 through 30. Mr. Crowhurst starts with one tube generating only 2nd harmonic, adds a second tube in series (resulting in 2nd, 3rd, and 4th), and then makes the whole thing push-pull (resulting in 3rd, 5th, 7th, and 9th), and last but not least, adds feedback to the circuit, which creates a series of harmonics out to the 81st. All of this complexity arises from theoretically-perfect tubes that only create pure 2nd harmonics!"
I believe there are studies showing that humans can hear well down into the noise floor.
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