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Re: On a different note: Response to Morricab on Amplifiers

Posted by morricab (S) on February 8, 2007 at 07:12:49

In Reply to: On a different note: Response to Morricab on Amplifiers posted by theaudiohobby on February 8, 2007 at 05:07:29:

Well, until I can read the whole article and see exactly what they are talking about I have no comment. FWIW, I have never said that SET is the only valid amplfication choice. If they are poorly done then they can sound bad or at least overly colored (I have heard many like this). I don't like the sound of coloration, which is why I champion ribbons and electrostatic speakers because overall they seem to be lower in coloration than most of other speaker types. I know of at least one or two hybrids that give fantastic sound and are complementary designs but with no feedback. I also know of a couple of push/pull Class A triode amps that sound fantastic...also with no negative feedback.

"The resulting alteration may sometimes be pleasing, but is it fidelity"

Keith Howard addressed this in an article in stereophile where he digitally added distortion with software he developed. He found that no additional distortion was more "euphonic" than the original unadultered recording. He did find that high and odd order harmonics added to the recording were even LESS pleasing than a regular decreasing progression (and no high order) or only even harmonics. So much for the myth of euphonic distortion. There is only distortion that is audible and distortion that is inaudible. If it is inaudible then it really doesn't matter how high it is, does it? Only to the point it becomes audible then it matters.

If we then assume that all audible distortion is bad then the quest is to finally and comprehensively determine which distortion and how much is audible to humans and not to an oscilloscope, which is useful to verify the levels but not to set them.

http://stereophile.com/reference/406howard/

Here are some interesting quotes from this article:

"The harmonic amplitudes specified in the table may seem surprisingly high, but the first surprise awaiting you when you experiment with distortion synthesis is just how large the distortion must be to become audible."

"Pattern 1 represents an ideal Hiraga pattern of declining harmonic amplitudes, albeit at high level. Pattern 2 mimics Pattern 1 but with all the harmonics (down to the self-imposed –100dB limit) at 20dB lower amplitude. Patterns 3 and 4 comprise, respectively, the odd- and even-order harmonics only of Pattern 1. THD, Shorter-weighted THD, and GedLee distortion metric (footnotes 5, 6) (Gm) figures are given for all three patterns. "

What does he say about the sound of these added patterns?

"But after a number of listening sessions—kept short to keep my ears fresh—over both headphones and loudspeakers, I did feel that I could detect differences. I found two of the recordings—of the violin and the harpsichord—particularly insightful, I surmise because both instruments have a rich enough harmonic structure to generate significant numbers of intermodulation products."

"The most important finding was that none of the different patterns of nonlinearity sounded in any way preferable to the undistorted reference. They all sounded worse, albeit in different ways."

"Pattern 1 added a distinct "dirtiness" to the sound that was not unpleasant but did change the instrumental timbre and diminish the sound's sense of fidelity—there was something clouding the sound. Pattern 2 was much better, with the closest sound to the undistorted reference. But I thought I could also detect it just beginning to muddy the presentation. Pattern 3 was unpleasant, adding an edge to the sound that would surely become fatiguing over extended listening. Pattern 4 wasn't as bad, but there was still something unnatural about it. Although Patterns 3 and 4 both introduce less distortion than Pattern 1, it was Pattern 1 that proved less subjectively annoying. While I could hear its effect, it did not threaten to send me screaming from the room after 10 minutes' listening. "

"There is a little support in these findings for Hiraga's thesis, in that Pattern 1—with its full complement of even- and odd-order harmonics—indeed proved preferable to the denuded harmonic Patterns 3 and 4."

So in the end he found all distortion bad but some clearly less offensive than others. I for one subscribe to this hypothesis. Now if you assume that most amps are running at very low power for most listening (say 1 or 2 watts) then most measurements that make SETs and other no feedback amps look bad are overestimating the distortion they produce at normal listening levels. For example Soundstage tends to measure SETS near their peak power (around 10 watts for most of them). One look at the THD vs. power tells you this is not anywhere close to their best performance. A high feedback amp usually measures best just before clipping. Comparing them both at 1 watt or 100 mW gives a quite different picture.

Here are some measurements from the KR audio VA340 hybrid SET amp. It has no feedback and is Class A
http://www.vi-fi.nl/assets/s2dmain.html?http://www.vi-fi.nl/xtrartikelen/testen/100000982d090451e/index.html

Notice first of all that it has a bandwidth of over 30Khz and that at 1 watt it has basically only to 5th harmonic distortion (2nd at -50 db, 3rd at -75 db, 4th at -95 db, and 5 at -108db and a tiny 7th at -120 db). At 10 watts it is worse with harmonics out to 15th but still overall not bad as above the 6th harmonic they are all below -110 db). At 1 watt the pattern closely resembles the pattern that Keith Howard found to be the least annoying in his test, pattern 2.

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