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RE: Vacuum tube amplifiers as a measuring stick for modern, high current, low feedback SS amplifiers?

Posted by morricab on October 11, 2010 at 10:24:40:
"How can you compare the performance you get out of modern, ultra-fast bipolar/FET devices to what you get out of an output transformer, esp. SE? Not to mention the sound?"

This depends on what you define as performance. I define it as how the amp sounds not how it measures because, as I am sure you are aware, there is not a good correlation between percevied sound quality and test measurements like THD. It seems that how linearity is achieved has as much or more of an impact than the raw number in terms of % THD.

"I like my frequency response flat, not looking like a bell curve, but that's just me"

As long as your damping factor is more than 5 there will not be a large amplitude swing in the FR curve. The flatness of most speakers will be far worse.

"I realize alot of people like the euphonic 2nd-order harmonic distortion"

Oh this old chestnut. Well seeing as it has been demonstrated that it takes at least 1% of 2nd order distortion to be readily audible I don't think that there is much in the way of "Euphonizing" the sound. The real problem is that we are very sensitive to high order harmonics that are present in high feedback and Class AB designs. These destroy the sound, whereas little old 2nd order is essentially sonically invisible.

The real problem with many tube amps is the output transformer but not at the high frequencies (A good one is quite linear up to 20Khz). The real problem is at low frequencies and core saturation. This is one of the main reasons for "tube" sound. If you have ever heard good OTL amps you will note that they sound distinctly non-tubey in that golden way but also distinctly non-transistory.

"However, that's not what we're discussing here"

What exactly are we discussing then if not perceived sound quality as THE benchmark for amplifier performance. Clearly the numbers, at least on the surface, don't support subjective findings. See papers by Geddes and CHeever and you will see what I mean.