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Re: Where did you get your electrical engineering degree?

I have a Ph.D in analytical Chemistry. If you want to see the instrument I helped to design and build during my graduate school experience look here: http://www.tsi.com/documents/3800SeriesPN1933798RevD.pdf
in the references at the end you will find my name on two or three papers regarding design and usage for experiments (B.D. Morrical)


"Harmonic distortion of typical solid state amplifiers remains extremely low right across the audible frequency spectrum"

Think so? Look here:
http://www.soundstagemagazine.com/measurements/classe_ca2200/

The amps distortion at HF increases by approximately 50 times (figure 3).

http://www.soundstagemagazine.com/measurements/accustic_arts_amp_iiac_high_performance/
Nearly 100 times increase.

http://www.soundstagemagazine.com/measurements/threshold_s5000e/
about 10 times

http://www.soundstagemagazine.com/measurements/anthem_statement_p2/
also 10 times

These are the latest in modern amp design and they all have sharp HF distortion increases when feedback stops working.

Read Cheever's thesis.

Now on to the subject at hand. THD is NOT the only important parameter in distortion. The harmonic spectrum and distribution of those harmonics affects what we hear and what we don't hear. The brain recognizes unnatural patterns, soundmind. This is why high order harmonic distortion is so unappealing, your ear doesn't make it except under extremely high SPL levels.

"High damping factors are not needed at high frequencies."

The drop in damping factor is merely a symptom of the bigger problem, rising THD and the failure of feedback. And not just any THD but High frequency THD. It is a sign that the feedback is not working at these frequencies (you knew that feedback doesn't work over the whole audible range, didn't you?). It is therefore, a little window into how linear the amp is more or less open loop. What do we see with most feedback amps?? Increase in distortion 10-100 times what it is with feedback.

There is another problem with the feedback not being effective at High frequencies, phase shift and a change in dynamic behavior. I can illustrate the dynamic behavior with a speaker example: I used to own the Infinity IRS Beta system. This system had a servo feedback loop for the bass towers that was very effective in extending the bass down to 15 Hz and gave a flat extremely tight sound.

Now this system would also work without the servo control just fine. You sacrificed a bit of bass depth and control but it also changed the dynamics of the system. Even though it now sounded a bit less tight (still like a good sealed box system) it also sounded more alive and free breathing, ie. more dynamic. Now what happens when you have this heavily controlled sound with a free breathing, albeit more highly distorted sound? Discontinuity. In practice you hear this all the time about SS amps. "Great bass but the highs are bright and grainy." Now I have heard SS amps that don't sound this way (but many more that do) and all of them used very low amounts of negative feedback and NO loop feedback.

As to transformers, well I don't have any in my signal path at the moment. My amp is hybrid and the rest of the system is tubed (including the phonostage but excluding the DAC). If I go to a full tube amp again (I find the zero feedback hybrid to be more neutral than most pure tube amps I have heard and yet much more tonally correct than any of the pure SS amps I have heard) it will most likely be OTL (the best of which are the best amps I have heard). I know some designers though that would rather have a transformer in the signal path than capacitors, claiming their effects are less audible. I agree that no transformer is better when possible.

What you don't seem to get very well is that the electronics are in the service of humans and not the other way around. The stuff is meant to be listened to not measured. Now the scientist in me would like both, if possible and generally when they are both in accord the sound is very good indeed. But not always.

However, if it sounds good, and I mean long term listening good, then if basic measurements say its bad, so what? Either I can't hear well and it doesn't matter anyway, or I hear very well and the measurements are inadequate to capture what is relevant to good sound. You cannot ignore psychoacoustics in this regard.

If a design measures bad but all the problems are in areas where humans either don't hear well or are masked by our own hearing mechanisms then that design will likely sound good. On the other hand, if the amp that measures nearly perfect has all of its (much smaller) problems right where the human hearing is most sensitive and they will then be laid bare. Think this is fantasy? This is the current state of affairs in high end audio. I will never dispute with you that SS amps measure better but are these measurements well correlated with sound quality? I don't think they are terribly well correlated.

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