Since I was not invited (for good reasons as you will see) to the seminar I'll just make this post-
It should not be a debate. If you have a tube amp that 'measures poorly' yet seems to sound just fine (as has often been seen in the pages of Stereophile) and an amplifier that measures just fine and is really 'neutral' but not particularly musical then you have two amplifiers that sit at the opposite spectrum of the same problem, which is distortion and what to do about it.
In this regard with **all** amplifiers the sound of them is all about how they distort.
In this regard each has chosen a different path. On the one hand, a tube amp that 'measures poorly' is probably not running any feedback to suppress distortion, and one might want to know why not? The answer is that feedback adds distortion of its own, and what it adds is highly audible in the form of higher ordered harmonics (at low levels) and IMD. The human ear is keenly sensitive to both types! OTOH, the ear **isn't sensitive to the lower orders (2nd, 3rd and 4th), which is why that tube amp measured so poorly as it probably has much more of these in exchange for keeping the higher ordered stuff inaudible.
The solid state amp employs feedback as it has to to prevent it latching to the power supply rails (which would cause it to have a lot in common with a rock) and to allow it to be linear. So it inherently is much lower distortion, but now most of that distortion is of the form to which the ear is keenly sensitive, and comes off as brightness and harshness, since the ear converts all forms of distortion into tonality.
Again this should not be a debate! The common problem here is that neither amplifier has enough gain bandwidth product and so is a set of compromises. Tube amps, unless OTL (Output TransformerLess) usually have poor gain bandwidth product. Solid state amps usually do much better in this regard. But neither has enough- and this is what that looks like: without enough gain, you can't apply enough feedback so that the application of feedback allows the amp to correct not just for simple distortion but also the distortion caused by the application of feedback! Did you get that? Its sounds recursive because it is. Feedback can correct for itself if **enough** is applied. That value seems to be a minimum of about 35dB and 40dB is better.
But that means that the amp has to have a lot of gain so that once you blow 35dB or more away you still have enough to work with the preamp and speaker, so the open loop (no feedback) value should be at least 60dB!
Now we come to the other bit of that gain bandwidth product thing: bandwidth. Sure you can make a tube or solid state amp with that much gain, but when you run feedback around it there is an enormous possibility that it will oscillate. The reason is that when there is gain there is also phase shift caused by limited bandwidth- the more gain you add, the more the bandwidth is compromised. The phase shift can thus cause negative feedback to become positive feedback at some high frequency- and then it oscillates. A related idea in amplifier design is that of 'phase margin' which is to say that if by the application of feedback the amp oscillates, it has insufficient phase margin. OTOH phase shift causes the feedback to become positive rather than negative.
So one way to deal with this is avoid feedback. You avoid brightness and harshness because the lower ordered harmonics mask the higher ordered harmonics. But the amp will measure poorly unless extreme steps are taken to suppress it by other means.
The other way is to stare down the gun barrels and add the feedback- now it 'measures' well but it has a brighter and harsher presentation than real life.
IOW **neither** amp is right! So this shouldn't be a debate. Instead we should be looking at the measurement standards- its obvious that since the ear is insensitive to lower ordered harmonics that they should not have the same value in the measurement as higher orders; IOW the various harmonics should be weighted. But in addition, the 2nd and 3rd in particular can mask the presence of the higher orders, so those harmonics should be in attendance in order to allow for something that sounds like music instead of electronics. But our current measurement regime takes little of that into account. In a nutshell, in this regard one of the most important aspects of human hearing (the fact that the ear senses higher ordered harmonics to gauge sound pressure) is ignored in order for the spec sheets to look 'nice'.
So this should not be a debate so much as a seminar on informing the public what is really going on. Sadly, it will probably just be a debate.
