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As I said below, I'm reading the thesis again.

The thesis focuses on a single idea, that there it is possible to create a measurable figure of merit the author calls a Total Aural Disconosnance (T.A.D.) which relates the "like-ability" of the sound resultant from selection of an amplifier to this measurable parameter related to a weighted harmonic distortion figure where the higher the order of the harmonic, the greater the weighting and the rate of weighting rises rapidly. The rationale behind this is a series of very highly contorted arguements which are pressed to fit the need of a pedetermined conclusion. I'm getting ahead of myself not having finished rereading the paper but the purpose seems to be a justification for lauding non feedback vacuum tube SET amplifiers. BTW, Cheever's ideal (best ever) amplifying device is the type 45 vacuum tube. He built a 1 1/2 watt two stage amplifier using this tube and a type 26 tube as a driver. He gets around much of the problem of the nonlinearity of the output transformer core due to hysterisis inertia in the BH curve around the zero crossing point by applying sufficient bias current to the plate through the primary winding to keep the induced field away from zero at all signal strengths. He spends some time comparing this amplifier with an inexpensive 10 watt ss amplifer which is unsurprisingly awful by comparison and a Hafler DH500 255 watt per channel MosFET amplifier which was highly regarded at the time of this writing.

Originally, transformers had the advantage of interstage voltage boost and bias voltage isolation at a time when tube gain was typically around 10. This of course is not of any real signifigance today anymore. Even MC cartridges are boosted with active gain stages, the old step up transformers used in the 60s and 70s being largely a relic.

The value of a high damping factor is determined by the woofer/enclosure system connected to the amplifier. Woofers having high mass and therefore potentially high inertia and inherently less than critically damped mechanically can benefit by having the amplifier exercise greater control over the cone's motion by damping out the woofer's tendency to act as a generator delivering current to the amplifier output stage. In motor control design, this same principle is called dynamic braking and is used to help stop motors connected to rotating machinery taking some of the strain off of mechanical brakes. In other woofer/enclosure designs, the value of low damping factor is much more dubious. BTW, I think you'd agree that a very conservative and stable power supply is important for many amplifiers and certainly helps to resist instability caused by reverse emf from some loudspeakers and recovery from overload.

"If the output were truly a low resistance then, when driven with an in band square wave, placing a capacitor across the output will show the rounding off of a first order RC low pass filter.

Instead, in most amplifiers, placing a cap across the output produces ringing to varying degrees and never look like an RC filter.
Ringing is a result of “higher order” stuff going on, clearly when driving a reactive load, it is NOT simply a faithful low resistance voltage source as imagined."

Many do show a rolloff. However, a ringing square wave is identical to a resonant peak in FR. The capacitor can form an LC circuit with some amplifier output stages even sending them into spontaneous oscillation and destroying them. This was seen and reported by technicians when some amplifiers were connected to high capacitance audiophile type speaker wire. I think many audio amplifier designers came to realize that FR well beyond the audible range was not only unnecessary and of no practical use but had many pitfalls. Many believed early on that response into the ultrasonic region was valuable (University sound, Harman Kardon, Audio Fidelity Records to name a few) but this later proved invalid and flies in the face of what is known about waveforms.

"One real problem designing passive crossover filters is dealing with the fact that the load (the driver it's connected to) is NOT a simple resistance where you can simply look up values"

I could hardly agree with you more and even Cheevers sites an early paper recommending using a reactive load to test audio power amplifiers. SS amps go into protection mode because early experience with them showed they are inherently fragile and easily damaged by careless or inadvertent mistakes such as short circuits. Therefore, every effort was made to make them bulletproof. Vacuum tubes do have parasitic shunt capacitance but that is usually shielded and overwhelmed by the inductance of the output transformer. Connecting a reasonably sized capacitor does little to alter the inhernet inductive reactance of the load the plate circuit sees.

"Are there faithful SS amps? Yes, I think so, are there colored “euphonic” tube amps, sure but there are faithful tube amps too."

True and I am impressed by some highly skilled OTL designs like Tom Dawson's and Julius Futterman's (even though bias adjustment on his was a nighmare.) But most tube amplifiers are not accurate, at least not by measurement standards most commonly used. I have a great deal of difficulty seeing the technical value of "euphonic amplifiers" although for this niche segment of the market they may have some commercial value at the moment. As I said elsewhere, I have very specific technical criteria for judging an audio amplifier an how it makes a particular sound system perform subjectively is not one of them. Besides, most of the time at least for ss designs, their differences are usually very subtle if they are audible at all.



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