2. Hopefully Ralph will weigh in but unfortunately I suspect @atmasphere is
referencing his own Class D amplifier that is currently under
development. I'm sure it'll be wonderful, but not yet available and
perhaps outside your budget.
We are working on a class D amplifier which has reached beta production. Its intended pricepoint is about 5000.00/pair. Although I think our amp is doing quite well as far as being musical is concerned, I was not talking about it specifically. I was more talking about how feedback is troublesome if improperly used (which is about 99% of the time). The Benchmark is an example of an amp that does feedback properly. Self oscillating class D amps are another example.
Feedback has gotten a bad rap and deservedly so because if you don't run enough of it (and that's been the case in the last 60 years), it will generate higher ordered harmonics and IMD at the feedback node (unless you are very careful about the latter). This causes harshness and brightness, and this is the main reason transistors are harsh and bright. If you run feedback in tube amps, they get harsher and brighter too (and this will be in spite of the fact that the frequency response is actually **flatter**, because the brightness is caused by distortion, not a frequency response error).
So we've seen designers, including myself, building zero feedback amplifiers to prevent this brightness and harshness. You can think of the use of feedback as being on a bell curve, use a tiny bit and its not bad but doesn't do a lot, use more and the distortion issues increase (all the while suppressing the innate distortion of the circuit, so it will measure well but not sound as good as the specs suggest). But finally you get to the other side of the curve, which is at about 35-40dB (meaning that if the amp had no feedback, it would have to have at least 65-70dB of gain!). At this point there is so much feedback that it allows the amp to compensate for the distortion generated by the feedback, and corrects phase shift too.
One thing I forgot to mention is how distortion can increase at higher frequencies if feedback is insufficient. You really want the distortion to be the same regardless of frequency. Our OTLs can do this, which is part of why they are musical, and the smaller the SET, the more they can do that as well (this is part of why the lower powered SETs are held in such high esteem). To cover up this problem, the distortion specs you see are usually done at 100Hz! If you measure the distortion spectra at 1KHz you get far more meaningful information- look at what the amp is then doing at 7KHz, which is in the area where the ear is most sensitive (birdsong frequencies). The 7th harmonic is unpleasant! If you measure at 100Hz, the 7th harmonic can often appear benign. This is why many solid state amps seem to do bass just fine, because at bass frequencies, they really **are** doing just fine because there's enough feedback. But as frequency is increased, the amp can run out of Gain Bandwidth Product, which is a way of saying that the feedback is being reduced. This is a very common problem!
My experience with monostrapping and bridging amplifiers isn't good. The amps almost always get less musical (higher distortion). Lower impedance loads can be an issue as well because the current expected of the output devices might be exceeded, such as into 2 ohms- that leads to failure and shipping costs...
I miss Al as well. He was a friend, a good egg and always steady on.
