What could I expect to hear from a Class A amp?

Tubes have their problems too! It's very hard to find good "tubes" that will last like they should "these days". New TUBES can go BAD "at any time"...at any price! That's a BIG Problem.....!

Bearotti: You may want to further your knowledge by looking at texts and articles from other than manufacturers of audio equipment. Those sources are generally biased to promote their product and do not provide a complete picture. Keep in mind that there is nothing particularly difficult in processing audio signals. Techniques that work in one area of signal processing work in other areas. For eg. in the mid 1990's I spent considerable time on a design project which applied signal processing ideas initially developed in radar target identification to extracting late potentials in ECG signals as an early predictor of cardiac problems.

Also keep in mind that in some 'perfect' sense there are tradeoffs in such things as class a vs class ab amps. However, those tradeoffs are meaningless because - as I indicate above - properly designed AB amps will not result in signal degradation that is in any way noticeable (unless of course one uses the magic 'audiophile ear').

A good beginning point for gaining more knowledge are the books by Douglas Self. These have very little mathematics in the explanation and are readily understood by anyone with a high school math background. The same goes for amateur radio publications. (Engineering texts and publications require a minimum of a formal course, or the equivalent, in differential equations and some background in Fourier analysis - which takes a considerable time investment - generally not a good investment unless one expects to make a living as an engineer)

I am familiar with Nelson Pass's very best current products. I have the pleasure of knowing a dealer who employs Pass components powering the towering Apogee Full Range speakers.

There are 600.5 amps handling the huge bass panels. The XA.5 100 finesses the mid and tweeter ribbons.

Through all types of music, and loudness, I never saw the power needles ever move on any of the amps. That would mean they are all running class A.

So, do both the 600.5 and the XA amps sound the same? The answer is no. I don't know why they sound different, but they do. The class A amps are clear, and easy listening amps. The 600.5 is forceful, and less open.

The question here is what is the difference between a wholly class A amp, and a powerful Class AB biased towards running in class A for around a fourth it's power rating?

Ferrari, I must agree with you about the exemplified work done by Nelson Pass on Class A design. I own the XA60.5. Based on his paper, it appears that primary difference between the X.5 and XA.5 is less on circuitry, and more about higher current bias setting for the XA.5. Secondly, people have contrasted the 2 series just like Muralman1 did. Thirdly, nobody receives such level of praise when it comes to bridging the gap between SS and tube amplifiers the way Pass amplifiers do. Yet with good end results we suspect Class A design is responsible for giving SS that wonderful tube sound without the overkill. Thus for these observations I tend to think Class A design is the way to go on amplifiers, as it provides the superior foundation. However there are enough unknowns that it's up to the astute designer to finish the job. There is only so much mathematics, theories, and the oscilloscope can explain.

Well it sure sounds like I am advertising Pass amplifiers, am I? It would be nice to hear comments from people who own non-Pass Class A amplifiers, especially when the author of this thread was wondering about Krell. It would be nice to balance personal bias when hearing contrasting opinion.

Over the course of 50 years in this hobby have heard or owned or near owned damn near all the high end amplifiers produced. Such as Levinson, Threshold, Krell, Classe, Coda and many others to numerous to menetion here. But when it comes to Class A ampd I have always gravitated back to the Nelson Pass Threshold and Pass Labs units. At present I do not have a Pass unit in the system. Moving shortly to the X350.5 in a few months. Now running a PS Audio HCA 2 Class D amp. Very happy with it, but no where near the level of the X350.5 While it is not a pure class A amp such as the Aleph series, it delivers the signature I want, with less heat and drain on the power bill.

As far as Levinson and Krell go I have never kept any of them longer than 3 months. Impressive at first, but in the long run just have not pleased my ear.

As far as tubes go, when I got into this hobby that was all there was and mono to boot. So from 1957 to 1977 was in the tube camp until the Threshold gear came out. Dumped the tube gear in 1977 and haven't looked back. Twenty years of fiddling with tube gear was entirely enough for me. And that was during a time when you could buy new good tubes made in U.S. or great tubes from Europe. I have heard the Russian and Chinese tubes, to me pure junk as opposed to what we had in the tube era.

One of the great Class A/AB amplifiers I have heard recently was the Musical Design D150B signature unit. Now thats one Class A/AB amps I could live with nicely and totally recommend.