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

Al - read also this http://stereophile.com/news/10065/

Class A is probably the best SS. The real question is what is the best amp for the money one has assigned to spend. For me it was class D - maybe not at the level of top class A amps but way better than my previous class AB amp.

The usual total harmonic and intermodulation
distortion figures do not reveal the abrupt output stage distortions accurately
because of the averaging factor involved in such measurements. A spike of
crossover distortion may reach 2 per cent, but if it occurs only over 5 per cent
of the waveform, it averages out to a respectable 0.1 per cent distortion
figure.

What he doesn't say which is important is that the crossover distortion is
fixed (it is not amplified as you crank up the volume, at least that is my
understanding) - therefore Class A becomes critically important at low power
output => this is what makes designs with sliding bias or several stages that
include a Class A stage attractive. (it allows distortion to remain low at low
output levels or quiet passages but then still gives you the ooomph needed
for loud passages....the crossover distortion is less critical during a loud
passage as it is a relatively small amount on a good Class AB design
compared to a big signal output)

This also makes an active speaker topology attractive. Since the distortion is
high order then some of the worst stuff is going to come out the tweeter. In a
biamplified active speaker design you simply feed clean Class A for the feeble
power requirements of the tweeter whilst you can power the mid/woofer with
a beefy Class AB design without much worries over high order harmonics or
intermodulation distortion being all that audible

Shadorne: Correct as to the unamplified nature of crossover distortion. It is only the ouput, or final, stage at which the bias point is placed near cutoff and saturation, respectively rather than centered - in a class B amp. How hard that stage is driven makes no difference as far as the amplitude of the crossover distortion since there is no amplification following that stage. But since the excursions will be greater away from the crossover point, when driven harder, any change at the crossover point (which will be insignificant anyway in an AB design) will be a smaller percentage of the overall signal. Hence, at low values of output signal any such distortion would be a larger percentage of the output signal. As to your comment regarding active speakers - absolutely true - it always makes sense to design the electronics for the load and the more known about that load allows for an easier design with less compromises.

It would be nice to know of non-Pass Class A amplifiers that perform well, and why so. Bearotti asks about Krell. At this point I am curious in just about anything Class A. The whole discussion thus far is very helpful to me as a Pass XA60.5 owner, but thus far continues to confirm what's heard through the months, in that nobody remotely comes close to Pass Lab on Class A amplifiers. Otherwise there would have been well-informed and fairly objective challenges left and right.

It would be nice to know of non-Pass Class A amplifiers that perform well, and why so.

ATC make amplifiers with what is probably a sliding bias - you get 2/3 power Class A before it reaches Class AB operating point for the final 1/3 power. I have not compared them to Pass but ATC are relatively unknown as an amplifier manufacturer whilst Pass is legendary. You are safe sticking with Pass, IMHO.

Coda makes some very good Class A amps in pure Class A and biased Class A. Recently heard the Coda S5.5 a real stunner. Also if you can find the Coda 12.0 it is a heavy contender in Class A amps.

Most of the Coda staff is former Threshold staffers, that formed their own company. So the Pass legacy is clearly evident in their product line.

"...nobody remotely comes close to Pass Lab on Class A amplifiers. Otherwise there would have been well-informed and fairly objective challenges left and right."

I'll submit Clayton Audio as a strong competitor to Pass Labs performance-wise, but they're obviously a much smaller company that sells far fewer amps so they're not close in that regard.

Spatine, As I've said I'm a big fan of Nelson Pass but in my experience I think ( and this is just my opinion) Gryphons class A amps are superior for some applications. The Antileon signature is $31,000 the Colosseum is $50.000 and the Colosseum monos are ( I think) $75,000. What makes them more universally useful is that they continue to increase class a power down to 1/2 ohm impedance. ie. 150w/8 ohms, 300w/4ohms, 600w/2ohms, 1200w/1ohm, 2400w/.5ohms. They have 2 power cords and I plug each into a dedicated 20amp outlet. Now in practice they would rarely exceed 600w output but they are impervious to load. This won't matter to people with efficient speakers with benign loads but it mattered to me in my case. That said Pass represents better value and if you don't need the extra oomph then I would go with them. If however you really dig class A but have challenging speakers or money is no object then Gryphon is a better way to go IMHO. This is not a put down of Pass. I really like their electronics and I especially like their people and business philosophy. - Jim

I did listen to Clayton's at a show. It blends in among the good amplifiers. It's interesting to note that big players have not caught on to Class A in big ways. May be they figure the R&D it takes to compete against Pass take so many years that it's not worth the risk. The little guys are still at Square One and thus don't have much to loose. Just pondering here.

The 'big players' prefer to sell convenience. IMO, their equipment sounds like it is built for that too :)

So there is plenty of room in the market for smaller companies that base their marketing and products on higher performance features like Class A operation.

Thanks for the knowledge guys! Feel free to keep writing your thoughts. I would love to get my hands on ANY class a stuff just to see what observations I could make...