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

Hey, I already said, the XA-100.5 monos did 115 dB without ever choking up on
any dynamic swing, and the X-600.5 never gassed out on heavy bass runs. The
needle never moved, staying class A. It blows your hair back, and to think they
are working a 1 ohm load throughout. The long ribbons never get warm.

I'm not sure I understand, though, why Class A would run cooler when outputting signal into a load. It's tempting to think that could result from a portion of the current that would otherwise flow through some of the output transistors (at zero signal conditions), being diverted through the speaker instead.

Yes that is the reason. At no load (or I meant to say no input signal) you are actually running at 50% current output and all that current has to be dissipated internally in the amp somewhere, across the bias circuit if you will (it is the high bias that makes the current flow at 50% when at zero input signal). When you play music the +ve signal requires more current (but it all dissipates in the speaker) and on the -ve cycle it actually diverts current from the bias circuit to the speaker (meaning the amp generates less heat).

The amp may use more power from the AC but it will likely run a bit cooler - no enormous difference and I gues it may vary a bit on the exact topology and type of output devices.

Thanks, Shadorne. I subsequently found this article by Nelson Pass, which appeared in "Audio" magazine in 1977. Figures 1 and 2 enabled me to visualize what you were saying:

http://www.passdiy.com/pdf/classa_amp.pdf

The article also contains the following interesting statements, relating to the original poster's questions:

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. Considering this error factor, it is easy to see why two amplifiers with the same specifications can sound so different. To properly evaluate the distortion, peak distortion and harmonic distribution must be considered. Typical class A amplifiers will exhibit low order harmonics, and their peak distortion is less than twice the average distortion. In class AB amplifiers, very high orders of harmonics occur, and the peak distortion can be as much as thirty times the average distortion.

Another problem common to class B and AB output stages is due to the unequal turn-on/turn-off times of the transistors. Because the turn-off time is greater, both transistor sets can conduct uncontrollably under high slew conditions, making it dangerous to operate the amplifier at high frequencies, a particularly bad problem with some quasi-complementary designs. In a class A output stage, however, there are no abrupt nonlinearities and no turn-on/turn-off delays. The smooth transfer characteristic yields low order harmonic distortions, and these harmonics can easily become unmeasurable at low power levels.

Regards,
-- Al

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