what is diff: Class "A", "A/B", and "B"

Class A is non-switching. It produces a full power envelope whether you are using it or not,that's why they get so hot.
Class A/B runs in Class A during low power operation and then switches to Class B for more demanding power needs. Class B amps adjust the power envelope to the power required for the music. Less heat. This is a very simplified explanation, but should give you a general idea.

I don't know what a "power envelope" is, but the difference between class "A" and "B" is how the signal is split when it is amplified.

In a class A amplifier, the entire signal, split into parts, + and -, is run through two identical amplifying circuits that do NOT split the signal at all. The whole signal is put through a transistor group, but it is never split into smaller components.

In a class B amplifier, the signal is split in two, which are in phase. There are now four amplifying circuits, two for both + and -. It is then recombined after amplification, and put across the binding posts.

Both of these have their advantages and disadvantages. Class A is the purists method. The signal is never tampered with, so it will remain cleaner. Class B splits the signal and then recombines it, which can create crossover distortion. But in a class A amp, the transitors are made to be driven much harder, which generates more heat. The transistors in a class B design do not have to be run at such high gain levels, which in turn allows them to run cooler.

A class "A/B" amp is simply one that runs class "A" up to a certain power level, where the amp switches from A to B operation, allowing for lower heat levels. The catch is that most A/B amps set the power level of this switching very low, making the amp, in essence, a class B amp (unless one is using high-sensitivity speakers). Hope this helps. Cheers!

Pass labs, Krell and Gryphoon are are some of the Class A crowd,FYI. Just becuase its class A doesn't mean its superior though. A well done class B can have lower distortion than a poorly done class A-although different kinds of distortion, or at least proportions of. There's about 8 key sources of distortion in a class B circuit (I'm using Douglas Self as my reference), whereas class A is freed of many of these. So in essence Class A does hold the potential for the lowest possible distortion. The price to pay for class A? Very inefficient: 50% is the maximum theoretical efficiency for a Class A ampilfier, and in real world operating conditions its more like 20%. So a 100wpc stereo amp is going to have to dissipate/waste an additional 800 watts of power in the form of heat. And heatsinking and the bigger power transformer are expensive. Hence the attraction to going with class B=more efficient=cheaper to build and if done well can still have very low distortion levels. As the others have pointed out, "class" a/b is just an amplifier that switches between the two classes, and in so doing usually has more distortion than if it was well optimized and run in B mode only. "class" a/b isn't really a good idea, but there's still alot of'em out there.

Preamps on the other hand are usually, or at least far more commonly done in class A. Its financially practical and easier to do. It just with those big powerful solid-state amps does the parts cost bill start racking up real quick. I bet some of those big Krell's with all the heatsinking--there's probably $1,000 to $2,000 for the casing and heatsinking alone, before the guts/actual circuit are even put in it. I don't know what manufacturer's pay, especially w/ bulk purchasing, but Class A can get expensive, for what some would consider only marginal gains in performance. There are other classes, but most aren't used in home audio. Class D is about the only other one you'll see pop-up now and then. The rest are for radio frequency use and what not, and a few oddballs that are used in home audio--I think there is/was a class T somewhere. Class C,G,H, and S are the others off hand.