Check this: http://www.passlabs.com/zenamp.htm
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Class AB amps use "2 sets" of transistors where one set amplifies the positive signal swing and one set the negative signal swing. There is a point near the centre of the signal swing where both sets amplify at once causing some distortion. With Class A amplifiers the transisitors are "ON" all of the time and amplify the entire signal. Because of this Class A amplifiers are generally less efficient than Class AB. Krell uses "Plateau Biasing" which sets the level of transistor bias or "on current" to the proper level just prior to the signal arriving thus being efficient while at the same time utilizing the sonice benefits of Class A operation.
What you're describing is single-ended vs. push-pull. Class A means that all of the output transistors or tubes ARE BIASED TO IDLE AT FULL POWER ALL THE TIME (unless their bias is computer controlled, like the Krell FPB series). EITHER SINGLE ENDED, OR BALANCED CIRCUITS CAN OPERATE IN CLASS A. Class AB means that the output devices are partially (but not fully) powered up all the time. BOTH "Emily" and "SteveM" NEED TO READ "THE COMPLETE GUIDE TO HIGHEND AUDIO"...