Krell’s amplifiers have always been based upon class-A operation, which is the most linear, lowest distortion operational mode.
Krell employed a technology called Sustained Plateau Bias. The KAS tracked the musical signal in relation to the amplifier’s bias. The bias level was set in several discrete plateaus, so that when the musical signal required a higher bias level to remain in class-A operation, the amplifier would elevate the bias to that plateau long enough to produce the signal needed before ramping back down to a lower bias point. This kept the amplifier operating in class A no matter what the signal requirements, but it reduced the amount of wasted energy.
The FPB, or Full Power Balanced, line of amplifiers employs Sustained Plateau Bias II, a refinement that employs microprocessors to monitor not only the signal level, but also the loudspeaker’s impedance. That way, both signal and load are monitored and constantly adjusted so that the amplifier constantly operates in the class-A bias that yields the lowest possible distortion with the lowest possible heat dissipation at all times.