Within power amplifiers, there are electronics elements that are employed to amplify music signal to levels sufficient to drive a loudspeaker; these elements can be either large transistors or thermionic vacuum valves (tubes). Power amplifiers can be constructed in two basic ways: to have all amplifying elements operate simultaneously, or to have amplifying elements pass the amplifying task from one to another. The first arrangement is named Class A, and is obviously a better choice for accurate music amplification. The popular term Class A is often abused by applying it to amplifiers that are really not Class A designs, so sometimes people use term Pure Class A to highlight they really mean Class A. However, there are disadvantages in class A design: they need a lot of energy to operate and they release a lot of heat, which makes them very, very costly. That is where the other arrangement becomes attractive - it is a lot more cost effective. Amplifiers designed in that way do not need as much energy and they do not heat up as much as Class A amplifiers because while some amplifying elements operate, the others are at rest. The trouble there is the distortion generated during passing the task of amplification from one device to another, and although small, ideally it should be avoided. One way of reducing this distortion is to have all devices operate simultaneously in class A at low levels, and at louder levels change to pass-on-the-job configuration. This combination is called class Ab and it is a neat compromise between cost and performance.

There are several other variations of avoiding the expense of pure Class A design, all with notable merits and essentially excellent in their own right, but sonically not as good as pure Class A.

Foundation #6: Class A power amplifier must be used to achieve the best audio performance. Class Ab power amplifiers must be used instead of class A amplifiers in otherwise the same loudspeaker design, in order to make a more affordable constructions that still produce sonically outstanding and satisfying results. Also, class Ab must be used in amplification of the lowest portion of the audio spectrum, where using class A design would not produce any audible improvements.