TABLE OF CONTENTS Most types of audio power amplifier are less efficient than Class-B; for example, Class-AB is markedly less efficient at the low end of its power capability, while it is clear that Class-A wastes virtually all the energy put into it. Building amplifiers with higher efficiency is more difficult. Class-D, using ultrasonic pulse-width modulation, promises high efficiency and sometimes even delivers it, but it is undeniably a difficult technology. The practical efficiency of Class-D rests on details of circuit design and device characteristics. The apparently unavoidable LC output filter-second order at least can only give a flat response into one load impedance, and its magnetics are neither cheap nor easy to design. There are likely to be some daunting EMC difficulties with emissions. Class-D is not an attractive proposition for high-quality domestic amplifiers that must work with separate speakers of unknown impedance characteristics.
There is, however, the Class-G method. Power is drawn from either high- or low-voltage rails as the signal level demands. This technology has taken a long time to come to fruition, but is now used in very-high-power amplifiers for large PA systems, where the power savings are important, and is also making its presence felt in home theatre sytems; if you have seven or eight power amplifiers instead of two their losses are rather more significant. Class-G is firmly established in powered subwoofers, and even in ADSL telephone-line drivers. It is a technology whose time has come.
Music has a large peak-to-mean...
