ABSTRACT
Most types of audio power amplifi er are less effi cient than Class-B; for example, Class-AB is markedly less effi cient at the low end of its power capability, while it is clear from Chapter 10 that Class-A wastes virtually all the energy put into it. Building amplifi ers with higher effi ciency is more diffi cult. Class-D, using ultrasonic pulse width modulation, promises high effi ciency and indeed delivers it, but it is undeniably a diffi cult technology, and its linearity is still a long way short of Class-B. The practical effi ciency of Class-D rests on details of circuit design and device characteristics. The apparently unavoidable LC output fi lter – second order at least – can only give a fl at response into one load impedance, and its magnetics are neither cheap nor easy to design. There are likely to be some daunting EMC diffi culties with emissions. Class-D is not an attractive proposition for high-quality domestic amplifi ers that must work with separate speakers of unknown impedance characteristics.
