Above: Bang & Olufsen’s BeoLab 9 loudspeakers plut Class D amplifiers at the centre of a piece of art.

Surely an amplifier is just an amplifier? Actually, there are a multitude of designs, based on a dozen or more fundamental principles. That’s a pretty impressive achievement, given that the triode, a vacuum tube that allowed the very first amplifier designs, only appeared a little over a century ago in 1908.

The major categories have class names, of which the most familiar to audio enthusiasts are Class A, Class AB and Class D. Each has its advantages… and its disadvantages.

Class A is supposed to be the best sounding design because it eliminates something called ‘crossover distortion’, but it is horribly inefficient, wasting more than half the power used by the amplifier.

Class AB is somewhat more efficient, but still wastes plenty of electrical power as heat.

Class D is completely different.

Now let’s get something straight: though the ‘D’ conjures the concept ‘Digital’, technically, a Class D amplifier is not necessarily a digital amplifier. In fact, depending on the source, the Class D amplifier originated in the early 1930s, or the early 1950s. And back in those days, digital had nothing to do with it.