Time Warp: Class D amps

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.

Class D is otherwise known as a ‘switching amplifier’. The reason that Class A and Class AB amplifiers are so inefficient – waste so much of the power which feeds them – is because their output transistors spend most of their time only partly on. Some of the electrical power is being allowed through to the speaker terminals, while some is being used by the transistors themselves to heat up.

That is never the case with a Class D amplifier: its output transistors are either switched fully on, or fully off. In either case, the amplifier wastes hardly any power. Theoretically it uses none at all, but in the real world electronic devices aren’t perfect. Nonetheless, better than 90 percent of the power drawn from the wall socket is used in a Class D amp to drive the speakers, instead of around 60 percent for Class AB.

In addition to keeping your power bills lower, this means that less heat is dissipated, so smaller lighter heat sinks can handle matters.

The result is smaller and lighter amplifiers, or more power for a given size and weight.

Class D amplifiers have had a relatively long history in professional audio, particularly sound reinforcement, given their virtues of relative compactness (which equals transportability) and high output power. But they have made their ways increasingly into homes in recent years, in such implementations as the IcePower amplifier modules used in many higher quality devices, Sony’s ‘S-Master’ home theatre receivers, and various subwoofer amplifier modules.

Given the need for prodigious quantities of power in high performance, compact subwoofers, this isn’t surprising. And with increasing use of digital signals, the shift from Class D to ‘Digital Amplifier’ is easy to understand.

Class D is especially suited to being a digital amplifier. Like digital, it is on or off, and never halfway between the two. The technical details can be a bit tedious, but in short it uses Pulse Width Modulation to define the signal (Pulse Density Modulation for Sony’s S-Master system). Wide pulses indicate a high point in the signal, and narrow ones indicate a low point.

The beauty of this is that Pulse Code Modulation – the ubiquitous workhorse of digital signals – can be readily converted to PWM, which can then be used to drive the speakers. All that’s needed then is a passive filter at the loudspeaker terminals to remove the ultrasonic noise inherent in such signals, and you have a highly efficient, highly powerful amplifier, perfectly capable of producing glorious sound.