In many descriptions of active loudspeaker systems and amplifiers, amplifiers are often said to belong to “class D“ or “class AB” But what does that mean and how is class D different from A, B, AB and C? Are certain amplifiers better for hi-fi audio than others?
As it turns out, the class indicates the type of technology used to amplify a signal. When it comes to hi-fi, class A, Class AB and Class D are the most common.
The classification of amplifiers actually continues right down the alphabet to E, F, G and H but these are not used for music reproduction. We therefore won’t be going into them in this article. Curious which class of amplification your system uses? You can usually find it written on the enclosure, although you might have to turn the device around a bit.
Class A – A real bundle of energy
Let’s start with the first letter of the alphabet. Class A amplifiers use transistors to amplify the incoming signal and require a base voltage of 0.7 volts. If the voltage sinks beneath this level, no amplification will take place and no sound will be produced. In order to prevent this from happening, a small trick is employed. A small amount of voltage is continuously sent to the transistor in order to achieve exactly this base voltage at all times. The name for this permanent voltage is quiescent current, and it is the defining characteristic of the class A amplifier.
The advantage of this type of construction is that it produces by far the lowest levels of distortion and therefore the best sound. On the downside, class A amplifiers really gobble up energy and generate a lot of heat while in operation.
In order to cool down class A amplifiers, heat sinks are often used. In spite of such technologies, class A amplifiers have been known to get so hot that it’s actually possible to burn one’s hand on them. The high energy use with their maximum efficiency of just 50% can really make itself felt on one’s electricity bill as well. In other words, although they offer the best sound, class A amplifiers have a few features that would discourage many people from using them. The vastly more efficient and yet similarly constructed class B amplifier are an interesting alternative to examine.
Class B – The efficient alternative
Compared to class A, class B amplifiers use very little electricity, making them markedly more efficient. The reason for this is a very low quiescent current characteristic of this type of amplifier.
With a class B amplifier, the audio signal is separated into a negative and positive half-wave. Each of half-waves is then separately passed on through its own transistor and is amplified. However, since class B amplifiers first start to work at 0.7 volts, one has to live with a bit of distortion. This sort of amplifier is therefore not suitable for hi-fi uses. Class B amplifiers are usually found in systems that amplify the human voice, such as megaphones, for instance.
Class AB – the optimizer
Some class B amplification technology can be found in the world of hi-fi, at least in part, namely in AB amplifiers. As the name would suggest, these are nothing other than a combination of class A and B amplifiers. This means that these amplifiers can switch between technologies in order to achieve an optimum of performance and efficiency. They therefore do not incur the distortion problems of a Class B amplifiers.
The next letter in the alphabet, C, is also represented with class C amplifiers. These work entirely without quiescent current, but are generally only used for the treble range.
Class D – the digital
The youngest of the amplification classes is class D. This works digitally and has very low power loss. It also develops much less heat than a class A amplifier. But the most revolutionary aspect of class D amplifiers is not their efficiency but their size. A class D amplifier with a power output of 50 watts can still be made small enough to fit inside a pack of cards. These makes it possible to integrate amplifiers into smaller devices such as headphones, smartphones and MP3 players, but also everywhere where space is at a premium. Active loudspeakers, that is, those with integrated amplification, use class D amplifiers.
Coda: Amplifier basics everyone should know
The typical consumer of electronics devices need not think too much about different classes of amplifier. Mid-class types all operate along similar principles. However, as a hi-fi enthusiast, there’s no getting around the topic. One is always confronted with the different classes A, B, C and D. Naturally it helps to understand the underlying principles behind each amplification technology. Anyone who’s truly passionate about hi-fi — and is lucky enough to have an affordable electricity provider — should consider a class A amplifier as it truly offers superior sound. For a true audiophile, this can be worth the trade-off of overheating devices and large electricity bills. For the rest of us, there are plenty of great amplifiers that manage to balance sound quality and energy efficiency.