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All About Amplifier Classes: What Is Class A, AB, D and More?

"This amplifier is class A!" You have heard it already, but what exactly does it mean? In this article we explain everything about amplifier classes.
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“This amplifier is class A!” In the store it is called out to you, but what exactly does it mean? And is it relevant for the home cinema fan? In this article we explain everything about amplifier classes.

Introduction amplifiers

Hopefully we shouldn’t tell you that an amplifier is an important part of any audio system. Amplifiers ensure that an audio signal is loud enough to be played on speakers or headphones. An amplifier does exactly what it promises: it amplifies.

Now we keep it very simple, because in practice it can become much more complex. Take that audio signal: maybe it is an eight-channel soundtrack or a stereo signal with dynamic orchestral music. That amplifier can be a classic hi-fi part, but also a much more complicated AV receiver . In addition, you come across them everywhere, from small to large. Bet you now have an amp in your pocket? No, no, we are of course talking about your smartphone. It has a small amplifier to convert that Spotify stream into sound that comes out of your ears. Your television also has one to control the built-in speakers. Sound bars, sound bases, active speakers, multi-room speakers, Bluetooth speakers… They all contain some form of amplification.

Why different letters?

Class A, AB, D, etc. indicate how a weak signal (= music or a movie soundtrack) is amplified so that it can come out of the speakers. After all, a typical source such as a Blu-ray player, a network player, CD player or turntable delivers a very weak and quiet audio signal. This signal is far too weak to drive loudspeakers directly, because it takes more. Some power is required to make those speakers vibrate so that they move air. The louder, the more air has to be set in motion. And the more power you need.

 Amplifier Classes: What Is Class A, AB, D and More?

Anyone who dives into the hi-fi world will often encounter indications such as class A, class D, and so on. In home cinema, there is less screen with such indications, because some techniques are simply not interesting for multi-channel applications. For example, a Class A AV receiver would have power consumption expressed in several kilowatts.

At home theater you almost always speak of class AB or class D, with a clear lead for the latter. Why? That becomes clear when you consider the pros and cons of each.

Amplifier Classes: Class A amplifiers

You should not doubt it: Class A delivers the very best results. Without diving into the technical details, a class A amplifier does the purest form of amplification. It is perfectly linear, which means that the input signal is perfectly translated into higher power at all points.

However, there is a big ‘but’. This is only possible by continuously energizing the relevant parts, even if there is hardly anything or even nothing to amplify in terms of sound. A class A amplifier that you forget to switch off consumes just as much as under a heavy load. In addition, class A is very inefficient. 70 to 90 percent of the electricity consumed is converted into heat! If you want 100 watts of music power, you use up to 1,000 watts of electricity. That’s a lot of light bulbs – especially if you go to sleep without pressing the power button.

 Amplifier Classes: What Is Class A, AB, D and More?

From a practical point of view, it is therefore very difficult to build a powerful class A amplifier. For home theater purposes, it becomes almost impossible, because for a 5.1-channel system, for example, you don’t need one amplifier – just five (the .1 LFE channel does not need an amplifier, as it is in the subwoofer itself).

Amplifier Classes : Class AB amplifiers

With class AB, an attempt has been made to overcome the disadvantages of class A, without losing the advantages. This is done by using more parts and having each amplify part of the signal. When not needed, these parts (usually transistors such as MOSFETs) turn off. This greatly reduces power consumption, although it is not to be underestimated. The efficiency will be around 50 to 70 percent. You will notice that with the rare multi-channel class AB receiver, because they also get nice and warm. There are many hybrid designs that, for example, function in Class A at low volumes, but switch to Class AB when it gets louder.

Amplifier Classes :Class D amplifiers

The ‘D’ does not stand for digital here, as is often thought. With Class D amplifiers, the output stage (that is, the part that amplifies the signal) is switched on and off a lot, as needed. This is done via a control that works on the basis of the incoming signal. That control can be digital, which creates confusion for many. Either way, a class D amplifier can be extremely efficient because of that super fast switching. 90 percent is feasible, in theory even 100 percent. Efficiency also means that these amps don’t heat up quickly and that they stay compact. And that is exactly what is needed with an AV receiver and TV audio devices such as sound bars. Active subwoofers are also very often class D.

 Amplifier Classes: What Is Class A, AB, D and More?

However, class D is hard to get right. The power supply must be of a higher quality and filters must be installed after the amplification hatch to remove high-frequency interference caused by the control. This means that a great difference in quality can be experienced with class D appliances. But it is certainly possible that you will experience absolute top quality with a class D receiver. The chances are that a very good class D device will also be pricey.

The nutrition

As mentioned, the choice is a bit more limited for someone who builds a home cinema. For efficiency reasons but also to reduce costs, class D amplification is often chosen. However, it is important to realize that there can be a lot of difference in the quality that class D can deliver. An important role remains for nutrition. Good amplification stands or falls with nutrition. It must be powerful enough and also stable, so that it can absorb large dynamic changes (such as an explosion in a movie). This certainly applies to AV receivers, because these devices contain five to eleven amplifiers, each of which requires a clean, stable power supply.

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