What are amplifiers? What are their types

Class A, B, C or D amplifier? The amplifier classes

Actually, you don't have to worry about the different amplifier classes, because modern mid-range integrated amplifiers work very similarly anyway. As a hi-fi enthusiast, it is of course an advantage if you understand the basic principle of the amplifier classes. We'll give you a brief overview.

Amplifier classes - the basics

Basically, all electronic amplifiers are divided into different classes, including hi-fi amplifiers. The class says nothing about the quality of the amplifier, only something about the technical structure. In the hi-fi area you put on above all Class A and Class AB as well as Class D amplifiers.

Although the letters C, E, F, G and H were also assigned to the division into different amplifier classes, they designate amplifier types that are not relevant for the hi-fi sector. So they can be neglected at this point.

Note: If you don't know exactly what kind of amplifier you have, you can simply check the housing. There should always be a class name there. Sometimes you can find them right on the front, sometimes on the back, and sometimes on the bottom of the device.

Class A - the bundle of energy

Let's start with amplifier class A. In order to explain the peculiarity of a Class A amplifier, you have to understand the basic principle of a transistor. You can see exactly how it works here, or you can refresh your memories with this graphic.

The transistor amplifies the incoming signal, but it requires a base voltage of 0.7 volts. If the tension is lower, it cannot intensify and you hear absolutely nothing. Unfortunately, our audio signals also have voltages below 0.7 volts, which theoretically would not be amplified. A Class A amplifier would not make much sense because it would distort the audio signal.

A little trick is used to prevent this from happening. You simply send a low voltage permanently to the transistor in order to be exactly on its base line. This in turn increases the collector current and signals with a lower voltage are also amplified. One calls this permanent tension the Quiescent current and that's what makes the class A amplifier a class A amplifier. The advantage of this design is that you have by far the lowest distortion and thus also the best sound. However, the Class A amplifier is a real power hog and also generates a lot of heat.

In order to cool down the Class A amplifier, there are usually quite large cooling fins inside, similar to the illustration. Even so, a Class A amplifier can get so warm that you can burn your fingers on it. And the high electricity consumption is definitely noticeable on the electricity bill. The efficiency of a class A amplifier is also only 50 percent - but it delivers the best sound results. The similarly structured Class B amplifiers are much more energy efficient.

Class B - the economical one

A class B amplifier consumes very little power compared to the class A and therefore has one significantly higher efficiency. This is simply due to the very low quiescent current of this type of amplifier. With a class B amplifier, the audio signal is divided into a negative and a positive half-wave. These half-waves then each go to their own transistor and are amplified. Because such a class B amplifier only starts working at 0.7 volts, you have to live with distortions. Such an amplifier would therefore be unthinkable for the hi-fi sector. However, this amplifier technology is used more often when it is only a matter of amplifying speech - with a megaphone, for example.

Class AB - the flexible one

A bit of Class B technology can also be found in the hi-fi world, at least in part. Namely with Class AB amplifiers, which - as the name suggests - are nothing more than a combination of the first-mentioned amplifier architectures. That said, these amplifiers can switch between the two techniques and work relatively efficiently without distorting too much.
Class C amplifier work completely without quiescent current, but are only used in high-frequency environments - very different from the latest class of amplifiers, Class D. It works purely digitally, has very low power dissipation and does not generate as much waste heat as a Class A amplifier.

Loudspeaker from the devil

Class D - the digital one

The revolutionary thing about modern class D amplifiers is not only their efficiency, but also their size. Even with an output power of more than 50 watts, you can still use a class D amplifier build in the size of a cigarette pack. As a result, today we find these amplifiers mainly in small devices. For example in headphone amplifiers, in smartphones or in MP3 players. So wherever there is hardly any space. Class D amplifiers are also used for active loudspeakers, i.e. those with built-in amplifiers.

The amplifier is already in here: Teufel Effekt


  • ▶ The Teufel effect loudspeakers have a built-in Class D amplifier, are wireless and easily transform your system into a surround set. With their full levels, the active ones do an excellent job in the rear area. Not only does the wireless communication with the rest of the speakers make it easier to place them: they are also inconspicuous and small.

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Conclusion: Class A amplifier or something else?

  • Class A amplifiers can also fully amplify weak signals - and are therefore ideal for enthusiasts who want to enjoy the full range while listening to music.
  • In terms of power consumption and heat generation, however, others are ahead: for example the class AB amplifier or the small, often directly built-in class D amplifier.
  • If you have a cheap electricity provider, you should definitely check out a few Class A amplifiers. Because there's a reason these devices are so popular - even if they're wasteful and run hot.