The basic design of an amplifier, tube or solid state, is quite simple unlike its power supply. Amplifiers should always be fed by an adequate power supply. In some consumer products this theory isn’t applied, resulting in a reduced head-room. This is particularly notable when an amplifier is pushed to its limits. Audio amplifiers are classified on their circuit and power supply classes for analog designs which are A, B, AB and C, whereas class D is equipped with a switching power supply. There are however more differences between these classes. Class A amplifiers are operating over the entire range of the input cycle, while Class B only amplifies half of it. With this in mind, it is obvious that a Class B generates more distortion than a Class A amplifier. Class A amplifiers are therefore quite inefficient thereby needing a lot of power. To get the best of both worlds, a Class AB amplifier was developed. It operates mostly as a Class B amplifier, amplifying only half of the input cycle, but also conducts a tiny bit on the other half. Analog modern amplifiers are almost always Class AB amps. There is a Class C, but it is rarely used in audio equipment because of its high distortion at the output stage.
Class D amplifiers are very different when compared to the previously mentioned classes. In both classes the transistors can either be on or off. The analog signal is converted into a signal by a pulse width modulation technique. The main advantage of these classes is their power efficiency. Since the switching transistors are on or off there’s no need for large and expensive power supplies. Sometimes this class is described as digital but there is no digital signal processing going on in this class. So, which class is the best? For audiophiles, only Class A will do, but Class D amplifiers are getting better and better. They will probably soon be equal to Class A amplifiers.
The output power of an amplifier is measured in Watts. When buying an amplifier, please make sure that the maximum power of your amp never exceeds the RMS power handling of your speakers. It is also important to be aware of the minimum impedance load of your amp. For instance, if your amplifier requires an minimum impedance of 4 Ohms you should never connect speakers to the amp with a lower impedance than 4 Ohms! Otherwise you could short-circuit the amplifier.
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