Amplifier

In your HiFi setup, the device with the best performance is probably the audio amplifier, since the audio amplifier has to increase a low-power audio signal into a level that’s suitable for driving speakers. Even amplifiers of poor quality are able to do this with an high SNR (Signal to Noise Ratio). This is quite important because the output of an amplifier can reach up to tens of thousands of Watts! A noise signal which is multiplied by a factor of 20,000 would be very annoying. Generally, an amplifier is the final piece of electronic equipment in an audio playback chain. Many amplifiers are fitted with pre-amplification functions such as equalization, mixing and effects. Some amplifiers are built into speakers where the amp is perfectly matched to the characteristics of the individual drivers.

Back in the early days

In 1909, Lee De Forest invented the first audio amplifier after he invented the vacuum triode tube (valve). Lee was actively involved in designing the wireless communication equipment. Until the invention of the triode tube, it wasn't possible to listen to radio through an speaker. This problem was solved by the vacuum triode tube, which would be the first step of the tube amplifier. Even today, people are still using tube amplifiers, and it wasn’t until the 1960s before they were slowly replaced by solid state amplifiers. Tube amplifiers are largely known for their warm and characteristic sound and are therefore commonly used in guitar amps, studio equipment and hi-end audio systems. However, a tube amplifier introduces a small amount of harmonic distortion which is quite audible, therefore many listeners prefer the linearity of an solid state amplifier. John Bardeen invented the first transistor in 1947, a semiconductor that is used to amplify and switch signals. In the 1960s these components became widely available which contributed to the popularity of the solid state (transistor) amplifier. The first transistor amplifiers didn’t match the sound quality of the tube amps. It appeared that when the output voltage of the amplifier increased rapidly, it led to intermodulation distortion. By modifying the slew rate (change of output voltage) this problem didn’t occur anymore.

Amplifier classes

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.

Output

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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