A Short Primer For Music Amplifiers

By Author: Corey Gusa
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None of latest audio systems would be possible without the aid of modern audio amps that try to satisfy higher and higher requirements regarding power and music fidelity. It is challenging to select an amp given the large number of models and designs. I will explain a few of the most common amp designs such as "tube amplifiers", "linear amps", "class-AB" and "class-D" along with "class-T amps" to help you comprehend several of the terms commonly utilized by amp producers. This article should also help you figure out what topology is best for your precise application.

An audio amplifier will convert a low-level audio signal which often originates from a high-impedance source into a high-level signal which can drive a speaker with a low impedance. Determined by the type of amp, one of several kinds of elements are used to amplify the signal like tubes and transistors.

Tube amps used to be widespread some decades ago. A tube is able to control the current flow according to a control voltage which is attached to the tube. Unfortunately, tube amps have a rather high level of distortion. Technically speaking, tube amplifiers ...
... will introduce higher harmonics into the signal. On the other hand, this characteristic of tube amplifiers still makes these popular. A lot of people describe tube amps as having a warm sound as opposed to the cold sound of solid state amplifiers. A different drawback of tube amps, however, is the small power efficiency. The bulk of power which tube amplifiers consume is being dissipated as heat and only a part is being converted into audio power. In addition, tubes are rather expensive to produce. Hence tube amps have generally been replaced by solid-state amplifiers which I will look at next.

Solid state amps replace the tube with semiconductor elements, generally bipolar transistors or FETs. The first type of solid-state amplifiers is called class-A amps. In a class-A amp, the signal is being amplified by a transistor which is controlled by the low-level audio signal. If you require an ultra-low distortion amp then you may wish to explore class-A amplifiers since they provide amongst the smallest distortion of any audio amps. Class-A amplifiers, on the other hand, waste most of the energy as heat. As a result they typically have big heat sinks and are quite bulky.

Class-AB amplifiers improve on the efficiency of class-A amplifiers. They use a series of transistors to split up the large-level signals into 2 distinct regions, each of which can be amplified more efficiently. The larger efficiency of class-AB amps also has 2 other advantages. First of all, the required number of heat sinking is minimized. Therefore class-AB amps can be manufactured lighter and smaller. For that reason, class-AB amps can be manufactured cheaper than class-A amps. Nonetheless, this architecture adds some non-linearity or distortion in the area where the signal switches between those areas. As such class-AB amps generally have higher distortion than class-A amplifiers. Class-D amplifiers improve on the efficiency of class-AB amplifiers even further by employing a switching transistor which is always being switched on or off. Thus this switching stage barely dissipates any energy and therefore the power efficiency of class-D amps usually surpasses 90%. The switching transistor, that is being controlled by a pulse-width modulator generates a high-frequency switching component which needs to be removed from the amplified signal by making use of a lowpass filter. Both the pulse-width modulator and the transistor have non-linearities that result in class-D amplifiers having larger audio distortion than other types of amps.

More modern audio amps include some type of means to minimize distortion. One approach is to feed back the amplified audio signal to the input of the amp in order to compare with the original signal. The difference signal is then used to correct the switching stage and compensate for the nonlinearity. One type of audio amps which utilizes this kind of feedback is known as "class-T" or "t amplifier". Class-T amplifiers feed back the high-level switching signal to the audio signal processor for comparison. These amplifiers exhibit low music distortion and can be manufactured very small.