Music amplifiers are at the very center of each home theater product. As the quality and output power demands of today’s speakers increase, so do the requirements of power amplifiers. It is tough to choose an amplifier given the large number of models and concepts. I will explain some of the most popular amplifier designs including “tube amplifiers”, “linear amplifiers”, “class-AB” and “class-D” as well as “class-T amplifiers” to help you comprehend some of the terms commonly utilized by amp manufacturers. This article should also help you figure out what topology is best for your specific application.
The basic operating principle of an audio amp is quite clear-cut. An audio amplifier will take a low-level audio signal. This signal usually originates from a source with a fairly large impedance. It subsequently translates this signal into a large-level signal. This large-level signal can also drive speakers with low impedance. Determined by the kind of amplifier, one of several kinds of elements are utilized in order to amplify the signal including tubes and transistors. Tube amplifiers used to be popular a couple of decades ago. A tube is able to control the current flow according to a control voltage which is attached to the tube. Sadly, tube amps have a somewhat high amount of distortion. Technically speaking, tube amps will introduce higher harmonics into the signal. These days, tube amplifiers still have a lot of fans. The primary reason is that the distortion which tubes bring about are often perceived as “warm” or “pleasant”. Solid state amplifiers with small distortion, on the other hand, are perceived as “cold”.
Moreover, tube amps have quite small power efficiency and consequently radiate a lot of power as heat. Moreover, tubes are rather expensive to manufacture. As a result tube amplifiers have mostly been replaced by solid-state amplifiers which I am going to look at next.
Solid-state amps use a semiconductor element, like a bipolar transistor or FET instead of the tube and the earliest kind is known as “class-A” amps. The working principle of class-A amplifiers is very similar to that of tube amps. The key difference is that a transistor is being utilized rather than the tube for amplifying the music signal. The amplified high-level signal is sometimes fed back in order to reduce harmonic distortion. In terms of harmonic distortion, class-A amps rank highest amongst all kinds of power amplifiers. These amplifiers also regularly exhibit quite low noise. As such class-A amps are ideal for very demanding applications in which low distortion and low noise are crucial. Though, similar to tube amps, class-A amplifiers have quite low power efficiency and most of the energy is wasted.
In order to improve on the small efficiency of class-A amps, class-AB amps employ a number of transistors which each amplify a distinct area, each of which being more efficient than class-A amps. As such, class-AB amps are typically smaller than class-A amplifiers. Though, this topology adds some non-linearity or distortion in the region where the signal switches between those areas. As such class-AB amps usually have higher distortion than class-A amps. Class-D amps improve on the efficiency of class-AB amplifiers even further by using a switching transistor that is constantly being switched on or off. Thereby this switching stage barely dissipates any energy and consequently 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 using a lowpass filter. The switching transistor and also the pulse-width modulator generally exhibit quite big non-linearities. As a result, the amplified signal will have some distortion. Class-D amps by nature have larger audio distortion than other kinds of audio amps.
Newer amps incorporate internal audio feedback in order to minimize the level of music distortion. One type of mini audio amplifiers that employs this type 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 small music distortion and can be made extremely small.
A Brief Primer For Music Amps 