None of latest audio systems would be possible lacking the help of today’s stereo amplifiers which strive to satisfy higher and higher demands concerning power and audio fidelity. There is a large quantity of amp concepts and types. All of these vary in terms of performance. I will describe some of the most common amplifier terms such as “class-A”, “class-D” and “t amps” to help you figure out which of these amplifiers is best for your application. In addition, after understanding this essay you should be able to comprehend the amplifier specs that suppliers issue. An audio amp will convert a low-level music signal that often comes from a high-impedance source into a high-level signal which may drive a loudspeaker with a low impedance. In order to do that, an amplifier utilizes one or several elements that are controlled by the low-power signal in order to produce a large-power signal. These elements range from tubes, bipolar transistors to FET transistors. A few decades ago, the most common type of audio amp were tube amps. Tube amplifiers make use of a tube as the amplifying element. The current flow through the tube is controlled by a low-level control signal. Thereby the low-level audio is transformed into a high-level signal. One drawback with tubes is that they are not very linear whilst amplifying signals. Aside from the original music, there are going to be overtones or higher harmonics present in the amplified signal. Thus tube amplifiers have quite high distortion. 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 amps. A downside of tube amps is their small power efficiency. In other words, the majority of the energy consumed by the amplifier is wasted as heat instead of being converted into audio. As a result tube amps are going to run hot and require sufficient cooling. Moreover, tubes are pretty costly to manufacture. As a result tube amplifiers have generally been replaced by solid-state amplifiers which I will look at next.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 earliest type of solid-state amps is generally known as class-A amplifiers. In class-A amps a transistor controls the current flow according to a small-level signal. A number of amps use a feedback mechanism to reduce the harmonic distortion. In terms of harmonic distortion, class-A amps rank highest amongst all kinds of music amplifiers. These amplifiers also regularly exhibit quite low noise. As such class-A amps are perfect for quite demanding applications in which low distortion and low noise are important. Class-A amps, though, waste the majority of the energy as heat. Therefore they typically have big heat sinks and are quite heavy.
By making use of a series of transistors, class-AB amplifiers improve on the small power efficiency of class-A amps. The operating area is divided into two distinct regions. These two regions are handled by separate transistors. Each of these transistors operates more efficiently than the single transistor in a class-A amp. Due to the larger efficiency, class-AB amplifiers do not require the same number of heat sinks as class-A amps. Consequently they can be made lighter and cheaper. Class-AB amplifiers have a drawback however. Every time the amplified signal transitions from one region to the other, there will be some distortion generated. In other words the transition between these two areas is non-linear in nature. Consequently class-AB amps lack audio fidelity compared with class-A amplifiers.
Class-D amps are able to attain power efficiencies higher than 90% by making use of a switching transistor which is constantly being switched on and off and as a result the transistor itself does not dissipate any heat. The switching transistor, that is being controlled by a pulse-width modulator generates a high-frequency switching component that needs to be removed from the amplified signal by using a lowpass filter. The switching transistor and also the pulse-width modulator typically exhibit quite large non-linearities. As a consequence, the amplified signal is going to contain some distortion. Class-D amps by nature exhibit larger audio distortion than other types of mini audio amplifiers.
To solve the dilemma of large music distortion, modern switching amp designs incorporate feedback. The amplified signal is compared with the original low-level signal and errors are corrected. “Class-T” amplifiers (also called “t-amplifier”) utilize this sort of feedback mechanism and therefore can be made extremely small whilst achieving low audio distortion.
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