Audio amplifiers are at the very heart of every home theater system. As the quality and output power requirements of today's loudspeakers increase, so do the demands of audio amps. It is hard to pick an amplifier given the large number of models and designs. I will explain some of the most common amplifier designs such as "tube amps", "linear amps", "class-AB" and "class-D" as well as "class-t amps" to help you understand some of the terms commonly used by amplifier manufacturers. This guide should also help you figure out which topology is ideal for your particular application.
The basic operating principle of an audio amp is fairly straightforward. An audio amp will take a low-level audio signal. This signal usually comes from a source with a fairly high impedance. It then converts this signal into a large-level signal. This large-level signal can also drive speakers with low impedance. The type of element used to amplify the signal depends on which amplifier architecture is used. Some amps even use several types of elements. Typically the following parts are used: tubes, bipolar transistors and FETs.
Tube amps were commonly used a few decades ago and utilize a vacuum tube which controls a high-voltage signal in accordance to a low-voltage control signal. Tubes, however, are nonlinear in their behavior and will introduce a fairly large amount of higher harmonics or distortion. A lot of people prefer tube amps because these higher harmonics are often perceived as the tube amp sounding "warm" or "pleasant".
One drawback of tube amps is their low power efficiency. In other words, most of the energy consumed by the amp is wasted as heat rather than being converted into audio. Therefore tube amps will run hot and need sufficient cooling. Tube amps, however, a fairly expensive to make and therefore tube amps have mostly been replaced with amps using transistor elements which are less expensive to manufacture.
The first generation models of solid state amps are known as "Class-A" amps. Solid-state amps use a semiconductor rather than a tube to amplify the signal. Usually bipolar transistors or FETs are being used. In a class-A amp, the signal is being amplified by a transistor which is controlled by the low-level audio signal. In terms of harmonic distortion, class-A amps rank highest amongst all types of audio amps. These amps also usually exhibit very low noise. As such class-A amps are ideal for very demanding applications in which low distortion and low noise a crucial. Class-A amps, however, waste most of the energy as heat. Therefore they usually have large heat sinks and are fairly heavy.
Class-AB amps improve on the efficiency of class-A amps. They use a series of transistors to break up the large-level signals into two separate areas, each of which can be amplified more efficiently. As such, class-AB amps are usually smaller than class-A amps. However, this topology adds some non-linearity or distortion in the region where the signal switches between those areas. As such class-AB amps typically have higher distortion than class-A amps.
Class-D amps are able to achieve power efficiencies above 90% by using a switching transistor which is constantly being switched on and off and thus the transistor itself does not dissipate any heat. The switching transistor, which is being controlled by a pulse-width modulator generates a high-frequency switching component which has to be removed from the amplified signal by using a lowpass filter. Both the pulse-width modulator and the transistor have non-linearities which result in class-D amps having larger audio distortion than other types of amplifiers.
More recent audio amps incorporate some sort of mechanism to minimize distortion. One approach is to feed back the amplified audio signal to the input of the amp to compare with the amplified signal. The difference signal is then used to correct the switching stage and compensate for the nonlinearity. "Class-T" amps (also called "t-amp") use this type of feedback mechanism and therefore can be made extremely small while achieving low audio distortion.
The basic operating principle of an audio amp is fairly straightforward. An audio amp will take a low-level audio signal. This signal usually comes from a source with a fairly high impedance. It then converts this signal into a large-level signal. This large-level signal can also drive speakers with low impedance. The type of element used to amplify the signal depends on which amplifier architecture is used. Some amps even use several types of elements. Typically the following parts are used: tubes, bipolar transistors and FETs.
Tube amps were commonly used a few decades ago and utilize a vacuum tube which controls a high-voltage signal in accordance to a low-voltage control signal. Tubes, however, are nonlinear in their behavior and will introduce a fairly large amount of higher harmonics or distortion. A lot of people prefer tube amps because these higher harmonics are often perceived as the tube amp sounding "warm" or "pleasant".
One drawback of tube amps is their low power efficiency. In other words, most of the energy consumed by the amp is wasted as heat rather than being converted into audio. Therefore tube amps will run hot and need sufficient cooling. Tube amps, however, a fairly expensive to make and therefore tube amps have mostly been replaced with amps using transistor elements which are less expensive to manufacture.
The first generation models of solid state amps are known as "Class-A" amps. Solid-state amps use a semiconductor rather than a tube to amplify the signal. Usually bipolar transistors or FETs are being used. In a class-A amp, the signal is being amplified by a transistor which is controlled by the low-level audio signal. In terms of harmonic distortion, class-A amps rank highest amongst all types of audio amps. These amps also usually exhibit very low noise. As such class-A amps are ideal for very demanding applications in which low distortion and low noise a crucial. Class-A amps, however, waste most of the energy as heat. Therefore they usually have large heat sinks and are fairly heavy.
Class-AB amps improve on the efficiency of class-A amps. They use a series of transistors to break up the large-level signals into two separate areas, each of which can be amplified more efficiently. As such, class-AB amps are usually smaller than class-A amps. However, this topology adds some non-linearity or distortion in the region where the signal switches between those areas. As such class-AB amps typically have higher distortion than class-A amps.
Class-D amps are able to achieve power efficiencies above 90% by using a switching transistor which is constantly being switched on and off and thus the transistor itself does not dissipate any heat. The switching transistor, which is being controlled by a pulse-width modulator generates a high-frequency switching component which has to be removed from the amplified signal by using a lowpass filter. Both the pulse-width modulator and the transistor have non-linearities which result in class-D amps having larger audio distortion than other types of amplifiers.
More recent audio amps incorporate some sort of mechanism to minimize distortion. One approach is to feed back the amplified audio signal to the input of the amp to compare with the amplified signal. The difference signal is then used to correct the switching stage and compensate for the nonlinearity. "Class-T" amps (also called "t-amp") use this type of feedback mechanism and therefore can be made extremely small while achieving low audio distortion.
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You can get further information regarding t-amp models as well as stereo amplifiers from Amphony's website.
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