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".
Another drawback of tube amps, though, is the low power efficiency. The majority of power which tube amps consume is being dissipated as heat and only a fraction is being converted into audio power. Also, tubes are quite expensive to make. Thus tube amps have mostly been replaced by solid-state amps which I will look at next.
Solid-state amps use a semiconductor elements, such as a bipolar transistor or FET in place of the tube and the earliest type is known as "class-A" amps. The working principle of class-A amps is very similar to that of tube amps. The main difference is that a transistor is being used in place of the tube for amplifying the audio signal. The amplified high-level signal is sometimes fed back in order to minimize harmonic distortion. If you require an ultra-low distortion amplifier then you might want to investigate class-A amps since they offer amongst the lowest distortion of any audio amps. However, similar to tube amps, class-A amps have very low power efficiency and most of the energy is wasted.
By using a series of transistors, class-AB amps improve on the low power efficiency of class-A amps. The operating working area is divided in two separate areas. These two areas are handled by separate transistors. Each of these transistors works more efficiently than the single transistor in a class-A amp. The higher efficiency of class-AB amps also has two other advantages. Firstly, the required amount of heat sinking is reduced. Therefore class-AB amps can be made lighter and smaller. For that reason, class-AB amps can be made cheaper than class-A amps. Class-AB amps have a drawback though. Every time the amplified signals 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. Therefore class-AB amps lack audio fidelity compared with class-A amps.
To further improve the audio efficiency, "class-D" amps use a switching stage which is constantly switched between two states: on or off. None of these 2 states dissipates power inside the transistor. Therefore, class-D amps regularly are able to achieve power efficiencies beyond 90%. The on-off switching times of the transistor are being controlled by a pulse-with modulator (PWM). Typical switching frequencies are between 300 kHz and 1 MHz. This high-frequency switching signal has to be removed from the amplified signal by a lowpass filter. Typically a simple first-order lowpass is being used. The switching transistor and also the pulse-width modulator usually have fairly large non-linearities. As a result, the amplified signal will contain some distortion. Class-D amps by nature have higher audio distortion than other types of audio amplifiers.
Newer amps incorporate internal audio feedback to minimize the amount of audio distortion. One type of audio amps which uses this type of feedback is known as "class-T" or "t amp". Class-T amps feed back the high-level switching signal to the audio signal processor for comparison. These amps have low audio distortion and can be made extremely small.
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".
Another drawback of tube amps, though, is the low power efficiency. The majority of power which tube amps consume is being dissipated as heat and only a fraction is being converted into audio power. Also, tubes are quite expensive to make. Thus tube amps have mostly been replaced by solid-state amps which I will look at next.
Solid-state amps use a semiconductor elements, such as a bipolar transistor or FET in place of the tube and the earliest type is known as "class-A" amps. The working principle of class-A amps is very similar to that of tube amps. The main difference is that a transistor is being used in place of the tube for amplifying the audio signal. The amplified high-level signal is sometimes fed back in order to minimize harmonic distortion. If you require an ultra-low distortion amplifier then you might want to investigate class-A amps since they offer amongst the lowest distortion of any audio amps. However, similar to tube amps, class-A amps have very low power efficiency and most of the energy is wasted.
By using a series of transistors, class-AB amps improve on the low power efficiency of class-A amps. The operating working area is divided in two separate areas. These two areas are handled by separate transistors. Each of these transistors works more efficiently than the single transistor in a class-A amp. The higher efficiency of class-AB amps also has two other advantages. Firstly, the required amount of heat sinking is reduced. Therefore class-AB amps can be made lighter and smaller. For that reason, class-AB amps can be made cheaper than class-A amps. Class-AB amps have a drawback though. Every time the amplified signals 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. Therefore class-AB amps lack audio fidelity compared with class-A amps.
To further improve the audio efficiency, "class-D" amps use a switching stage which is constantly switched between two states: on or off. None of these 2 states dissipates power inside the transistor. Therefore, class-D amps regularly are able to achieve power efficiencies beyond 90%. The on-off switching times of the transistor are being controlled by a pulse-with modulator (PWM). Typical switching frequencies are between 300 kHz and 1 MHz. This high-frequency switching signal has to be removed from the amplified signal by a lowpass filter. Typically a simple first-order lowpass is being used. The switching transistor and also the pulse-width modulator usually have fairly large non-linearities. As a result, the amplified signal will contain some distortion. Class-D amps by nature have higher audio distortion than other types of audio amplifiers.
Newer amps incorporate internal audio feedback to minimize the amount of audio distortion. One type of audio amps which uses this type of feedback is known as "class-T" or "t amp". Class-T amps feed back the high-level switching signal to the audio signal processor for comparison. These amps have low audio distortion and can be made extremely small.
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