Demands regarding audio power and audio fidelity of today's speakers and home theater systems are constantly growing. At the heart of these systems is the audio amplifier. Today's audio amps have perform well enough to meet these ever growing demands. With the ever increasing amount of models and design topologies, such as "tube amps", "class-A", "class-D" and "t-amp" designs, it is becoming more and more difficult to pick the amp which is ideal for a particular application. This article will explain some of the most common terms and clarify some of the technical jargon which amplifier manufacturers often use.
An audio amp will convert a low-level audio signal which often comes from a high-impedance source into a high-level signal which can drive a loudspeaker with a low impedance. Depending on the type of amp, one of several types of elements are used to amplify the signal such as tubes and transistors.
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 Thus tube amps have mostly been replaced by solid-state amps which I will look at next.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.
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.
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.
To solve the problem of high audio distortion, newer switching amplifier designs incorporate feedback. The amplified signal is compared with the original low-level signal and errors are corrected. A well-known architecture which uses this type of feedback is known as "class-T". Class-T amps or "t amps" achieve audio distortion which compares with the audio distortion of class-A amps while at the same type offer the power efficiency of class-D amps. Thus t amps can be made extremely small and still achieve high audio fidelity.
An audio amp will convert a low-level audio signal which often comes from a high-impedance source into a high-level signal which can drive a loudspeaker with a low impedance. Depending on the type of amp, one of several types of elements are used to amplify the signal such as tubes and transistors.
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.
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.
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.
To solve the problem of high audio distortion, newer switching amplifier designs incorporate feedback. The amplified signal is compared with the original low-level signal and errors are corrected. A well-known architecture which uses this type of feedback is known as "class-T". Class-T amps or "t amps" achieve audio distortion which compares with the audio distortion of class-A amps while at the same type offer the power efficiency of class-D amps. Thus t amps can be made extremely small and still achieve high audio fidelity.
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You can get further information about t amps and stereo amplifiers from Amphony's website.
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