Requirements regarding audio power and audio fidelity of latest loudspeakers and home theater products are always increasing. At the center of those products is the power amplifier. Latest power amplifiers have to perform well enough to satisfy those always growing requirements. It is tough to pick an amplifier given the large number of products and concepts. I am going to clarify a few of the most widespread amp designs such as “tube amplifiers”, “linear amps”, “class-AB” and “class-D” along with “class-T amps” to help you understand several of the terms frequently used by amp makers. This guide should also help you figure out what topology is perfect for your precise application.
An audio amp will convert a low-level audio signal which frequently originates from a high-impedance source into a high-level signal that may drive a speaker with a low impedance. As a way to do that, an amplifier utilizes one or several elements which are controlled by the low-power signal in order to generate a large-power signal. These elements range from tubes, bipolar transistors to FET transistors.
Tube amps used to be widespread some decades ago. A tube is able to control the current flow in accordance to a control voltage which is attached to the tube. Tubes, however, are nonlinear in their behavior and will introduce a rather large amount of higher harmonics or distortion. These days, tube amps still have many fans. The primary reason is that the distortion that tubes cause are often perceived as “warm” or “pleasant”. Solid state amps with small distortion, on the other hand, are perceived as “cold”. Also, tube amps have fairly low power efficiency and consequently dissipate a lot of power as heat. Yet another drawback is the big price tag of tubes. This has put tube amplifiers out of the ballpark for the majority of consumer devices. Consequently, the bulk of audio products nowadays makes use of solid state amplifiers. I will describe solid state amplifiers in the subsequent sections. Solid state amps replace the tube with semiconductor elements, usually bipolar transistors or FETs. The first type of solid-state amps is often known as class-A amps. In class-A amps a transistor controls the current flow according to a small-level signal. Several amps use a feedback mechanism to minimize the harmonic distortion. In terms of harmonic distortion, class-A amps rank highest amid all kinds of music amplifiers. These amps also regularly exhibit very low noise. As such class-A amplifiers are perfect for extremely demanding applications in which low distortion and low noise are essential. However, similar to tube amplifiers, class-A amps have quite low power efficiency and most of the energy is wasted.
Class-AB amps improve on the efficiency of class-A amps. They make use of a series of transistors to split up the large-level signals into two distinct areas, each of which can be amplified more efficiently. The higher efficiency of class-AB amps also has two further benefits. Firstly, the required amount of heat sinking is minimized. Consequently class-AB amplifiers can be made lighter and smaller. For that reason, class-AB amps can be manufactured cheaper than class-A amplifiers. Class-AB amplifiers have a downside though. Each time the amplified signal transitions from one region to the other, there will be some distortion produced. In other words the transition between these 2 regions is non-linear in nature. Consequently class-AB amps lack audio fidelity compared with class-A amps.
In order to further improve the audio efficiency, “class-D” amps use a switching stage that is continuously switched between 2 states: on or off. None of these 2 states dissipates power inside the transistor. Therefore, class-D amplifiers regularly are able to achieve power efficiencies higher than 90%. The switching transistor is being controlled by a pulse-width modulator. The switched large-level signal needs to be lowpass filtered in order to remove the switching signal and get back the music signal. Both the pulse-width modulator and the transistor have non-linearities which result in class-D amps having larger music distortion than other kinds of amplifiers.
Newer amplifiers incorporate internal audio feedback in order to reduce the amount of audio distortion. A well-known topology that makes use of this sort of feedback is generally 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 time having the power efficiency of class-D amplifiers. Thus t amplifiers can be manufactured extremely small and still achieve high audio fidelity.
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