Stereo amps are at the very center of each home theater system. As the quality and output power requirements of modern speakers increase, so do the requirements of mini stereo amps. It is tricky to pick an amp given the big range of models and concepts. I am going to clarify a few of the most widespread amp designs including “tube amplifiers”, “linear amps”, “class-AB” and “class-D” and also “class-T amplifiers” to help you understand a few of the terms regularly utilized by amplifier producers. This essay should also help you figure out which topology is perfect for your specific application.
Simply put, the purpose of an audio amplifier is to convert a low-power audio signal into a high-power audio signal. The high-power signal is great enough to drive a speaker sufficiently loud. As a way to do that, an amplifier makes use of one or more elements that are controlled by the low-power signal in order to create a large-power signal. These elements range from tubes, bipolar transistors to FET transistors.
Several decades ago, the most common kind of audio amplifier were tube amps. Tube amplifiers employ a tube as the amplifying element. The current flow through the tube is controlled by a low-level control signal. In that way the low-level audio is converted into a high-level signal. Tubes, though, are nonlinear in their behavior and will introduce a quite large amount of higher harmonics or distortion. Many people favor tube amps since those higher harmonics are frequently perceived as the tube amp sounding “warm” or “pleasant”.
A different drawback of tube amplifiers, though, is the low power efficiency. The bulk of power which tube amplifiers use up is being dissipated as heat and merely a fraction is being converted into audio power. Yet one more downside is the big price tag of tubes. This has put tube amps out of the ballpark for many consumer products. Consequently, the bulk of audio products these days uses solid state amplifiers. I am going to describe solid state amps in the subsequent sections. The first generation versions of solid state amplifiers are generally known as “Class-A” amps. Solid-state amplifiers make use of a semiconductor instead of a tube to amplify the signal. Generally bipolar transistors or FETs are being utilized. In a class-A amp, the signal is being amplified by a transistor which is controlled by the low-level audio signal. Class-A amps have the smallest distortion and usually also the smallest amount of noise of any amplifier architecture. If you need ultra-low distortion then you should take a closer look at class-A types. The main downside is that similar to tube amps class A amps have extremely small efficiency. Consequently these amplifiers require large heat sinks to radiate the wasted energy and are frequently quite large.
Class-AB amps improve on the efficiency of class-A amps. They use a series of transistors in order to split up the large-level signals into 2 distinct areas, each of which can be amplified more efficiently. As such, class-AB amps are generally smaller than class-A amps. Nonetheless, this architecture adds some non-linearity or distortion in the area where the signal switches between those areas. As such class-AB amplifiers normally have larger distortion than class-A amps. Class-D amplifiers improve on the efficiency of class-AB amps even further by utilizing a switching transistor that is constantly being switched on or off. Thereby this switching stage barely dissipates any power and thus the power efficiency of class-D amps frequently exceeds 90%. The on-off switching times of the transistor are being controlled by a pulse-with modulator (PWM). Typical switching frequencies are in the range of 300 kHz and 1 MHz. This high-frequency switching signal needs to be removed from the amplified signal by a lowpass filter. Generally a straightforward first-order lowpass is being used. Both the pulse-width modulator and the transistor have non-linearities which result in class-D amps having larger music distortion than other types of amps.
In order to resolve the problem of high music distortion, newer switching amplifier designs include feedback. The amplified signal is compared with the original low-level signal and errors are corrected. One kind of audio amps which employs this type of feedback is called “class-T” or “t amplifier”. Class-T amplifiers feed back the high-level switching signal to the audio signal processor for comparison. These amps have low music distortion and can be manufactured extremely small.
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