Stereo amps are at the very heart of each home theater system. As the quality and output power demands of modern loudspeakers increase, so do the requirements of audio amplifiers. It is tough to select an amplifier given the large number of types and designs. I am going to clarify a few of the most common amplifier designs like “tube amps”, “linear amps”, “class-AB” and “class-D” along with “class-T amps” to help you comprehend some of the terms commonly utilized by amp producers. This essay should also help you figure out which topology is best for your precise application.
The main operating principle of an audio amp is rather straightforward. An audio amplifier will take a low-level audio signal. This signal generally comes from a source with a fairly large impedance. It then converts this signal into a large-level signal. This large-level signal can also drive loudspeakers with small impedance. In order to do that, an amp employs one or more 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.
A couple of decades ago, the most common kind of audio amplifier were tube amps. Tube amplifiers utilize a tube as the amplifying element. The current flow through the tube is controlled by a low-level control signal. Thereby the low-level audio is transformed into a high-level signal. Tubes, on the other hand, are nonlinear in their behavior and are going to introduce a rather large amount of higher harmonics or distortion. However, this characteristic of tube amps still makes these popular. A lot of people describe tube amplifiers as having a warm sound versus the cold sound of solid state amps.
A different downside of tube amplifiers, though, is the small power efficiency. The bulk of power which tube amps consume is being dissipated as heat and merely a fraction is being converted into audio power. Tube amps, though, a rather costly to manufacture and for that reason tube amps have by and large been replaced with amplifiers using transistor elements which are less costly to manufacture.
The first generation types of solid state amps are often known as “Class-A” amps. Solid-state amps use a semiconductor instead of a tube to amplify the signal. Usually bipolar transistors or FETs are being used. In class-A amps a transistor controls the current flow according to a small-level signal. A number of amps employ a feedback mechanism to minimize the harmonic distortion. If you need an ultra-low distortion amp then you may want to investigate class-A amplifiers since they offer amongst the lowest distortion of any audio amps. Class-A amplifiers, though, waste the majority of the power as heat. Therefore they generally have large heat sinks and are quite heavy.
To improve on the low efficiency of class-A amps, class-AB amplifiers make use of a series of transistors which each amplify a distinct area, each of which being more efficient than class-A amplifiers. As a result of the larger efficiency, class-AB amps do not need the same number of heat sinks as class-A amps. Therefore they can be made lighter and less expensive. Class-AB amps have a disadvantage however. Each time the amplified signal transitions from one region to the other, there will be certain distortion created. In other words the transition between these two regions is non-linear in nature. Consequently class-AB amps lack audio fidelity compared with class-A amplifiers. Class-D amps improve on the efficiency of class-AB amplifiers even further by utilizing a switching transistor that is continuously being switched on or off. Thus this switching stage hardly dissipates any energy and therefore the power efficiency of class-D amps usually surpasses 90%. The switching transistor is being controlled by a pulse-width modulator. The switched large-level signal needs to be lowpass filtered to remove the switching signal and recover the music signal. The switching transistor and also the pulse-width modulator frequently have rather large non-linearities. As a consequence, the amplified signal will have some distortion. Class-D amplifiers by nature have higher audio distortion than other types of audio amplifiers.
More recent small audio amplifiers incorporate some type of mechanism to minimize distortion. One method is to feed back the amplified audio signal to the input of the amp in order to compare with the original signal. The difference signal is subsequently used to correct the switching stage and compensate for the nonlinearity. One type of audio amplifiers which makes use of this kind of feedback is known as “class-T” or “t amp”. Class-T amplifiers feed back the high-level switching signal to the audio signal processor for comparison. These amps have small audio distortion and can be made extremely small.
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