A Short Overview Of Power Amplifiers

Demands concerning audio power and audio fidelity of latest speakers and home theater systems are always growing. At the center of those products is the stereo amp. Latest power amps have to perform well enough to meet those always growing demands. It is tricky to choose an amplifier given the big range of products and concepts. I am going to clarify a few of the most popular amplifier designs such as “tube amplifiers”, “linear amps”, “class-AB” and “class-D” in addition to “class-T amps” to help you understand several of the terms regularly used by amplifier makers. This article should also help you figure out which topology is best for your particular application.

An audio amp will translate a low-level music signal which often originates from a high-impedance source into a high-level signal which may drive a loudspeaker with a low impedance. Depending on the type of amplifier, one of several types of elements are utilized in order to amplify the signal like tubes in addition to transistors. A couple of decades ago, the most common type of audio amplifier were tube amplifiers. Tube amplifiers utilize 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. One problem with tubes is that they are not extremely linear when amplifying signals. Aside from the original audio, there are going to be overtones or higher harmonics present in the amplified signal. For that reason tube amplifiers have rather high distortion. Though, this characteristic of tube amplifiers still makes these popular. Many people describe tube amplifiers as having a warm sound versus the cold sound of solid state amps.

An additional disadvantage of tube amplifiers, though, is the small power efficiency. The majority of power that tube amplifiers consume is being dissipated as heat and only a fraction is being transformed into audio power. Yet another disadvantage is the big price tag of tubes. This has put tube amps out of the ballpark for a lot of consumer devices. Because of this, the bulk of audio products these days employs solid state amplifiers. I am going to explain solid state amplifiers in the next sections. Solid-state amps make use of a semiconductor element, such as a bipolar transistor or FET as opposed to the tube and the earliest kind is generally known as “class-A” amps. In a class-A amp, the signal is being amplified by a transistor which is controlled by the low-level audio signal. In terms of harmonic distortion, class-A amps rank highest amongst all types of audio amplifiers. These amps also regularly exhibit very low noise. As such class-A amps are perfect for very demanding applications in which low distortion and low noise are important. The main downside is that much like tube amplifiers class A amplifiers have quite low efficiency. Consequently these amps need large heat sinks to dissipate the wasted energy and are typically fairly bulky.

In order to improve on the small efficiency of class-A amps, class-AB amps utilize a series of transistors which each amplify a distinct area, each of which being more efficient than class-A amps. Because of the higher efficiency, class-AB amplifiers do not require the same number of heat sinks as class-A amps. As a result they can be made lighter and less expensive. Class-AB amps have a downside however. Each time the amplified signal transitions from a region to the other, there will be certain distortion produced. In other words the transition between those 2 areas is non-linear in nature. As a result class-AB amplifiers lack audio fidelity compared with class-A amps.

In order to further improve the audio efficiency, “class-D” amplifiers use a switching stage which is constantly switched between 2 states: on or off. None of these two states dissipates energy inside the transistor. As a result, class-D amps frequently are able to achieve power efficiencies beyond 90%. The switching transistor, that is being controlled by a pulse-width modulator generates a high-frequency switching component that needs to be removed from the amplified signal by using a lowpass filter. The switching transistor and also the pulse-width modulator frequently have fairly large non-linearities. As a result, the amplified signal is going to contain some distortion. Class-D amplifiers by nature have larger audio distortion than other types of audio amplifiers. More recent audio amplifiers incorporate some sort of mechanism in order to reduce distortion. One method is to feed back the amplified music signal to the input of the amplifier in order to compare with the original signal. The difference signal is subsequently used to correct the switching stage and compensate for the nonlinearity. “Class-T” amps (also called “t-amp”) make use of this sort of feedback method and thus can be made very small while achieving low music distortion.

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