Stereo amplifiers are at the very core of each home theater system. As the quality and output power demands of modern speakers increase, so do the demands of power amplifiers. With the ever increasing number of models and design topologies, like “tube amplifiers”, “class-A”, “class-D” along with “t amp” types, it is becoming more and more demanding to pick the amplifier which is best for a specific application. This article will describe a few of the most popular terms and clarify a few of the technical jargon that amplifier makers often utilize.

The basic operating principle of an audio amplifier is fairly clear-cut. An audio amp is going to take a low-level audio signal. This signal regularly comes from a source with a rather high impedance. It subsequently translates this signal into a large-level signal. This large-level signal may also drive speakers with small impedance. Depending on the kind of amplifier, one of several types of elements are used in order to amplify the signal such as tubes as well as transistors.

Tube amplifiers were commonly used a number of decades ago and use a vacuum tube which controls a high-voltage signal in accordance to a low-voltage control signal. Regrettably, tube amplifiers have a fairly high amount of distortion. Technically speaking, tube amplifiers will introduce higher harmonics into the signal. A lot of people prefer tube amplifiers because those higher harmonics are regularly perceived as the tube amplifier sounding “warm” or “pleasant”.

Moreover, tube amplifiers have rather small power efficiency and therefore dissipate much power as heat. Tube amps, on the other hand, a fairly costly to produce and thus tube amplifiers have mostly been replaced with amps employing transistor elements which are less expensive to produce.

Solid-state amplifiers use a semiconductor element, like a bipolar transistor or FET rather than the tube and the earliest type is generally known as “class-A” amps. The working principle of class-A amps is quite similar to that of tube amps. The primary difference is that a transistor is being used rather than the tube for amplifying the music signal. The amplified high-level signal is sometimes fed back in order to lessen harmonic distortion. Regarding harmonic distortion, class-A amps rank highest among all types of music amplifiers. These amps also usually exhibit very low noise. As such class-A amps are ideal for quite demanding applications in which low distortion and low noise are essential. The main disadvantage is that much like tube amplifiers class A amplifiers have extremely low efficiency. As a result these amplifiers require large heat sinks to dissipate the wasted energy and are usually rather large.

To improve on the low efficiency of class-A amps, class-AB amps make use of a number of transistors that each amplify a distinct area, each of which being more efficient than class-A amplifiers. As such, class-AB amps are generally smaller than class-A amps. When the signal transitions between the two distinct regions, though, some amount of distortion is being created, thus class-AB amps will not achieve the same audio fidelity as class-A amps.

Class-D amplifiers improve on the efficiency of class-AB amps even further by using a switching transistor that is constantly being switched on or off. Thus this switching stage hardly dissipates any power and consequently 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). Standard switching frequencies are in the range of 300 kHz and 1 MHz. This high-frequency switching signal has to be removed from the amplified signal by a lowpass filter. Typically a straightforward first-order lowpass is being utilized. Both the pulse-width modulator and the transistor have non-linearities which result in class-D amps exhibiting larger music distortion than other kinds of amps.

In order to solve the dilemma of high audio distortion, modern switching amp designs incorporate feedback. The amplified signal is compared with the original low-level signal and errors are corrected. A well-known architecture which makes use of this sort 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 time having the power efficiency of class-D amplifiers. Consequently t amplifiers can be made extremely small and still attain high audio fidelity.

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