A Quick Guide For Understanding Stereo Amplifiers

None of today's music products would be achievable without the help of today’s miniature amplifiers which try to satisfy higher and higher demands concerning power and music fidelity. It is tough to pick an amplifier given the huge range of products and designs. I am going to clarify a few of the most common amp designs like "tube amps", "linear amplifiers", "class-AB" and "class-D" in addition to "t amps" to help you comprehend some of the terms normally used by amp suppliers. This guide should also help you figure out what topology is perfect for your precise application.

The fundamental operating principle of an audio amp is rather basic. An audio amp is going to take a low-level music signal. This signal typically comes from a source with a rather large impedance. It then converts this signal into a large-level signal. This large-level signal may also drive loudspeakers with low impedance. Depending on the type of amp, one of several types of elements are used in order to amplify the signal such as tubes and transistors.

A couple of decades ago, the most popular type of audio amplifier were tube amplifiers. Tube amps employ 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 converted into a high-level signal. One dilemma with tubes is that they are not very linear whilst amplifying signals. Aside from the original audio, there are going to be overtones or higher harmonics present in the amplified signal. Consequently tube amplifiers have moderately high distortion. Today, tube amps still have many fans. The primary reason is that the distortion that tubes bring about are often perceived as "warm" or "pleasant". Solid state amplifiers with low distortion, on the other hand, are perceived as "cold".

Another disadvantage of tube amplifiers, however, is the small power efficiency. The majority of power which tube amps use up is being dissipated as heat and only a part is being converted into audio power. Moreover, tubes are fairly costly to produce. Hence tube amplifiers have by and large been replaced by solid-state amps which I will look at next.

Solid-state amplifiers make use of a semiconductor element, like a bipolar transistor or FET rather than the tube and the first type is called "class-A" amps. In class-A amps a transistor controls the current flow according to a small-level signal. A few amps make use of a feedback mechanism to minimize the harmonic distortion. Regarding harmonic distortion, class-A amps rank highest amongst all kinds of music amps. These amplifiers also regularly exhibit very low noise. As such class-A amplifiers are ideal for extremely demanding applications in which low distortion and low noise are important. The main disadvantage is that much like tube amps class A amps have quite small efficiency. Consequently these amps require big heat sinks in order to dissipate the wasted energy and are usually fairly large.

In order to improve on the small efficiency of class-A amplifiers, class-AB amps utilize a number of transistors which each amplify a distinct area, each of which being more efficient than class-A amplifiers. Because of the higher efficiency, class-AB amplifiers do not require the same amount of heat sinks as class-A amps. Therefore they can be manufactured lighter and less costly. When the signal transitions between the 2 separate areas, however, a certain amount of distortion is being generated, thereby class-AB amps will not achieve the same audio fidelity as class-A amps.

Class-D amps improve on the efficiency of class-AB amps even further by using a switching transistor which is constantly being switched on or off. Thus this switching stage hardly dissipates any energy and therefore the power efficiency of class-D amps typically exceeds 90%. The switching transistor, that is being controlled by a pulse-width modulator generates a high-frequency switching component that has to be removed from the amplified signal by using a lowpass filter. The switching transistor and also the pulse-width modulator generally exhibit rather large non-linearities. As a consequence, the amplified signal will contain some distortion. Class-D amps by nature exhibit larger audio distortion than other kinds of audio amps.

To solve the dilemma of high audio distortion, new switching amplifier designs include feedback. The amplified signal is compared with the original low-level signal and errors are corrected. A well-known topology that makes use of this type of feedback is called "class-T". Class-T amps or "t amps" achieve audio distortion that compares with the audio distortion of class-A amps while at the same time exhibiting the power efficiency of class-D amps. Therefore t amplifiers can be made extremely small and yet achieve high audio fidelity.