A Brief Primer For Audio Amplifiers

Requirements regarding audio power and audio fidelity of recent speakers and home theater products are continuously increasing. At the center of those systems is the audio amp. Modern audio amplifiers have to perform well enough to meet those ever growing requirements. With the ever growing number of models and design topologies, like "tube amplifiers", "class-A", "class-D" in addition to "t amplifier" types, it is getting more and more complex to pick the amplifier that is perfect for a specific application. This guide is going to explain some of the most widespread terms and spell out some of the technical jargon that amp suppliers regularly use.

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 adequately loud. The kind of element utilized to amplify the signal depends on what amp architecture is utilized. Some amplifiers even use several kinds of elements. Generally the following parts are used: tubes, bipolar transistors and FETs.

Several decades ago, the most widespread 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 converted into a high-level signal. Tubes, though, are nonlinear in their behavior and will introduce a quite large level of higher harmonics or distortion. A lot of people prefer tube amps because those higher harmonics are frequently perceived as the tube amplifier sounding "warm" or "pleasant".

Also, tube amplifiers have quite low power efficiency and consequently radiate a lot of power as heat. Furthermore, tubes are rather expensive to manufacture. Hence tube amplifiers have by and large been replaced by solid-state amps which I am going to look at next.

Solid state amps replace the tube with semiconductor elements, generally bipolar transistors or FETs. The earliest kind of solid-state amplifiers is known as class-A amps. In a class-A amplifier, the signal is being amplified by a transistor which is controlled by the low-level audio signal. In terms of harmonic distortion, class-A amplifiers rank highest amongst all kinds of power amps. These amps also usually exhibit quite low noise. As such class-A amplifiers are perfect for very demanding applications in which low distortion and low noise are important. Class-A amplifiers, however, waste most of the power as heat. For that reason they frequently have big heat sinks and are quite bulky.

By making use of a number of transistors, class-AB amps improve on the small power efficiency of class-A amps. The operating area is split into two separate regions. These 2 regions are handled by separate transistors. Each of these transistors works more efficiently than the single transistor in a class-A amp. As such, class-AB amps are typically smaller than class-A amplifiers. Though, this topology adds some non-linearity or distortion in the area where the signal switches between those regions. As such class-AB amplifiers typically have larger distortion than class-A amps.

In order to further improve the audio efficiency, "class-D" amplifiers make use of a switching stage which is continuously switched between two states: on or off. None of these 2 states dissipates power within the transistor. As a result, class-D amplifiers frequently are able to achieve power efficiencies beyond 90%. The switching transistor, which is being controlled by a pulse-width modulator generates a high-frequency switching component which has to be removed from the amplified signal by using a lowpass filter. Both the pulse-width modulator and the transistor have non-linearities which result in class-D amps exhibiting larger audio distortion than other types of amps.

More modern audio amplifiers incorporate some sort of mechanism to minimize distortion. One approach is to feed back the amplified music signal to the input of the amp to compare with the original signal. The difference signal is subsequently used in order to correct the switching stage and compensate for the nonlinearity. One type of audio amps which uses this type of feedback is called "class-T" or "t amp". Class-T amplifiers feed back the high-level switching signal to the audio signal processor for comparison. These amplifiers exhibit low audio distortion and can be manufactured very small.