Music amplifiers are at the very center of each home theater product. As the quality and output power demands of today’s speakers increase, so do the demands of mini audio amplifiers. It is challenging to pick an amplifier given the large range of types and concepts. I will explain some of the most popular amp designs such as “tube amplifiers”, “linear amplifiers”, “class-AB” and “class-D” and also “class-T amplifiers” to help you comprehend some of the terms frequently utilized by amp manufacturers. This article should also help you figure out which topology is best for your precise application. Simply put, the use of an audio amp is to translate a low-power audio signal into a high-power music signal. The high-power signal is large enough to drive a speaker adequately loud. As a way to do that, an amp uses one or several elements that are controlled by the low-power signal to create a large-power signal. These elements range from tubes, bipolar transistors to FET transistors. Tube amplifiers were commonly used a number of decades ago and utilize a vacuum tube that controls a high-voltage signal in accordance to a low-voltage control signal. Regrettably, tube amplifiers have a somewhat high amount of distortion. Technically speaking, tube amplifiers will introduce higher harmonics into the signal. 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.
One disadvantage of tube amps is their low power efficiency. In other words, most of the power consumed by the amplifier is wasted as heat rather than being converted into music. For that reason tube amps will run hot and require adequate cooling. Yet another drawback is the big price tag of tubes. This has put tube amps out of the ballpark for the majority of consumer devices. Consequently, the bulk of audio products today employs solid state amplifiers. I will explain solid state amps in the subsequent paragraphs.
Solid-state amplifiers utilize a semiconductor element, like a bipolar transistor or FET instead of the tube and the earliest type 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. If you need an ultra-low distortion amplifier then you might wish to investigate class-A amplifiers as they offer amongst the lowest distortion of any audio amplifiers. The major disadvantage is that similar to tube amps class A amps have quite small efficiency. Consequently these amps need big heat sinks to dissipate the wasted energy and are frequently quite heavy.
Class-AB amplifiers improve on the efficiency of class-A amplifiers. They employ a number of transistors to break up the large-level signals into two distinct regions, each of which can be amplified more efficiently. Because of the larger efficiency, class-AB amps do not need the same amount of heat sinks as class-A amps. Therefore they can be made lighter and less expensive. Nonetheless, this topology adds some non-linearity or distortion in the region where the signal switches between those areas. As such class-AB amps normally have larger distortion than class-A amplifiers. Class-D amps improve on the efficiency of class-AB amplifiers even further by employing a switching transistor that is always being switched on or off. Thereby this switching stage hardly dissipates any power and therefore the power efficiency of class-D amps generally surpasses 90%. The switching transistor is being controlled by a pulse-width modulator. The switched large-level signal has to be lowpass filtered to remove the switching signal and recover the audio signal. Due to non-linearities of the pulse-width modulator and the switching transistor itself, class-D amps by nature have amongst the largest audio distortion of any audio amp. More recent audio amps incorporate some kind of means to minimize distortion. One method is to feed back the amplified audio signal to the input of the amp to compare with the original signal. The difference signal is then used in order to correct the switching stage and compensate for the nonlinearity. “Class-T” amps (also called “t-amplifier”) utilize this kind of feedback method and for that reason can be manufactured very small while attaining low audio distortion.