Requirements concerning audio power and audio fidelity of today’s loudspeakers and home theater products are constantly growing. At the core of these systems is the power amp. Recent power amplifiers have to perform well enough to satisfy those ever increasing requirements. With the ever increasing amount of models and design topologies, such as “tube amps”, “class-A”, “class-D” as well as “t amplifier” types, it is getting more and more complex to pick the amplifier that is best for a specific application. This article is going to explain a few of the most widespread terms and spell out a few of the technical jargon which amp producers frequently employ. Simply put, the principle of an audio amp is to translate a low-power music signal into a high-power audio signal. The high-power signal is great enough to drive a speaker adequately loud. To do that, an amplifier makes use of one or several elements that are controlled by the low-power signal in order to make a large-power signal. Those elements range from tubes, bipolar transistors to FET transistors.
Tube amps used to be widespread a few decades ago. A tube is able to control the current flow in accordance to a control voltage which is connected to the tube. One dilemma with tubes is that they are not extremely linear when amplifying signals. Aside from the original music, there are going to be overtones or higher harmonics present in the amplified signal. Thus tube amplifiers have rather high distortion. On the other hand, this characteristic of tube amplifiers still makes these popular. Many people describe tube amps as having a warm sound as opposed to the cold sound of solid state amps. Another downside of tube amplifiers, however, is the low power efficiency. The majority of power that tube amps use up is being dissipated as heat and only a part is being transformed into audio power. Yet an additional drawback is the high price tag of tubes. This has put tube amps out of the ballpark for many consumer products. As a result, the majority of audio products today makes use of solid state amps. I will describe solid state amps in the following paragraphs. Solid state amplifiers replace the tube with semiconductor elements, typically bipolar transistors or FETs. The earliest kind of solid-state amplifiers is generally known as class-A amplifiers. In a class-A amp, the signal is being amplified by a transistor which is controlled by the low-level audio signal. Class-A amps have the lowest distortion and typically also the smallest amount of noise of any amplifier architecture. If you require ultra-low distortion then you should take a closer look at class-A types. The major drawback is that much like tube amps class A amplifiers have extremely small efficiency. As a result these amplifiers require large heat sinks to radiate the wasted energy and are frequently rather heavy. To improve on the small efficiency of class-A amplifiers, class-AB amps utilize a number of transistors that each amplify a distinct area, each of which being more efficient than class-A amps. The higher efficiency of class-AB amplifiers also has two further advantages. First of all, the required number of heat sinking is reduced. As a result class-AB amps can be manufactured lighter and smaller. For that reason, class-AB amps can be made cheaper than class-A amplifiers. When the signal transitions between the two separate regions, though, some amount of distortion is being generated, thereby class-AB amplifiers will not achieve the same audio fidelity as class-A amps.
Class-D amps are able to attain power efficiencies above 90% by using a switching transistor which is constantly being switched on and off and thereby the transistor itself does not dissipate any heat. The switching transistor, which is being controlled by a pulse-width modulator generates a high-frequency switching component that has to be removed from the amplified signal by making use of a lowpass filter. The switching transistor and also the pulse-width modulator typically have fairly large non-linearities. As a consequence, the amplified signal is going to contain some distortion. Class-D amps by nature exhibit larger audio distortion than other types of small stereo amplifiers.
To solve the dilemma of large audio distortion, modern switching amplifier designs incorporate feedback. The amplified signal is compared with the original low-level signal and errors are corrected. “Class-T” amps (also called “t-amp”) utilize this type of feedback method and therefore can be made very small whilst attaining small music distortion.
A Brief Primer For Music Amps 