Music amplifiers are at the very heart of every home theater system. As the quality and output power demands of modern loudspeakers increase, so do the demands of audio amps. With the ever growing amount of models and design topologies, such as “tube amplifiers”, “class-A”, “class-D” as well as “t amplifier” types, it is getting more and more demanding to pick the amp that is best for a particular application. This post will describe a few of the most common terms and spell out some of the technical jargon which amp producers regularly employ.
The basic operating principle of an audio amp is rather simple. An audio amp will take a low-level audio signal. This signal regularly originates 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. To do that, an amp uses one or several elements which are controlled by the low-power signal in order to create a large-power signal. Those elements range from tubes, bipolar transistors to FET transistors.
Tube amps used to be widespread several decades ago. A tube is able to control the current flow according to a control voltage that is connected to the tube. Tubes, however, are nonlinear in their behavior and are going to introduce a quite large amount of higher harmonics or distortion. Though, this characteristic of tube amplifiers still makes these popular. A lot of people describe tube amplifiers as having a warm sound as opposed to the cold sound of solid state amplifiers.
One disadvantage of tube amplifiers is their small power efficiency. In other words, most of the energy consumed by the amplifier is wasted as heat rather than being transformed into audio. For that reason tube amplifiers will run hot and require sufficient cooling. Moreover, tubes are quite costly to build. Therefore tube amplifiers have mostly been replaced by solid-state amps which I will look at next. Solid state amps replace the tube with semiconductor elements, usually bipolar transistors or FETs. The earliest type of solid-state amplifiers is called class-A amplifiers. In class-A amps a transistor controls the current flow according to a small-level signal. A number of amps use a feedback mechanism in order to minimize the harmonic distortion. Regarding harmonic distortion, class-A amplifiers rank highest among all kinds of audio amps. These amplifiers also regularly exhibit very low noise. As such class-A amplifiers are perfect for extremely demanding applications in which low distortion and low noise are crucial. Class-A amps, on the other hand, waste the majority of the energy as heat. For that reason they frequently have big heat sinks and are fairly bulky. By making use of a series of transistors, class-AB amps improve on the low power efficiency of class-A amplifiers. The operating area is divided into two distinct regions. These two areas 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 amplifiers are generally smaller than class-A amplifiers. When the signal transitions between the 2 separate regions, however, some level of distortion is being created, thereby class-AB amplifiers will not achieve the same audio fidelity as class-A amps.
In order to further improve the audio efficiency, “class-D” amplifiers utilize a switching stage which is continually switched between 2 states: on or off. None of these 2 states dissipates energy inside the transistor. As a result, class-D amps frequently are able to achieve power efficiencies beyond 90%. The on-off switching times of the transistor are being controlled by a pulse-with modulator (PWM). Usual switching frequencies are in the range of 300 kHz and 1 MHz. This high-frequency switching signal needs to be removed from the amplified signal by a lowpass filter. Commonly a simple first-order lowpass is being used. Both the pulse-width modulator and the transistor have non-linearities that result in class-D amps having bigger audio distortion than other types of amplifiers. More recent mini stereo amps include some sort of mechanism in order to reduce distortion. One approach is to feed back the amplified audio signal to the input of the amplifier to compare with the original signal. The difference signal is subsequently used to correct the switching stage and compensate for the nonlinearity. “Class-T” amplifiers (also referred to as “t-amplifier”) use this type of feedback mechanism and for that reason can be manufactured very small whilst attaining low audio distortion.