Power amplifiers are at the very center of each home theater system. As the quality and output power requirements of modern loudspeakers increase, so do the demands of music amplifiers. There is a big quantity of amplifier styles and models. All of these differ regarding performance. I will describe a few of the most common amp terms including “class-A”, “class-D” and “t amps” to help you figure out which of these amplifiers is best for your application. In addition, after understanding this essay you should be able to understand the amp specs that producers publish.
The fundamental operating principle of an audio amp is fairly simple. An audio amp will take a low-level audio signal. This signal typically originates from a source with a comparatively high impedance. It subsequently converts this signal into a large-level signal. This large-level signal can also drive loudspeakers with small impedance. To do that, an amplifier makes use of one or more elements that are controlled by the low-power signal to produce a large-power signal. Those elements range from tubes, bipolar transistors to FET transistors.
A number of decades ago, the most common kind of audio amp 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 transformed into a high-level signal. Unfortunately, tube amplifiers have a rather high amount of distortion. Technically speaking, tube amplifiers will introduce higher harmonics into the signal. Many people prefer tube amps because these higher harmonics are regularly perceived as the tube amp sounding “warm” or “pleasant”.
A disadvantage of tube amplifiers is their small power efficiency. In other words, most of the energy consumed by the amplifier is wasted as heat as opposed to being converted into music. Thus tube amps will run hot and require enough cooling. In addition, tubes are fairly costly to produce. Hence tube amplifiers have by and large been replaced by solid-state amplifiers which I am going to look at next.
Solid state amps replace the tube with semiconductor elements, usually bipolar transistors or FETs. The earliest kind of solid-state amps is called 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. Class-A amps have the lowest distortion and typically also the smallest amount of noise of any amplifier architecture. If you need ultra-low distortion then you should take a closer look at class-A models. Class-A amps, on the other hand, waste most of the energy as heat. As a result they typically have large heat sinks and are fairly heavy. By utilizing a number of transistors, class-AB amps improve on the low power efficiency of class-A amplifiers. The working region is divided into two distinct regions. These two areas are handled by separate transistors. Each of those transistors works more efficiently than the single transistor in a class-A amplifier. As such, class-AB amps are generally smaller than class-A amplifiers. When the signal transitions between the 2 separate areas, however, some amount of distortion is being generated, thereby class-AB amps will not achieve the same audio fidelity as class-A amplifiers. In order to further improve the audio efficiency, “class-D” amplifiers use a switching stage that is continuously switched between 2 states: on or off. None of these two states dissipates power within the transistor. Consequently, class-D amplifiers frequently are able to attain power efficiencies beyond 90%. The on-off switching times of the transistor are being controlled by a pulse-with modulator (PWM). Standard switching frequencies are between 300 kHz and 1 MHz. This high-frequency switching signal has to be removed from the amplified signal by a lowpass filter. Generally a straightforward first-order lowpass is being used. The switching transistor and also the pulse-width modulator generally have rather big non-linearities. As a result, the amplified signal will contain some distortion. Class-D amps by nature exhibit higher audio distortion than other kinds of mini amplifiers. New amps include internal audio feedback in order to minimize the level of audio distortion. A well-known architecture 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 offering the power efficiency of class-D amps. Consequently t amps can be made extremely small and yet achieve high audio fidelity.