Requirements regarding audio power and audio fidelity of latest loudspeakers and home theater systems are continuously increasing. At the core of these systems is the audio amplifier. Recent music amps have to perform well enough to meet those ever growing requirements. It is tricky to select an amp given the big range of models and concepts. I will describe a few of the most common amp designs such as “tube amps”, “linear amplifiers”, “class-AB” and “class-D” in addition to “class-T amplifiers” to help you comprehend a few of the terms frequently used by amplifier makers. This article should also help you figure out what topology is perfect for your particular application.
An audio amp is going to convert a low-level music signal that often comes from a high-impedance source into a high-level signal that can drive a loudspeaker with a low impedance. As a way to do that, an amplifier utilizes one or more elements which are controlled by the low-power signal to create a large-power signal. These elements range from tubes, bipolar transistors to FET transistors. Tube amps used to be common 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. A lot of people favor tube amplifiers since those higher harmonics are frequently perceived as the tube amp sounding “warm” or “pleasant”.
One downside of tube amps is their small power efficiency. In other words, most of the power consumed by the amplifier is wasted as heat rather than being transformed into audio. As a result tube amplifiers are going to run hot and require sufficient cooling. Yet another disadvantage is the big price tag of tubes. This has put tube amps out of the ballpark for many consumer products. Because of this, the majority of audio products these days makes use of solid state amps. I am going to describe solid state amplifiers in the following sections.
Solid state amplifiers replace the tube with semiconductor elements, typically bipolar transistors or FETs. The earliest type of solid-state amplifiers is known as class-A amplifiers. The working principle of class-A amps is quite similar to that of tube amplifiers. The primary difference is that a transistor is being used rather than the tube for amplifying the audio signal. The amplified high-level signal is sometimes fed back to minimize harmonic distortion. Class-A amps have the lowest distortion and usually also the lowest amount of noise of any amplifier architecture. If you require ultra-low distortion then you should take a closer look at class-A models. Though, similar to tube amps, class-A amps have extremely low power efficiency and most of the energy is wasted.
Class-AB amps improve on the efficiency of class-A amps. They use a number of transistors to split up the large-level signals into two separate regions, each of which can be amplified more efficiently. Because of the higher efficiency, class-AB amps do not need the same number of heat sinks as class-A amplifiers. As a result they can be manufactured lighter and less costly. Class-AB amplifiers have a disadvantage however. Each time the amplified signal transitions from one region to the other, there will be some distortion created. In other words the transition between these two regions is non-linear in nature. Consequently class-AB amplifiers lack audio fidelity compared with class-A amplifiers.
To further improve the audio efficiency, “class-D” amps utilize a switching stage which is continuously switched between two states: on or off. None of these two states dissipates power inside the transistor. Therefore, class-D amplifiers regularly 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). Typical switching frequencies are in the range of 300 kHz and 1 MHz. This high-frequency switching signal has to be removed from the amplified signal by a lowpass filter. Typically a straightforward first-order lowpass is being utilized. The switching transistor and in addition the pulse-width modulator frequently exhibit rather large non-linearities. As a result, the amplified signal will have some distortion. Class-D amps by nature exhibit higher audio distortion than other types of mini stereo amps.
Newer amps incorporate internal audio feedback in order to reduce the amount of audio distortion. One type of audio amplifiers that uses this type of feedback is called “class-T” or “t amplifier”. Class-T amps feed back the high-level switching signal to the audio signal processor for comparison. These amplifiers have low music distortion and can be manufactured very small.
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