UrWealthwizard's Blog

Audio amplifiers are at the very heart of every home theater system. As the quality and output power requirements of today’s loudspeakers increase, so do the demands of audio amps. It is hard to pick an amplifier given the large number of models and designs. I will explain some of the most common amplifier designs such as “tube amps”, “linear amps”, “class-AB” and “class-D” as well as “class-t amps” to help you understand some of the terms commonly used by amplifier manufacturers. This guide should also help you figure out which topology is ideal for your particular application.

An audio amp will convert a low-level audio signal which often comes from a high-impedance source into a high-level signal which can drive a loudspeaker with a low impedance. Depending on the type of amp, one of several types of elements are used to amplify the signal such as tubes and transistors.

Tube amplifiers used to be common a few decades ago. A tube is able to control the current flow according to a control voltage which is connected to the tube. Unfortunately, tube amplifiers have a fairly 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. Many people describe tube amps as having a warm sound versus the cold sound of solid state amps.

Another drawback of tube amps, though, is the low power efficiency. The majority of power which tube amps consume is being dissipated as heat and only a fraction is being converted into audio power. Also, tubes are quite expensive to make. Thus tube amps have mostly been replaced by solid-state amps which I will look at next.

The first generation models of solid state amps are known as “Class-A” amps. Solid-state amps use a semiconductor rather than a tube to amplify the signal. Usually bipolar transistors or FETs are being used. In a class-A amp, the signal is being amplified by a transistor which is controlled by the low-level audio signal. In terms of harmonic distortion, class-A amps rank highest amongst all types of audio amps. These amps also usually exhibit very low noise. As such class-A amps are ideal for very demanding applications in which low distortion and low noise a crucial. Class-A amps, however, waste most of the energy as heat. Therefore they usually have large heat sinks and are fairly heavy.

By using a series of transistors, class-AB amps improve on the low power efficiency of class-A amps. The operating working area is divided in two separate areas. These two areas are handled by separate transistors. Each of these transistors works more efficiently than the single transistor in a class-A amp. The higher efficiency of class-AB amps also has two other advantages. Firstly, the required amount of heat sinking is reduced. Therefore class-AB amps can be made lighter and smaller. For that reason, class-AB amps can be made cheaper than class-A amps. Class-AB amps have a drawback though. Every time the amplified signals transitions from one region to the other, there will be some distortion generated. In other words the transition between these two areas is non-linear in nature. Therefore class-AB amps lack audio fidelity compared with class-A amps.

Class-D amps improve on the efficiency of class-AB amps even further by using a switching transistor which is constantly being switched on or off. Thereby this switching stage hardly dissipates any power and thus the power efficiency of class-D amps usually exceeds 90%. The switching transistor is being controlled by a pulse-width modulator. The switched large-level signal has to be lowpass filtered in order 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 highest audio distortion of any audio amplifier.

To solve the problem of high audio distortion, newer switching amplifier designs incorporate feedback. The amplified signal is compared with the original low-level signal and errors are corrected. A well-known architecture which uses this type of feedback is known as “class-T”. Class-T amps or “t amps” achieve audio distortion which compares with the audio distortion of class-A amps while at the same type offer the power efficiency of class-D amps. Thus t amps can be made extremely small and still achieve high audio fidelity.

You can find further information concerning class-t amps and stereo amplifiers from Amphony’s website.