Car Audio Class System – Size Matters

Car Audio Class System – Size Matters


When studying the technical features of a car amplifier, you will come across a strange parameter called “class”. A full explanation requires a good knowledge of how transistors work but here we try to give you a guide that may help you when comparing different products. Class A, class B, class D, class AB, what is that all about? Why are these strange terms quoted in the sales literature of in car amplifiers and what is the average customer supposed to do with this information.

The terms refer to the electronic architecture, of an amplifier. They describe how the components inside are configured. A basic knowledge of performance expectation is always handy in order to help you compare products so we will take each of the above in turn and try to explain the pro’s and con’s of each.

Class A

Class A – Inside an amplifier, the components that actually do the most work are the output transistors. A transistor is a solid-state (no moving parts) switch that turns off and on to differing degrees. With a class A architecture, the output transistors are kept turned on all of the time. This ensures that both positive and negative parts of a signal are amplified by the same devices avoiding any anomalies being introduced due to the switching action of a circuit featuring an array of transistors. So, why are not all amplifiers class A? By having the output transistors turned on constantly, current is taken from the power supply at all times even when there is no input signal. This makes this architecture extremely power hungry often requiring an upgraded battery, alternator etc. The amplifier needs to dissipate unused power as heat requiring fans or a huge heat sink.

Class B – Transistors are used in pairs with one transistor looking after the positive half of a signal cycle and another the negative half. This is much more efficient than class A (about 50% rather than 20%) but due to the switching characteristics of the devices used, introduces distortion at the point where one transistor switches off and the other switches on. This is known as crossover distortion and can sound quite nasty! Class B amplifiers are rare in car audio these days and tend to be used only for low fidelity applications.

Class AB – As the name suggests class AB combines the aforementioned topographies. The output devices are still dedicated to either the positive or negative half of the audio signal but transistors are kept switched on for more than half the cycle. This minimises crossover distortion while maintaining an acceptable efficiency of around 50%. This is the most common architecture for in car and home hi-fi amplifiers.

Class D – Here, instead of amplifying the input signal as a positive or negative waveform, the input signal is carried on a high frequency signal of fixed amplitude (height) but with a varying width between each cycle. This is known as pulse width modulation (PWM) and is extremely efficient when compared to linear amplification achieving efficiencies of more than 90%. This is because output devices are switched on for varying amounts of time rather than with varying amounts of effort. Initially, class D was the reserve of subwoofer amplifiers but as the technology has developed so has the integrity and now can be used in reasonably high quality full-range audio amplifiers. Because of the remarkable efficiency of class D circuitry, there is less heat generated which means amplifiers of significant amounts of power can be fitted into relatively small, light-weight cases.

Hopefully, this blog goes someway to explaining the difference between different amplifier designs. We do however apologise to those scientists and electronics experts amongst us who may find my necessarily brief descriptions somewhat lacking in detail. A little knowledge is better than being made to look foolish at the point of purchase but more importantly, could save you being exploited for want of some insight.

A good independent specialist such as a FOUR MASTER will be able to help you further and it is important to stress that there is no substitute for the knowledge gained through hands-on experience.

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(Content supplied by Driving Sounds Magazine –