Hi nouse and everyone else,
I wrote an article on this very topic a few years back, it is edit to be easy enough for anyone to understand the basics of amplifer classing.
Introduction:
Amplifier classification (A,B,AB,C,D etc.) can be misleading due to the fact that manufacturers have been known to periodically invent their own classes or, as a selling point, misapply existing classes. Additionally, the subject matter is quite technical. The premise of this article is to simplify the theory and to help you to understand and define the audio amplifier classification system, which basically describes circuitry types, as well as the advantages/disadvantages of each. I will only deal with the more popular classes, as the others are not generally utilized in our industry.
Amplifiers are divided into several classes of operation which are determined by two attributes: How inherently linear (low-distortion) they are and how effectively they convert voltage into power to drive speakers (efficiency).
Modes:
The nomenclatures of the most common amplifier classes (Class A, Class A/B and Class D) do not necessarily pertain to sonic quality like a test score, in which one is better then the other. In fact, high quality amplifiers are manufactured using all modes of operation. These modes are commonly known in the field as “linear” and “switched”. Amplifiers in classes A, B, and AB operate their output transistors in “linear” mode. Class D amplifiers operate their outputs in “switch” mode.
Class Examples:
Class A: This class is the least efficient of the categories I will cover, (about 25% efficient). One advantage of a class A amplifier is that it offers very low distortion. However, its inefficiency inherently causes it to use a large amount of current, which in turn generates a large amount of heat. With this class A “linear mode” amplifier the output transistors never actually turn off and will even consume full current with no audio signal, possibly as much as with a speaker connected to it when operating at maximum power. This means that a class A amplifier will operate hot even when playing at very low volume, as well as at high levels. Because of all the heat generated, class A amplifiers require very large heat sinks on the amp casings and usually incorporate fan cooling.
Many audiophiles claim pure class A amplifiers sound better than other classes. Although at one time this may have been true, in actuality amplifiers should not produce any “sound”. They are designed to increase power only hence any quality amplifier should perform that function well enough so that even the most critical listener will have difficulty telling one design from another by ear.
Class B: There is no reason to delve into class B amplifiers since it is not generally used for audio. In a class B amplifier, there is a small part of the waveform that will be distorted. Distortion, when describing audio equipment specifications, is any unwanted variation in a signal (compared to the original signal).
Class AB: We realize that a class A amplifier is very inefficient. In most cases this is not the best choice for a car audio amplifier. (It will use excessive power and operate hot). Class B amplifiers will have too much distortion, which is not good for any audio amplifier. The class AB amplifier was designed to be a compromise. A class AB amplifier is more efficient than a class A (about 50%-60% efficient) but without the distortion of the class B.
Some car audio amplifiers that claim to be a class A amplifier are just a “high bias” class AB design. These amplifiers are only class A at very low power output levels. This is not to say that true class A amplifiers do not exist. There are a few high quality (and expensive) mobile amplifiers that are a true class A design.
Class D: Class A amplifiers are very inefficient. Class 'AB' amplifiers are also inefficient but are more efficient than class A amplifiers. Class D, also called switch mode or digital amplifiers, is the most efficient method (up to 90%) and is the least linear, unless it employs sophisticated corrective signal processing. It is predominantly used in less critical applications where fidelity is not as much of an issue but power output is, such as with dedicated subwoofer amplifiers or “mono blocks”. However, there is a new breed of digital amplifiers that use Class D (or variations thereof) for high-end applications.
Summery:
Although I did not cover some of the more technical aspects you should now at least have a basic understanding of the differences between amplifier classes. This general information can come in very handy to promote to the customer your professionalism and product knowledge. However, keep in mind that the ear of the subjective listener is the final judge of what sounds good, not the specification sheet.
Copyright © 2003 by Ron Gold. All rights reserved including reproduction in any form, whole or in part, without expressed permission of the author.*
*any or all of this info may be shared publically or privately without recourse for educational and/or non-profit purposes only.