While reading some reviews of amplifiers on another guitar site, I was reminded of a peculiar scenario I encountered before. A few weeks ago, I was in a music store and overheard a middle-aged man talking to a sales representative. He insisted to the worker that he did not, under any circumstances, want anything but a “Class A” amplifier because he “didn’t want something cheap”.
While I didn’t intervene, I did think that this man’s misunderstanding of amplifier classes is widespread and not limited only to him. In fact, most people don’t even know that there are different amplifier classes, and knowing what type of amplifier you need and which class suits you can help you make a better purchasing decision. Classes are not grades of quality; they are classifications based off how they work.
In essence, an amplifier’s class is based upon how much of the original signal is used through the circuit. More often than not, the percentage used is notated as an angular degree, or the “angle of flow”. Therefore, È = 360° means that the full signal is used, and È = 180° would be half of the signal. The angle of flow is also closely related to the efficiency of the amplifier. Let’s take a look at each individual class:
Class A
In this form of amplifier, 100% (È = 360°) of the original signal is used throughout the whole circuit. The result is an upscale version of the original signal, unclipped and effectively amplified to a more intense, usable signal. These amplifiers are very energy inefficient however; because the amplifying element is biased to constantly be conductive, power is drawn from the source even when no signal is being input. In layman’s terms, the amplifier is drawing power even when you’re not playing.
Up to one Watt is dissipated for every Watt used to amplify the signal. This 1:1 ratio means that as much energy is wasted as is used when managing the linear signal. To many players, this inefficiency is worth it; the main reason for a Class A’s linear signal function is the use of tubes. Tubes have asymmetrical output, resulting in even and odd-numbered harmonics. While this is chalked up to opinion, many players agree that tubes producing those forms of complex harmonics result in a higher-quality sound.
Class B
In these amplifiers, only half of the signal (È = 180°) is used, resulting in a lot more clipping (distortion) but a more efficient system. The system only operated half the time, processing half of the signal, so it naturally uses less power. It is unusual, however, to find amplifiers using single Class B elements due to unusual output signal, and are more often found in personal radios and battery-operated devices than . Instead, they are quite often paired with with a matching push-pull element, resulting in a Class AB system.
Class AB
Relying on the use of two Class B units, a Class AB system is a pair of complementary push-pull devices, each amplifying ~55% (È = 198°) of the original signal and combining them afterwards, resulting in a full signal. The reason why each device takes more than 50% of the signal is to ensure that the signals crossover and match up, and no device is completely shut off at any time. However, Class AB amplifiers are still extremely efficient. There is the risk of crossover distortion, where the mismatched signal ends clip once combined; at most performance volumes, the distortion is not easily noticed and a the power efficiency of the amplifier is considered to be worth it.
Class C
Class C amplifiers conduct less than 50% of the original signal. This results in an unusually high level of sound clipping and signal distortion. Class C amplifiers are extremely efficient, boasting up to about 90% efficiency. However, they are more complex than normal amplifiers and are not usually found in guitar amplification systems, but instead have vocal and other instrumental practicality. A Class C amplifier has both a “tuned”, or “clamped” mode of operation, and an “untuned” mode. When tuned, the amplifier is biased so that only one-half of the input voltage is utilized, resulting in less power dissipated and wasted after amplification. Again, in layman’s terms, only half of the signal is input, but it retains its form after processing. It is possible to bias the amplifier to end up producing a signal that is reactive to very specific harmonics, for instruments such as bells or tuned idiophones.
Class D
These amplifiers operate similarly to Class AB units, running two separate signals (~È = 198°), but instead use switches at each transistor that can turn on and off when there is no signal input. The result is a moderately clean signal that is amplified using very little power. This class is usually only found in batter-powered mini-amps which rely on weak power sources and need as much life longevity as possible.
Class G and Class H
Most players will never play through these types of amplifiers, yet they are quite unique. Usually used for high-volume performing such as in a stadium or other large venue, these amplifiers have the ability to run off multiple voltages. A series of power supply rails with adjustable voltages run along the signal, allowing a different voltage to be used depending how far through the device the signal is. The advantage to adjustable voltages is a very low (almost 0%) amount of wasted power at the output transistors or after the signal passes through the tubes. These classes of amplifiers are usually costly enough that any less of an efficient design wouldn’t be worth the price.
So there’s a basic explanation of power amplification. Hopefully this clears up a little misunderstanding about amplifier quality and classes. Depending on your situation, buying an amplifier of a specific class can be very beneficial. Next time you try out some amplifiers, ask to try a few different classes and get a feel for each; you may end up liking a new sound.
By Kevin Heiland
/ my profile 
See the Zen amplifier at passdiy.com for an example of such an amplifier with 4% (yes four percent) efficiency. Almost all solid state power amplifiers would be push-pull types, and most likely some variation of class AB.