Understanding Amplifier Classes

The components used in power amplification are different from unit to unit and are described using the "Class" system. This system is often misunderstood for a marking of quality; this article stands to clear that up.

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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, = 360 means that the full signal is used, and = 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% ( = 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 ( = 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% ( = 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 (~ = 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

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    jean_genie
    Here's an easier way to understand A vs. A/B: Think of a sine wave. Class A amplifies the entire waveform. This type of amp tends to get overworked, which results in easily overdriving. Class A/B amps split the waveform before processing. Half the tubes take the positive side of the waveform, have take the negative side. Because of the shared duty, these amps have more clean headroom before they go into overdrive. Because they have a cleaner signal, they're also much better for extremely high-gain playing, as they don't tend to muddy up as much. Extremely simplified, but explains the basics without any enginerring understanding. I usually tell my customers that it they want really clean or REALLY distorted, they want a Class A/B amp. If they want something in the middle, they want a Class A amp. Obviously I encourage all my customers to play any amp of any class in any price range that interests them, but 90% of the time my explanation holds true. Even after an hour of trying half a dozen amps, my customers generally agree with where I've pointed them. And yes, I always encourage them to try different amps, in case they disagree with me
    PussyPunk182
    Awesome, thanks. I too thought that the classes were grades on somethign to do with warmth or how good the tubes that are used were. haha. Thanks alot, although alot of the numbers confused me, I got the jist
    RSmarties
    A good start but just a few comments and details: Class A, B, AB, C (and I suppose D) can all be single-ended or push-pull. Single-ended class B, AB, C and D are not really suited for general audio applications of any power rating (even low power) because of their tuned/resonant nature for full cycle reproduction, or bad fidelity without tuned/resonant load circuits. Single-ended class C is often used with resonant(tank) circuits in radio amplifiers. Valve amplifiers are easily operated in single-ended class A as long as the output transformer is suitably designed and rated. The high voltage / lower bias current and impedance transformation of the output transformer makes it practical and an attractive option. Single-ended solid state amplifiers are another story where the minimum bias currents are essentially half that of the maximum load current required. this makes for a heater that also does some amplification 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. Class D amplifiers have switching output devices and are typically configured to control the output signal using PWM (Pulse Width Modulation). The switching behaviour results in very high efficiencies, but requires high operating frequencies (100's of kHz) and very good output filters to remove those high frequencies. Class D amplifiers are often used in power and space critical applications (like PC Notebook audio amplifiers), or in very high power applications where inefficiencies (heat) along with size (heatsinks and power supply) and weight (especially the PSU transformer) must be minimised. A 1kW class AB amplifier with, say, 75% efficiency would dissipate up to 300W in heat, where a 90% efficient class D would only dissipate just over 100W of heat. Such amplifiers are usually the preserve of solid state electronics because of the switching speeds involved. Clipping is a result of the one of the amplifier stages (usually the final/output stage) saturating i.e. the voltage swings to one of the supply rails (less the output device's saturation voltage). Where clipping occurs "softly" (in MOSFET and valve amplifiers the peaks of the waveform are rounded rather than squared-off) primarily even-harmonics (think octaves) are generated which are less objectionable and a part of the desired valve sound. Where the clipping is "hard" then odd-harmonic rich distortion is created, that undesired, harsh character of bipolar (NPN/PNP transistor) amplifiers. Crossover distortion is a result of imperfect and mismatched switching between the upper and lower half devices. Crossover distortion is minimised by biasing an amplifier in Class AB, but never eliminated completely. Push-pull stages need not be complementary - in the case of valves they use centre-tapped transformers and identical devices on each end. Often for reasons of cost, simplicity and efficiency only N-Channel/NPN devices are used in solid-state amplifiers almost always resulting in degraded audio quality.
    rbrad1962
    Can a home audio amp (Class t) be used to build a guitar amp/speaker combo ? Thanks.
    mity88
    Okay,i get it,but ive got some questions, how do u tell whether an amp is A or B or watever? can u distinctly hear a difference? and what role do effects play on the different classes? thanks
    Zigioman
    A very educational artical I learned some stuff, Thank for writing this.
    the*matt
    Kevy Absolution wrote: animesh_joshi wrote: That is a very well written article. Gives one a great understanding. A few examples would have been good. Examples of what?
    Maybe a few brands and models that the average player might be familiar with?
    Kevy Absolution
    animesh_joshi wrote: That is a very well written article. Gives one a great understanding. A few examples would have been good.
    Examples of what?
    wesselbindt
    I'm going to do an essay on this(for school, not for ug), and this is pretty helpful, thanks!
    rhoadsfan92
    i strongly approve. i only knew about class a, class b, and class ab. and honestly, i thought b was ab, and had no clue what ab was...
    Oosh.
    Very helpful, it has cleared up the whole class thing up for me!
    strat0blaster
    Helpful, but kinda overly complex IMO. Still, if you sit and wrap your head around it, it's very helpful.
    animesh_joshi
    That is a very well written article. Gives one a great understanding. A few examples would have been good.
    Vabolo
    Interesting, I've never heard of the energy efficiency side of amp classes that much. There is now an inner battle in me, musician vs environmentaslist, althouugh I doubt an amp driven by hydroelectricity will really make such a big impact on the environment, wether it's a class A, B or AB. Anyhow, good job on the article!