Beginner’s Guide to Amplifier Class

Overview

Assuming you've been looking at the various amplifiers available for purchase, you may be a little overwhelmed by the different classes on offer. You've probably seen amplifiers labeled as Class A, B, A/B and D, and have wondered what each letter means exactly.

​You're definitely not alone.

​Like you, we've been there. We've also managed to put together the following guide which aims to walk you through the various classes of amp and help you understand how each class works, and ultimately inform you which class suits your audio components best.

Table - Amplifer Class Pros and Cons

AMP TYPE

TYPICAL
EFFICIENCY

PROS

CONS

Class A

~15-35%

- Clean output

- Purer sound

- Low distortion

- Low efficiency

- High levels of heat generation

- Larger size

Class B

~70%

- High efficiency

- Typically of a smaller size

- Less heat generated

- Compromised reproduction of sound

- Potential signal distortion

Class A/B

~50-70%

- More efficient than Class A

- Distortion practically nil

- Typically less efficient than Class B

Class D

>90%

- Extremely efficient

- Light weight

- Typically less clean output

Class A Amplifier

The key feature of this amp is that it is able to conduct through the full cycle of a waveform. What this means in simpler terms is that the amp is able to replicate and amplify 100% of the input signal, effectively being "on" at all times.

This class of amplifier has a very simple design in relation to other classes. The main advantage of this configuration is that there is less distortion compared to other class of amp. They also tend to be less susceptible to hum and, above all, produce a cleaner output. These positive attributes tend to make Class A amplifiers highly rated among regular audiophiles.​

Class A Amp

Class A amplifier signal

There are drawbacks to this design though. For one, because the amp is constantly "on", the Class A tends to suck significantly large amounts of energy. The power generated in turn produces in a huge amount of heat as a result. This alone is reason why Class A amplifiers are not common in cars - simply too much heat generation for car audio requirements.

In conclusion, it's also worth noting that Class A amplifiers tend to be a larger size than the other classes, hence take up more space.

Class B Amplifier

​Next we come to Class B amplifiers which, in contrast to Class A, are configured in such a way that the waveform is shared among two output devices. In most cases, this is achieved via a 'push/pull' configuration, meaning the amplifier switches the current between the two outputs.

Effectively, this means that Class B amplifiers are much more efficient than their Class A counterparts, using less energy, and producing less heat as a result. Furthermore, they also tend to take up less space than Class A.

Class B Amplifier

Class B amplifier signal

The downside of this design is that there is signal distortion during the switching between the output devices. This distortion is not negligible, and impacts the quality of sound reproduction - particularly noticeable at very high volumes.

Class A/B Amplifier

​The Class A/B is the most common/popular class of amp in the car audio market.

​This class of amp is the result of an effort to combine the strengths, while minimizing the weaknesses, of both Class A and Class B amps. This is achieved by allowing each output device to slightly overlap, with regards to the part of the waveform they share. This means there is far smaller chance that both outputs are "off" simultaneously, and as a result, crossover distortion occurring.

Class AB Amp

Class A/B amplifier signal

​This resulting "hybrid" manages to deliver the best of both worlds, resulting in a class that is vastly more efficient than Class A, and with a lot less distortion than Class B. In fact, if your average listening volume is not particularly high, audible sound quality is more or less in the Class A range.

Because of the relatively clean output, the Class A/B amplifier is most commonly used to drive speakers.​

​Class D Amplifier

Class D's major draw-card is it's efficiency - rated much higher than A/B, and even B. This results in an amp that draws less power from your car than usual. Consequently, as very little energy is wasted in comparison to other classes, the unit can function with much less heat syncing, less power, and smaller components in general. This also results in a unit that relatively light weight.

The internal configuration of a Class D amp can be quite complex, as it operates fairly differently to other classes. While Classes A, B and A/B produce an analogue signal and are always "on", Class D amps produce a square wave, which effectively translates to "off" and "on" states.

This ultimately results in a signal that difficult to clean up, and thus, Class D amplifiers are best used to drive devices where quality is not of importance, namely subwoofers.

Class D Amp

Class D amplifier signal

​Finally, it's worth nothing that the D in Class D doesn't infact stand for "Digital", as is sometimes mistakenly believed (although there are some digital units out there). Quite simply, the letter C was already taken.

Other Class of Amplifier

​In recent years, what appear to be variations of the above classes have made their way into the market. The most common, such as bD and GH, are typically a combination of some of the above mentioned classes (ie. B and D), or slightly tweaked versions of existing classes, used by particular brands.

Conclusion

So, which class amplifier do you choose?

If we stick to the common classes above, this is actually a pretty simple question to answer:​

Typically, in most cases it makes sense to pair A/B Class amps with (coaxial or component) speakers.​

On the other hand, Class D amps are usually best suited for powering subwoofers.

We hope the above guide has been helpful.  If you have any questions, or if there is any class of amp we've missed, feel free to leave a comment!​

Comparison Table

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