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Lead image

Which amplifier class is the best?

How Class D amplifier technology differs from Class A and AB

What is amplifier class? The way an amplifier combines power and signal defines its Class. Which class is best depends on your needs:

  • Class A design is the least efficient but has the highest sound fidelity.
  • Class B design is a little more efficient, but full of distortion.
  • Class AB design offers power efficiency and good sound.
  • Class D design has the highest efficiency but isn't quite as high-fidelity.

Amplifier basics

Typically, a car amplifier works by taking the 12-volt DC power coming into the amp, turning it into AC, and increasing the voltage via a transformer. Then it combines that high-voltage power with the audio signal coming from the stereo to create a high-voltage, high-current output version of that weak input signal.

Amplifier basics diagram.

Amplifier Class is the system for combining power and signal. Amp class differs from amp to amp with efficiency and sound fidelity dependent on which design gets used. In all designs, banks of output transistors, each a little amp by itself, add their collective power together to provide the amplifier's final output.

The inside of a Rockford Fosgate T500-1bdCP amplifier

The inside of a Rockford Fosgate T500-1bdCP showing the transformer (the coil of red and green wires) and output transistors (black rectangles glued to the heatsink with white thermal paste).

Heat is the enemy

Amplifiers always put out less power than they consume. An amplifier's efficiency is the ratio of what it puts out divided by what it draws from the electrical system. No amp is 100% efficient, putting out exactly what it draws, nor can an amp put out more power than it draws. The power that doesn't make it to the output terminals is wasted energy that turns into heat. Too much heat will destroy the amplifier's output signal and internal components.

The different amplifier classes produce different amounts of heat. See each class description below.

Watch the amplifier classes video

Amplifier classes — what's the difference?

There are different ways to design an amplifier and heat is the enemy. Got it. Now let's talk about the different classes of amp design. 

Class A amplifier setup diagram.

Class A amplifiers — the high-fidelity heat source

A Class A amplifier's output transistors run with "constant bias," meaning they always run at full power whether there's an input signal or not. When there's no signal, the transistors' power turns into heat. When there is a signal, the power goes out the speaker terminals. Also, each Class A output transistor amplifies both the negative voltage and the positive voltage parts of the signal's AC waveform, adding to the workload and generating more heat. Class A amps usually operate around a 25% efficiency level. That means that 75% of their power is turned into heat.

Highest fidelity amplifier Class

Because each output stage transistor is always on, there's no turn-on, turn-off, warming, or cooling cycles affecting the signal flow. In fact, the transistors perform in their most linear fashion, distortion-free, under this condition. And because there're no switching going on, there's no induced high-frequency interference either. Pure Class A amplifiers are rare, expensive, and never used in car audio.

Class B amplifier setup diagram.

Class B amps — the two transistor solution

Class B amplifiers lighten the workload of each output stage by replacing the single transistor there with two transistors set up in what is called a "push-pull" arrangement. One transistor amplifies the negative voltage parts of the signal's AC waveform and the other takes care of the positive voltage, and then the two are combined into a unified output. Each transistor is on half the time and off the other half.

Efficient with low fidelity

Class B amplifiers are much more efficient than Class A amps — 50% or so — but produce distortion as the two transistors switch on and off. This "crossover distortion" is so bad that very few if any manufacturers offer or produce an amplifier of pure Class B design.

Class AB amplifier setup diagram.

Class AB amplifiers — higher fidelity and efficiency

The push-pull pair of output transistors in a Class AB amp are each on more than half the time, and they don't turn on and off suddenly either. This gives the amp the characteristics of a Class A amp when the signal's at low power and conducting through both transistors, and a Class B amp when the power is high. For each amp, there's an optimum bias current, the amount of time when both transistors are passing current, that minimizes the crossover distortion of Class B design.

Class AB amps are everywhere

The result of this design is that Class AB amplifies have much higher efficiency than Class A amps, up to about 60%, and much less distortion than Class B amps. Most home theater and stereo amplifiers are Class AB.

Until recently, using an AB amp was the only practical choice for attaining high-fidelity, full-range amplification, but now Class D amps are being built that are just as accurate. (Class A is still the winner for accuracy, though.)

Class D amplifier setup diagram.

Class D amplifiers — popular kings of efficiency

Class D amplifiers operate in a unique fashion. Onboard circuitry creates very high-frequency (often over 100K Hz) pulses of DC current. The width of each pulse is then modified by the input signal — the wider the pulse, the louder the signal. This is called "pulse width modulation" or PWM. These DC pulses are run through the amplifying output transistors creating the high-power output.  Because they are getting DC pulses, not analog signals, the transistors, also called MOSFETs, are either on full power or off with no power. This is the most efficient way of running these transistors — as much as 90% efficient in some cases.

D does not stand for digital

Although making a signal by rapidly switching transistors on and off resembles digital processing that uses zeros and ones, Class D amplifiers are not digital devices. Some of them might have digital control circuits, but the amplifier circuits will be strictly analog.

Less-than-highest fidelity

After amplification, a low-pass filter smooths the output signal so the amp won't put out pulses of power but rather, a continuous analog power output. It also removes the interference generated by those high-frequency DC pulses. Because of this, most audiophiles won't use Class D amplifiers in their systems, citing that need for filtering out generated distortion.

Class D has become the go-to choice

On the other hand, in professional PA systems and car audio applications where perfect fidelity isn't as important, Class D amplifiers have become much more popular because they're smaller, lighter, and run cooler than the other Classes of amplifiers with the same amount of power. And these are big advantages when you have to fit an amp in a vehicle or carry one around for gigs.

Other amplifier Classes

There are other technologies used in amplifier output stages, usually enhancements to Class AB design, like Class G, Class H, Rockford Fosgate's Boosted Rail, and Alpine's Dynamic Peak Power. They usually manipulate the amplifier's own power supply in various ways in order to add power on demand more efficiently. Hybrid combinations of all of these technologies can be found in many amplifiers manufactured today.

A class D Alpine amp and a class AB Rockford Fosgate amp

Two 50 watts RMS x 4 amplifiers: the Class D Alpine KTA-450 (left) and a Class AB Rockford Fosgate 50x4 (right). And that's Car A/V web editor Robert FV contributing his expertise.

Confused and looking for an amp? Give us a call.

For today's car amplifiers, you probably won't hear any differences between the different Classes of amplifiers, especially while driving. But it's good to know what those differences are so you can make educated decisions on which amplifier will work best for you.

Check out Crutchfield's entire selection of car amplifiers. Once there, you can narrow your focus by filtering the selection for each amplifier Class. And if you have any questions or want some help, contact our expert advisors via phone or chat.

  • Todd from Northville Michigan

    Posted on 8/2/2022

    Great article and simple explanations of the different amps. Thank you for this. I did want to correct the comment on "Class A never used for car amplifiers". Not true. Back around Y2K - the height of Car Audio IMO, Soundstream had a full line of pure Class A car amplifiers in their Reference line. I have several of these and they are truly amazing in both power and fidelity. I now understand why they run so damn hot though! Can you help explain why they also had such incredible headroom - both for burst power and essentially true power doubling when dropping from 4 to 2 or even 1 ohm? Thx again!

  • Jeremy from Milford

    Posted on 7/30/2022

    What is the approximate efficiency of a Class BD amplifier?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 7/30/2022

    Jeremy, Every individual amplifier has a different efficiency, no matter what Class the amp is. You measure efficiency by comparing the amplifier's current draw from its power source to its output current capability. Without knowing precisely what amp you're referring to, we can't help you with advice. If you want a question answered about an amplifier, you must identify it by brand name and model number, so we can get the right information to you.
  • Charles from Akron

    Posted on 7/17/2022

    When calculating rms wattage for amps in different classes, should you calculate the inefficiency? Example: if my components have rms of 200w each and I want an AB class amp, should I look for a amp also at 200w rms, or look for one closer to 300rms factoring in the efficiency.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 7/18/2022

    Charles, The only spec relevant, in this case, is the amplifier's output power, not how it got it or how efficiently. The sub doesn't care if it's being driven by an AB or a D amp, only the amount of power matters. A sub rated for 200 watts RMS should be driven by an amplifier with up to 200 watts RMS of power.
  • Ethan from Bellefontaine

    Posted on 7/7/2022

    When trying to match an amplifier to my subs should I get the amp closest to the rms of the sub or higher close to peak power?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 7/8/2022

    Ethan, You should only use RMS specs when planning a system..
  • AB from Minneapolis

    Posted on 6/10/2022

    I have ordered the Infinity Reference 3004 which I believe is a Class H amp, thought there seems to be some conflicting information on the Crutchfield website. The Details tab says it is a Class D. The main overview page says it is a Class H. I noticed that the signal to noise ratio on this amp is only 80db which has me worried a bit, but that would probably be related to it being a Class H and not Class D. I know Class H is a variation of Class AB but with a tracking voltage to make it slightly more efficient. But what about that signal to noise ratio? Should I be concerned? I know I can get a Class D amp in the same power and price range over 100db signal to noise ratio. But I am taking a little gamble on this one, hoping the AB variation to Class H actually gives me a richer, or full audiophile amplification....

    Commenter image

    Buck Pomerantz from Crutchfield

    on 6/13/2022

    AB, You should judge an amplifier, or any piece of audio gear, by how it sounds, not by its published specifications. If you don't like the sound of an amplifier purchased from Crutchfield, you can return it.
  • Chris Batchelor

    Posted on 6/7/2022

    My impression of this article is that the author wanted to provide an objective narrative comparing Class A and Class D amps. However, the objective needs the subjective for appropriate comparison. Class D amps have gotten extremely good, but a subjective comparison may find them less satisfying because they don't have the noise and distortion characteristics of class A tube amps which are desired features of the sound. This is why some architectures combine tube pre amps with solid state or Class D amplifiers.

  • Christoph Kaywood from Buffalo Junction

    Posted on 5/30/2022

    My question is if you don't mind giving the best advice of your knowledge, what amp would you use to push a sundown audio zv6 10" 2500rms 2ohm ?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 5/31/2022

    Christoph, A Memphis Audio VIV2200.1V2 ought to work well for that DVC 2-ohm 2500 watts RMS rated sub.
  • Junathan Carolissen from Cape town

    Posted on 5/15/2022

    Are class D 4 channel better amplifiers in sound quality output then a class AB 4 channel amplifier..?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 5/16/2022

    Junathan, Most people would think otherwise.
  • C. M. Barons from Bergen, NY

    Posted on 5/8/2022

    It seems this article is aimed at personal users in a home or vehicle listening environment. It would be useful to compare class D and H in the industrial milieu, specifically high power subwoofer applications. Nowadays many amplifiers are rated on a burst Vs continuous power rating. Recovery speed needs to also be considered. I've heard arguments for both D and H as better suited for subwoofer powering. For instance a D class amp rated at 2000 watts burst Vs an H class amp rated at 1700 watts continuous, both with 4 ohm loads- which would have the advantage?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 5/9/2022

    CM, I don't know what you mean by "industrial milieu" but for all devices you should only use RMS ("continuous") power ratings for comparisons.
  • Antony Alumkal from Denver

    Posted on 5/6/2022

    The comments on Class D are out of date. The Marantz PM-10, PM-KI Ruby, and Model 30 are all Class D and all have excellent sound. The Model 30, the least impressive of the bunch, got the top rating from Stereophile Magazine.

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