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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.


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.

  • Dwstudeman from Desert Shores

    Posted on 10/16/2021

    So far I have seen nothing but anecdotes by people who seemingly do not know that amplifiers do not amplify anything, they merely reshape current available on the negative and positive DC rails to match as closely as possible the shape of the input signal. In the home you are just reshaping wall current to copy the input signal on a larger scale, no electrons from the input ever reach the speakers. Class AB can be prone to crossover distortion very easily as they switch between the positive and negative halves of the waveform and this is purely in the audio band for every single wave cycle you hear. Judicious use of forward bias is what this delicate see-saw rests on as it can fall to class B very easily. Class A is always on but it takes 300 watts for every 100 watts to reach your speakers. By the late 1990s Class D amps that rivaled and even bested the best high end amps existed from the likes of Spectron. Not surprisingly Arnie Nudel from Infinity was at the helm and he made the first Class D in the 1970s. Bruno Putsey's designs for Hypex use global feedback that goes outside the filter to correct differences between the input and the output. It is easier and cheaper to make a decent class AB design as there are many good examples to copy. They too vary in sound quality based on feedback topology. A good class D amp is more challenging to design but in the end, some are great, some are irritating. I like the new Gallium Arsenide amps myself.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 10/18/2021

    DW, In spite of your detailed description and endorsement, amplifiers do not "reshape current." They amplify, increase, a signal's strength - voltage and power. How they do it is the "special sauce" of each manufacturer's design and engineering, and is the basis of amp class distinctions.
  • Caleb from Indianapolis

    Posted on 10/13/2021

    Whoa. Class D has made some of the most amazing advancement in amplifier technology. The old adage that amplifiers still use the same technology, so don't need to be updated is now only half right. A good amp doesn't need to be updated, but amplifier technology in Class D is rapidly changing and is nowhere near it's full capability. Companies like Legacy Audio, the same company who makes the Legacy Valor's at $86,000, Red Dragon, D-Sonic, Bel Canto, Hypex, ATI (maker of Outlaw and Monolith, all use Class D). NAD, Mola Mola. McIntosh. The main man at audioholics, Gene Dellasall said that Class D is the future. I'll admit, the things said here used to be true. But, that's because they were making Class D like you would a Class A/B. They had to learn how to play to it's strengths. And there are many. Check out Gene Dellasalla's review of the new Legacy iV series amps. After all, you guys sell really good Class D amps. Oh, JL Audio. The best name in car audio. Granted, it's sub amps, but if they got better fidelity from a Class A/B, they would be selling those. Look forward to the friendly debate.

  • Steve Middleton from Long Eaton

    Posted on 9/13/2021

    Finally, an excellent explanation for how different amplifier classes operate

  • Mohave Joe from Montclair

    Posted on 4/26/2021

    I purchased a monitor from Seismic Audio. About nine months later I purchased another that were identical with one exception. The first was a Class AB and the second they had changed to a Class D. When I daisy chained them the sequence made a huge difference. With the AB first and the D second the performance of the D was pretty bad. When I reversed them and put the D first and the AB second the performance was fantastic. Why?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 4/27/2021

    Mohave, Without knowing the make and model of your speakers, nor the application you're trying to accomplish - why and how you're daisy-chaining speakers together in the first place - we can't help you with advice.
  • Daniel from Quinlan

    Posted on 3/2/2021

    Thank you for the information,that was very helpful...

  • Eugene Gaudreault from Danbury

    Posted on 12/29/2020

    I would be pleased to work with anyone to develop an input stage for a Class AB (or G or H) amplifier to eliminate the drawback of crossover distortion. GeneHG

  • Eugene Gaudreault from Danbury

    Posted on 12/29/2020

    Adding to my previous comment on AB amplifiers: Class AB amplifiers can be designed to have a input stage that provides local feedback to eliminate crossover distortion, but that doesn't seem to be practiced. When playing music, there should be a quiet background without buzzing. It is amazing that if the volume is turned down, the buzzing remains. Until specialized input stages become widely available, I prefer the class D amplifier, and look forward to pulse generating clocks of 500 Khz in order to lessen distortion effects. A 20 Khz signal sampled at 500Khz has only 500/20 = 12.5 samples per 1/2 cycle, creating distortion as the phase angle changes. Second harmonic would be at 40 KHZ, likely filtered out and not audible. I recently bought a pair of JBL Party box 100's based on a listening test. They were awesome with low buzzing. Subsequent comparisons with the JBL 300 showed the 100 to be inferior. I suspect that the difference was due to differences in the crossover characteristics. Apparently these are not tightly controlled. I returned the speakers and am looking forward to combining a class D amplifier with JBL Lancer 44 speakers (with renewed cone). Instead of the class D amplifier, I would prefer to use an AB amplifier that has a crossover minimalization circuit (whenever that becomes available). Adding class G or H is good if there is a circuit to minimalize crossover distortion. GeneHG

  • Eugene Gaudreault from Danbury

    Posted on 12/29/2020

    AB is apparently the standard many use for judging power amplifiers. AB amplifiers are inherently distorting (the crossover problem), just like many tube amplifiers (grid voltage to plate current characteristic follows a square law ). Apparently, many have become so accustomed to the distortion that it is now accepted as a standard. In a world of "Audiophiles", that seems absurd. This is similar to the effect of walk-man transistor radios in the 60's. The walk-man was a marvel, but the amplifiers were under powered. Many ran the volume so high that the output was nearly a square wave. This obviously distorted sound became the standard that was used for judging the quality of everything else. High quality audio for the massed disappeared because the public preferred distorted sound. To me, an amazing step backward. For class AB music, there is an accompanying buzzy sound (listen carefully). This buzzy sound is absent if all processing is done in class A. The buzz results from crossover distortion that produces a dead spot of lower gain at the crossover point. Traditionally, this crossover was thought to be trivial because output voltage is at zero only a small fraction of time. In practice, the crossover is at each reversal of current. Voice is provided from the vocal cords that produce square wave bursts of air. The resulting sound is shaped by resonances in the vocal tract. Voice is inherently buzzy and sounds good when processed by basic AB ampli

  • Vincent from University place

    Posted on 12/22/2020

    I've used many different brands of car audio amplifiers in the last 25 years. Some cheaper, some expensive (all name brands). Class ab sounds the best for mids and highs. Class d is great for subwoofers but nothing else. But for 90% of people, class d would work fine for their mids and highs.

  • Antoine Bercy from Metairie

    Posted on 12/17/2020

    Hi Mr. Buck Pomerantz, hope all is well, I've heard that Class D amplifiers cause issues with the vehicle's radio FM signal. Is this still the case for class D amps on the market today? I've also heard that class AB amps don't have FM signal issues, and I was also wondering if the class AB+D amps have FM radio issues due to the D section? Thanks

    Commenter image

    Buck Pomerantz from Crutchfield

    on 12/21/2020

    Antoine, Class D amps have transistors that switch on and off rapidly. This used to cause havoc with nearby FM radios, but nowadays it isn't much of a issue due to improvements in Class D output filtering.