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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.

  • 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.
  • Joe from Roswell, GA

    Posted on 12/2/2020

    I have a new(er) Alpine class D amp. I just bought a set of Focal speakers to match against my Alpine 6×9 3-ways, and was SO disappointed in the bass (penetration and dispersion) AND treble (symbols especially) until I switched to an old Crunch A/B amp - same RMS wattage output (never thought much of Crunch before, but "loved" Alpine). The difference is night and day! Symbols are now brilliant, and the bass can be heard outside my motorhome, but it is not "louder". The "pure" sine-wave does not "break up" when it passes through walls or is interfered with by other sound waves. Don't feed me any bull about class D being audiophile quality. It was OK for my (now retired) 4×4 truck where I had A LOT of road, motor, and gear noise, and very little mounting space. But in my motorhome, it sounded like a cheap boom-box. No warmth, detail. Vocals especially sound like a "recording" even with hi-fi quality recordings. Those sound near "live" with the AB amp. It is no wonder that Alpine's subwoofers are now 600-watt RMS, to match their new class D subwoofer amps. That is to make up for the facts above, methinks.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 12/7/2020

    Joe, It sounds like you listen to music when parked. In that quiet environment you can hear the subtle differences between D and AB processing. Now you know why the makers of expensive high-end amplifiers only use AB technology.
  • Andrew hill from New york

    Posted on 11/12/2020

    Hi can I use a 4 channel amp with 2 power base subs 8" 200w rms each?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 11/12/2020

    Andrew, Without knowing precisely what amp and subs you're referring to, we can't help you with advice. If you want a question answered about a system, you must identify the gear by brand names and model numbers so we can get the right information to you.
  • Xcalibur from Illinois

    Posted on 10/26/2020

    I have 2 questions. I'm looking at getting a taramps ts2000x4 to run my mids and highs. I can't find the power input gauge size, do yall happen to know? Also it has a terminal for the + of one speaker, a - for another speaker and a third terminal for -/+ of both speakers. How is the possible? Both speakers would share one terminal. Weird

    Commenter image

    Buck Pomerantz from Crutchfield

    on 10/27/2020

    Xcalibur, You can go to that amp manufacturer's website and download the owner's manual for that amp, and all your questions will be answered.
  • David St. Clair from COLLEGE PLACE

    Posted on 10/18/2020

    I love the class D amps. I have 2 crown pa amps that just wont get hot after hours of playing music at dances. I put a 500 watt class D rockford fosgate in my truck to run my 12 inch sub. It blast away and barely gets warm. Class D is the way to go. More power with less heat.

  • Scott Saunders from Phoenix

    Posted on 10/11/2020

    Actually, there is a company [Mosconi] that makes a Class A car amplifier. [link removed] for $2,400 bucks!

  • Antoine B from Slidell

    Posted on 10/3/2020

    Hi, I'm interested in a 5 channel car amplifier. The JBL club 5505a has a Class AB+D amplifier. AB for the high/midrange channels and D for the subwoofer channel. I know that a full class D amplifier would be the most efficient, but, will the ab+d technology be efficient as well if crossed over at around 60hrz with that sub channel on the D channel? I was also wondering why? Is it the best of both worlds? I couldn't find any information on the combined tech used together. Thanks Antoine

    Commenter image

    Buck Pomerantz from Crutchfield

    on 10/7/2020

    Antoine, Manufacturers of AB/D amps claim higher fidelity on the front and rear AB channels and higher efficiency for the D section, but there are no audible differences.
  • Tom from NY

    Posted on 9/30/2020

    As an aside question. I have an old hafler 220 power amp. What modern preamp should I paid with it?

  • Lyle from Redondo Beach, CA

    Posted on 8/29/2020

    Thank you for the extremely clear overview of amplifier classes. Class D is interesting. It has been gaining acceptance from the HiFi industry and may be the future since it can deliver lots of power with very good sound at a relatively low cost-at least by audiophile standards. Stay well.

  • Richard kinyanjui from Palm Desert

    Posted on 8/7/2020

    Mr. Pomerantz, you are a really gifted teacher. I've been confused by the classes for a long time but not anymore. You've made the classes distinctions as clear as crystal. I'd pay for my new knowledge if you sent an invoice. Please consider writing a book on electronics 101/for dummies. It will help many including myself. Question. How great are amps with more than two channels? I've seen a 7 amp chanel specified as 7x 80. Does the number of channels affect amps fidelity? How do i know if a home amp is class A, AB, or D? Where is the class specified? Thanks and keep informing. Also please include the button for printing this articles so we can refer to it many times> Thanks and G bless!!

    Commenter image

    Buck Pomerantz from Crutchfield

    on 8/7/2020

    Richard, The only way to find out the class of an amplifier is to refer to the manufacturer's manuals and sales brochures - and even there, they often don't publish that information.
  • Jeff from san diego

    Posted on 7/8/2020

    hi, you classification of class D as less than high fidelity, really is inaccurate. Perhaps this is true in car amplifiers, but certainly not in home audio. PS Audio, Nad, Devaliet, even McIntosh, has embraced class D version from Hypex and Icepowers top of the line technology.  Class D, especially when coupled with a class A input stage like Nad Master Series M22 are exceptional amplifiers.  You really should correct this distinction, its misinformation and insults the gifted engineers, like Bruno Putzey, who helped to really bring this technology out of simply subwoofer only use from the early 2000's to mainstream full audio high fidelity.

  • Asghar

    Posted on 6/11/2020

    What is class fd ?!!!

    Commenter image

    Buck Pomerantz from Crutchfield

    on 6/12/2020

    Asghar, I'm not exactly sure, but I think the Class F element adds a dual transistor push-pull circuit to the Class D amp stage. In other words, it's magic that only an electrical engineer will truly understand.
  • eric from Hong Kong

    Posted on 4/13/2020

    Good for me more understanding kinds of amplifier class, it can also apply for home hifi knowledge as well.

  • Ron from Greenbelt, MD

    Posted on 3/6/2020

    Why doesn't Crutchfield specify the amplifier class in the Features & specs of the Details tab?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 3/9/2020

    Ron, We most certainly do publish an amp's class when known. Perhaps you didn't scroll down far enough on the features tab to see it.
  • Chris Christensen from Grass Valley, CA

    Posted on 3/5/2020

    I am questioning your description of Class D operation. You state that the width of the waveform controls the volume. If this is the case, where is the frequency term encoded?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 3/5/2020

    Chris, After modulation, the frequency of the pulses convey the frequency of the output signal.
  • DMac from Burlington, ON

    Posted on 12/9/2019

    The transistors in an AB configuration are not 'mostly on', they are biased to the point that the transition from the positive to the negative device is smooth. At best, they would be considered 'slightly on' except in the case of a high-end amplifier like the ARC Audio SE series of the D'Amore Engineering A Series.

  • Alex Mendez from Whittier, California

    Posted on 11/16/2019

    Informative, thank you.

  • Jay from Nyc

    Posted on 10/27/2019

    Great article. Thank you.