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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 and smallest footprint. It's the modern marvel of audio amplification.

And be sure to watch our video, down below.

We use a few car audio references below, but the amplifier design principles are the same for all audio amplifiers - car, home, and portable gear.

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. You'll sometimes find them in home audio and guitar amps.

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 amplifiers 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 and many car amplifiers are Class AB.

Until recent years, 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.

Efficient ... and smooth

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.

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.

Class D has become the go-to choice

When they first hit the market, Class D amps were only used for bass because of the high amount of noise they generated. But that has definitely changed over the years.

Modern Class D amplifiers are popular because they're smaller, lighter, and run cooler than the other Classes of amplifiers with the same amount of power. These are big advantages when you have to fit an amp in a vehicle or carry one around for gigs. You'll even find Class D amps in high-end home audio gear where they're tuned for incredible sound reproduction.

Small size, very little heat, and lots of power. Class D is where the action is.

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.

  • Martin from Corona

    Posted on 1/15/2023

    Which amplifier or receiver delivers good power to Epic CF-3s? They have efficiency of 100 dB SPL @ 1 Watt/ Meter and 250 watts RMS.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 1/16/2023

    Martin, If you're asking about Klipsch floor-standing speakers - almost any home theater or stereo amplifier will work. Give us a call so an Advisor can help you choose the perfect amp for your system.
  • paul g breda from Northville

    Posted on 1/2/2023

    i had to pull out the class D amp in my jeep , when it turned on , all FM signal was lost . A-B amp solved this 100%

    Commenter image

    Buck Pomerantz from Crutchfield

    on 1/3/2023

    Paul, The amplifier should not have that effect on a source's signal - unless there's something wrong with the amp and it's radiating interference.
  • John from LKN

    Posted on 12/20/2022

    I've always been a fan of the class H amps. High power with low distortion works for me. Soundcraftsmen, Haffler, and even Dynaco made some great sounding amps and the Soundcraftsmen amps are bulletproof.

  • Joseph Morris from North Chicago

    Posted on 12/15/2022

    Great info even after years of dealing with amplifiers it's still good to learn new technologies as they progress

  • John from Bend

    Posted on 11/29/2022

    You should have mentioned the incredibly difficult design of the output filter for any switch-mode i.e. class D amplifier. This is not trivial. We have to do that for class D/E plasma generators as the plasma doesn't like distortion either. While class D might sound OK for car audio where other noise is there anyway, it's not on my plate for home audio. The losses and phase distortion in an audio class D amplifier do not, at this time, make me want to buy one. I actually did, and sold it as it sounded like a first generation CD.

  • Brandon from Aurora, CO

    Posted on 11/20/2022

    First, thank you for the article. I have 2 10" Kicker Q-Class Model 42CWQ102. I am looking for the best amplifier for quality as well as the boom! A lot of amplifiers boast about max output power, but I am looking for quality power distributed for the best quality sound. I know with quality comes price, and so I am planning *saving* for the quality product. Rockford has the T1500-1bdCP which seems to have what I am looking for, but the reviews of the amp talk about the heat. Can you provide any feedback on other amps that would deliver 700 RMS at 2 ohms with great efficiency as the Rockford has at 2ohm load? A follow-up question I have is my vehicle is a 2023 Hyundai Palisade with an amazing sound system from the factory. Would you know if this factory system has integration to plug and play? If not can you please direct me to a product that I will need to install? I am just now coming to a maturity in my financial life to get back into building a quality audio system. Although I am exited about it, the installers in my area don't seem to have the same quality in mind. I am going to venture this on my own! Thank you kindly for the read, and I look forward to your response! With love from Colorado. Brandon

    Commenter image

    Buck Pomerantz from Crutchfield

    on 11/21/2022

    Brandon, Two DVC 2-ohm subs rated for 700 watts RMS each will work best with an amplifier that can put out up to (but not more than) 1400 watts RMS at 2-ohms, like a JL Audio HD1200/1 or a Morel MPS 1.1100.
  • David

    Posted on 11/14/2022

    No mention of amps using output transformers or even tube amps

    Commenter image

    Buck Pomerantz from Crutchfield

    on 11/16/2022

    David, Amplifiers that use vacuum tubes and output transformers are not used in car audio because the vehicle's vibrations would most likely break the tubes' delicate internal components.
  • Ricky from Palm Desert

    Posted on 11/12/2022

    Heyyz, how about the subwoofer.Does the switching slow down when the frequency is lower? And, does the switching performance give equal robustness to all audio frequency's outputs in lower price units ?

  • Bill Hurst from Carbon Hill Alabama

    Posted on 11/6/2022

    I like a quiet room, at least 100w per channel, clean and clear to hear each instrument in the piece I am listening to.

  • Matt

    Posted on 10/16/2022

    Hello, For 400W RMS sub one voice coil is best which amp A/B or D? Currently i have 4 chanell A/B for front speakers and for subwoofer as bridged 400W RMS for sub which sub require 400W.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 10/16/2022

    Matt, There is absolutely no reason for not using a Class D subwoofer amplifier in a car audio system.

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