What's the difference between amplifier classes? | Crutchfield

Class D, Class AB, Class A...what's the difference between these amplifiers, and why would you want one over another for different applications?

Amplifiers use different designs to perform their primary job: to increase voltage in order to combine with an audio signal to play over a set of speakers. There are different ways to achieve this, and depending on what you're asking your amp to do, one design may be better for you than another.

Amplifier class designation refers to how that particular amp combines the power and signal. The classes differ on how efficient they are, how much room they take up, how they handle the heat generated by amplification, and the fidelity of sound they produce.

Class A amps offer the highest audio fidelity, but they're inefficient and rare, and due to the heat they generate they are not used in car audio.

Class B amps run much more efficiently than Class A amps, but due to their two-transistor design they produce crossover distortion. Pure Class B designs are exceedingly rare.

Class AB amplifiers take the best of Class A and Class B to produce a fairly efficient amp with high fidelity sound. Most home theater amps and stereo amps use a Class AB design.

Class D amps are very efficient, and can take on a much smaller form since they are not producing as much waste heat. This makes them great for car audio and PA systems, though Class D amps don't output as high-fidelity a sound as Class AB. The trade-off for car audio is well worth it, though, because Class D amps don't draw as much power and you don't need to worry as much about keeping them cool.

Which one's best? That depends on what you plan to do with your amp. As always, if you have questions, just call Crutchfield or chat online with one of our product advisors. They'll be happy to recommend something for you.

0:00 Intro
0:20 Amplifier basics
1:06 Class A amplifiers
1:48 Class B amplifiers
2:10 Class AB amplifiers
2:58 Class D amplifiers
4:19 Other amplifier classes
4:45 Should I care?
5:18 Recap and contact info

[aegisofhonor]

class D uses a 'constantly switching" PWM (pulse width modulation) to maintain the output stage of signal while allowing as much of the input power to be converted to sound. In practice, you will never get 90% efficiency out of even the best Class D amps as the efficiency comes from the output levels matching what power is required to keep the output stage running at all times, which happens between 40% and 100% of maximum output power mattering on the type of amplifier and it's max power rating and still use a good chunk of power even at idle (NOT standby, idle; it's on, just playing music). Class A for instance will often use more then 25% (sometimes 50% for smaller amps) of it's max power input even on idle and never gets above 50% efficiency even at maximum output. AB is slightly better at about 20% input power at idle and gets to about 70% efficiency at around 80% of it's output power rating or higher. Class D often uses about 10-15% of it's input power at idle and hits it's maximum efficiency at around 40% power. Because most people rarely "push" their amplifiers anywhere close to their limits, their amp's effency is never as good as it's "max efficency" because you have to output enough power and use that power to really measure any respectable amount of "efficiency".

[makstaz]

Finally someone explaining the amplifier classes in a way that everyone can understand.Great job!

[nighttow]

One thing I can say about a/b amps from reputable manufacturers, they certainly last a long time. I have an old Kenwood kac-7201. This amp in combination with some Fosgate Subs, have been in service in my various Vehicles since I was 19. I'm 41 years old now. And just like when I was 19, I pushed these things hard every single day. The amp is underrated as well as the subs. The amp is supposed to be stable with 4-ohm load only, and I've been driving a 2 ohm load in excess of 20 years. I have yet to own a class-d amp. I have an Optimus 4-channel Class AB that I purchased at RadioShack some 22 years ago driving 4 full-range speakers. This amp is capable of driving Subs, but its built-in, non-adjustable subsonic filter starts cutting off at 32Hz. Which is absolutely unacceptable for driving Subs. But it does pretty darn good for 6 by 9 coaxials and six and a half inch components. I do have to be careful though, at Full Tilt, this setups current draw exceeds alternator output.
Edit: actually now that I think about it, this Kenwood amp that I'm so fond of came from Crutchfield all those years ago. I miss the days when a phonebook sized Crutchfield catalog would arrive at my door. For you kids that don't know what a phone book is, ask mom or dad

[zoroluva]

Very good quick educational video. Thank you!
A couple of things to note::
1. The claimed efficiency of A/B design is realized at over 85% output... Meaning your ears would be bleeding before you get anywhere close to it's peak efficiency.
2. Class D hits peak efficiency quickly. 30-40% of output you'll almost be at max efficiency. I think most people live in this range majority of the time and this is why Class D is the future.

[cgreenfield]

Crutchfield is such a great company. Always quality info, reviews and products. Glad to see they survived the Amazon invasion.

[dougmurray]

A breakdown of how each of these classes work, how each reduces or increases distortion, etc., would be appreciated.

[itisjustacomment]

I remember building a mosfet amp some 600w single output at college 28 years ago .
I still have it, but not the 15" guitar bass speaker and larger box I had set up to my stereo with I was young .
Back then, I built with a normal stereo amp, mulplie crossover, plus 3 class D's one for bass and two for normal full range and the regular amp for highs only.
The sound was unmatched to what I even had two, which costs 3k new just for speakers and pre plus power amp .
It's amazing what building class D's yourself can achieve or trying out different chips in Morden versions at low cost.
The only issue with building is the boxes, which cost more than all the components plus buying an external power supply. Luckily, I had already had multiple outputs .
I wish some 30 years ago we had class D's to these days with such quality, controls, and power at around £100 pound, which can get you a 150-watt at 8ohms easily.
It's kind of perfect for recond decks or phone jack to input to play YouTube music or other apps.

[tomkocur]

Anyone calling a basic class D amp "digital" should go to jail. Thank you for calling things the right way.

[adrianbennett]

Still love the sound of my 18 year old Panasonic (Technics) Class H+ AV receiver. Have tried replacing it but I always come back to it. Can't find any info on the modules inside it, but can see two distinct voltage rails feeding them.

[thewoodlife]

This is the first time I understood what different class amplifiers are. Nice explanation. Thank you

[nanocash]

Quick question, would it help showing showing a car audio tech thru headphones/earbuds, what kind of sound you want in your car by showing them what type of music you listen to ?

[CoolDudeClem]

I built a powered speaker with class B amplifier (yes, class B). It uses negative feedback to provide a "corrected" waveform to the output transistors (opposite to how they would distort) and it sounds pretty decent.

[jjc]

Can you explain how the H class amplifier works? And why only Bob Carver uses it and no one else? Thanks.

[BrandonBuckaudioanarchy]

Amplifiers convert input POWER to output POWER. ALL electrical circuits are comprised of voltage and current, the combination of which is where we derive power.

Saying amps convert input voltage to output power makes no sense. You could say they convert input voltage to output voltage but that's only half the equation. They also convert current... which again, is the second component of power.

To measure efficiency you can't compare input voltage and output power... you have to compare input power to output power. Hope that helps clear this up.

[Siddharth_Matada]

Hi sir, whether class AB amps drain car battery more quickly than compared to class D amps when we have stop the car for 1 or 2 week? Also whether we need to face car starting problem as battery gets older in case of class AB amps?

[akeemjackman]

What’s the best set up for a motorcycle. 2008 Hayabussa. I’m thinking 2x 6x9 speakers and then either subwoofer under the tail. What are your thoughts?

[PMantle]

I am now exclusively using apps like Youtube Music to a Chromecast Audio/AV receiver/4 way selector to patio and in celling speakers. How do I decide on a new amp?

[raymondchapman]

So, an A/B amp is better for mids/highs than a D amp as far as sound quality ?

[jasonwilke]

I noticed both of the Sony MEX-XB120BT stereos we bought from crunch field don’t seem to have very good quality of sound they get loud but not an enjoyable loud probably the class D amp it says it has. Do most car stereos have these class D amps now because I really want to replace them with some thing that doesn’t and also the 10 band EQ is a pain to use.

[jrgumex]

hi, knowing class D amp full range are more eficient than AB in term of fidelity is D supeior than AB or AB is has more fidelity than D in car audio? I need to make a decision for my two pair component speaker system