Class A? Class AB? Class C? Class D?

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n4d5

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Sep 7, 2008
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Hi

I been looking at a few manufacturer websites and there are many amplifiers out there saying they are Class A, or Class AB or Class C or even Class D. What are there differences? is one suited to a particular application?

Thank you for your advice.
 
Class A is regarded by many as the best, it being a push-pull design. It also heats the room up nicely!

Love them. Here is an article that I have found that explains things.

The Different Classes of Amplifier.

There are a number of different classes of amplifier categorised by the behaviour of the
output devices when a sinusoidal test signal is applied:

Class A - Both output devices are continuously conducting as there is some bias current
flowing in the output devices. This topology is the most linear (resulting in the lowest
distortion) but also the least power efficient, around 20% (That is why they sound beautiful!)

Class B - In this case there is no continuous bias current flowing in the output devices with
one device conducting in the positive region and the other in the negative region. This
improves the efficiency to around 50% but generates crossover distortion due to the time it
takes to switch one device off and turn the other device on. This is particularly evident at low
output levels where the non-linearity at crossover is a larger proportion of the output signal.

Class AB - This type of amplifier is a combination of the above two types which provides an
acceptable level of performance (typically 0.1% to 1% distortion) at similar efficiency to class
B. Here both output devices are allowed to conduct at the same time, but just a small amount
near the crossover point (i.e. each device conducts for slightly more than half a cycle, but less
than a whole cycle). Class AB is the most popular amplifier topology for portable audio.

Class C - Is not generally used for audio applications as it adds too much distortion to the
audio signal

All the above are termed linear amplifier topologies as the output devices remain "nonsaturated"
(do not operate near the power supply rails). Unfortunately most amplifier power supply rails do saturate the audio signal to some degree, this is why great benefits are achieved on mains conditioning as it reduces this interaction.
 
n4d5:...I been looking at a few manufacturer websites and there are many amplifiers out there saying they are Class A, or Class AB or Class C or even Class D. What are there differences? is one suited to a particular application?

Class A / AB are both "push pull".

Class A keeps the output transistors turned on permanently (hence the higher dissapation) where as class AB maintains a small amount of bias current, keeping the transistors turned on at a very low level. Class AB designs used to suffer from "switching distortion" although that has largely been minimised / eradicated with modern designs. A 100w Class A amp will dissapate about 200w of heat even when it's sat idle. A Class AB design will dissapate approx. 1watt depending on bias.

Class C used to be the preserve of RF transmitters, and class D is digital (way too complex to explain here).

Take a look at this article on Wikipedia for a more detailed explanation.

Scroll about half way down the page to the heading "Power amplifier classes".

As for application - Class A / AB is largely still regarded as the best for general audio use. Class D is good for subwoofers.

Class C still best for RF IMHO.

EDIT: It would appear Trevor posted during me composing mine!
 
n4d5:Thank you for the prompt advice. Can anyone recommend any Class A amplifiers?

Don't be misled into thinking that Class A is the holy grail of amplifiers. You get bad Class A designs just like you get bad Class AB designs.

When HiFi Choice tested the Marantz PM7200 (a switchable Class A / AB amp) they couldn't tell the difference between the two classes. This was probably because it's a good class AB design in the first place.
 
Also bear in mind that many (but not all) valve amplifiers are class A, but usually quite low power (unless you're prepared to spend ££££).
 
Well the idea was to buy Plinius, my budget is £5000 in total for an amplifier or a pre amp and pre amplifier.setup.
 
Andrew Everard:
Terryff:Excellent post Trev.

And nice one Wolfson...

Andrew, where did 'Wolfson' contribute to this thread?
 
n4d5:Well the idea was to buy Plinius, my budget is £5000 in total for an amplifier or a pre amp and pre amplifier.setup.

You are looking to spend serious money here. It is neigh on impossible to give you specific advise.

I would say that you have to have a focused idea on what needs and requirements are, also what the rest of your system is comprised of (or will be). Then you need to do some careful auditioning.

Many years ago (1988 ish) I had 2 x Marantz MA6 monoblocks (special additions) run with a Massive 3kw Russ Andrews mains transformer (Pure class A) cost £8k in total.
Do you know I couldn't live with them, the sound was too brittle/harsh with the sound stage so wide the music was on the verge of being disjointed.
I quickly tired when listening to music on them, and was loosing the pleasure. The funny thing was that I had a Cyrus 6 and PSX unit (highly modded) which was more musical - I went back to that!

With my current AV system (which I have done various modifications to from new) it gives me almost the same musical performance as any hi end gear I have owned or listened to over the years. The point I am making is that you can achieve far better sound today for less money than it cost years ago, and with a few tweaks can make well designed equipment sound sublime.
 
Thanks for the advice Trevor I see what you are saying.

My initial systems was going to be either a Cyrus setup consisting of the Mono X amps, pre amplifier, and the DAC cd player, or the Quad 909 setup with the cd player.

Then I started reading up on Plinius and thought my budget can expand to buy the pre amp M8 and power amps P10 and buy the CD101 later. I was going to auditons these, but a friend of mine put the spanner in the works and started to talk me about Class A.

Basically I'm confused now.

My existing system dates to the 1970's. For example A Pioneer SA-8100 integrated amplifier 40 watts RMS, matching tuner and a NAD C541I CD player. The amp is faulty now. gave me many years of good service and loved the sound. Now it cannot be repaired due to parts not available etc.

I only want to spend this kind of money once and not keep on upgrading. Therefore I'm after a CD player, and amplifier setup (Being intergrated or pre, power combination).

Speakers will come later as I have some MS908i's which I want to get some use out of before I upgrade those.

The type of music I listen to from Classical, R&B, Jazz, and Bhangra. I would like a clear detailed sound, with a nice hard hitting controlled bass line where you can feel the music and enjoy it, ie the bass gliding through the floor boards etc. (Wife and Neighbours not a problem 🙂)

Any thoughts?
 
If you're looking for a sweet sound with large dynamics, you could follow the Musical Fidelity route. Buy an A1 integrated and add some KW550 Superchargers. These are mono blocs that connect between the speakers and amp and boost the sound but keeping the original signal quality. Might an idea.

You have a big budget so look carefully! Forget class A or not, just audition.
 
Now many hifi system using class D and some of them are sound good enough.

but still below the Class A.

some say that class D will not be good / just enough.

What do you think ?

and also about the power supply. my friend say switching powersupply will not as good as toroid ones.
 
utomo said:
Now many hifi system using class D and some of them are sound good enough.

but still below the Class A.

some say that class D will not be good / just enough.

What do you think ?
Click to expand...

I think that hoping for Class to define the absolute performance is pretty pointless. It's a bit like saying all 4 x4 cars are better than all estate cars. They are just different designs for different reasons.

There are various exceptions too. The Devialet are part Class D and I'd struggle to tell you what sounds better!
 
Tony_R said:
When HiFi Choice tested the Marantz PM7200 (a switchable Class A / AB amp) they couldn't tell the difference between the two classes. This was probably because it's a good class AB design in the first place.
Click to expand...

That's not quite what my memory serves of their review. As far as I remember they could hear a difference, and also note it was considerably lower powered in Class A mode.
 
nopiano said:
The Devialet are part Class D and I'd struggle to tell you what sounds better!
Click to expand...

Yes, but the Class D part is slaved to the Class A part. The sonic signal is entirely produced by a (low-powered) Class A amp.

:cheers:

Matt
 
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