What is the difference between Class A and AB

[comeup]

I currently have a class AB amp that sounds great but keep hearing that class A is better, is this true what's the difference? Comments please.

[ThomasW]

There are many different amplifier 'topologies' This is the design of the amplification circuitry itself. And they're differentiated by class. A, A/B, D, H, G, etc.......

Class "A" solid state runs a much higher bias current than A/B, and IMO they certainly sound better, all other things being equal.

Note that class A amps run as hot as tube amps, and even at idle draw a TON of current.

Google 'power amplifier topologies' and you'll find a bizillion links. It would take a post the length of a book to fully answer your question.

[locomk]

here is a web site that gives a decent explanation of what Class A is and how it compares to a Class AB amp. I have a parasound Amp that is Class A/AB, it's class A for about the first 6 watts and than it switches over to AB. many Amps do this and yours may as well.

http://sound.westhost.com/class-a.htm

what kind of amp do you have anyway?

[Raul Benavides]

to oversimplify:

A class A amp draws the same current no matter whether there is no input signal or when there is a signal (music, etc). When there is no signal, all the current is dissapated simply as heat. When there is current, it goes to the speakers. (that is is why is it said that class A amps actually run COOLER when music is playing vs silence), It is said that the advantage is that when a sudden peak in sound appears, that the power is available instantaneously vs having to draw in exra current. Class AB just means that this applies up to a certain point (i.e. 6 watts of power, etc.), and anything above that additional power is switched on. In short, at decent volumes, you are class A, above a certain volume, you switch to class B.

[comeup]

Thanks for the help guys make a little sense to me I guest since the class A draws so much current you have to baby it a little more because of heat. I have a B&K Ref 50 preamp with a Monster power amp that is A/B, I like it better than the B&K poweramps that are Class A amps. The B&Ks didn't sound that great with music like the Monster did maybe its the Kef speakers that matched better with the Monster amp I don't know, but I keep hearing that class A is better. Not to me.

[BTB]

Hi comeup

I had a Marantz PM 7200 integrated which could manually be switched between Class A and Class AB mode, for practical reasons (like the enormous amounts of heat generated in normal use) power dropped from 105w per channel (Class AB) to 25w per channel (Class A). Like many amps that operate in dual mode only the "first" 25w (in the case of this Marantz) was within the class A envelope and thereafter (as peaks in the music require) it exceeds this threshold and switches into Class AB, then dips back into Class A when the peak has ended and so on...

Honestly, I battled to here any difference (between modes) at all, perhaps the Marantz just wasn't the best quality example of the "Class A" breed to make any kind of judgement at all. Also, if this amp was anything to go by, you could fry an egg on top of one of these things, it gets that hot. I'm no electrical expert, but essentially using a Class A amp, the incoming signal is constantly powered as opposed to a kind of "alternating" ouput of a Class AB amp. Apparently, as this (Class A) technique effectively doesn't have to account for "crossover distortion" as the signal passes the "0" volt line it is sonically more "pure" than Class AB. Perhaps another scenario, common in high end audio it seems, where a good theory has minimal benifit? After all perhaps the sound is fractionally better, but on the down side you have low power output, high heat signitures and large current demand (even when idle). Maybe something will come along to change my mind one day but in the meanwhile I'll stick with "AB".

[comeup]

BTB I heard about the Marantz 7200 and wondered about the A/AB on it thanks for the info I actually I have a co-worker interested in the PM 7200. I notice most of the power amps out there are AB I guest with good reason. I wish they would upgrade the tube amp family I love the way they sound.

[BTB]

hi there

No, problem. That Marantz is a great value amp, if you can still find one as it's just been discontinued. Aside from running warm to hot (not a problem with proper ventilation) it sounds very good...pretty "tube" like. And just like a real tube amp it's just got a little hint of "soft focus" and not the greatest low frequency control you ever heard (I loved it when I used little standmounts but it didn't cope that well with the floorstanders I bought soon after), but I guess for the asking price... The upside of course are beautifully rendered vocals, in fact just excellent midrange in general, quite unforced and natural. I too am slowly being seduced by the tube sound, have a friend with a really good tube based system, got such an "in the room" sort of presence, specially on the sort of intimate acoustic jazz stuff I enjoy listening to.

Coming back to the original post, yes Class AB is favoured for many practical reasons, high efficiency, high power output, etc.