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Class A into Class AB

I’m still a little confused about power amplifiers and integrated amplifiers that are class A & class A/B. Like when they say the first 8 watts are class A then it goes into 400watts class A/B. But the same amplifier can be biased to put out 12watts class A then go into 250watts class A/B. It can be biased again for 18watts class A and 150watts class A/B. etc. Ive read that these amplifiers, ones that can be biased like that...and in general all the "first X amount of watts are class A before going into class A/B"...that those first X amount of class A watts is NOT true class A. If that’s true...what is it then? What’s "kind of" class A mean? What’s the point of a "first X amount of watts are class A" then?

tmac1700
atmasphere's avatar
atmasphere
11,800 posts
 

No. There are also amps that have a sliding class A bias system, where the bias is increased with the signal level. This allows them to idle without much heat. Krell used a system like this.

In tubes, you have class A1, class A2 (where grid current exists during part of the cycle of the waveform being amplified), class A3 (which was patented by Jack Elliano), class AB1 (no grid current) and class AB2 (grid current exists during part of the waveform cycle).

To be clear though:

There are amps that slide from class A to class AB (e.g. Pass XA25, Luxman).

There are amps that can be produced with different amounts of class A before transitioning to class AB (e.g. Coda).

-these are both simply class AB.

 

jjss49's avatar
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jjss49
6,612 posts
 

now that we are all confused and mystified, to add further complexity to matters, nelson in his well and densely written owners manual material for my pass int 150 says - (i am paraphrasing) - this balanced topology amp produces 150 wpc class ab into 8 ohm, but the first 15 watts are run in class a, and the first 2-3 watts are single ended (as opposed to push pull) class a

does that make sense? it must cuz nelson usualluy doesn’t fool around and b-s much, but i am wondering how this works

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separately, it is certainly my experience that within the power envelope of the class a amp, they sound purer, sweeter than their class ab counterparts - evidenced by my own comparisons between the belles sa30 vs sa150, schiit aegir vs vidar... 

holmz's avatar
holmz
2,328 posts
 

does that make sense? it must cuz nelson usualluy doesn’t fool around and b-s much, but i am wondering how this works

Well the link made it clear that a biogases amp with be running as Class-A until teh signal level exceeds the bias voltage (or bias voltage - 0.6V).

If one is really distressed with crossover distortion, then they could run Class-A on tweeters, Class-0AB on Mids,. And Class-D on woofers.
Maybe in a 10,30 and 200W range.
Of course they need an active crossover then.

But then there are quality Class-D amp that also have low distortion and do not have crossover distortion.

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