Class A into Class AB

Mostly it is marketing lingo, for a biased amp.

I agree!

My understanding is a class A /AB amplifier, will have a switch to differentiate between the two classes. The switch changes the amplifier quiescent current. My amplifiers have a switch on the front panel in class A the quiescent current is 250 watts, in class AB quiescent current is 90 watts. I don't know if that answered the OP's ?

Every aspect of sound quality was improved by bias increases, especially at low listening levels. I ended up at 3.3x the factory setting with heat buildup being the limiting factor. Hard to believe that a no-cost adjustment could offer such an improvement.

Usually you have a certain amount of dissipation that occurs in the output section. Something like the Bryston I would expect to not be heavily biased, but quite often when you increase the bias current, its also helpful to reduce the Vcc+ and Vee- voltages so as to prevent the output devices from being damaged. The heatsinks are probably designed to present a certain 'thermal resistance' which in plain parlance means you may overheat the amp in certain circumstances. So be careful!

So let me get this straight.

You can design amps that only operate in class A.  When pushed to their limit they simply distort.

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).

There are amps that have a switch that allows you to choose between class A and class AB operation (example?).

Does that cover it?

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.

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...