DIYHiFi.org

Ok, let see if I get this right:

So a class AB amp is a class B push-pull amp with bias applied so that there is no hard crossover distortion point. One side of the push pull continues to work briefly work as the other side turns on.

Now, you can vary the amount of bias on that AB amp so that for a given load, it is always operating in its Class A region. Given a higher load, it will still move into class B operation (where one side of the push-pull turns off).

So my question is that is a Class-A amp, that is really a Class-AB amp biased really deep into Class-A that is expected to always run in Class-A, really a Class-A amp? Or is it just a Class-AB with creative marketing?

My basic electronics background tells me it is a Class-AB.

huh....you start on the end (class B) and progresing toward beginning (class A)....
Better to start on the beginning (A) and go toward end (B)

IMHO, Class AB is Class A with too low bias
This leads to funny question, what is really creative marketing: Class AB or Class B

Well yes, single transistor is Class A, is it not? So yes, class A is the beginning.


But two transistors in push-pull would be called Class-B, so class-AB must be the lie then.

Interesting thought though. Challenges my assumption on step further. Now I can be more confused.

Don't mix number of devices with Class A or AB or B.
Some time ago there was a nice document on Passdiy site, maybe is still there.

'Class AB called Class A?"

Humph. I've seen this used * lot* by makers of mosfet -based amps who somehow think that running merely enough idle current to get good crossover behaviour amounts to running in 'class A' (*cough* even pro-orientated ATC for one *cough*).

OK,100mA/device or so might be enough for 'Class A" at late-night domestic listening levels, it just might. But it's rather disingenuous as a practice!

Or class B with high bias?
Depends at how you look at it.
It's more "marketable" to call it "class A" or "class AA" or whatever they come up with. Low bias is in the details.

Yes, like optimistic/pesimistic view: glass half full/glass half empty

The term in this case was "push-pull Class A" which I called a marketing term to sell more amps. It didn't go over well but my opinion of the term still stands.

Erik,
are you asking us if single ended amplification is the only version that can be defined as ClassA?
This could be valve/tube or BJT or FET or some new technology.

Why should single ended be the only ClassA?

No.

I'm just saying that calling a Class AB stage running in its Class A area as pure Class A a bit misleading.

Maybe I don't know enough but it would seem that there is a difference between a stage fully designed as Class A and one operating in Class A

Hi,
"fully designed as ClassA".
Are you able to expand on your proposed substandard definition of ClassA.

Single end is basically a native class A, as active component never turns off (it would be disaster otherwise).

Push pull "class A" is always a "class A up to xx Watts at given load". At one point, every push pull "class A" amp leaves the class A, unless you limit it elsewhere, intentionally or not.

Colloquially, however, class A is not only about the lack of crossover distortion, it is also about the "constant" current, and that's what this disco is usually rather about.

As someone with not great technical skill, I have to rely on definitions.

We generally call single-ended Class-A. Some that I have had this discussion with state that it even leaves Class-A when it starts to get overdriven.

Push-Pull without bias is Class B. Again, a traditional definition.

Class-B with some bias to remove crossover distortion has been called Class-AB.

Add "heaps" of bias to a Class-AB amp and operate it in the bias range and we get push-pull Class-A. If it moves outside its bias range, we move into Class AB (or Class B?)

So what would I call "fully designed as Class-A"? An amp that cannot leave Class-A. Single Ended Class A; push-pull Class A limited to Class A operation only.

Maybe I'm just being overly specific, but I see stuff listed as "pure Class A" when it is just an AB circuit operating in its bias range. And there is a clear omission of the fact that the amp may ever leave Class-A.

Truth in advertising? I've already lost that one. But at least I understand some more of the aspects of this whole Class thing now.