Is there an actual difference between AB and A/AB ?


Almost all high end linear audiophile power amps which are not pure Class A sell some story about being Class A up until a certain power output, then switching to AB.

Excuse me, but isn't this actually AB to start with?

The whole point of AB is that you run with some bias current to prevent complete transistor shut off at the crossover frequency. This whole "our amp is Class A up until n watts" seems like hype.

Am I wrong?

Best,
E
erik_squires
No- you got it. If its not class A, then its AB (or in some rare cases, B). There is also AB1 and AB2 (which mostly applies to tubes depending on whether grid current exists in the output tubes during part of the waveform).
But there is no such thing as A/AB; that is strictly a marketing term.
Are you saying that amps like Pass Labs do not  operate in class A up to a certain wattage or are you just saying that those amps should be called AB not A/AB?
It’s simply the bias on the output devices that determines the level of power afforded in Class A. Such bias allows, or not, whether the output device can swing a given power level through the entire 360 degrees of the duty cycle. The limiting factor is heat and this must be managed effectively, otherwise, the device will fail, sometimes catastrophically.
I don't think it can be called hype when the product of the claims is measurable and audible differences in sound quality. Practically all push-pull class A amps are going to transition to AB at some point, so does that mean push-pull class A amp are just a myth? They're just high bias class AB amps? That sounds like silliness. And what about Pass X series amps that operate single ended class A up to a point? That's obviously more complicated than a simple push-pull amp with high bias, both electronically and sonically. 
Are you saying that amps like Pass Labs do not operate in class A up to a certain wattage or are you just saying that those amps should be called AB not A/AB?

The latter. I mean, the whole point of AB is to avoid the crossover distortion of B, without being as power hungry as A.

You can even dial the distortion up and down with an AB amp. More bias = less distortion = more power at idle.

We used to just call this Class AB.
Lately however we call it "Class A until x watts, then AB"


Best,
E


You're not wrong, but what's your point?  Knowing how many class A watts an amp can provide is useful info.
You're not wrong, but what's your point? Knowing how many class A watts an amp can provide is useful info.

Hey @onhwy61 - I'm not really making a point, I'm fine with a manufacturer making a claim  to Class A at low power if accurate.


Really I am asking for help fixing a disconnect in my own mind and knowledge. Ages ago, when I took a course in the subject, you had A, or B or A/B.


This new fangled combined labeling of audio amps as A/AB is rather new to me, I just wanted to know if it involved actual engineering differences from AB.

Best,
E

Kosst: you can have push/pull Class A if the devices operate their 360 degrees of duty cycle out of phase of one another.
I guess that's the nice thing about my Cary 120 (and other Carys) - it's a Class AB, push-pull amplifier, but operates in Triode (Class A) or Ultralinear (Class AB) mode with a switch.  No ambiguity or point of departure that isn't deliberate.
Yes and no. Some AB amps are capable of practically zero power as class A.
@stevecham 

I'm well aware of how class A push-pull amps work. The one I built is warming up right now. 
Perhaps audio manufactures should drop the term AB and adopt A/AB to give consumers more info on how the amp operates.  What are the benefits/drawback of two identical 100w amplifier but one is biased for the first 10w in Class A (A/AB = 10/90) whereas the other is class AB from the get go (A/AB = 0/100). 
Perhaps audio manufactures should drop the term AB


They have, I think. They say something like "high bias AB" or "A/AB"
The curmudgeon in me wants them to just stick to calling it AB. :)

Let's have a fight over it! ;)

Best,
E


Perhaps audio manufactures should drop the term AB and adopt A/AB to give consumers more info on how the amp operates. What are the benefits/drawback of two identical 100w amplifier but one is biased for the first 10w in Class A (A/AB = 10/90) whereas the other is class AB from the get go (A/AB = 0/100).
The problem is that there is not established standard for A/AB... its purely marketing. Its not misleading if the manufacturer says it transitions from A to AB at a certain power level, but the result is its still class AB.

The curmudgeon in me wants them to just stick to calling it AB. :)
And that curmudgeon would be 100% correct.


No such thing as A/AB. It’s simply AB. Bias determines how many watts of A you’ll get. And things get real hot and big with even several watts of A. All of those supposed Pass class A amps, I doubt they put more than 10-15 watts in A.

invictus005
"
No such thing as A/AB."

Wrong!
 "All of those supposed Pass class A amps, I doubt they put more than 10-15 watts"
Wrong!
Post removed 
Wow... Pass class A amps are a scam, huh? Good luck trying to back up that claim. Typically his class A amps have a class A envelope double what they're rated at. 
The last time I looked Pass amps are rated class A or class AB. Never seen them rated class A/AB!
In ss gear it’s all about bias current.   my 2 mono 100watt class a amps cost about 125 bucks a month towards the power Bill   Why.  Cuz that’s what it takes to have such high bias .   If it’s real class A they will be very heavy and get very hot.  It’s a horribly inefficient .  Mine are basically like having a hair dryer going 24/7. At idle.
In a Nelson pass design the A to  AB  kicks in once the output current goes more than double the bias current.     that’s right off passlabs web page as to how they explain their rating system.    ,It’s misleading to say they are class A,   They are AB.    Step up to  Amps like Boulder which are true class A and they weigh hundreds of pounds and will set you back the price of a sports car  for a small one or house for higher powered amps.   It takes heavier components to maintain such high bias current in ss gear. 

   Which is why tube amps amps are awesome and can be low wattage and give out true class A albeit just a few watts and still be in most peoples realm of affordability.