CLASS A POWER RATINGS

While I do not understand their magic, this guy does all class A only, I believe his theory is, "if the first watt sucks, why continue".  His go up and down from 8ohm to 4ohm, in either direction.  All I know is, the stuff sounds pretty awesome.  While I don't go to magic shows, their stuff is pretty magical. 

https://www.firstwatt.com/prod.html

The only amps that you can be sure are always class A are single ended amps because their topology makes them always class A; they cannot go into class B since the output devices must be on all the time.

Push/pull amps are different and what you read is true. Most 'class A' push/pull amps go into class B at some point, especially into lower impedance loads. Look at the Pass class A push/pull amps. They are rated at 30 watts and they are class A at 30 watts but all the tests show they actually develop about 70 watts but by then they are class B They are rich AB amps and Pass is honest since he only calls them 30 watts where they are class A.

It's hard, expensive and difficult to make a push/pull amp of any power that stays class A essentially all the time. The only one I can think of was the Mark Levinson ML2 amp of decades ago It was huge and yet only rated at 25 watts 8 ohms. But it had huge bias that kept it class A all the time even doubling into low impedance loads. It was 50 watts 4 ohms and even 100 watts 2 ohms and I believe it still stayed class A.

But stop and think even an 85 dB speakers only needs 1 watt to play that loud and 10 watts for 95 dB. So the Pass 30 watt class A amp is still class A when it's so loud you shhould not be in the room. It's only split second peaks like banging a piano where the amp goes out of class A and then for such short times the amp is in class A probably over 98% of the time and you don't notice the spit seconds it's class B. Most of us were super happy when we got test scores in school over 95.

@atmasphere

The technology hasn’t advanced in this regard. When a class A amp can double its power into half the load impedance, its simply because its built to be able to do that. The technology to do that has been around for decades

+1

This makes absolute sense. Either it’s a legitimate true class A designed and built circuit or it isn’t. Amplifiers that claim generous class A power and are barely warm to the touch are suspect to me. Perhaps some degree of class A biasing but not genuine full  “A” circuit.

Charles

@dynamiclinearity

It’s hard, expensive and difficult to make a push/pull amp of any power that stays class A essentially all the time. The only one I can think of was the Mark Levinson ML2 amp of decades ago It was huge and yet only rated at 25 watts 8 ohms. But it had huge bias that kept it class A all the time even doubling into low impedance loads.

 

An excellent example of a pure class A push-pull amplifier. It was/is a beast.

?? Doesn't an amp's topography determine?