In well designed SS class A power amps I was told that although power could increase to double in class AB mode from speaker demands of  8 ohms to 4 ohms, class A suffered to be cut in half.

For example a class A denominated amp rated at 50 watts class A into 8 ohms would double to 100 watts class AB  into a 4 ohm load but only have 25 watts of class A power before switching to class AB.

But I've noticed of late claims of manufacturers stating their amps double their class A power as impedance demands are halved.

So is the explanation that technology has advanced ?


@charles1dad .. but class B operation is push/pull, not? Does not push/pull imply switching in between?


No. Class “B” and push-pull aren’t synonymous. @atmasphere has given a clear (At least to me) example/explanation. His OTL amplifiers and the Mark Levinson mentioned earlier are examples of pure class A push-pull topologies. There’re others as well.


but class B operation is push/pull, not?

@sngreen  Class B only means that the amplifier is conducting for half of the waveform. You can build a single-ended amplifier that is class B (not that you would want to listen to it).

IOW Its not an amplifier, its a class of operation. You can make single-ended class D amplifiers too.

So, if I compare, say a Pass Labs XA 30.8 (30W class A) vs a Pass Labs X260.8 (1st 34W are class A assuming 8 ohm loads) at regular listening levels on efficient speakers (so, at 1-5W power at listening levels), are they two different levels of Class A quality?

@clearthinker  the krell ibias amps do not run in class B, they are class A the circuit used increases the class A bias as more class A power is needed. Krell's old sustained plateau bias was also class A, in fact if you drive those old amps hard they will generate enough heat that the top two bias positions will drop down to class B until the temperature of the amp comes down.