CLASS A POWER RATINGS
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 ?
@reg19 Possibly. More will depend on other aspects of the circuit, such as how feedback is implemented, the construction of the power supply and so on. |
@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. |
@sngreen No. The Dynaco SCA35 is an EL84-based push-pull amplifier that is class A to full output. This is so because both output tubes are fully class A, only operating out of phase with each other. In this way the amplifier generates lower distortion than would ever be possible with a single-ended amplifier, and easily has wider bandwidth (and more power) too. No downside, except for the heat. We've been building class A push-pull OTL tube amplifiers that are class A for decades on now (nearly 50 years). Our largest model makes over 500 Watts in class A. FWIW though, class D has seriously eroded any reason to go class A. The reason a class A amplifier can sound good is because of its distortion spectra, which tends to be more benign than an AB amplifier. However, the way the feedback is handled has a tremendous effect on the distortion spectra as well, which is why merely being class A isn't always enough. Feedback can be poorly applied (and often is), resulting in a harsh sounding amp with measurably low distortion, on account of non-linearities in the feedback loop introducing higher ordered harmonics. A benign distortion product will be mostly the 2nd and 3rd harmonics, with enough amplitude that they can mask the presence of any higher ordered harmonics (the latter of which, if unmasked, are responsible for an amplifier sounding dry, bright and/or harsh). Turns out that class D, if designed properly, has a similar distortion spectra to a really good class A tube amplifier, and sounds similar, since the distortion signature of any amplifier is also its 'sonic signature'. |
@rost The confusing bit here is that any amplifier that goes to 'AB mode' is a class AB amplifier and is not class A. By definition. If you are conflating a class AB amplifier with a real class A amplifier, I can see why the question. 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. |
