i had an interesting thought (maybe a moment of insanity?). an amp with a class D PWM as an input/VAS stage, and class B outputs. while not as efficient as a completely class D amp, it would (at least as i thought of it) not have the shortcomings of GBW rolloff and phase lag instabilities, and the "output" filter of the PWM (placed between the PWM and the output stage) would not require high current inductors. maybe i'm just blowing smoke here (or maybe such an amp would tend to do so
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not the same as what i was thinking..... that's a class G design with a variable (adaptive) rail voltage. what i was thinking of is a class D input stage and a class B output stage. i'm beginning to think it was not such a good idea. i'll keep thinking about it, but just as something to tinker with, nothing more. the whole idea of class D is increased efficiency, which my idea is not. it's probably too complex to be a workable idea anyway, but then again, that's never stopped a lot of engineers that bring a concept to market, not because it's any better, but "just because i can".
It's an interesting idea all right, but as you mentioned you lose the efficiency of the class d output stage.
One thing worth thinking about regarding this design is that you could integrate volume control into the VAS stage. Since the amp would basically be a filtered PWM stage feeding a unity gain current buffer you could vary the PWM voltage to change the voltage level. This would be easier to do with a low current VAS stage than a high current output stage (like TI does by varying the voltage of the open loop output stage for volume control).
One thing worth thinking about regarding this design is that you could integrate volume control into the VAS stage. Since the amp would basically be a filtered PWM stage feeding a unity gain current buffer you could vary the PWM voltage to change the voltage level. This would be easier to do with a low current VAS stage than a high current output stage (like TI does by varying the voltage of the open loop output stage for volume control).
i'm going to have the PWM and output stage within a feedback loop. amps themselves shouldn't need volume controls. control of signal levels should be a function of the preamp. some pro-audio amps have level controls, even though mixers all have level controls. with road equipment, it's just something else that can get broken. consumer audio doesn't get much abuse, but a separate volume control for the amp really isn't needed (but then again, a few years ago, my attitude towards remote controls was that remotes really aren't necessary for audio, and now you are hard pressed to find any consumer equipment that doesn't use one).
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