One area where I often find myself with a bit of confusion with low-cost class D amps is with the output impedance. I recall that in some cases Amir has measured the output impedance and mentioned it in the text overlaying the graphs. Perhaps I missed it, but I didn't see it with this one. I wonder if there is a reason why it isn't relevant and whether I'll end up feeling stupid when someone explains it. As Amir and others have variously pointed out, if the output impedance is not sufficiently lower than the nominal speaker impedance, the frequency response of the speaker will be altered, due to frequency-dependent variation in the speaker's proportional share of the voltage output by the amplifier output device. (The "damping factor" is the usual way of expressing this, however the damping factor can be misleading since it is calculated using an assumed nominal speaker impedance rather than the true nominal impedance of actual speaker.)
The output impedance is of course primarily a consequence of the low-pass filter customarily used to block the ultrasonic frequencies that are inherent with class D. This is why the output impedance of a class D amp is characteristically much greater at 20 kHz than at 1 kHz. Eight or ten months back I spent a little time one evening looking over TI's spec sheets and literature for several of their class D chips. I recall that TI advocated "filterless Class D", omitting the low-pass filter altogether. This begs the question of why this filter has customarily been used if it isn't needed.
In any case, I struggle with the understanding of why the output impedance wouldn't be a fundamentally important consideration for class D amplifiers in general, and especially with low-cost class D amplifiers, since the way in which this fundamental concern is addressed by the amplifier designer seems to be a major differentiating factor for low-cost class D amps vs. the expensive ones. Someone here may be willing to shed some light on this. I suppose that it probably has to do with the feedback topology. While any explanation that is half-decent will be welcome, I hope that it will not draw attention away from the salient question of why it would not be fundamentally important, with class D amps generally and particularly, to pay close attention to the output impedance.
