We've discussed some of the basics problems Class D amplifiers have when used as AC regenerators. The last problem I will discuss is one that applies not only to Class D amplifiers in regenerators, but in audio amplifiers as well. It concerns not what the speaker or equipment AC input sees, but what your wall socket sees.
It's instructive to think of your system in a holistic big picture view, rather than a smaller one focused only on specific areas. We plug a piece of equipment into our system's mix, it sounds this way or that way, and we assume that sound is a result of what's going on inside the product. Rarely do we consider what affect a particular product has on the other elements in our system. I will recant a good story that illustrates this principle. Over a decade ago, when the art of designing digital audio equipment was emerging from its infancy, I was at a reviewer's home and we were listening to the stereo. He asked me if I would be willing to offer him an opinion on a strange observation he had made. I, of course, agreed. We listened to a few cuts on the reference setup and built an image of its sound in our memories. He then went behind his system, did something mysterious and returned. We played the same tracks and I was stunned at the difference. Worse, in all respects. The sound had gone from musical, lifelike and open to cardboard and dull. I asked him to put it back to where we started. He did and the magic and life returned. Fascinating.
"What did you do?"
"Plugged a new DA converter into the wall."
"Huh? Was it plugged into the system?"
"Nope. Completely isolated and sitting on the carpet alone. Come take a look."
I walked behind the shelf of equipment to see a well respected DA converter sitting on the carpet, its unconnected AC cable sitting nearby. Nothing else was attached. All he had to do was plug it into the wall socket where the rest of the equipment was powered and the soundstage collapsed.
This is a classic case of a noisy power supply kicking back onto the AC power source it started from. Imagine what a rack full of such equipment plugged into the same circuit does to the system's performance. Nothing good, I assure you.
Why should this matter with a Class D amplifier or regenerator? You can probably guess where this is going, but let's visit again tomorrow.