This
deep dive into the MA12070 is a little over my head, but this part was interesting:
"Absent some inductance to slow the switching slew rate across the load resistor, the amount of switching noise induced on the output is enormous and that makes measurement difficult. The solution, according to Infineon, is to use use
22 uH inductors in series with the output when making measurements. This will tame the fast edges, and make the signal more palatable to the signal analyzer. Of course, when driving into a real speaker, you'll get the inductance for free which helps to keep your EMI edges under control.
...
So, what does it look like if we make a noise measurement with the QA450 and QA401 with and without the 22uH inductors?
In the plot below, we can see the yellow (left) trace and the red (right) trace. The left channel has a -60 dBV signal going into the MA12070 board, and with the default 20 dB gain of the amp, the output is -40 dBV. The right channel is muted.
We can see the N+D measurement for both channels. This measurement is all of the energy from 20 to 20 kHz with the signal subtracted. For the left channel, that is
-83.7 dBV (A-weighted). This is 65.3 uVrms.
For the right channel, this is
-70.9 dBV (AW) which is 285 uVrms. Clearly, the inductor is making a big difference. The noise levels are almost unchanged for both 4 and 8 ohms. But the 13 dB gap is substantial."
Maybe just a coincidence, but those numbers don't seem too far away from the measured differences between analog and digital sources.