Then the "switching noise output filter" can also be set higher and so eliminates most of the switching frequency noise left at the speaker terminals. Without introducing phase shifts back down into the audio band, like what happens with lower 600-800khz switching frequencies/filters.
This is fake news. Let’s, once again, unpack and debunk this rubbish to set the record straight.
The ’switching noise’ of any class D amplifier is a sine wave at the switching frequency. If the filter is properly designed, the sine wave is quite small and low power- it has to be so because the amplifier can’t interfere with other services, like AM broadcast (1.5MHz is at the top of the AM band). But an amp that is switching at 600KHz will have the same amount of noise and 600KHz is in the AM broadcast band too- the same rules apply.
Phase shift is indeed a consequence of using a filter. But it is incorrect to assume that because a filter is used, even one of a low frequency like 80KHz, that there will be phase shift in all examples of class D amps that have a filter set this low. This is because some class D amps are of the self-oscillating variety, and self oscillating class D amps run a prodigious amount of feedback. They do this as part of their switching scheme but that is not why so much feedback is used.
The thing about feedback is that its application causes distortion while suppressing the innate distortion of the circuit in which its used. The distortion generated by feedback tends to be higher ordered harmonics, which is audible as harshness and brightness, due the ear being keenly sensitive to these harmonics. It is that sensitive because it uses them to sense sound pressure.
But if you use enough feedback (north of 35dB) you can not only suppress the distortion caused by the application of feedback, but you can also correct phase shift. This is why self oscillating class D amps with a switching frequency of 500KHz and a filter set at 70KHz can have less than 1 degree of phase shift at 20KHz- quite unlike the amp depicted at the link which George put up in his last post- and I might add, knew that this was the case.
Amps that have less then 35dB (which includes about 99% of all solid state amps ever made) will have some brightness and harshness due to that lack of sufficient feedback. This is why class D.
@mijostyn We’ve been working on a class D project of our own design for about 4 years. In November of 2019 we were awarded a patent in the field. Our prototypes make 100 watts into 8 ohms with 200 watts into 4 ohms and 400 watts into 2 ohms (sorry George) no worries, if given enough current in the power supply (the GaNFET output devices are rated 35 amps). When testing traditional solid state amplifiers the FTC requires that the amp be preconditioned at 1/3rd power for an hour and then the amp can be tested for as short a time as possible to get the full power specs. This is done because traditional solid state amps will overheat if you run them at full power for a long period of time and usually that’s only a few minutes. Our amp (and I assume for most class D amps this is true) can be operated at full power all day long and you can hold it in your hand- they get warm but not hot. We don’t know where this will lead, but we have a goal and now we’re seeing if it can be achieved.
