Atmasphere, I’d like your opinion on Bruno Putzey’s latest class D amp design philosophy. I’ve read that he once held the same view as you do regarding negative feedback producing random upper order harmonics, but he’s now broke with that orthodoxy. First, he claims that negative feedback needs to be evenly applied across the frequency spectrum and not disproportionately at lower frequencies. Second, Putzey claims that as you apply more and more negative feedback the odd harmonics disappear and distortion levels are low and smooth. I believe he is talking about 40 to 60 dB loop gain.
There is not a simple answer to this. First, to be clear, I don’t agree about the bit of ’random upper order harmonics’; they are a bit more predictable as you will see below. Also, I think you run into some classic issues with gain and phase margins if you do as Bruno proposes (see Norman Crowhurst; this book is an excellent primer, look on page 3-15 and read from there:
http://www.tubebooks.org/Books/crowhurst_basic_3.pdf)I’ll go with Norman Crowhurst any day- math has a way of not going bad as it gets older.
There is more to loop feedback though. One of the bigger problems that is rarely dealt with is the issue of Radio Frequency Interference (RFI). The speaker cable tends to act like an antenna, and since the feedback loop takes information from the output of the amp and feeds it back into the input for correction, RFI is also thus injected into the amplifier.
This can cause havoc- RFI is not helpful in audio circuits! It might be why an amplifier might sound really bright in one situation and in another with the same speakers, might seem pretty good. RFI can be an unknown variable and susceptibility.
Yet you rarely see feedback loops that deal with this problem.
Bruno is correct that as you add more feedback you can eventually decrease the higher ordered harmonics. However, as Nelson Pass has pointed out in this excellent article on feedback:
https://passlabs.com/articles/audio-distortion-and-feedbackQuoting from that article, Nelson puts the problem in a nutshell:
Negative feedback can reduce the total quantity of distortion, but it adds new components on its own, and tempts the designer to use more cascaded gain stages in search of better numbers, accompanied by greater feedback frequency stability issues.
The resulting complexity creates distortion which is unlike the simple harmonics associated with musical instruments, and we see that these complex waves can gather to create the occasional tsunami of distortion, peaking at values far above those imagined by the distortion specifications.
If you want the peak distortion of the circuit of figure 13 to remain below .1% with a complex signal, then you need to reduce it by a factor of about 3000. 70 dB of feedback would do it, but that does seems like a lot.
By contrast, it appears that if you can make a single stage operate at .01% 2nd harmonic with a single tone without feedback, you could also achieve the .1% peak in the complex IM test.
I like to think the latter would sound better.
As we know class D reacts the same way to feedback that any other amplifier does, being an analog process. Generally its been my conclusion that while Bruno is right in theory (except where he isn’t, see Norman Crowhurst), he’s wrong in practice.
