It has been known since the mid-1950s that loop feedback enhances odd ordered harmonics and there were cautions expressed that long ago about excess use of Global negative feedback due to this problem . . . How do you square that reality against what you have stated?
Atmasphere, this is not reality, it is rather a myth -- there are two soruces that I am aware of. First is Norman Crowhurst's 1957 AES paper on feedback in amplifiers (where he refers to "regenerative distortion"), and the second is from Peter Baxandall's 1978 series of articles in Wireless World (where he discusses the theoritecal possibility of an amplifier with only second-harmonic distortion generating higher orders through intermodulation with feedback). While I greatly respect both authors and recommend especially the Baxandall works for reading, this particular theory simply doesn't hold up in practice.
First of all, the point isn't about even-order distortion products becoming odd-order . . . it's about lower-order products becoming higher-order. In a push-pull topology (which cancels even-order products through another mechanism), that may be a supposition, but it's not part of the theory. Here's how it's supposed to work: If an amplifier has a strong second-harmonic distortion product, the addition of negative feedback causes the distortion and the fundamental to intermodulate and become third-harmonic . . . then the second and third intermodulate and become fifth-harmonic, the first and third become forth, etc. etc.
The counterpart to this is simply that negative feedback on the whole is so much better at eliminating distortion than generating it. I think Douglas Self put it concisely and eloquently in his book on power amplifier design (commenting on Baxandall, of which he is a huge admirer), so I'll quote him:
All active devices, in Class A or B (including FETs, which are often erroneously thought to be purely square-law), generate small amounts of high-order harmonics. Feedback could and would generate these from nothing, but in practice they are already there.
The vital point is that if enough NFB is applied, all the harmonics can be reduced to a lower level than without it. The extra harmonics generated, effectively by the distortion of a distortion, are at an extremely low level providing a reasonable NFB factor is used. This is a powerful argument against low feedback factors like 6dB, which are most likely to increase the weighted THD.
In addition, I've spent some time personally with the math behind this supposition, and done some SPICE simulation to back it up. The beauty of SPICE for this kind of application is that we can examine the feedback itself in its most pure, basic form, where it can exist without bandwidth limitations, stability problems, or any kind of loop "Propegation Delay".
First, I created a voltage stimulus with a controlled voltage source in series, to allow me to easily apply any amount of negative feedback, then another for open-loop gain. I then created another controlled voltage source, that adds a huge amount of pure second-harmonic distiortion (only 34dB below the fundamental!). No other distortion products exist, down to the FFT limits of about -200dB. I then applied various amounts of negative feedback, by changing the amount of open-loop gain. For a loop gain of 4 (12dB feedback), we see the 2nd harmonic drop to -52dB, a 3rd appear at -88dB, a 4th at -122dB, and a 5th at -155dB, and the 6th at -188dB.
So what does this mean? First, virtually any amplifier that's so ill-conceived as to have enough second-harmonic distortion as to be only 34db below the fundamental, will almost surely have some higher-order harmonics as well. But for even such an amplifier, adding just 12dB of feedback puts the third harmonic at -88dB, which will almost always be buried in the noise floor. And the rest (at <-120dB) will certainly always be undetectable and inaudible. But the improvement by knocking down the second harmonic to -52dB will be certainly be audible, and for the better. I think this supports Self's conclusion very nicely.
But there's another aspect of looking at this in SPICE -- these results exist in a world without any phase shift ("Propegation Delay") . . . meaning that they are equally valid for both local and global feedback! And the phase shift evident in real-world circuits can indeed introduce instability and transient-response problems (ringing), but it doesn't change the distortion-reducing effectiveness of feedback. So if you're truly worried about "regenerative distortion" . . . you'd better avoid all forms of local feedback as well. (Good luck with that!)
So again, if properly implemented, in the real world . . . negative feedback reduces ALL manners of distortion.
