No even order THD?

[Anthony C Smith]

If a nonlinearity is present in a stage you will have harmonic distortion on a singel static signal like a sine wave, feed a two-tone signal and you will see modulation between the two, IMD.



/Peter

for class AB possibly; for classd the mechanisms are totally different- you can have a poor thd but passable imd;
curiously also vice versa- the delay element because class D is discreet time rather than continuous time has differing effects;
you can also have alaising errors in classd where the sample clock demodulates high frequencies present in the input audio back into the audio band.

 

[Pafi]

you can have a poor thd but passable imd

I don't really know what is poor and passable in numbers, but in an amp harmonic distortion cannot exist without intermodulation distortion.

Harmonic distortion and intermodulation are different faces of the same nonlinearity(s). An amp doesn't know about harmonics or trigonometric components. An analog amp basically have 2-3 state variable. It cannot react to harmonics, since there can be infinite numbers of harmonics. It can react only to the apparent values of signal, its derivatives, and to the values of state variables. So distortion happens in time-domain, it is the same mechanism for IMD and THD.

If you wanted to generate independent IMD and THD, you should do an FFT, make the unwanted components, and inverse FFT is again. This is impossible to be done with such a few state variables as many exists in an amp.