Pretty much as expected. I'm mildly surprised the opamp sans clipping diodes was not more abrupt, but then opamps do odd things.
Actually, now that I think about it, I guess that's not all that odd. It does rise very abruptly, but then rounds off as the input signal rises, pushing it past the early clipping where the forward gain is still enough to enforce steadily rising distortion. Once it's clipping, it gets harder to make it clip more and the amplifier gets less sensitive. Very different case from clipping diodes, where what is going on is exploring the V-I curve.
Something I've always intended to do now that I'm using a software oscilloscope pretty much entirely is run a similar test, but with the spectrum analysis function to capture the curves not just of THD, but of second, third, and further harmonics. Curves of THD capture the abruptness of the transition into distortion, but on top of that the harmonics curves would capture the change in clipping tone as well, at least as much as any visual curves can capture a sound.
How about what happens if you wire a potentiometer in series with the diodes as a "softness" control?
I suspect that it
stretches the curves out laterally to the right, as what that does is to add a constant fraction of the undistorted signal to the clipped signal. This doesn't work for the "no-diodes" case, obviously, but one could cobble that up with a mixer. Hmm. Now that I think about THAT, that's what the most recent products from my day job do. I had been looking at it a different way.
