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RE: Long-term DBT example

Assuming perfect op-amps and diodes, it's just a perfect half-wave rectifier driven by the input with it's output partially summed back in to provide even-order distortion. The voltage series of the distortion is that of a half-wave rectifier which is 1/2, 2/3pi, 2/15pi, 3/35pi... It's the same series as a full wave rectifier, just half the absolute amplitude. If the op-amps and diodes aren't perfect then odd-order products also occur due to slew rate and stored charge in the diodes, but it's probably safe to ignore them as they should be ~-80dB.

Yeah, I think if you assume ideal op-amps, the input-output characteristic Vout vs. Vin should look as follows. For positive input voltages, it will be a line from the origin with slope S1, and for negative input voltages it will be a line from the origin with slope S2, where S1 is not equal to S2. So it's just a slope discontinuity at the origin giving rise to even-order harmonics only (in the ideal case).

The interesting thing is the choice of distortion for this test. If I believe what I read on AA (I can, can't I?)...

Well, you can, but it's risky :-)

...a mild case of even-order distortion is not so much a problem as a blessing on many systems, and if it doesn't actually improve the sound of low feedback tube amps, it doesn't cause much harm.

This isn't your ordinary even-order distortion though. It's a slope discontinuity at the origin. That could potentially be very different than having a smooth nonlinearity, but I can't say for sure. That's what I was alluding to earlier. A smooth nonlinearity should have a Taylor series expansion, which in turn implies a percent distortion that cannot be constant with changing signal level as this one is.

Being correlated to the signal amplitude just about guarantees that it won't be an annoyance since the ear's internal distortion rises in a similar manner and it's masked psychoacoustically.

That's a big assumption. I don't see how it's possible to look at the schematic of something and determine whether or not its distortion is pleasant. In addition, I think such observations may be strongly dependent on the individual. This article in Stereophile by Keith Howard concludes that there is no such thing as euphonic distortion. I'm not sure I agree that this is universal, but I don't doubt that this is the way he truly feels about it. To take a somewhat silly example, the example on this page of second harmonic distortion sounds better to me than the original on my cheesy computer speakers. The third-order one sounds to me like something out of the Munsters or something :-).

I believe that other forms of distortion such as crossover distortion and IFM (jitter for instance), are much more likely to be distressing in casual listening but may actually be harder to put your finger on in testing.

It's hard to say, really, without building the circuit, setting the pot to the point where it's just audible, then trying to put some qualitative value judgment on its sound. One could argue that the grunge circuit is a (rather strange) variant of crossover distortion due the the slope discontinuity at the origin.

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