Wow. Great paper, that must have been a lot of work! :shock: This one's definitely going into my archives for future reference. Sincere thanks for posting this! :thumb:
Doug Self's work is very interesting. His series of articles on power amplifier distortion, published in Electronics World & Wireless World around '93-'94 was my first deep dive into the inner workings of the conventional amplifier circuit. Seminal work, I believe (for me, anyway). Apart from exposing some of the more subtle distortion mechanisms within the amplifier circuit, it was for me, a great way to simply learn about how amplifier circuits work... He breaks the amplifier circuit down into pieces and discusses in some depth how each piece functions. Those articles, along with a few others, were later compiled and published together as "Self on Audio"... I would recommend it if you can find a copy. Dr. Self has a more recent book, which probably covers much the same material though I haven't read it.
On the subject of IC opamps, I have been interested in 'transient' distortion mechanisms for a few years now. As Samuel notes, static THD measurements certainly have their value, but modern high-performance amplifiers have all but eradicated THD as a factor in audio performance, and "a single sweep just ain't gone tell the whole story". IC's in particular may be subject to thermal distortions, given the tiny mass (milligrams) of an opamp wafer, and the fact that the power-dissipating output transistors are integrated onto that die. The thermal time constant can be very short... as low as 1ms from what I've read.
But information on this topic seems a bit scarce, and I lack the means to appropriately measure these effects. What I would love to try, is a burst-stimulus measurement, where a low-amplitude distortion measurement is taken on the 'cool' die. Then apply a high-amplitude burst into a low-Z load, followed immediately (within milliseconds) by a very quick low-amplitude distortion measurement to compare with the initial measurement. In this way, one might observe the effects of the wafer's thermal tail - changes in the distortion level / spectra due to parametric changes in the on-die transistors, etc. Seems to me this kind of effect would be highly relevant to music signals, which are typically high dynamic range. If true, it might help explain the apparent subjective superiority of discrete circuits vs. IC opamps. One day, I'll get to this.
Yet another interesting area is microphonics. IC wafers can indeed be piezoelectric, and I've seen at least anecdotal evidence that IC opamps can be highly microphonic. One report posted to a forum somewhere was illustrating an example where an opamp was generating substantial electrical output (tens of millivolts, if memory serves), with the tap of a pencil eraser. The author of that post solved his problem by soldering the leads to short flexible wires which suspended the chip up off the PCB. Yet another fascinating area to explore.
I'm curious what your thoughts are on this (Sam, or whomever else is reading)...
Anyway, sorry to ramble... I love this stuff! Thanks again, Sam.