OK, Chapter 2 in the tale of making a REALLY low distortion audio oscillator...
At the end of the tale before at
https://imgur.com/a/Buj8WfD the oscillator works fairly well but had two issues.
First, there was some 60 Hz power line crap leaking into the signal and was being displayed on my HP 3580A spectrum analyzer. I had to find out where this was coming from and do something about it.
Secondly, the THD of the oscillator was so low that it completely disappeared below the noise floor of the SA 90 dB down. So I couldn't tell just how far down it was. The solution to that, for me, was to make a notch filter to pass the signal through to suppress the fundamental of the signal. That would allow me to crank the sensitivity of the SA up to bring the distortion products into view. But the filter had to be tuneable and tuneable over pretty much the whole audio range.
Here is my attempt at solving those two issues:
https://imgur.com/a/6C434zjI might as well say it again, but my motivation in building this was more academic than pragmatic. Yeah, I'll use it if when (or if) I build another stereo amp but, mostly, it was just an intellectual exercise in seeing what I could do with glory-box proceeds and little to nothing in the way of out-of-pocket spending.