I built a second ULDO-Nacho board and made some 1 M point 50 sample average FFTs.
The Y-axis in the following FFTs have offsets to reflect the post notch filter gain (+40 dB) and the "carrier" (output level).
The result is dBc and does not reflect the insertion loss of the notch filter.
The Left channel level text display is the actual notch filter output level in dBu.
THD results, calculated from a spreadsheet, reflect notch filter insertion loss.
+20 dBu balanced open circuit load. THD is about -142.6 dBc, 0.074 ppm, or 0.0000074%.
+20 dBu balanced 604Ω load. THD is about -141.6 dBc, 0.083 ppm or 0.0000083%.
Other than matched resistors no other devices were hand-picked.
The op amps, THAT340 and 2180 VCA were randomized by virtue of being the next one in line at the end of the tube.
Keep in mind that the results are not just the distortion of the oscillator but also the cumulative distortion of the passive notch filter, post-notch amplifier and A/D converter. Some terms add, some subtract.
The active circuitry following the notch filter are working at very low levels and I would encourage anyone working with this circuit to get to know the low-level distortion signature of their A/D converter.
Fortunately most of the active devices we would measure have distortion at least an order of magnitude higher than the combined distortion of ULDO-Nacho and the converter.