Audio Signal Generator

Best buy would be generator with following schematic:

Unfortunately they are no longer available from the author. There are only a few forum threads about his hardware:
https://www.diyaudio.com/forums/equipment-and-tools/205304-low-distortion-audio-range-oscillator-21.html
https://www.eevblog.com/forum/testgear/looking-for-ultra-low-distortion-1khz-sine-generator/


There are many sellers on eBay selling board like this:

It is a low distortion 1KHz signal generator, THD spec: 0.1% (-60dB).

LT AN67 based signal generator (PCB REV 2 from diyaudio) did not work properly (oscillation not stable).