With tweaking of resistors, and keeping oscillator output at 1V RMS, I managed to get THD down to 0.01%. But it should be possible to get much lower.
Jim Williams wrote:
From that I assume that this should be achievable on a solderless breadboard. As I had already built the oscillator from this article:
https://sound-au.com/project174.htm- I shortened all leads, fitted it with ceramic 100nF power bypass caps, and tested. Disappointing; 0.1% THD, way worse results than with my finished PCB.
I assume that harmonic distortion is mainly caused by semiconductors and other, nonlinear components. The parasitic resistances and capacitances on a breadboard may add (non-harmonic) noise, affect amplifier bandwidth and cause spurious oscillations, but not increase THD, right?
When it comes to the nonlinearities of a JFET, and in particular the channel-length modulation, I have only read that Vds should be kept as low as possible, but nothing about currents and/or whether some JFETs are better than others. Would I eg. gain anything by replacing the BF245C with a (genuine) 2SK170BL?
Next up is breadboarding of Jim Williams' final circuit:
I don't have a VTL5C10, only a DIY optocoupler, and I don't have the opamps he used, but I hope LM4671/4672 works all over.