Yes! The hardware is finally working!
And of course, there were a couple of problems along the way.
The first problem was an oscillating opamp (1.5MHz) right at the end of the signal chain. During the design I decided to remove the filtering capacitor across the inverting opamp's feedback resistor as I didn't want anything more than a 0.2% decrease of the 20kHz response. However, I didn't expect the opamp to completely take off in the way it did. To correct this a ~25pF was soldered to the top of R22 which seemed to completely fix the problem. 25pF is not ideal (it was all I had on hand) and 10-20pF would be more suitable to preserve the 0-20kHz frequency response. See the filtering capacitor "CF" below:
The second problem is that there is too much heat and it can't get out. The two tiny transformers are simply too inefficient and are generating more heat than I anticipated. Additionally, the 18% infill of the 3D printed PLA enclosure is acting as a perfect insulator with its internal air pockets. Fortunately, the slow heat build-up allows the PCB to conduct thermal energy from one end of the case to the other without any significant temperature gradient. This means that more heat transfer out of the case at one end will effectively cool the other end as well. This works well for me as I like to insulate the dangerous end of the thing. Here is the only viable 3D printed solution without active cooling:
Yes, trypophobia is a thing.
Besides not running it continuously and letting it cool down, there is also my other solution:
And now, here are a few pictures:
Back without the panel installed:
Just after getting home after a day spent fixing it:
Finally, a use for my variac!
The grid-2-audio case (Autodesk Inventor design) fits in quite nicely next to my headphone amp (TinkerCAD design). Oh how I've improved my case making skills.
Yes, I know there are no waveforms to show yet (sorry!) - This will change very soon!