So finally an update on this project. I received the redo (done and shipped at CustomPCB's expense), and it is of acceptable quality. The soldermask is 'weird'; it makes a plastic-y smell if you get it too hot, and it seems very weak as there are some spots on my board where it scratched off rather easily. I wouldn't recommend this fab house, but the boards are working.
Due to the (unknown to me at design time, since it doesn't seem to be in the datasheet) rather large input bias current for this amp, it has a pretty serious DC offset as designed. Swapping 5K resistors for the 100K ground reference resistors in the initial design gets the DC offset down to approximately 20mV, which I find acceptable. Input impedance is definitely on the low side, but both my portables and AlienDAC seem to do just fine driving it.
Summary of issues I ran into building this board:
- 100K resistors should be replaced with 5K to get DC offset to reasonable levels
- Drill holes for RK097 pot are too small, the part will fit but must be forced fairly hard. I recommend straightening the crimped pins first or they will likely crumple instead of being forced into the hole. I was able to fit the pot properly without modifying the board.
- I forgot to remove the soldermask from the ground plane under the thermal pad on the TPA chip; you could scrape this away if you wanted to try to solder it down.
- Pin 8 on the OPA551 should be removed, if you use this chip
Other than that, I think it sounds pretty fantastic considering the size. It's smaller than the last protoboard CMoy I built was, and sounds far better. I don't detect any hint of oscillation, but I don't have a scope to verify this for certain.
If anyone wants to build this up, PM me. I have a few boards I'll part with at my cost + $1 shipping = $9/board.