0.13V offset is perfectly acceptable for the pupDAC ... it may be that the OPA2835 got damaged somehow. Sorry you had to go this route, but you'll be happier with the OPA2836 anyway - it's a far superior opamp. JMHO, but the 2835 was just a silicon prototype for the 2836. When TI/Burr-Brown felt the design was vetted, then they really turned up the performance on it for the 2836. The ringing pulse could be caused by anything, but if it's in the MHz range, it's not audible. The only other thing I would suggest checking is the lead length on the mica capacitors. Once again, it's the parasitic effects of the lead lengths, but it takes some effort to really get those micas down low to the PCB (like pliers pulling the leads through - carefully). As for the 50Hz, that's probably just a USB-power artifact.