Hi ldc,
I also talked to our product expert regarding this and he gave few suggestions to be able to see the cause of the problem.
Can you do the following:
Remove the comp cap, and the feedback cap, C12, C42 and retest.
Reduce the feedback resistor a lower value, maybe somewhere in the 200-500 ohm range and retest.
Have you tried replacing the amplifier? If not, can you replace it and see if distorted output goes away?
Lastly, can you remove all load from the amplifier output. This might mean cutting the PCB trace from the amplifier output to everything else except the feedback resistor.
We are thinking that maybe you either has a bad part or a very heavy load. The heavy load can result from a bad PCB and that is why cutting the trace is a good way to ensure no load.
Let me know the results of your evaluation and let's see if its gonna make your circuit working.
Best regards,
Emman