Quote: Originally Posted by Garbz You're welcome. Though the thought that the wallwart is putting a hum on the ground plane is worrying. You sure you didn't connect it to a supply rail , sorry had to be said, stupider things have happened. The voltage coming from the wall wart has something like a 0.04mv-0.08mv AC component between the ground pin and either of the rails when there is no load. There is a roughly equivalent AC component between the two rails. When actually plugged into to PH-1, the ripple between the two rails increases to 0.175mv. I assume when there's an actual signal going through, it gets even worse. After more investigation, it looks like the hum is louder in one channel than the other. I made a makeshift battery pack for the PH-1 and now the noise has gone down quite a bit, but I can still hear a slight hum in the same channel. Next step is to check the solder joints on the inputs. Any ideas what else could be causing this hum? Quote: Originally Posted by Garbz When it's all up and running apply a Class-A bias to the chips. The circuit can be copied directly from the PPA or MINT designs. All you need is 2 FETs. Personal opinion is it made it well worth it. I'll probably look into this once I've built the PSU. How much bias current do you recommend for OPA627s?