From the input I received last time, I set about designing and building a 2-stage, single-supply amplifier based off two BB OPA2134A op amps, being powered off a USB port and fed audio from a PC soundcard. All in all, the project has been a resounding success - much improved from the basic CMoy design of its predecessor. However, I've encountered a bit of a problem. When I power the device off my PC and provide it audio from another source (such as my phone), it's crystal clear. If I power it from an outside source (my phone's wall charger) and feed it audio from my PC, it's again perfectly clear. If I power and provide the signal from my PC, though, I encounter a noticeable amount of static in the background, whether there's sound playing or not. My immediate thought is: ground loop. Problem is, I'm not entirely sure what I can do about it. As I mentioned in my previous thread, I'm piecing this together mostly from educated guesses and I've learned almost as much from this second attempt as I did from the first. So while I'm of the impression that isolating the earths would work I don't know how I would go about that. The following are the basic schematics for what I've built: Power supply Fairly standard (from what I can tell) active voltage divider. Most of the values chosen are, if I'm perfectly honest, fairly arbitrary - I realise they're important but I don't know enough about them to choose correctly. L1, for example, just happened to be the biggest Inductor I could get in PCB-mount form, which I chose mostly because it was the same price as the smallest (and I figured the bigger one would offer a lower LPF frequency). Amplifier (mono) If I'm right, this is a fully AC-coupled cascading amplifier, using the second op amp as a buffer/voltage runner. As they're both OPA2134s I'm not entirely sure whether it's actually achieving anything, but at least I know the design works if I should get different amps and try again. With that, what have I missed? Should I be connecting the input and output earths directly to each other, separate from the supply earth? Should I AC couple the in/out earths to the supply earth? Am I going to somehow have to fully isolate both in and out signals using transformers? Or is there another solution out there that I simply don't know about? Ideally I'm hoping the solution is something I can easily splice into the existing circuit. If not, I guess I can always take another try - quite possibly one without three HPFs (do away with C3 and R4 entirely) if I can help it, and potentially using the second op amp in parallel with the first, rather than in series, to double the output amperage capacity.