text: Massdrop Objective 2 Headphone Amp: Desktop Edition Discussions | Drop | Hi guys, I am new...
  Hi guys, I am new to Amp. I just bought this with 6xx headphones, after I plugged in the RCA and the headphone start to have very serious da da da sound, it still there even if I pull out the RCA cable connecting the amp and my PC, wasn't sure what's going on
  I've seen a number of other commenters stating this issue (with various headphones, but HD 6XX most often because that is the bestseller here). They described it as a popping sound, but I wonder if yours is like small frequency sweeps... the former would be a grounding issue (even if RCA is disconnected, maybe the connection to ground needs improving overall), while the latter happened to me one when a phase switcher in an amp got fried. Drop a comment if you find an answer. Others will want to know. I bet it's a grounding issue with the amp.
  I just bought the O2 unit and it seems I am having the loud buzzing/pop issue. No music plays when the buzzing starts. I've tried multiple headphones and it randomly works.
  I'm not 100% sure, but I've heard that before (on any amp, but especially on amps with tight sockets) when the headphone plug isn't inserted all the way deep. If pushing the plug in harder solves it, great! If not, the socket on the amp may be out of spec. It's also worth checking that the analog inputs are plugged all the way in as well, and if the O2 has an input select that the selection is set to something plugged in. Best of luck, hope it's something common and easy like this!