A while ago I custom built some 2 channel modules from an Amek Angela console (2 channel-strips into one metal enclosure with its own independent psu. As these channel-strips have no ballanced output I made one for each module by using a NE5534 opamp). Even in my workshop witch has a lousy AC power condition they all sounded great and even more so in the customers professional recording studio. Now the customer moved into another location and suddenly those modules are making a strange noise that sounds more or less like a buzzing that increases the farther the faders are turned down and vice versa. I tried switching the NE5534 to a TL072 opamp (different schematic, same output) but the noise remains. I'm starting to think that it might be a condition given by the AC power conditions at the customers new location, since my workshop is located in the basement of my home and doesn't induce any noise, while the customers new location is in a mixed commercial - residential area and maybe some neighbors with 2.4gHz cordless phones or cellphones (or microwaves, or TV sets, or whatever comes to mind) could be the cause for this? Does anyone know how you could shield these modules against any kind of possible interference? Or do you think this "buzz" might have a completely different origin? Any help is welcome.