Did some testing with headphones plugged into the out jack and using a "Silence" patch:
- Magus powered by USB alone: Very pronounced 60 cycle hum, Macintosh can talk to it
- Magus powered by DC alone: Basically silent but see below
- Magus powered by DC, USB plugged in: Sounds same as DC alone, Macintosh can talk to it
- Magus powered by DC, USB plugged in to Adafruit isolator: Sounds same as DC alone, Macintosh cannot talk to it
So, neat! I can power from wall and plug into USB with no audible noise. I can't use the Adafruit isolator, but it doesn't help, so who cares.
Then I discovered something weird…
I did some testing with this patch. This patch takes its input and plays it to the output after a delay; the idea is you can create a delay-echo or looper by connecting the right output to the right input. If you turn down parameter 16 "Monitor output mix" to 0 and listen only to the left output, what you are hearing is the left input fed directly into the left output.
If I run on DC power only, run this patch and plug in nothing except headphones into the "OUT" output, I hear a very loud 60 cycle hum, almost as loud as the USB hum. In other words, when powered from DC power there's no hum in the headphone OUT, or if there is it's so quiet I can barely hear it, but if I play the inputs to the outputs with headphones plugged in it appears there is quite audible 60 cycle hum on the inputs. (This is ruinous for the delay looper because when you feed back it layers on itself louder and louder). The hum gets noticeably louder whenever one of the buttons (that is the press-down knobs) is held down.
If this pronounced hum is not seen with other units, one possibility is that the power in my building is just very dirty. The building is very old and there's only one power plug in this room into which are plugged many electronics. I'm also hearing a pronounced 60 cycle hum on my electric bass and I was assuming there was something wrong with the bass, but maybe the problem is the ground in this building is just noisy. It is also maybe possible(?) that I messed something up when I disassembled and reassembled the unit to upgrade the firmware. I'm using the "mass power" AC adapter that came with the Magus.
Do you have any advice on this? Does the loud input hum with the "pure delay" plugin reproduce on your own units? If the problem is the Magus is failing to clean dirty building power, is there anything I can do about that?