Jbags,
I don’t have specific experience with the DSP20, but recently had a similar experience with my system and traced it to my digital source - Oppo Sonica DAC. I am not sure which streamer you run, and how it is connected, but perhaps my experience will add another troubleshooting option to the arsenal.
I recently added a new Bryston preamp and amp to my system, all powered out of a Bryston BIT15 and had a nagging hum introduced. At first I thought it was the new equipment, and was upset. Of course, I got frustrated. I kept troubleshooting and searching until exhaustion, with the patient help of a couple other forum members (my thanks to Questortapes74 and Ivan), and seem to have found an expected, but unexpected answer. It turns out it was not the Bryston at all. I tried just about every system configuration I could think of, with seemingly inexplicable results. The answer did lie in “a hum is usually a ground loop problem...”. Ivan seems to have been on target when he pointed that axiom out to me.
I got a “cheater plug”, in which a male three prong cable connector is converted to a two prong “ungrounded” male connection. It cost about 79 cents at a local home improvement store. When I plugged it between the streamer power cable and power source, the hum was completely eliminated. Something about ungrounding the digital source, eliminated the ground loop hum.
There are a couple of things I learned while searching for the solution I found in my system.
1. It seems certain digital equipment sometimes is more compatible when ungrounded & connected to a properly grounded system. This can be system specific. Seems there may be a lot “floating grounds” in some digital connection streams. A few cable manufacturers offer cables with either absent or removable ground prongs - PS Audio being one, and Pangea, and VH Audio are the ones I have found. I am sure there are more. At some places it is mentioned this is specifically for digital components.
2. In my troubleshooting it seemed the hum was worse while using balanced connections than unbalanced. I don’t know if you are using balanced or unbalanced connections? I could briefly eliminate the hum by switching to unbalanced connections, unplugging all equipment from power, and replugging them in a specific sequence. It was as if this ritual eliminated some charge, voltage differential or eddy current. At times I would get a gradual worsening, or reintroduction of the hum again. It seemed unexplainable at the time. Another possible contributor may have to due with the way different manufacturers and designers approach connections on their balanced connections. Whether or not they differentiate signal ground from chassis ground in the balanced part of their component connections can contribute to ground loop hum between components. This can be a specific compatibility issue. The note on this site helped me understand it better.
http://rane.com/note110.html
The ground loop seemed to not only create speaker hum, but also component toroidal hum in other components (including power conditioner, pre-amp and amp) as red herrings. This made it especially difficult to locate the source. This may be what you are hearing.
I am early in the process, but the solution seems to stand so far. I don’t really like the “cheater plug” in my system, but the hum is gone. Looks like I will be investing in a power cable that can be ungrounded as a band-aid solution. Ultimately I would prefer a digital source that does not require this solution. Needless to say, ungrounding a component is not desired, especially from a safety standpoint. I agree with the “unacceptable” sentiment, though have not made up my mind how it may relate to design quality. Essentially using your interconnects to ground a component in your system would not be the safest choice. Even if I don’t plan on spilling liquid on my system, a freak incident could occur, and possibly a fire or electrocution. Maybe someone with more experience and background can comment better.
Bottom line, for troubleshooting your system, to try and isolate the issue, you may try the 79 cent “cheater plug” trick to locate a specific component grounding issue. Start with digital components. Not sure it will work, but hope this helps in some way.
Best,
Shrinky