I had this unusual problem that took me a about a week to fix with help. I want to share this experience just in case others may end up having the same problem. And I want to know why the solution worked.
I just purchased a used Virus TI keyboard off of Ebay. Every time I plug it into the wall power socket, I hear a humming noise on my monitor speakers. It is very distracting. The synth can be turned off and still this happens. I did not think this is normal. The synth is plugged directly into the powered monitors. And I noticed both the power cable to the synth and to the monitor speakers do not have a third prong on the plug end for ground.
Guess what the solution is? A DI box and balanced cables did not fix the problem. I found that the problem went away when I grounded the chassis of the synth! Is this because the sheilding to the power supply works better at filtering out EMI when there is a good ground? Or is there just some sort of basic grounding problem, particularily with both the synth and the monitors not having a good ground? I do not know, but this is how I solved my problem.
A more interesting question comes to mind. Why was the Virus TI designed without a good ground through its power cable? Most electronic systems that I am aware of use a three pronged plug, effectively grounding the chassis and the internal electronics. IMO this is apparently good desigh practice, particularily for audio components.
Maybe a person much more experience than I can comment on this. I really want to understand what happened.
Thank you for your time.
Bob Graham