Power supply hum on monitors

    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

    No ground issues with My Polar or Desktop.


    Polar and Keyboard have the same internal power supply which should be earthed internally..


    My guess is either a earth loop somewhere else in your studio setup or internally in your Ti keyboard.


    DI with ground lift in probably didn't fix it as the earthing issue could travel back down the power line to other devices.


    Try unplugging other items in your setup one by one to see one stops the earth issue.. If not its the Ti and you should get you local Access distributer to look at the power supply/internal wiring.


    The old bush method is to remove to earth pin from a double adapter so the device isn't earthed at all. If the internal wiring has the power and earth mixed up the chassis could be live and you may get a nasty shock but its saved many a festival!!!


    Cheers


    spanda

    I must say I'm sometimes afraid of touching the chassis of my Virus.
    I once had a defective power supply, that made my fuses blow immediately when I plugged a MIDI cable into the Virus. :thumbup:
    (the other end of the MIDI cable was already connected to my PC soundcard, which is of course grounded)


    If I plugged the MIDI cable first, then the power supply, it went fine ... for a few hours. Then, my fuses would eventually blow.
    The store where I bought the Virus got me a replacement power supply for free, and now it's working fine.
    Except that I sometimes feel some electricity in my fingers when I'm plugging the power supply (if I touch the Virus and the metallic part of the power supply connector).


    I have the feeling that in some ways, plugging a MIDI cable grounds the synth. AFAIK the MIDI connections are supposed not to electrically connect devices, but maybe it does not stand for shielding?

    In general I would recommend to use a power conditioner for studio setups like the Furman devices. Not only do these devices protect from a power spike, but also do the "clean" the power and stabilize voltage differences.


    Midi signals are also transferred as voltages of course, but very very low ones, so a Midi cable or another audio connection can ground a unit, however the report by r010159 sounds like a general grounding issue with the setup. The TIs in general do not cause this.


    Best wishes,
    Jörg Hüttner