Hum is a vibration (though not a good one). A distant inSync reader battles a different kind of hum.
“I have my American voltage MIDI studio here with me in Belgium. I have been using a step down voltage converter from 220v to 110v to run my setup. I have a hum that can be audible at low levels. It’s the natural mechanical hum the converter makes itself. What would it take to even out this irregularity? Would a 110-volt line conditioner placed after the transformer help, or maybe a 220 volt line conditioner before the converter?”
Unfortunately there is no clear-cut answer to this. The problem is that when AC current passes through a transformer it generates a magnetic field. This magnetic field can not only induce hum into other nearby devices, but it also turns the transformer into an oscillating electromagnet. As it oscillates the magnetic field expands and collapses causing the transformer to try to vibrate physically against the metal case it is attached to. Under the right circumstances the physical vibration can become audible. These circumstances are just the right combination of line frequency, harmonics, load, physical construction of the case, how the transformer is attached, and current draw (though some of these may have much more to do with it than others).
You can try placing conditioners before or after the transformer, but odds are they will not help. Given that there isn’t very much you can do to change the characteristics of the load (the gear you have is what you must use), your best hope is to either change the transformer or isolate it. Some transformers just do vibrate like this. You may wish to try another one. Your unit could be defective or just not designed to high enough standards to behave quietly, though I’ve seen the best do this on occasion (remember all those variables?). On the other hand (and you probably should do this anyway) you could just isolate it away from your work area so you don’t hear it. In the big picture that is probably the best plan anyway because it also keeps it away from your sensitive electronic gear.