We had a Dude in here for repair a while back and found that the ground setup was peculiar. In a nutshell is would not pass UL/CE certification. I understand later units are grounded differently.
I think there are several things going on with the various responses we have seen on this thread. I am certain that in all cases a transformer is not needed, and I suspect that the reason that it eliminated hum has to do with ground isolation that transformers can provide. But we must keep in mind that there are variances in the production of the Dude, so what may be causing hum in one system may not be what is causing it in another.
So let's start with the AC ground. It is possible to have the Dude chassis tied to the ground connection of the AC power cord, but in some cases this will result in a buzz if the amps are also grounded. This will vary with amplifier, for example some amps (like Classe) are grounded correctly while others (older Cary) often have the chassis and circuit ground tied to the AC ground (which is incorrect).
As you can see, its not just the Dude that has variance, so this can be a tricky matter! It is possible to test the amps and preamp without plugging them in to see if they are grounded correctly. If you measure from the AC ground pin of the power cord, it should have a direct connection to the chassis. However the audio input should not- you should be able to measure a resistance (under 200 ohms but more than zero!) between the audio input and the chassis. This is also true of the preamp, however since the preamp is the heart of the system, it can be alright if its audio circuit is the same as chassis ground, so long as the amplifiers are either grounded into the AC as above **or** not grounded at all (in the case of improperly grounded amps like the older Carys mentioned).
Now let's examine cables. If you are running from the Dude, which is single-ended, it is a good idea to check the resistance from the shield connection of the RCA to the same at the other end of the cable. It should measure nearly zero ohms. If you are using an adapter to convert to XLR, there should be zero ohms between the RCA shield connection and pin 1 **and** pin 3 of the XLR. The center pin of the RCA is thus pin 2 of the XLR.
Now I have also seen amplifiers that purport to have balanced inputs that have some really strange stuff going on- a year and a half ago we discovered a well-known solid state amp that used an input transformer (which allowed it to theoretically operated balanced or single ended with ease) was set up so that the input transformer ignored pin 3! On another occasion we found an Audio Note DAC that used pin 2 for ground instead of pin 1 of the XLR!
So if you want to be scientific about it the connections at the input of the amp should be tested to see that they are correctly wired. Many high end audio manufacturers are not aware that there are standards for the balanced operation...
If the above issues are satisfied, there will be no hum/buzz. This is of course assuming that the preamp is working correctly in the first place (hum and buzz was a problem with the sample we saw). So its a good idea to test the preamp with a known-good alternative amplifier, or to replace it temporarily with a known-good preamp (quite often one of your sources like a CD player can be sufficient for this).
Good Luck- keep posting.
