Boomzilla - are your speakers shielded? Reason I am asking is that I had a similar situation when I placed my speakers within a close proximity to the television. Drove me nuts trying to figure out what the problem was. Just a thought.
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I have a Yaqin MS22B phono stage. The factory, for some stupid reason tied the negative (grounds) of right and left channel inputs and the right and left channel outputs together. It was a pretty easy fix.
Read post number 68.
I realize that you have a tube buffer, but maybe since it's the same company, they followed suit with other pieces of gear.
Disconnect and unplug everything. Now working backward from your amp, one component at a time, connect RCA from pre-amp to amp, turn it on - any hum? If yes, try a cheater plug on the pre-amp to see if the hum goes away.
Assuming not, now connect a source, etc to the preamp. Any hum - same drill as above. Sooner or later this process will lead you to the hum's source and help you determine if floating the ground is a solution.. Don't overlook the antenna as a source of the hum - that is very common. Don't overlook the possibility of a hum occuring because of cable's being too close to transformers in your components (very common source) or other electrical sources. Don't just turn off components, actually unplug them and disconnect all of the connecting cables.
This may not solve the hum problem but you will have at least identified the source and then be able to figure out how to solve it.
Very good advice, @newbee - My gut feeling is that the chassis ground from the Yaqin is the source. When that is unplugged (despite all the interconnects being hooked up), there's no hum. Therefore, my next move will be to use the cheater cable on the Yaqin. If that doesn't get it, then your "full court press" solution is next!
I'm not surprised that that worked for you, however the actual source of the grounding problem might still be in a source ahead of the Yaqin. I'm not against the idea of lopping off the ground on the Yaqin's PC to solve the problem like a lot of folks are, IE's included if not principally, however on some boring/rainy day you might need something to break up the tedium and my suggestion will give you something to do. FWIW, in an almost identical situation I sourced a hum to one of my CDP's. I used a cheater on it and the hum went away.
Now there can be a number of things that can cause hums in a system, but the most common ground loop issue is from two or more components in a system that are different phases of the AC line and sharing a common ground. Your home has 220v coming from the pole to your panel. It is then split 110v to each side of your panel, but each side is 180 degrees out of phase. So if you have equipment that are plugged into different outlets and any of the AC outlets are from different phases, AND your sharing a common ground you will get a ground hum.
So you always want to have your equipment on the same phase, and that is important when running dedicated lines, and good idea to have 20 amp lines. Now if that is not possible or you don't want to go to that extent, you can simply lift grounds. I have done it for years, never ever heard of any issues, in fact many systems I have installed I lifted either all grounds or left just one connected typically the preamp. Other issues can be dimmer switches, unshielded interconnects, power cords running parallel to speaker cables, refrigerators or appliances on the same circuit, just to name a few.