Best Grounding

[marce]

Or the chance of a fire if there is a short. I once got blasted for mentioning something theoretical that was no where near as bad on this site...

 

[LuisMCP]

Can you post some more info, photos, schematic?
The ground loop may be through the I/V converter. Can you try a separate +/- 9V supply (having its own ground) for the I/V?
Even a pair of 9V batteries for a short test would work, if you don't have any other supply available.
Disconnect completely the other 9V supply (including its ground) from the I/V.

You was near to final solution I made:

1) Circuit was divided in two different sections: analog and digital, being de DAC the joint point of analog and digital GND.
2) An aditional PSU for Analog GND was added.

The hum was eliminated completely (don't hear it)

 

[LuisMCP]

The tube tester is not trying to get noise floors 100+dB down on peak signal at 20Khz.....

Still a ratsnest, but there are places you can get away with it, google "Cable Lacing" for that sort of thing done right.

Regards, Dan.

Cable lacing has nothing to do with the problem here.
The solution adopted is a different matter than cable lacing.
It seems to me that cable lacing seems an aesthetical matter.

When fixed, cables was much more "ratsnested" than in the begining, and no hum was here.

 

[rayma]

You was near to final solution I made:
1) Circuit was divided in two different sections: analog and digital, being de DAC the joint point of analog and digital GND.
2) An aditional PSU for Analog GND was added.
The hum was eliminated completely (don't hear it)

Glad to hear that, best wishes.