Thank you Wayne (I think you are Wayne isn't it?)
First, sorry I forgot it in my first post, have a happy new year, you and all people here.
Here is the story: In my 20 years in the 80' I built a preamp and amp according to Elektor's magazine, namely the "Crescendo" and "The Preamp". Later I moved to more little house where I did not had room enough to host my home made boxes. Then I recovered room and last year I decided to plug again the boxes to the mains. In the lap time, a huge change happened; do you guess?... Internet arose all around
. Do eldest remember and can youngest imagine how making one's first steps in electronics was more difficult without it?
OK, enough with the long story, the more recent one now: when I plugged the boxes I got huge hum that a gentle French guy in the diy forum helped me to remove after I dug around for ground loops without any success. Once done, I found my passive home-made LS filters were say... rubbish.
Unfortunately, a lower hum remained, I could not say it was LS filters' fault, so I decided to tweak the amp and preamp, with the help of affordable hardware we can find nowadays, active LS filters, so the way went: go active + go balanced. After digging the net around, I first discovered there were dedicated OAP for this, SSM and DRV things (OAP and few build-out components got my preference because of room in the boxes and also because in the mid 2000's I lost my job in the electronics facility of my 20's, which was a big help to do my own PCB and find/buy components, and also because my audio system and skills do not pretend to reach far-end quality), and in the end I found the THAT's chips would be the definitive ones. 120x and 1646 would be the ones, 1646 instead of 1606 because I do not dare SMD.
So I made my own PCBs design and built a bunch of them, and tested successfully a pair of balanced outputs in a recent preamp I offered to my daughter this very last Christmas (she already has pair of bal/unbal amplified shelf-speakers).
For my own preamp I have an issue with the add-on line drivers. I designed the pcb driver to be able to choose to install or not install an additional OAP (OPA227) depending on the genuine outputs impedance, to reach the THAT1646 source requirements. For my daughter's preamp, because of the 1kΩ outputs, I used the OPA227. In mine I cut PCB to remove the unwanted OPA227 as the outputs are direct from the original OP27 through a 15µF film capacitor.
When I bring my hands near to the metal box I get a growing hum (up to huge) on the bal outputs although the case is earthed and grounded. I draw a picture of the output, that shows how the supplies and zero are wired.
When the add-on input signal is picked directly from OP27 output instead of directly on RCA after the 15µF, the hands effect is no more there. But if I plug a phone amp (Sennheiser 170 transmitter) in unbal outs, I get hum in bal outs. A very strange thing is that even if I just touch one preamp's out RCA ring with one Sennheiser's input RCA ring (RCA tips untouched at preamp and phoen amp) I get similar hum as when RCA are plugged the normal way.
Sorry for this long description.
Please, I would really appreciate any suggestion. thank you in advance.