Thanks for all the suggestions. I did not try lifting the power ground as it would not be a permanent solution considering the potential danger in this. I think that the signal ground on one of my components is injecting hum into the system or the cable configuration has introduced a ground loop. Which, I don’t know but I suspect the preamp which seems obvious since it is the component common to both channels.
I experimented with different cable configurations and discovered that the recommendation by van Den Hul with the additional connection from screen to the unbalanced return fixed the hum. A 3rd conductor from the RCA return to pin #1 on the XLR connector as in option #1 in the diagrams I supplied must cause a ground loop in my system.
I asked the supplier of my Krells about the pin #1 to #3 strapping and was informed that there was no such strapping in the version of my amps just the jumpers to short out or leave in the input capacitances in the event a preamp injects dc into the amp, though some day I shall take a look inside.
BTW, various commentators have proposed the pin #1 to #3 short whilst others instead prefer to connect the screen to pin #1 and then shorting them out at the RCA connector. This second way works for me. I presume the screen has greater effect minimising RFI & EMC interference by keeping it at the preamps ground potential. This is only conjecture on my part. Important thing is it works.
As for the suggestion to use an isolation transformer, this is something that I shall try in future. Matching the impedance will be an issue. The transformer will also add some phase shifts to the signal at high frequencies. Will this be detrimental to the sound?
Thanks again everyone.
I experimented with different cable configurations and discovered that the recommendation by van Den Hul with the additional connection from screen to the unbalanced return fixed the hum. A 3rd conductor from the RCA return to pin #1 on the XLR connector as in option #1 in the diagrams I supplied must cause a ground loop in my system.
I asked the supplier of my Krells about the pin #1 to #3 strapping and was informed that there was no such strapping in the version of my amps just the jumpers to short out or leave in the input capacitances in the event a preamp injects dc into the amp, though some day I shall take a look inside.
BTW, various commentators have proposed the pin #1 to #3 short whilst others instead prefer to connect the screen to pin #1 and then shorting them out at the RCA connector. This second way works for me. I presume the screen has greater effect minimising RFI & EMC interference by keeping it at the preamps ground potential. This is only conjecture on my part. Important thing is it works.
As for the suggestion to use an isolation transformer, this is something that I shall try in future. Matching the impedance will be an issue. The transformer will also add some phase shifts to the signal at high frequencies. Will this be detrimental to the sound?
Thanks again everyone.