Connecting a unbalanced output preamp to Krell FPB350 balanced amp

I have tried Option #1 with the screen connected to pin#1 at the XLR end and as a second experiment with the screen connected as above and also tying the screen to the return at the RCA end. This second method helped to drop the hum but it was still noticeable. I don't like this option as it seems to add some extra capacitance to the cable and has an effect of flattening  the "3D" imaging.

This should have worked.

What happened when you lifted the ground connections on the power cords?

How were the Lundhals connected? The link you provided does not give information about impedance. Your preamp may have had troubles driving that impedance. Jensen makes transformers for this application that do not present problems to tube preamps in general.

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.

Hello, 
audio Research made line driver in past for this reason with unity gain. I use one to connect arc sp11 tp arc ref 150. It works and no hum.  Audio Research BL1 line driver is its name. Hope this helps

@llira,

Thanks. It seems the BL-1 is not available anymore. Yours must have given you many years of service.

As I said, call Jensen. They have great customer service. When I contacted them, I got a fast response to all my questions and even a discount on the regular price.
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