Connecting a unbalanced output preamp to Krell FPB350 balanced amp

Several things to try:

1) Using the regular XLRs, if the center pin of the RCA is pin 2, make sure pin 3 is tied to pin 1 (ground) and see if that shuts it up.

2) you may have to float the ground pin of the amp's power cords (this should only be done for testing).

3) If *that* does not do it, then the CMRR of the amp is insufficient to suppress noise pickup. At that point, you'll have to use transformers at the input of the amps to convert from single-ended to balanced.

If you use the transformers, then you won't have to float the amplifier's grounds on the power cords.

Regarding point #2, if floating the amps does the trick, you have to get an isolation transformer to run the preamp. This is not expensive; you can find such transformers on eBay for about $120.00.

A pair of Jensen Transformers would be my choice:
http://www.jensen-transformers.com/product/db-2px/
I found Jensen to be very responsive to queries. Just tell them what you want to do and they will give you the proper product- and sometimes offer a discount.
B

Thanks for your prompt replies.

I tried using Lundahl Problem Solvers and the hum/buzz was as loud as ever. To make matters worse, there was a matching problem with the Lundahl LL156X-PHF3MX and everything sounded bad with no bass, no high end and attenuation was very high.

I know I should persist in working out the best cable configuration but there appears to be a few options one of which is that suggested by "Atmasphere".

It seems its not possible to include graphics or pictures in postings. I was wondering if I could upload some drawings to a dropbox (https://www.dropbox.com/s/uy1lr5ph97rhq47/RCA-XLR%20connections.pdf?dl=0)  for you to view and let me know what you think of the various cable connection options as a prime candidate solution. Option #2 in the drawing is the "rane.com" recommended configuration #17. I am sure I've not included every possible alternatives but the ones in the drawing seem to be the most common.

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.

I will try the Jensen option per "gdnrbob" as a last resort if I don't find a cable solution. Unfortunately, this will be an expensive option as I will need extra pair of XLR-to-XLR  interconnects as well.

Thanks again for your help.

Did your Krell amplifiers not get shipped with the Krell Shorting Pins - (Jumpers) for running in single ended mode ? 
As Atmasphere said these should be inserted in slots 1 and 3 to short them out.

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.
B 

@swcng2001 
I know this is some time later but I have an extra ARC BL-1 if you're still interested in going that route. I used one between a phono pre and preamp. Worked well for me.

Get a used Krell KCT preamp...dead quiet and extremely musical!  Krell amps are designed to be run fully balanced.  You will not hear what the amps can do unless you run Balanced!

dave_b
are you still enjoying your Krell Vanguard?  Happy Listening!

Yes, longest I’ve owned a piece of audio...no plans to change or sell!  It is a gem and a superb DAC to run my BlueSound through.  I have found a worthy replacement for my previous Krell Cipher for my vast CD/SACD collection.  I bought a Yamaha CD/SACD player.  Built like a tank...32bit resolution and discrete components hardwired.  Amazingly clear, sweet dynamic sound with tremendous extension.

$6000 for KBDigital At Audioadvisor!

Thank You - dave_b
I actually remember when you bought, posted the Vanguard some time back.I also remember you owning the Krell Cipher (I could not believe that you sold it).  Agreed, when companies like Pioneer, Sony and Yamaha want to build statement products (S2100 player) they are the best in the marketplace.  Usually priced less than the competition as well.

Happy Listening!