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Connecting a unbalanced output preamp to Krell FPB350 balanced amp

Hello everyone,

I own a Conrad Johnson 17LS and a pair of Krell FPB350 amps. The CJ has only RCA unbalanced outputs and the Krell has balanced inputs (either in their proprietary CAST configuration or a standard XLR connection). I've been trying to upgrade my interconnects and have been experiencing buzz and hum. I was wondering if anyone has a cable configuration that best minimises these problems. I have seen the "rane.com" suggestions and tried their configuration #17. I've tried other configurations such as 3 cores with a shield that is connected at the XLR end to ground and the RCA end to the return conductor/pin. I've also tried with only one end (XLR balanced end) connected to ground. None of these solved the problem though the second option above seems to have the least amount of hum or buzz.

Regards.
swcng2001
gdnrbob
4,444 posts
 
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
swcng2001 OP
6 posts
 
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.

ct0517's avatar
ct0517
2,041 posts
 
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.

atmasphere's avatar
atmasphere
11,793 posts
 
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.
swcng2001 OP
6 posts
 
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.

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