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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
atmasphere's avatar
atmasphere
11,753 posts
 
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.
gdnrbob
4,441 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.

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