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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

Showing 2 responses by atmasphere

atmasphere's avatar
atmasphere
12,099 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.
atmasphere's avatar
atmasphere
12,099 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.
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