Unbalanced pre out to balanced amp in

I have a Modwright preamp with RCA outs and Channel Island amps with balanced inputs. I have always just made up cables with RCA outs and XLR connector inputs. But I have had to extend the distance between the amps and preamp and the new run has the cables going by a few power cords, a TV, an ethernet cable and other possible sources of noise. I would like to experiment with a transformer to convert the unbalanced output to balanced and then use 3 conductor cables for the run. Does anyone have any suggestions. And has anyone used a transformer they could recommend?

Post removed 
The Jensen transformers are excellent! But- like all transformers, if you want to really get the most out of them, make sure that the secondary side of the transformer is loaded to the value specified. For example if the output is 600 ohms, put a 600 ohm resistor across it (if you don't, the transformer will express the inter-winding capacitance, rather than the turns ratio). That way the transformer will be the flattest frequency response, and minimum coloration, which is to say it will be very transparent.
There's also excellent active component which is ATI "The Match Maker" often available at ebay. Virtually noise free and ideal impedance matching for lines of any length + the price is often bellow $100 for used.
Post removed 
From the data sheet, a fairly wide range of impedances is shown for the load, but they also show that the output impedance of the transformer is 80 ohms. There is a rule of thumb that suggests that the load would be about 800 ohm (IOW 10X the output impedance).

There is an optimal value- it looks to me (based on the specs) as if this unit is designed to support 600 ohm operation. I bet 600 ohms would do the job nicely. Have you tried loading it ever?

With the right load not only will you get broader, flatter frequency response, but the unit will be more transparent too.
Post removed 
By 'across' , I mean in parallel with the output.

If the input of the xformer is supposed to be 20K and the output 600ohms, then the xformer will express 20K to the input if there is a 600 ohm resistance across the secondary.

Its the same idea that you see in a conventional tube amplifier: if the tubes are supposed to have a 3000 ohm load, that will only be the case if the output (8 ohm tap for example) is loaded at 8 ohms.

You'll notice that in the test circuits that produced the graphs on the first page, that there is indeed a 600 ohm resistor (or a pair of 300 ohm units) loading the output of the transformer.
Ralph, I think Bob raises a valid concern. This is basically a 1:1 transformer, not a transformer with a significant step-down ratio (which would obviously be inappropriate for a line-level interface).

So while adding a 600 ohm resistor to the secondary would make the transformer perform better, it would very conceivably cause problems relative to the output stage of the preamp (or source component, if that is what it is used with). If the output impedance of the preamp or source component is a significant fraction of 600 ohms, and is not perfectly constant as a function of frequency, then a frequency response irregularity would be introduced. And, depending on its specific design, the output stage might be unhappy providing the current that a 600 ohm load would draw, as well.

Or are we missing something?

-- Al
The specs call for a source impedance maximum of 2Kohms. There are a lot of preamps that can do that, although some of them may have trouble making bass.

IMO/IME, you are better off with a step-down transformer, maybe 10K to 600 ohms. You loose some voltage gain, but a wider range of sources can drive it.

At the input of the amp could be another transformer that is 600 to 10K... then there would be no loss in gain. The bigger the step down/step up, the more transformer artifacts are introduced. That is one of the reasons I prefer to direct-couple :)