Long distance between preamp and power amp

No , no adapters, they degrade the sound and also might create electrical disturbances.
Some RCA cables like better Purist Audio will give you no or minimal signal loss. I don't know if it would make sense to use XLR/RCA cables.

my suggestion is to sell the amp and get one with balanced inputs

I agree with randy-11, but please make sure your preamp is a true balanced design, so you will truly benefit from the amp replacement. Some preamp use chipsets to provide balanced connectivity without the noise reduction which a true balanced design provides.

I considered replacing my amplifier, but since I love it so much (it's a Line Magnetic 219ia) and this situation will be temporary for 3 or 4 years (because then I will move again), I prefer to keep the amplifier and work out a solution around the cabling. 

I use 6-meter RCAs (AQ King Cobra) between my preamp in my listening room and my phono preamp in the next room, with no issues.

Rule #1 for single ended cable is to not use them for longer runs (I think rule #2 is don't eat them). Sqlsavior is a rebel rule breaker...

6 meters is not a very long run for an excellent RCA cable. Not a problem. I can speak of older Purist Dominus with full certainty.

There’s no reason to consider selling such a fine amp. If it proves to be necessary you could connect a suitably chosen Jensen transformer (ca. $275) between the preamp and the amp (close to the amp), to convert balanced to unbalanced. That would provide essentially all of the benefits of a balanced connection.

But it may very well not be necessary. What is the make and model of the preamp?

The approach that is best may very possibly depend on several different characteristics of the preamp. Including its output impedance; how its balanced outputs are generated (and therefore whether the sonics of its balanced outputs are likely to be better than, worse than, or similar to the sonics of its unbalanced outputs); and its internal grounding configuration (that may affect susceptibility to ground loop issues, which can be brought out by long lengths of unbalanced cables).

Regards,
-- Al

 I used a 7 meter cable for years between pre and power amp. It sounded fine  , but I always felt it HAD to be a weak link. When I was able to reconfigure my system I went to a much shorter and more expensive cable. Damned if I could hear the difference.

As many of you say, maybe the distance is not an issue after all - as long as I use a good cable. 
My preamplifier is an old (and mint) McIntosh C712. 

yes, my better suggestion is to sell the amp and get one with _true_ balanced inputs

monoblocks should be even better

you can certainly get away with a long RCA run - I did it as a student with Advent speakers and Dynaco gear, but if you want to maximize the SQ do the above

Agree with randy11,

Always been a long IC , short SC fan. Recently found a pair of 30' balanced ICs of my preferred brand and at a reasonable price. (Rare find and Rare price). SQ took the maximum leap.

RCA could be worked with, but was always a bit compromised.

 I begrudgingly worked with the RCAs ( equipment that I had and cost of ICs I wanted ) made it big dollar proposition to make a wholesale change. Thankful for A'gon. Pleased with system more than ever.

I couldn’t find an output impedance spec on the C712, or anything that would provide an indication of how its balanced outputs are generated. But given that it is a solid state design chances are its output impedance is relatively low. And given that all of the input connections it provides are unbalanced, chances are that one of the two signals in the pair of signals it provides to its balanced output, for each channel, is generated by putting the other signal through a simple op amp stage, ca. 1995. Which in turn suggests that the C712’s unbalanced outputs stand a good chance of providing better sonics than its balanced outputs.

Given those considerations, I would suggest simply using an RCA cable having relatively low capacitance, less than say 20 pf per foot, and providing good shielding. Blue Jeans LC-1 meets those requirements at a low price point.

I suspect that will work out fine. If not, though, a Jensen transformer can be considered, as I mentioned earlier.

Also, FWIW, personally I would much prefer to replace the preamp and/or add a Jensen transformer than replace your amplifier, which is a treasure based on all of the user comments I have seen about it.

Good luck. Regards,
-- Al

I run about 25' from a Zesto Leto, floating balanced preamp, to my Mcintosh MC60s. Output impedance 250 Ohms to Input impedance 500KOhms. Ran it through the wall in EMT conduit and don't have issues. Always wondered though, if I'm missing something in the frequency. No hum.
The Leto manufacturer says that floating balanced is his preferred topology. Curious to hear an opinion on that and the long run.

Dentdog, I don't see any issues with what you've described.  In fact the design of the Leto seems ideally suited to a situation involving a lengthy connection to an unbalanced input.

John Atkinson's measurements of the output impedances of the Leto were even lower than specified, and as I'd expect in the case of transformer coupled outputs did not have much variation as a function of frequency.  That will make the capacitance of the long interconnects a non-issue for any reasonably designed cable. 

Also, I don't know if you are using RCA cables or XLR cables + adapters to mate with the unbalanced inputs of your MC60s, but either way you are in effect running unbalanced interconnections since the load is unbalanced.  However given the transformer coupling and the provision of ground lift switches in the Leto for its unbalanced outputs you presumably are immunized from ground loop effects that can occur in some cases involving long unbalanced interconnections.

Also, actively driven balanced outputs on some equipment (as opposed to transformer coupled outputs) may have problems with XLR-to-RCA adapters, since most such adapters short the signal on XLR pin 3 to ground (XLR pin 1).  But again that won't be a problem with a transformer coupled output, and in some cases involving transformer coupled outputs it may actually be necessary for an adapter to connect those pins together.

Enjoy!  Regards,
-- Al
 

Thanks AL!

Thank goodness we have Al, the voice of reason. Getting rid of that amp just to change the mode of connection would be ridiculous. 

Thank you all for your inputs. I will definitely keep the amplifier, so the bottom line is I will run a good pair of RCA cables inside a shielded sleeve and under my floor - I believe I'll be safe from interferences. 
I work in a theater where we have all sort of professional grade cables (including underwater cables). I'm sure I'll find a good quality cable to perform this task. And as I said, it will be a temporary solution. 

The problem with long RCA interconnects is a loss of high frequencies and definition.

The way to deal with it is run balanced connections to the amp and convert from balanced to single-ended using the Jensen transformers Al suggested earlier. That way works!