You might be lucky. I used to have a 30’ run single ended from my preamp to mono blocks. With it running below the floor in the crawl space I used to have no issues. I then moved temporarily to a rental and had to run it on the floor and it was unacceptably noisy so I had to switch to balanced which I’ve been with ever since. You also want to make sure the output stage of your phono stage is up to driving a long cable (as cable LCR will increase with length) - what stage are you using?
Try running the AC lines from the same spot as the pre or amp, to reduce potential for hum. Ie, the same ground potential would be the result of the AC is from the same exact receptacle.
If you get some hum, try the ’same AC origin point’ trick. Play with the AC to try and reduce hum, is the deal.
We've made 6.5-7M RCA cables for people -- totally unshielded btw. No hum. That's a $35k pair of cables. Better hope there's no hum.....
OK that's a healthy 2mV output so the phono stage will be outputting 1.4V which is a good signal and should put you in as good a situation as you could hope to drive that long single ended run. If you had a .3mV output (i.e. typical low output MC) things might be more problematic -- anyway good luck!
So eventually ended up making my own interconnects....12.5 feet length is actually sufficient, I overestimated the initial length. Went with DH Labs Silver Sonic II cable and KLE Copper RCAs...total parts cost $200...all silver plated wires to interconnects...sounded fantastic at initial listen. Not a cable believer at all, Pink Floyd's 'The Wall' never sounded better through vinyl though...but who knows, may be the $20 Kabeldirekt sounds the same as well, I just can't be bothered to test. Wanted to make a shielded silver cable and here we are. Highly recommended. If I wanted to buy it, it'd probably cost $800 - $1200, you figure $200 (parts) + $400 (labor) + $600 (marketing + other overheads) = $1200.
Triple that number if you want to actually have a business that lasts longer than the initial musing.
The number that is openly stated by synergistic, in the TAS published cable bible section of an industry overview with many interviews with company owners... (pdf), is 9x over raw cost calculations, in the cable world.
The audio business is hard. Not straightforward - not like it seems .... from a safe distance. But hard. Very hard.
There is a lot of the moat problem that is recently being buzzed about, in this candy/moat concerned war of words between Elon Musk and Warren buffet.
The moat problem, something created by established players. they protect their Territory via territory denial via natural advantage coupled by sometimes (ok, almost always) very nasty behaviour... and the rest of their business connectivity is risk aversion oriented and plays it safe, as that is human nature.
It’s a human nature thing and is even deeply embedded into the sciences:
"How success breeds success in the sciences" (whether it is deserved, or correct... or not)
Part of the message in business, in order to have wide term success, some part of your plan and acts have to center around the psychology of sociopathology, psychopathology...... as the competition for market share involves that mentality, to some notable degree (extreme to fair in level) in all the extant players.
Which, for a reasonable human, when found to be, feels quite unsettling and sickminded. We find this effect everywhere, most notably (seemingly) in the corporate and political worlds. Which, of course, as birds of a feather, will begin to flock together...
they'll run the gamut, like SpaceX having what is now noted to be high levels of natural advantage, all the way over to extreme expressions of embedded pathological aspects being the main provider of a moat, as the product cannot compete in an open world..ie..Monsanto.
The longer a market is around (a given area of technology, etc) the more likely is is to have drifted from natural advantage being the driving force...into pathology and psychosis being the main component of market share attempts/control.
And politics is a very old thing. We note it is built almost completely out of pathology and psychosis.