Advise on Long Phono Cable run

Just to kick a dead horse even further down the road, you could choose a moving magnet cartridge that requires a high-capacitance load. For example some types are flattest in response when they have as much as 400 pF or even more  of capacitance on them. Then you could deliberately choose a cable that would give you the required amount of extra capacitance.But if we are now talking about a phono stage placed very close to the turntable, that is the best way to go. Hififan, microphones make a fair amount of signal voltage. I’m not sure what you meant by your remark.

“Pro sound operators” are dealing with line level signal voltages ( 1-2V). Here we are talking about 5-10mV at most, even from a high output cartridge. Different ball game.

Chakster made a good point. I recommended an MM cartridge, but as he notes, MM cartridges are capacitance sensitive. More so than either MI or MC types. So I agree with his recommendation for an high output MI type cartridge. Second, it goes without saying that you will have to have your cables specially made at that length. Several companies will do that. Among them I recommend AntiCables. In addition, going balanced would have no benefit unless you are running into a true balanced phono stage. A phono stage that just provides an XLR input but is single ended, will not get you all the benefits of a balanced connection. Balanced might be a bit quieter.

If you MUST do this, find the absolute lowest capacitance cable (per foot) that exists, use a high output cartridge (MM or MI recommended over HOMC), and expect to have to live with some sonic degradation.  If you live in a high RFI area, expect that too.