Why do most phono preamps lack XLR input even thought cartridges are naturally balanced?

Twisting a pair of wires works great against capacitive or electromagnetic pickup, exposing both wires evenly to electric or magnetic field.  That way induced noise currents are exactly even and cancel.  It works fine as long as twist is even and its pitch is much shorter than the wavelength of offending signal.  Shield in addition to twisting adds more protection against electric or magnetic field by being a Faraday cage, but even some induced currents flow on the surface (shield) only if frequency is high enough (skin effect) while some are lost as eddy currents.  Both Faraday cage and skin effect becomes less effective for electromagnetic radiation below 100kHz.  Combining both provides good protection against low frequencies (twisting) and high frequencies (shielding).  Of course keeping cables short is very important, since cable as an antena for electromagnetic field becomes very ineffective when shorter than 1/10 of the wavelength.  If shield creates ground loops, then I would at least ground it at the source end only.  It is much better, than not having shield at all, IMHO.

kijanki,

My last post was specific to the link luisma31 posted. I am not disputing anything you said, just not sure you were aware I was making a specific response to that.

roberttdid, Of course.  I posted only to state that shield, IMHO, plays important role in defense against electrical noise (when connected properly).  I also strongly agree with you that shorting open ends of the shield with wire creates large loop that picks-up electrical noise.  Noise currents in such loop produce magnetic field that will "transform" noise into signal wire.    

I was not saying that shorting the shield with a loop will pick up noise kijanki. It will act as a shorted turn and will direct an external magnetic field away from inside conductors to a degree. However, it will also act as a shorted turn to the current carried by the internal conductor, which could seriously impact it's performance.

roberttdid, Sorry I read it wrong and I understand argument about shorted turn, but transformer works both directions. Any closed loop will pick-up ambient electrical noise.  Induced electric charge will flow in this loop thru/along the shield producing magnetic field around signal wire, that will induce electric noise current in it.