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

I can see one possible problem with true balanced operation.   Gains of both amplifiers have to be exactly the same, otherwise common mode noise will get converted to normal mode signal.  It requires some form of cross-feedback to keep gains exactly the same.  Phono preamp in addition has RIAA equalization that changes frequency response within audio band by 40dB  (100 times).  I suspect, that it would be very difficult to match it in both "legs" of true balanced phono stage.

@lewm  One advantage of true balanced is to remove reference to GND in order to get rid of wire to shield capacitance, but the same can be achieved with input transformer.  The other is that even harmonics produced by both "legs" cancel, but some people looking for "warm" sound might not see it as particular advantage.

Matching resistors is unnecessary if cross-referencing of both "legs" is used (like in the input stage of instrumentation amp), otherwise it is almost impossible.  As for matching RIAA curve - it is very difficult. Typical good capacitors are 1%.  Matching them by hand is not practical for production, not to mention changes over time (aging).  Phono stage amplifies 60Hz by about 6 times vs 1kHz signal.  As a result of both you will get very poor CMRR @60Hz in order of 30dB only.  Good Instrumentation amp, like one in my Rowland amp, have 90dB CMRR @60Hz (and it is just plain line-in).  My current power amp (Benchmark AHB2) has CMRR=80dB (equivalent to 0.01% gain matching)

If they really don't make balanced inputs for such low level signals, they should.  True balanced is out of the question, IMHO, because of capacitors tolerances, but simple solution would be to use input transformer. Why not use instrumentation amp and drive shield with common mode signal?  It is very common in low level amplifiers.

Interesting subject.  Perhaps Almarg or Atmasphere can chime in?

With a cartridge, you only have a ground connection on one end. Equal currents induced in the same direction on each wire (common mode) "induce" the same voltage and cancel each other out (it is a loop), at least at practical audio frequencies. Differential noise, will of course look just like a signal. The cartridge is already "isolated".

That is true, as long as currents flow in the loop only.  If one of the wires go to input while the other goes to GND (unbalanced input), currents in both wires are not even anymore.  Perhaps, that's why floating (for audio frequencies) balanced output, like transformer is not enough and has to be connected to balanced input (not single ended).  Microphones also benefit from balanced input.  

As for Instrumentation amps, yes they have laser trimmed resistors, but front (two amp) differential section has always gain of 1 for common mode signal independently of resistor tolerance.  If you set gain of 100 for this section you get automatically 40dB CMRR independent of resistors tolerance.  It happens because each of two amps is referenced to input of another (instead of the GND). 

https://www.researchgate.net/figure/Fig-Two-Op-amp-Instrumentation-Amplifier_fig1_299514235

That is the only way I can see, for truly balanced amps, to function without converting common mode noise to normal mode signal.  Otherwise matching resistors and keeping them matched to some sensible number (like 60dB=0.1%) is not practical, while adding RIAA frequency correction and matching capacitors to 0.1% is next to impossible.

That's not how it works. I think you are confusing something. For a 40db CMRR, you must have gain matching of gain matching between the two channels to +/- 0.5% (1% total), and you must have perfect transistor matching (which they never are).

Look at the picture I provided.  Each of two amps operates in non-inverting mode and is referenced to inverting "-" input of another amp.  Since voltage at inverting input is the same as voltage at non-inverting input then voltage across R2 is V2-V1.  For common mode signal V2=V1 and voltage across R2 is zero.  It means you could remove R2 (no current flow) and without it you just have two amplifiers with perfect gain of one each (followers) even if R1s are a little different.  R2 plays role only for normal mode signals.  That way you can set any gain for this stage and gain for common mode will always be one. If you set gain of 100 then CMRR will be 40dB.

As for the balanced connection - it is not balanced anymore when you connect it to unbalanced input.  Noise currents induced in both wires will be different and noise will go thru.  The fact that cartridge is floating won't help, unless input is balanced.

The differential input of an op-amp does not have a gain of 1

Nobody said that.  Instrumentation amp is not an op-amp.

Roberttdid,  I would compare single ended phono preamp input to, for instance, oscilloscope.  When you short oscilloscope probe (tip to probe GND) there will be still some noise on the screen - bigger if you touch shorted inputs (in-spite of you "floating").  It is because for electrical noise one of the wires (GND) provides lower resistance path to GND.  Current thru this ground wire causes tiny voltage drop, seen by the input as input voltage (input is referenced to BNC GND and not to end of the cable GND).    Of course scope can be used in two channel differential mode but it suggests that second input is required.  Amp could be truly balanced, but it is overkill, IMHO - costs money and introduces more components in the signal path.  Remaining choice is likely between differential amp and transformer.  Transformer, in spite of some distortions at low frequencies (most likely inaudible) is a clean simple solution.  What do you think?

I had TT long time ago and always had problems with low frequency hum (single ended phono stage).

Stingreen - you stated that your Ayre phono is dead quiet.  It's likely because of balanced input.  XLR cable wires are usually twisted - a very strong defense against electrical noise.  Do you know by any chance what is inside (transformer, differential amp etc.)?

Ralph, very interesting.  As roberttdid said about differential filter "it seem obvious", but it escaped me completely, being stuck on "matching". 

luisma31, If it is not grounded at either end, then capacitance to what? To electrically floating object?

Almost none. I ran unshielded tonearm cable in my home system for some years. You could crank up the volume and no hum or buzz from the cable, even if grasping it or moving it around. You do still have to ground the tone arm and it works best if that wiring travels with the signal wires. One advantage here is this allows for much lower capacitance in the cable.

@atmasphere  Wire to shield capacitance should play role only if either output or input circuit is ground referenced.  I thought it was the advantage of fully balanced, not ground referenced (floating) configuration.  There is still wire to wire capacitance (increased by twisting), but there is no wire to shield capacitance, making even less expensive cables to sound better.  I agree that shield does not improve much, since twisting wires makes them very immune to electromagnetic or capacitive pickup, but it should not cause buzzing.  What lusima31 described sounds like ground loop.  XLR cable shield is grounded at both ends, but shells shouldn't be, since it might create such loop.  I've read that in recording studios they often have problems with that and fix it by cutting shield at one end.

luisma31, It might be true, but I've never heard of hum caused by high capacitance of the cable.  I've heard of output oscillations and frequency roll-offs, but not hum.

luisma31, Unplugging connector?  You probably mean - disconnecting shield?  If you had no hum with shield connected at both ends, then why to change it?  I would guess that disconnecting shield at the Phono side only left TT ungrounded with floating antena (shield), while unplugging on TT side or both sides should have less of an effect.  How is your TT grounded? 

luisma31, sorry, I got confused with initial post about TT problems.  You keep saying "unplugged from pre side" but you probably mean that shield was disconnected, otherwise what are you listening to if nothing is connected to pre?  I suspect that cable might create ground loop.  Lifting shield on pre side might still produce buzz if shield touches connector shell.  Anyway, there is very little chance that anybody can help you if Ralph couldn't.  I use XLR cable grounded at both ends between DAC and power amp and it is dead quiet.  

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.

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.    

Perhaps it works, but I would think that it can do more harm than good in comparison to plain grounded shield.  High frequency noise currents induced in the signal wire flows on the surface (skin effect) - shield to ground.  I'm not sure replacing it with shorted turn will be better, but who knows.  I'm just skeptical, but if it works it works.

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.