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

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.

I have XLR's out of my VPI table going into Ayre pre....dead silent.

Sorry kijanki, I thought we were talking about discrete level or discrete within an IC circuit, and that the 3 op-amp instrumentation amplifier you showed was a different part of the discussion. The circuit shown is missing the output buffer that would be on a practical circuit and those two output resistors have to be matched as they dominate CMRR in the calculation. I now get completely where you are coming from  :-)


If the noise current induced are not the same in both wires, then they are not common mode, they are differential mode, and a balanced connection only removes common mode noise.

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.

In a typical connection of two components, you have multiple paths for current flow which is how CM noise generates a signal. With a catridge/phono stage, you have a single loop so induced common mode noise does not generate a signal. Where it gets dicey, is when you start adding new current paths like shields. Fix your DM noise, and now create CM noise. I think this is perhaps was jcarr was referring too?

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.)?