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

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.)?

Hmm. Did I just open Pandoras box? Btw, some phono preamp designers prefer to refer to cartridges as a “floating differential” rather than balanced. 

@clearthinker, contrary to your statement of 6dB reduction with true balanced which I’m not sure applies to cartridges, the designer of BAT phono has claimed that RCA offer 6dB reduction in noise over XLR with cartridges. Don’t kill the messenger.