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

kijanki, To the degree that signal handling by the two phases of a balanced circuit is not exactly symmetrical, you are correct in saying that will reduce Common Mode Rejection, and it does for sure in tube-based balanced circuits, because exact matching of vacuum tubes is not possible or stable even if it's momentarily possible.  (Your argument about RIAA equalization does not hold as much water as does the argument about tube or transistor gain matching, because it is much easier to exactly match capacitor and resistor values, or to tweak the values for good balance, than to match the gain components.) To all of that I say, so what? CMR is reduced from some perfect value to some lesser value, but you still do get CMR.  Whereas, in an SE circuit, you don't. I have two fully balanced phono stages; neither of these ever drove me crazy with hum and noise problems that I read about all the time in relation to SE phono stages. But the OP asked "why" we don't have (more) balanced phono circuits, and I think it's mainly the profit motive.

As @lewm notes, that a phono preamp has XLR inputs doesn't necessarily mean that it is fully balanced. But the inverse is also true - there are differentially balanced phono preamps that use RCA connectors. ARC Ref Phonos, for example.

@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?

@kijanki instrumentation amplifiers use laser trimmed resistors internally and/or external precision resistors as well as significant open loop bandwidth to achieve high CMRR and fixed gain levels. If you mean the differential pair / long tailed pair on the input, they also need somewhat matched resistors and in op-amps they are trimmed to provide higher CM range. <<$1.00 op-amps have 100db+ common mode rejection, but not in practical circuit which would be about 70db with 0.01% resistors. Low cost practical instrumentation op-amps with high CM range readily achieve 90db CMRR. Your amps may have a few precision resistors, and/or they are calibrated.


The main point of differential connections in audio is to eliminate the noise from having ground references which are different at the receiver and the transmitter whatever they may be and how they are caused. In a home audio system, that is going to be predominantly through noise induced through the AC, with the capacitance of the power supply, even EMI caps completing the circuit.


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".