@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?
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?