The "Brand-Rex" cable I have measured is undocumented and I haven't measured resistance or inductance per foot.
I do have the published specs for Belden 9451 which is quite similar posted here: http://www.ka-electronics.com/images/jpg/Belden_9451_Signal_Characteristics.jpg
It shows 0.17 uH per foot and 14.1 Ohms per 1000 feet for the conductors.
"But doesn't not grounding at both ends leave the system open to EMI issues?"
Please refer to Muncy "Noise Susceptibility in Analog and Digital Signal Processing Systems" http://www.aes.org/e-lib/browse.cfm?elib=7945
And Whitlock "Common-Mode to Differential-Mode Conversion in Shielded Twisted-pair Cables (Shield-Current-Induced Noise)" http://www.aes.org/e-lib/browse.cfm?elib=12594
As well as Whitlock's "Balanced Lines in Audio Systems: Fact, Fiction, and Transformers" http://www.aes.org/e-lib/browse.cfm?elib=7944
"If you use that circuit you linked to earlier, that uses a 1MOhm to gnd T-network, then does that get you enough CMRR?"
It depends on the source impedance imbalance (not the shunt capacitance to ground if the shield is grounded at the source) and how much CMRR is enough. Do realize that the 1M Ohm could be made larger (e.g. 4M7) approaching the CM impedance of InGenius bootstrapped approaches. The value is primarily limited by the op-amp bias current and the allowable reduction in DC common mode range. For an LME49860 with a worst-case I bias of 72 nA per input, the maximum CM Vos that would develop is approx. 680 mV. Typically at 10 nA per input it would be less than 100 mV. This CM Vos is rejected by the following diff amp and the Inoise that develops across it also appears in common mode.
"Thinking the difference amp that follows the buffers can be anything you want, like good old INAxxx."
Yes it could be an INA134/137 THAT1240/1246 or be cross-coupled (INA2137/THAT1286) to provide a differential output. Alternatively a conventional op amp and precision resistors could be used to lower the circuit impedances and Johnson noise though the resulting CMRR - and the noise performance with a high value of Rcm - might not be as good due to resistor mis-match.
But doesn't not grounding at both ends leave the system open to EMI issues?
Interesting info on the capacitive imbalance over 1km. At 1kHz, the imbalance in the two impedances to gnd is about 277Ohms. If you use that circuit you linked to earlier, that uses a 1MOhm to gnd T-network, then does that get you emough CMRR? Thinking the difference amp that follows the buffers can be anything you want, like good old INAxxx.
Bill Whitlock points out in Ballou "Handbook for Sound Engineers" 4th ed. that by grounding the driven end shield, and not the receiving end, common mode to differential conversion at the receiver due to capacitive imbalance from the cable is avoided.
"The reason I singled out cable Z imbalance is because of the capacitance to ground variation of each wire in the cable. I'm thinking about impedance mismatch over frequency, not at dc."
Over long runs the capacitance to shield variations are significant. As a point of reference I measured approx. 1000 feet of "8451-type" two conductor foil shield with drain wire cable and found that the conductors measured 40.9 nF vs. 43.8 nF.
"I'm aware of the CM i/p Z problem of conventional audio difference amps and how the system CMR gets blatted by cable Z mismatch."
Yes the effect is well documented though I think you may mean source Z mis-match which also includes the cable as well as what is driving it.
What I've never seen in the wild however is an audio tieline that develops (say 20 Ohms) resistive imbalance. I've cleaned a lot of dirty patchbays and cords in my career and replaced switches that may have developed 20 Ohms or more imbalance, or fixed a bad Elco or "DL" connector pin but I was never called to fix it because it hummed due to reduced CM. The call or note from the engineer was that the patchcord or switch sounded "crunchy" or the circuit was open.
I'm also not sure how a "proper" balanced output could develop a 20 Ohm imbalance. Typical build-out resistors might be 47R 1% and have at most a 1 Ohm error. One would have to use 1K build-outs with 1% resistors to approach 20R imbalance.
So my question is how do we arrive at such large imbalances? I realize that with 9-25K Zcm inputs 1 Ohm imbalance is significant but it generally doesn't produce session-stopping hum.
"Can you explain the shortcomings of using e.g. INA317, its inputs buffered by 2 x op-amp stages like OPA1642 (to make a classic instrumentation amp), plus say 4.7MOhm resistors to gnd on each op-amp input for bias?"
I'll let Bill take that question. The circuit you propose is similar overall to this one only the linked citation refers to a bipolar op-amp with T-bias and AC-coupling. It's a simplified circuit:
There's a lot of flexibility in the value choice of input T-bias values and Cin. As shown, the differential -3dB point is 4 Hz. For common mode signals the LF cutoff is approximately 0.1 Hz.
"The i-related voltage noises in R1 and R2 will not be nullified by the following diff-amp."
Correct. That's why their values are relatively low.
"The two i-noises will add as the sq-rt of the sum of their squares in R7, creating a common-mode noise component that will be nullified in the diff-amp."
Also correct. That's why it's magic. Should also point out to our readers that a DC-term develops there also in common mode and is nulled out. For the THAT151X and NE5532 examples if Rcm is made too large there will be a reduction in headroom. That sets the upper limit. For a 5532 with a Ibias per input max of 1500nA, 3 uA total, a 1M can produce a Vcm of 3V. An LME49860, with an Ibias per input of 72 nA max (144 nA total) the Vcm with a 1M is less than 150mV.
I prefer to use the "T-bias" approach (and InGenius) with AC-coupled inputs to provide great LF CM because it reduces capacitor matching requirements considerably.
"Even if just two high-value resistors to ground supplied bias, I doubt they'd contribute significant noise because it will be seriously attenuated by the differential source impedance - somewhere in the 150 to 200 ohm range for a mic preamp."
In a mic preamp with coupling capacitors you'll never see a 150-200 source impedance at LF which gives rise to 1/f noise. At 20Hz 22uF (~47uF/2)you'll see an added 360 Ohms to the 150-200R source Z from capacitor reactance. Thus, in a preamp R1 and R2 still need to be kept relatively low.
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