From the above linked Doug Self paper:
6: INPUT/OUTPUT COMBINATIONS Taking five kinds of output (the rare case of floating output transformers being excluded) and the two kinds of input amplifier, there are 10 possible combinations of connection. The discussion below assumes output Rs is 100 Ohms, and the differential input amplifier resistors R are all 10k, as in Fig 9.
Case 1) Unbalanced output TO unbalanced input. The basic connection. There is no rejection of ground noise (CMRR=unity) or electrostatic crosstalk; in the latter case the 1mA notional crosstalk signal yields a -20 dBv signal as the impedance to ground is very nearly 100 Ohms.
Case 2) Unbalanced output TO balanced input.
Assuming the output ground is connected to the cold line input, then in theory there is complete cancellation of ground voltages- unless the output has a series output resistor to buffer it from cable capacitance, (which is almost always the case) for this will unbalance the line. If the output resistance is 100 Ohms, and the cold line is simply grounded as in Fig 4a, then Rs degrades the CMRR to -46 dB even if the balanced input has exactly matched resistors.
The impedances on each line will be different, but not due to the asymmetrical input impedances of a simple differential amplifier; the hot line impedance is dominated by the output resistance Rs on the hot terminal (100 Ohms) and the cold line impedance is zero as it is grounded at the output end. The rejection of capacitive crosstalk therefore depends on the unbalanced output impedance, and will be no better than for an unbalanced input, as at 1); the main benefit of this connection is ground noise rejection, which solves the most common system problem.
Case 3) Impedance-balance output TO unbalanced input.
There is nothing to connect the output cold terminal to at the input end, and so this is the same as the ordinary unbalanced connection at 1) above.
Case 4) Impedance-balance output TO balanced input.
In theory there is complete cancellation of both capacitive crosstalk and CM ground voltages, as the line impedances are now exactly equal.
The table below shows the improvement that impedance- balancing offers over a conventional unbalanced output, when driving a balanced input with exactly matched resistors.
Capacitive 1mA CMRR Conventional -20 dBv -46 dB Impedance-bal 99R -60 dBv -101dB Impedance-bal 100R Infinite -85 dB Impedance-bal 101R -60 dBv -79 dB
The effect of tolerances in the impedance-balance resistor are also shown; the rejection of capacitive crosstalk degrades as soon as the value moves away from the theoretical 100 Ohms, but the CMRR actually has its point of perfect cancellation slightly displaced to about 98.5 Ohms, due to second-order effects. This is of no consequence in practice.
Case 5) Ground-cancelling output TO unbalanced input.
There is complete cancellation of ground voltages, assuming the ground-cancel output has an accurate unity gain between its cold and hot terminals. (Which is a matter for the manufacturer) This is a very efficient and cost-effective method of interconnection for all levels of equipment, but tends to be more common at the budget end of the market.
Case 6) Ground-cancelling output TO balanced input.
This combination needs a little thought. At first there appears to be a danger that the ground-noise voltage might be subtracted twice, which will of course be equivalent to putting it back in in anti-phase, gaining us nothing. In fact this is not the case, though the cancellation accuracy is compromised compared with the impedance-balanced case; the CM rejection will not exceed 46 dB,even with perfect resistor matching throughout. Capacitive crosstalk is no better than for the "Unbalanced output TO balanced input" ie approx -21 dB, which means virtually no rejection; however this is rarely a problem in practice.
Case 7) Balanced output TO unbalanced input.
This is not a balanced interconnection. There is nowhere to connect the balanced cold output to; it must be left open- circuit, its signal unused, so there is a 6dB loss of headroom in the link. The unbalanced input means the connection is unbalanced, and so there is no noise rejection.
Case
Balanced output TO balanced input.
A standard balanced system, that should give good rejection of ground noise and electrostatic crosstalk.
Case 9) Quasi-floating output TO unbalanced input.
Since the input is unbalanced, it is necessary to ground the cold side of the quasi-floating output. If this is done at the remote (input) end then the ground voltage drop is transferred to the hot output by the quasi-floating action, and the ground noise is cancelled in much the same way as a ground-canceling output.
However, in some cases this ground connection must be local, ie at the output end of the cable, if doing it at the remote (input) end causes HF instability in the quasi-floating output stage. This may happen with very long cables. Such local grounding rules out rejection of ground noise because there is no sensing of the ground voltage drop.
Perhaps the major disadvantage of quasi-floating outputs is the confusion they can cause. Even experienced engineers are liable to mistake them for balanced outputs, and so leave the cold terminal unconnected. This is not a good idea. Even if there are no problems with pickup of external interference on the unterminated cold output, this will cause a serious increase in internal noise. I believe it should be standard practice for such outputs to clearly marked as what they are.
Case 10) Quasi-floating output TO balanced input.
A standard balanced system, that should give good rejection of ground noise and electrostatic crosstalk.
The hot and cold output impedances are equal, and dominate the line impedance, so even if the line input impedances are unbalanced, there should also be good rejection of electrostatic crosstalk.
