Firstly, it is commonly (but very much mistakenly) assumed that signal balance is important, but this is not correct. In reality, it doesn't matter at all if a balanced line has all its signal on one lead, and none on the other. What
is important is impedance, and the signal leads of balanced lines must have the exact same value of impedance to earth/ground/common at all frequencies of interest. Ideally, this should be as high as possible for both the send and receive circuits. The
InGenius® IC (licensed to THAT Corp.) is a solution that provides a much higher impedance to earth than normal balanced line receiver circuits, and it's worthwhile to have a look at the PDF to understand all the reasons.
In reality, balanced receiver circuits are extremely hard to get right, and all active solutions have a limited impedance, often made worse by the addition of capacitors to reduce EMI and/or high frequency noise. Bill Whitlock says "Transformers outperform all conventional input stages for one very simple (and, to me, obvious) reason: transformers have incredibly high common-mode input impedance. In the real world, simply matching these impedances is not enough - they must also be very, very high. With transformers, they are inherently in the area of 50 M-ohms at 50 or 60 Hz, and rejection is so high that it became taken for granted ... and seemingly, everyone who ever knew why either forgot or died! Ordinary balanced line receivers have common-mode input impedances in the area of 50 k-ohms ... a factor of 1000 less than a transformer (or the InGenius® input stage)."
Secondly, send circuits ('transmitters' if you like) should also have a high impedance to earth, and the circuit shown here does just that. However, it must be understood that the circuits shown are useful, but only to a limited degree. While there is no doubt that the circuit performs well, it is easily disturbed by cable capacitance and can become unstable. Because it uses positive feedback to achieve the high effective impedance to earth, it doesn't take much of an imbalance somewhere to cause oscillation - certainly something that should be avoided.
Ultimately, there is no active balanced send or receive circuit that can match a good transformer - this does not include $20 mic transformers you can purchase from retail electronics outlets! As noted above, the windings of (good) audio transformers have an extraordinarily high impedance to earth unless the centre tap is earthed - generally a very bad idea. If you need to provide phantom power via the transformer centre tap, this ruins the inherent high impedance, but the far end will be a microphone, and is floating. A low impedance to earth is not such a great concern then, as there is no earth reference at the far (microphone) end, other than the mic body which is connected back to the mixer anyway.
It is very important that the true principles of balanced lines are understood properly, but this is often not the case. Many people concentrate on signal symmetry, but neglect the requirement for a very high common mode impedance and/or impedance matching of the two inputs or outputs. Note that this does
not imply that input and output impedances be matched, because doing so reduces signal level by 6dB and may overload send amplifiers - whether opamp or transformer based. Transformers provide an almost perfect match when the input or output winding is floating, and will nearly always give the best results in harsh conditions.
Predictably, most people shy away when they see the prices, so opt for (often simple) active circuits instead. Under relatively benign conditions with no heavy interference sources this is often quite alright, and works just fine. Just remember though - just because you have a balanced line, this doesn't mean that you'll get no noise. Oh, and I must point out that balanced lines don't sound 'better' unless better is defined as lower noise or interference.
