If you’ve understood everything so far you can now see that you can drive an unbalanced output into a balanced input and still get much of the benefit of the CMRR (Common Mode Rejection Ratio) of that input.
In such a configuration, it’s common to tie the “cold” wire going to the balanced input directly to the ground at the source end of the wire. Any noise picked up along the wire can get canceled at the other end. However, because the cold wire is tied to ground the noise picked up is more or less killed right there, or “shunted” to ground, as is commonly said.
Consequently, most of it doesn’t show up at the balanced input, which never gives that input device the opportunity to use it to cancel the same noise that was picked up on the “hot” signal wire. Further, it opens up the possibility for noises on the ground line to get into your audio through the negative side of that balanced input.
In a ground compensated system that cold wire is not tied directly to ground. Instead a resistor is placed in between the cold and ground so that noises picked up in the wire are not shunted off to ground. Now they appear at the other end, just as they would if the line were a differentially balanced line. They get canceled and the noise mostly goes away.
The effectiveness of this scheme is largely dependent upon the CMRR ratio of the input device and the accuracy of the resistance in simulating a source impedance that’s the same as the hot wire. This is not a truly balanced system, but in terms of noise cancellation behaves similarly.
If you’re working with a device that employs impedance balancing you should connect it to other balanced devices just as if it’s a normal balanced output with hot, cold, and ground leads. If you’re connecting it’s output to an unbalanced device, you can use a standard unbalanced cable (which actually makes it easier than connecting a balanced output to an unbalanced input [what to do with that third conductor]).
Some equipment can use its “cold” output terminal as what is known as a “ground sense” line. Thus even when driving an unbalanced input (where you’d have the cold terminal tied to ground) this sense line is able to add any noise or hums from the ground back into the hot signal where they would get canceled at the other end.
They get canceled there because the same signal would be appearing at the ground of that device (since it came from the ground wire in the first place). In unbalanced inputs, ground serves as a reference against which the signal on the hot lead is taken.
If the hot and ground have the same waveform on them then nothing appears at the device to amplify, and again the noise ends up getting canceled. This isn’t as effective as a balanced input, but the performance is certainly better than a typical unbalanced line.
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