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Impedance Balancing

Sometimes also called pseudo-balancing, quasi-balancing, resistor balancing, ground compensated balancing, or any of a dozen other similar names. There are some differences between these, but basically it’s a method of creating an output that will function similar to a balanced output without having to employ all of the electronics normally required in a fully (differentially) balanced system. The benefit is reduced cost and sometimes increased overall flexibility without losing much performance in many situations.

Here’s what it is and how it works: In a truly balanced output there are two conductors and a ground wire. The two conductors each have the output signal on them, but at opposite polarities. Refer to our WFTD, Balanced, for more background on this. Having this signal on both wires is beneficial for several reasons, but one of the biggest benefits is just in having the two wires, even if signal is only on one of them. A balanced input is able to look at the hot and cold wire’s signal and compare the difference between them. Any differences get amplified; any signal that’s the same between the two gets cancelled. That means noise and hum picked up in the wires along the way will get cancelled, and any signal – even if it’s only on one wire – will get amplified. It’s a great system, but comes at a cost in terms of the components required. 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 is 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 cancelled 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 they say. 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 pickup 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 cancelled 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 are 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 are connecting its 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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