Tech note: Balanced lines-1

Why would one use balanced interconnects, and how can we make them work well?

Balanced lines came about at a time where very long signal lines were coming in use for telephone and later for large audio performance venues. If you use a single screened line for your signal, and the line is long, the ground current through the screen causes a voltage between the ground points of the cable ends. Since the signal send out (and received) is the difference between the voltage on the signal wire and the ground wire, the unwanted signal (noise, hum) is effectively added to the wanted (music) signal. We don’t want that.

The trick is to use TWO signal lines in parallel. You send the signal over the two lines in such a way that the signal you want to transmit is the difference between the signals on these two wires, and then at the receiving end you have an amp that reacts to the difference between the two lines, so your signal at the far end is the difference between the signals you send out. You use a third connection, the cable screen, to connect the grounds of the two pieces of equipment. But, and here’s the trick, the voltage on the ground wire is no longer part of the signal! That ground wire is more like a safety ground, and also to screen the signal wires from induced stuff. The screen is not perfect, so there will be some noise etc on the signal lines, but if we make sure that those are very close together, the induced junk on both will be equal. And since we know that the receiver amp also takes the difference between the noise and hum on the two lines, that noise effectively cancels. Neat, isn’t it?

A few observations.
For the cancellation to work, the induced noise & hum must be exactly the same in both wires. That means they have to be very close together, which can be done by twisting them. Secondly, and this is crucial, they must have the same impedance to ground so that the induced junk is attenuated exactly the same in each. An example: to attenuate the noise & hum at the receiving end by a factor of 1000 (which is 60dB) you must make sure that the impedances of the two lines are also balanced to 1/1000. This is a tall order especially at higher frequencies!

It is also clear from the above that the two signals in the wires need not be exactly equal. In fact, it doesn’t matter even if one wire carries no signal and the other does: still, at the receiving end you get the difference between the two.

There is however a good reason to make the two signals not only opposite in phase but also approximately equal: If your sending amp has a max amplitude of say 10VRMS (often limited by the available supply), and you send two opposite 10VRMS signals, the difference between the lines, which is what the receiver sees, amounts to 20VRMS. So this will let you have larger signals that can improve your signal to noise ratio by a few dB. BUT, for cancelling hum and noise, the signal levels are immaterial!

Sometimes the term ‘pseudo-balanced’ is used for connections that are impedance balanced but not signal balanced. That’s OK; for cancelling hum and noise, that’s all you need…..

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