Balancing act

Back to the music recording advice and an explanation of why it is sometimes necessary to have a soldering iron at a gig.

There are two ways to send an analogue signal down a cable: the standard (and common on consumer equipment) unbalanced audio and the more "professional" balanced audio. An unbalanced signal is very easy to understand. One line is the ground and the other is the so called 'hot' signal. A balanced cable requires three lines - one for ground, one for the 'hot' or '+' signal and one for an inverted, 'cold' or '-' signal. This makes more sense in the diagram below...

So, what is the point of a balanced cable? Basically for very long lengths of cable (over 20m is probably noticeable) it helps to reduce noise. The "balancing" refers to the fact that the impedances at each end of the cable are the same i.e. balanced. Any noise that is picked up along the cable should produce equal signals on both the hot and cold lines. Then a differential amplifier in your mixing desk or soundcard is used to compare the two lines and since the noise appears the same on both, it is not amplified. Another advantage of this system is that there is very little electromagnetic field around the cable as the two signals have opposite polarities and so crosstalk between different cables is minimised.

The problems come when you have a mixture of balanced and unbalanced inputs and outputs in your setup. Remember that a line needs balanced connectors at both ends for the noise cancellation discussed above so ideally you should use transformer adaptors to keep any long cables balanced. However most people find it necessary at some point to connect equipment of different types without worrying about having it balanced... and it is a real pain! Firstly there are three kinds of balanced output:

1. Impedance balanced - only the hot signal is driven and the cold signal is at ground (this works despite what I said above since noise is still removed by the differential amplifier but crosstalk is not reduced).

2. Transformer balanced - an output transformer is used to provide the opposite polarity signals with the centre 'tapped' to provide the ground.

3. Active balanced - hot and cold lines are both actively driven but with opposite polarity signals (this is what much modern equipment now consists of).

So now that that is clear (!) I have compiled a nice list of how everything should be connected (I've used both XLR type plugs and TRS jacks to demonstrate how you might need to use both, however they are of course interchangeable). Also note that often 2- or 3-conductor cable can be used. The advantage of using three is that the outer screen can be used as a shield, connected only at one end. Try to be consistent with which end you connect the shield so that they are all grounded to one point and there are no 'earth loops' which will pick up noise.


And that is why a soldering iron and multimeter can come in handy... two cables may look the same from outside but be wired up completely differently.