OK, many of us already know why balanced audio cabling is important, but for those that don’t know what we’re talking about, here’s a quick catch up.
We’re all surrounded by electromagnetic radiation (ER), hitting our bodies, going through us, bouncing all over the place. It’s everywhere. Of course, some of that ER is useful, like radio stations or cell phone signals, but a lot of it is stuff that’s not as useful, such as noise from lighting dimmers and electric motors and the like.
Regardless of which kind of ER we’re talking about, it’s all pretty much invisible to us until it encounters an antenna. And unfortunately, any metal wire naturally acts as an antenna, including the wires in all our audio cables.
Just like things that we want to be antennas, the wires in audio cables are going to be exposed to the same ER that is all around us and they’re going to produce electrical currents in those wires. Since none of those currents are things we want in our audio signal, we’re going to call those electrical currents “noise.”
So, if we need to run a signal wire from “Audio Thingy A” to “Audio Thingy B,” and we’ve established that that wire is going to act as an antenna and pick up noise along the way, how do we keep this noise from getting into our wire?
On The Defensive
Strategy one is defensive. We can try to keep our wire as far away as possible from things that radiate ER, but obviously that has certain practical limitations. Keeping things clear of radio station towers isn’t too difficult. Additionally, we can try to keep our audio wire a reasonable distance away from things that emit a lot of noise like power cables and lighting cables. But given that ER exists effectively everywhere, there’s only so much we can gain by keeping our distance.
We can shield wire using a flexible metal jacket that surrounds it, and that certainly helps. In fact, nearly all audio cables utilize some form of shield, whether it’s balanced or not. Yet while some cable shielding is quite effective, none of them are foolproof. Foil shields can crack. Braided shields can develop gaps. And these issues lead to noise leaking into the wire.
Finally, we can also try to create a signal that’s powerful enough to overcome the noise the wire picks up along the way. This is the reason why line level signals are inherently more noise resistant than mic level signals. This is also one of the reasons why we don’t put shielding on loudspeaker cables, because the signals they’re carrying are powerful enough that any noise picked up along the way is negligible.
All these strategies are valid but often one or more of them are impractical, and none of them eliminate the problem completely. So, what do we do?
On The Offensive
Well, what if instead of having one wire to carry our signal, we use two wires. And then what if we have a magic circuit that they both went into that would amplify only the things that are different from each other?
As long as the noise is present on both wires, this magic circuit would just ignore the noise. Turns out that there’s such a magic circuit and it’s called a differential amplifier because (wait for it…) it amplifies differences and ignores things that are the same.
OK, this is great, but another issue quickly crops up. How do we make sure that the noise on each wire is identical so that the differential amplifier will cancel it out?
First we make sure the two wires are precisely the same, including having the same impedance. Impedance is a fancy electronics word that has a ton of implications in the audio world, but for our purposes here it can be thought of as the “sensitivity” of the wires to the noise. Wires that are different length or thickness will have different impedances.
Further, a wire’s impedance is affected by what circuitry it’s attached to. To be clear, for the purposes of noise cancellation, the actual impedance of the two wires is not super critical, but it is critical that it be the same. All we need to do to accomplish this is to make sure our wires are the same size, same length, and attached to the same circuitry on each end.
There’s one last step needed to make sure that the noise on both these wires is the same. If we run the wires next to each other, and we have noise coming from a specific direction, one wire could end up with more noise on it simply because it’s physically closer to the source of the noise.
So, a really quick fix is just to twist the wires together. Now neither of them is closer to anything than the other. This little trick is called making a “twisted pair” and is found in all sorts of cabling including things like DMX and Ethernet cables.
Now we have two wires that are going from “Audio Thingy A” to “Audio Thingy B” and arriving with the noise canceled out. All we’ve got do now is put an audio signal on one of those wires and poof – Balanced Audio.
But What About… ?
Wait. What? Don’t we have to invert one side or do a polarity trick or something?
No. No, we don’t. The kind of balanced audio circuit that I’ve been referring to is commonly known as quasi-balanced or impedance balanced which implies that it’s not a “real” balanced circuit, but it is in fact a fully balanced circuit.
The more traditional form of balanced circuit that most of us were taught is what is known as a “differential circuit” and in this circuit it is true that one of our wires carries our signal and the other wire carries an inverted polarity copy of the same signal. In this type of connection, the differential amplifier still sees the same waveform, so the noise cancellation works in an identical manner. It’s important to note though that it’s still necessary for both of our wires to have the same impedance in order for the noise cancellation to work correctly.
There are some advantages in certain circumstances to using a differential balanced connection rather than an impedance balanced one. But for the majority of audio applications, they can essentially be used interchangeably. In fact, given that much of the gear we work with on a daily basis has different types of balanced circuitry, most of us have likely been using them interchangeably for a long time whether we knew it or not.
It’s important to note at this point that neither form of balanced circuit requires a third wire of any kind. While most balanced audio cables have a shield that is typically connected to ground, that wire is not necessary for the noise cancellation to work correctly.
In fact, there are times when having that shield connected between pieces of equipment can cause noise because of a phenomenon known as a ground loop. But that’s an article for another time.
To close, working with balanced audio was something that I thought I had a firm grasp on many years ago. After all, I’ve been an audio professional for decades. It wasn’t until recently that I discovered that there was much more going on with this topic than I had originally understood. And for me, learning something I thought I already knew is a very good thing. I hope it’s the same for you.