First off, lets start with an unbalanced audio connection. Every time you hook up a guitar to an amp, an iPod to a computer or mixer, or anything else that just uses a 2 conductor cable, you're using an unbalanced connection. This is what that type of cable looks like:
A balanced connection requires three conductors. If you're hooking up a professional mic to a mixer (especially if it's going down a long cable), then you're likely using a balanced connection. Here's what that type of wire looks like:
OK, we've identified the physical difference between the two types of cable. But what's going on here electrically, and why is one better than the other.
Let's look at the unbalanced (also called single-ended) connection first, since it's a little easier to understand. With unbalanced connections, the shield is performing two functions. It's serving as the return path for the signal (the "negative" side of the audio if you will), and it's shielding the center conductor (the "positive" side) from external noise. It does a pretty good job, but if the interference is strong enough, it's going to start bouncing the ground around...and you're going to hear that in the audio.
OK, so why is a balanced audio connection so much better? It has to do with the fact that there's a second line of defense against noise that's built into the architecture. It's a concept called Common Mode Rejection. No, I'm not talking about when you got turned down for dates back in high school. This is far more useful and much less painful.
Before we talk about common mode rejection, we're going to have to have to touch on a mathematical concept here (sorry...I've put it off as long as I can). That concept is Vector Addition. OK...let that sit for a moment...the sting will go away.
The classic (and simplified) definition of a vector is a ray with magnitude and direction. Putting that concept into a real world example...a car heading north at 50 MPH could be considered a vector. You have both magnitude (50 MPH) and direction (north).
So far so good, but how does vector addition work? For that example, let's use a tug of war game. Let's say that there are 4 people on each side of the rope pulling. Pulling from the left side, it's 4 Marines. On the right side, it's 4 members of the Austin Botanical Society. When we say "go" there's force being applied from both directions, but guess which way the rope is going to go. Yeah, I'm betting the that vector force coming from the Marines side is going to be a little greater.
Now, let's even this up. We'll send the Botanical Society home and bring in 4 Army Rangers to stand in for them. And now let's assume it's a dead even match and both teams are pulling with the same force. Which way will the rope go? Right, it doesn't move because the 2 forces are cancelling each out. (Now, in reality I know that one team will probably prevail...but I have friends who are ex-Marine Corp and ex-Army...so I'll just let them talk trash to each other).
Back to the world of electrons. A balanced audio connection works by running the same signal down 2 wires, but they're running opposite of each other. If you've heard the term "out of phase"...this is it.
By the way, here's another tech term you can throw around and impress people. Whenever you have two signals like this that are compliments (opposite) of each other, it's called a Differential Pair. That's going to turn some heads at your next beer bust.
I know...you're thinking "if those signal cancel out, then how do we hear anything?". Well, you're right, but this is just to show you what happens to two signals that are out of phase with each other when they're added together. I'll show you how we recover the signal in a bit.
Now, we've got our two out of phase signals running down the cable. All of a sudden, noise decides to show up and join the party. Now, our otherwise pristine signal looks something like this:
I want you to notice something very important here, because it's key to understanding how we're going to get rid of the noise. While our audio signals are out of phase with each other, the noise is the same (or in phase) on the 2 lines.
So what do you suppose would happen if we could flip one of those audio signals over so that is was in phase with the other one. What would happen to the noise?
Now our two audio signals are in phase with each other, but the noise is now out of phase! When we add those two signals together...the noise is cancelled out. And just to throw another tech term out there for you, the specification that describes how well a piece of equipment does this job is called the Common Mode Rejection Ratio, or CMRR. That's one of those specs where the bigger the number the better.
The "phase flip" and signal addition was done with a transformer in the olden days. Now, it's typically done with a ....wait for it...differential input amplifier. Electrically, what's going on looks like this:
Given this significantly improved method of getting rid of noise on audio lines (along with a couple of other electrical characteristics), we're able to run much longer cables without degrading our signal. How much longer? If you've got a good pro mic and you're plugging it into a decent mixer, you can typically run up to 2000' feet of cable before you start messing the signal up. Yeah, you read that right...about 4/10ths of a mile! This is why you have to used balanced audio connections when you have a console out front to mix the band. In that application, it's not uncommon to have 120 feet of wire between the mic and mixer.
So, can unbalanced and balanced connections ever mix? The short answer is yes. You can unbalance a balanced connection through wiring:
You would use a cable like this if you wanted to plug a balanced microphone into a guitar amp. You loose the noise cancellation and some signal, but it works. When you do this, though, the rules for unbalanced wiring apply...especially the length of cable limits.
Going the other way from unbalanced to balanced can't be done through wiring alone. You're going to have to have some electronics or a transformer. If you're a bass player or keyboardist, you've like already used a device that performs this function. You just probably know it by it's common name...a Direct Injection (DI) box.
The transformer does the job of converting the signal from single-ended (unbalanced) to differential (balanced). It also electrically isolates whatever you're hooking up to prevent ground loops. And again, this can be done with electronics to...which would make this an active DI box (as opposed to a passive one shown above).
I want to leave you with one final thought here. If you happen to have a balanced cable that's had one of the signal conductors fail, you will continue to get a signal through it, because that that point you've got an unbalanced connection. It's going to be quite a bit lower in level, and likely have more noise on it (since you've lost the ability to cancel it out). So, if you're setting up your PA and happen to notice that a mic is quieter than you expected, it's probably a good idea to bust out the handy dandy cable tester that I'm sure you have now and check that cable.
This is has been a pretty heavy installment, and I've only covered a small part of theory behind all of this. If you have additional questions, you're always welcome to email me.
Next week, as long as were talking about these connection schemes we might as well tackle another "mystery subject" to a lot of people, and that's phantom power.
Until then, keep the meters out of the red.
Ken
Ken Carver has been a musician and performer since the early 70's, and involved with live music production since the mid 70's. He worked for 15 years as a broadcast engineer, building numerous studios and transmitter sites around Texas. He's also worked in Critical Care Communications for the medical industry, R&D for an automated lighting manufacturer, and owned Project Lighting & Sound in the 80's. He currently heads up an R&D Hardware Technician Team at National Instruments in Austin, and still performs on the weekends in the Central Texas area. You can reach Ken at itsjustlogistics@gmail.com
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