Ok there were people who asked me how XLR and balanced signal transfer works and so this page has been dedicated to explaining this in as much detail as I see fit. I mean honestly I could pull all this up on a nice scope and have it take screen captures of things working in real time but I decided to go out on the net and find good enough pictures for examples to show you guys.
First off I will show you what XLR looks like. Keep in mind that this part pertains only to XLR which is only one type of balanced signal transfer. But the electronics I talk about later is global to all balanced signal transfer methods.

Ok [in the following pictures, on top] you see the male XLR connection from the end. On the [bottom] there is a somewhat lesser picture that shows both the male and female connectors.




Ok now I go into more about why later but XLR is not something that you just wire into an audio device and it works. There has to be a filter on the end that is receiving the signal. Here is a picture of a socket that would be put into an audio device and you can see the filter (which is an OpAmp wired to be a differential amplifier). There will be more on differential amplifiers later.




Ok now that you know that I'm talking about balanced signal transfer you are probably wondering what is unbalanced signal transfer and why do we have both.
Well that answer is simple. Unbalanced signal transfer is the RCA cable that people usually run in car audio applications. Often times RCA cables run in home theater applications are unbalanced as well unless they are used for digital signal transmission but that is a whole different ball of wax that I'm not going to cover here.

If you're wondering why we have two standards that are not compatible with each other its because at one time all consumer gear was unbalanced where as all professional gear was balanced. As a matter of fact balanced signal transfer is often referred to as professional inputs so as to distinguish them from consumer inputs.

Balanced signal transfer was previously never implemented in consumer applications because it was considered unnecessary and too expensive and for most users this is true. Also professional audio often has much longer cable runs than consumer audio and the longer the cable run the more noise prone it is.

Ok now you know that balanced is better but you still don't know how it works. First off I need to explain how unbalanced works.

Unbalanced signal transfer (like RCA for example) is run over two wires. RCA's wire is arranged in a coaxial fashion. There is one wire in the center and then the second wire is a shield that is wrapped around that center wire. The center wire in this type of cable carries the signal and the outer wire is grounded to give a reference point as well as to try to prevent noise.

The problem with unbalanced signal transfer is that it is just a simple wave and if it is run next to a fluctuating magnetic field such as an AC power wire for example this signal can be magnetically induced into the line and will be audibly heard as a hum or buzz in the speakers. Car transmissions because of their large ammounts of moving metal can cause these fluctuating magnetic fields.

Ok now that you have a better understanding of unbalanced and RCA we can move into balanced. In professional audio and for consumers with discerning tastes this hum or buzz on the speakers is far from acceptable. So a signal transfer that was immune to this type of noise needed to be developed. Thus the balanced interconnect was contrived and the world became a better place.

Sorry I feel like I'm writing a term paper for high school or something and I'm 10 credits from two bachelors degrees (one in computer hardware engineering and one in computer softward engineering) so you'll have to deal with the dry humor its the only thing that keeps me typing this!

Ok now balanced cables have three wires in them instead of two. In balanced two wires carry the signals and the third is the ground reference. Now its not exactly identical for both of the two wires one has been inverted and is 180 degrees out of phase with the other.

Ok now an illustration of what I've said so far so I can recap:




Sound Domain made this pic a bit small and hard to read by the top part is how a unbalanced signal would look if you sent a sine wave down it on an oscope. The center lead would have the sinewave on it and the ground lead would remain at a 0v ground potential.

The bottom portion of this graphic shows what would be on a balanced line if it were carrying the exact same signal. Don't worry about the amplitudes being different they are just drawn to different scales. Anyway you'll notice that the top wire is carrying the exact same signal that the unbalanced wire was but the bottom wire has the exact opposite. You notice this by looking at the peaks and vallies. When one is at max positive voltage the other is at max negative voltage. Strange eh? Well it makes PERFECT sense and it will to you too soon. that is hopefully it will....

Ok now hopefully you're wondering why magnetic noise doesn't get into a balanced wire when it does get into an unbalanced one. I mean honestly they are both wires and they both act like antennas right. Well you are correct in wondering this and the answer is that noise DOES get into balanced wires just as bad as it gets into balanced cables. But when you feed this signal into a unit with a balanced input all of the noise is rejected and cancelled out. Now I know you're wondering how we do that. This is where it gets cool!

Ok you noticed in that pic of the XLR jack that the jack had some electronics on it. Remember that differential amplifier that I mentioned before. Well finally you find out what it does. A differential amplifier amplifies the difference between two signals. Now being that the two signals are as different as can be (cause one is flipped and is therefore EXACTLY different) you get a doubling effect cause you are amplifing only the stuff that is differnt.

Ok now still one would think that this amplifier would amplify the noise as well but here is where you would be wrong. The two wires that carry the signal are twisted together and so they therefore go through exactly the same noisy environment and they therfore both get exactly the same noise induced onto both wires. Well if both of those wires have the exact same noise on them then that is just going to simply get cancelled out cause if its not different it doesn't get amplified by a differential amplifier so it never even makes it through the connector.

Now in really high end professional applications transformers are used to reject the noise and only pass the differential signal but in high end consumer and most professional equipment differential amplifiers are used instead of transformers but as far as concepts go its exactly the same. One just has a better Common Mode Rejection Ratio (CMRR) which is not something we need to talk about here.

Ok now lets look at another illustration:




Ok its tiny again so here is the explanation of what you are seeing. The top graph is the signal you want to send. Just a standard sine wave. Now the second graph is a noise that is induced onto your wire. This noise is summed with your transmitted signal in the wire electrically.
Now the next picture with the triangle in it is how unbalanced signal transfer works. That triangle is called a preamp and it simply amplifies the signal coming into it and you'll notice that you get a sinewave superimposed on a second sinewave. Thus noise and distortion and that annoying hum that we mentioned before.

The bottom picture shows balanced signal tranfer and you see that the preamp now has two inputs (because its a differential amplifier) and its only going to amplify the differences which is the signal that we put into it and then on the output you see a nice clean sinewave that has been amplified from its orignal. Now we just take this slighly amplified signal and we feed it to the input stage of our power amplifier and we bring it up to a voltage acceptable for driving speakers and we're are finally one with the world sitting blissfully listening to our clean audio signal.

Now for the morbidly curious there is this beast out in the electronics world called a Operational Amplifier or OpAmp and here is a schematic of one being used as a differential amplifier. Now there is a lot more to it than just wiring it but you can look that up if you're even more curious. Or sign the guestbook and ask.