On headphones three pins are used for stereo and two pins for mono, yet on microphones three pins are often used for mono; why? Also why is a oddly-shaped plug (XLR) used rather than a jack?
1An XLR connector is balanced, with a separate audio circuit (send and return have one connector each) and earthing for the shield. Headphones are unbalanced, the audio return for both stereo signals is shared with the ground.– jonrsharpeAug 28, 2015 at 12:36
Just for background, I wish to be able to link to a GREAT answer on this question from another question when it looks like the person has no concept of how electrical signals work. I am not able to write a great answer myself as I understand the engineering too well, so can’t put myself into the mindset of someone that does not.– Ian RingroseAug 28, 2015 at 12:40
@jonrsharpe Your comment looks like a good answer to me.– Todd WilcoxAug 28, 2015 at 12:41
1I think e.g. music.stackexchange.com/q/24065/12202 and music.stackexchange.com/q/24275/12202 address it better than I could - there are quite a few other posts on [un]balanced audio and connectors– jonrsharpeAug 28, 2015 at 12:41
3I tend to ask myself, why to so many instruments use those @#^$%^ unbalanced jacks, instead of decent XLR connections which would actually work reliably...– leftaroundaboutAug 31, 2015 at 0:33
XLR is designed to carry a "balanced" signal. Pin 1 is ground, Pin 2 is "in phase" or "+" and Pin 3 is "out of phase" or "-". See for example http://www.clarkwire.com/pinoutxlrbalanced.htm
The reason is the following: The microphone signals are very low so they are vulnerable to noise that gets injected into the cable especially if the run is long. Noise gets typically injected into both signals the same way. The receiver than forms the difference between the + and the - signal. That cancels the noise. Since the "-" signal is out of phase, the difference is actually twice the desired signal.
The"balanced" approach also helps with avoiding ground loops in larger systems. The actual signal is always the difference between "+" and "-" and therefore no ground is needed.
The choice of male vs female is guide by "no exposed pins should go into the amplifier". Pins can be touched which can create a lot of noise, pops or even damage a loudspeaker. Hence inputs are female and outputs are male.
And out of phase meaning exactly phase-reversed by 180 degree... ;-) Any noise that affects both signals is in phase. By turning the phase 180 degrees at the output end once more - back to 0/360 degrees, the dirty signal is now phase-inverted and gets totally eliminated and the mic-input gets doubled. If you listened to the signal without the second phase-inversion you would hear no original signal ( it's inverted now) only the dirty signal - doubled of course, because there are two wires...– mramoschAug 28, 2015 at 23:21
Signal/ground/shield are the three leads to a mono microphone. Or +/-/sheild. On other occasion there will be phantom power to the microphone to turn it on or to provide voltage to active electronics. Etc... Etc... The fact is that your microphone being mono is not the reason why it has three leads it can operate with two. There are three leads because the cable is shielded or insulated against interference. The 1/4 inch phono plug compatable with this is "tip-ring-sleeve" however more popular and standard for microphones is "xlr" the industry standard.