Headphone Plug

[quantum]

As much as I know, headphone plugs have three output terminals. The out most terminal is the neagitive, and the inner most is the positive, I believe. What is the middle terminal?

[audioguru]

Hi Quantum,
Headphone and their plugs don't have positive and negative, but instead have left channel, right channel, and a 3rd terminal where the common wire of each channel is connected togerther which is usually grounded.
On the outside of the plug, the "tip" is one channel, the "ring" is the other channel, and the "sleeve" (longest part) is the common.

[quantum]

I don't understand if the third channel is consisting of the right and left channel, wouldn't that short them. A picture would better explain this. ;D

[audioguru]

Hi Quantum,
You want a pic, OK, here's a pic.

Although headphones don't have Positive and Negative, because they operate on AC, I have shown Pos and Neg to indicate phase connection. The standard is that if Pos is applied to the Pos terminal and Neg to the Neg terminal, the cone will move outward. Try it with a speaker and a "D" cell battery.
Like speakers, headphones must have their channels connected with the same phase, so that with low-frequency signals they move together in the same direction.
If headphones have their channels out-of-phase (one channel wired backwards) then the sound is weird and it seems like your ears are being pulled apart.
If speakers are wired with one backwards, then the sound will lack bass, because the 2 channels will cancel. Try it with 2 speakers, with one wired backwards. If you put the speakers together with a small space and with their cones facing each other, then the bass will totally disappear!

[quantum]

Thanks Audioguru, the term of the "ring" made it seem that you were talking about the insulating ring that seperates the channel terminals, instead of the metal ring. Thanks for the picture!