I've got a pair of Sennheiser HD555 headphones and I'm trying to wire on a 3.5mm connector to the end, and there are 4 wires in the cable. Black, White, Red, Blue. Which ones to I connect to the tip, ring, and sleeve of the connector? I can't really find much around the internet, but I'm probably not looking in the right spot. All I can find is explanations of what T.R.S. are/is.
Take your multimeter, put it on the resistance setting, and figure out what is connected to what. Put one lead on one wire, and one on the other wire. You are looking for something that will measure somewhere between 8 and 120 ohms or so. Basically, anything that is not 0 ohms, and not completely open.
Chances are your white and black wires will be the speaker on one side, and the red and blue will be the speaker on the other. From there, it is likely that the black and blue wires are ground, and the red and white are hot.
Connect the tip to the L+, the ring to the R+, and both grounds to the sleeve.
The only thing wrong is that if this is wrong, you may either invert the polarity, or the L/R. Neither will break anything, but if one ear is of one polarity, and the other is of the other, it may sound interesting. That's why it is best to reference the original plug, if you still have it.