The way I’ve seen (or rather, heard) it:

• 3-prong plug w two 3.5mm & one 2.5mm(?), it is Left, Right and power/extra (the 2.5mm).
• 2-prong plug:
  • Two 3.5mm, then they are L and R.
  • One 3.5mm & one 2.5mm(?) prong, then it’s stereo & power/extra (the 2.5mm)
• 1 prong, 3.5mm, Stereo.

There could be other variants, but those are the only ones I remember coming across.


I know you can somewhat replicate stereo where the L & R channels are on separate 3.5mm prongs (both for a 2-prong separate channel plug & a 3-prong plug) by partially inserting the single 3.5mm prong from a personal headphones’ cord.

I believe it works because both the L & R connection points on the cord’s prong touch the (single channel, either L or R) connection point within the (L or R) socket, while the ground connections are still connected.

But, you’re really only getting either L channel sound to both ears, or R channel to both ears.




[EDIT]Oh, almost forgot these old-style pneumatic headphones… interesting ‘adapter’!(Essentially seems to be a sound funnel.)