[sdiy] Stereo to mono summing..

[RevTor at aol.com]

All, 

   a friend of mine is working on a earphone project where he has to take stereo headphone outputs and route the two signals (plus ground) to one 16ohm headphone speaker..  So we're wondering the simplest (cheapest, smallest) way to get both signals at the one driver with no loss of volume and with no distress to the amplifier.

This is surely an elementary query for the Synthgods, is it not?

thanks for any ideas!
~Steve