[jakdedert]

Ludwig77 wrote:
I played a gig using my laptop to play backtrack MP3s.

The MP3s were recorded in stereo. I was running out my laptop's
headphone out jack via a cable that converts 1/8" to 1/4'. I ran that
to a direct box and then to the board.

It seemed that some instruments were missing. Would I be right to
theorize that I would have been losing one of the two speaker's in the
stereo field via this routing?

If so, how would I be able to ensure that both stereo tracks would
have been routed to the mono signal?

One of two things could have been happening: You used a mono plug and
indeed lost one side of the stereo signal...or you used a stereo plug
and combined the left/right signals out of phase. Either could account
for your missing instruments.

The proper way (one at least) would be to use two DIs, or a stereo DI.
Lots of stereo 'av' DIs will properly sum the l/r channels and then
output to one balanced out. Otherwise, two DI's--with the proper cables
to split l/r--would work (into two channels of the mixer). I've used
this one very successfully:
http://www.directproaudio.com/product.cfm?directid=61283 but it's
pretty pricey.

Otherwise, you could use a summing network ahead of your single DI to
combine the l/r channels in a fashion that won't upset the stereo output
of your lappie. Many (most) don't like l/r to be just connected
together without some resistance to isolate them. Distortion is the
usual result. A couple of 1k ohm--1/8 watt resistors is probably enough
to make your output happy. Connect one end of each in series with the
left and right outputs and tie the other two ends together. Where
they're tied together is your summed mono out. You can build that into
the housing of a 1/4" plug....

jak