Any audio engineer worth his or her salt tests a mix for mono compatibility 
by summing left and right.  The only case I've ever encountered where 
summing caused nasty artefacts involved old mono-reprocessed-for-stereo 
recordings that had been fed through the MPEG-2 "joint stereo" codec.  Short 
answer: two resistors or a simple op amp summing amplifier should suffice.

RR


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