Passive 2-ch mixer or active?

[Rodgre]

I've been asked to make a specific stereo A/B switching box for a friend to use with a keyboard and Leslie/Keyboard amp setup. This would be straightforward if not for one thing. The outputs to the straight keyboard amp are just straight through: right channel to right, left channel to left. The Leslie, however, needs a summed mono feed. I need to sum the right and left at that output.

My question is can I just use two resistors with the right and left signal at one end, and shorting the resistors together at the other, then to the output jack, or should I build a simple amplifier circuit to blend them together at unity gain? A single op-amp? A transistor?

How would you go about this?

Thank you
Roger

[iainpunk]

i wouldn't send stereo mixed to mono to a leslie, i'd send only left or right.
but if i had to mix, i'd go with a dual buffer, which has two out's (L and R) and a simple summing amplifier for de-stereo'd signal.
something like this:


cheers