How to do it:
I designed this circuit myself first. The most desirable configuration would have been to have 4 output jacks - OUT1, OUT2, OUT3, and MIX OUT. Unfortunately, the overriding design criteria was to make it fit on a 1U wide panel, so 1 jack had to go. As a result, you can't use input 3 as a reversible attenuator and still have mixer functionality. However, you CAN use IN1 and OUT1 to get a reversible attenuator, and IN2, IN3 and OUT3/MIX to get a 2 input mixer at the same time, with none of the IN1 signal being present at OUT3/MIX. Just remember that inserting a jack into either OUT1 or OUT2 removes the corresponding input from the MIX output. I also originally planned to normal the input jacks to +5V, so that you could have 3 separate dc control voltage outputs when none of the inputs was used. However, that makes it hard to plug anything in without adding a dc bias, since it's hard to set the pots to exactly zero. Therefore, I am not going to build it that way. Anyway, here's the original schematic.
After I got this circuit the way I wanted it, I was browsing Tony Allgood's site and found his MultiMix. Great minds must think alike<g>, because here is a circuit providing identical functionality, with the advantage of a ready-made pcb. I chose to use a dual opamp front end instead of single, which may result in less offset voltage, but the MultiMix design offers some extras such as LED indicator and inverted outputs. You could take advantage of these (but you would have to use a wider panel!)
So when I designed the panel, I put the Oakley name on it, because I figured that the vast majority of people building this would choose to use the ready made Oakley pcb anyway, instead of building my circuit on an MOTM diy board.Notes:
To build it as configured by my panel, you must take the inputs to the mixer section from the normalling lugs of OUT1 and OUT2, whether you use my design or the Oakley pcb. The Oakley provides pads for you to do this.
This is optimized as a CV mixer, since I fully expect a deluxe 2U MOTM mixer to be released which will deal with audio in a more deluxe fashion. Therefore, I recommend using LT1013 opamps for the lowest possible dc drift and offset. If you don't plan to use it to create stable control voltages for your VCOs, save your money and use what Tony recommends instead.
One other possible mod: as configured, both circuits give a gain of from -1 to +1, so signal gain greater than unity is not possible. You might consider doubling the opamp feedback resistor value on one of the inputs, to have at least one input with a gain of -2 to +2. Of course, this will clip any attempt to amplify a signal beyond the +15/-15V rails (you might like that anyway!)