I have a drum machine that gives out separate outputs for each one of its 11 sounds. I've found that I don't really need 11 outputs and some of these can be grouped, e.g one signal that has the low/mid/high toms. So I was looking for a device that would take in multiple mono TS/TRS inputs and give one output that's the summation of these inputs. I had assumed that this will be a simple cheap device - a voltage adder basically - however, such a thing doesn't seem to exist so I may be missing some understanding here.
So my questions is if this can be done, then what are the options? Using an audio interface/mixer and outputting the mix signal is an option of course, but the whole point of this is to mix the signal before it reaches the interface.
If this is not as simple as it sounds which explains why there is no such device around, then could you explain why?