Originally Posted by
Nakatomi
The technical explanation of why you shouldn't just combine the outputs of audio devices is this :
The output impedance of a mixer or other playout device is generally quite low, say around 150 ohms. This is designed to plug into an input which might have an input impedance (in most pro kit) of around 600 ohms or 1000 ohms or more.
Plugging one output into another, you're potentially halving its drive capability (voltage level and headroom before distortion) because 150 ohms in parallel with 150 ohms is 75 ohms. This would mean the output stage of each device would have to work twice as hard as usual. They may cope, or they may not. They may be damaged but best case performance will likely suffer.
Passive (unpowered) combiners will use either transformers (slightly lossy but expensive ) or resistors in line to sum the signals (more lossy but much cheaper & less bulky than transformers).
Some kit, it's unknown which, you can likely get away with joining outputs of til the cows come home. That's usually real pro stuff like what they use in radio & telly though (where it's still analogue signals anyway). I remember reading sonifex cart machines' outputs were designed to be connected across others so a cart machine stack would only need one stereo pair of mixer inputs.