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You have to have the Audio I/O, yes. The reason there is because your incoming audio signals need to be boosted up from line-level to synth-level, and then back down again at the output. It's a decent interface for this sort of use, but you need to keep in mind that its inputs don't have envelope followers, so if you wanted to patch up something dynamics-dependant, you either have to add a couple of these or jettison the Audio I/O and go with separate inputs that have the envelope follower (the Doepfer A-119 is sort of the bog standard here) and then a stereo output with transformer isolation. That last bit is useful to keep noise and crud from passing from the synth to your mixer and vice-versa.