The Tascam DR-680 can be used without a mixer, though I wonder if people in the field will be happy about the fact that there aren’t separate level controls; it looks like you have to select the track and then use the single control to adjust level. Saves money I guess, but will mean spending more for a mixer for some applications..
Oh, and if that doesn't excite you, maybe you’d like a ribbon mic? There's a comparison of several ribbon mics in a separate article. These ribbon mics aren't cheap (the cheapest is about $700) but they certainly do a nice job recording a cello; there's quite a noticeable difference in the sound to a regular condenser mic.
- B & H article: Multi-Track Field Recording Gets Affordable with the Tascam DR-680
- B & H article: Testing Out Ribbon Microphones for Home Studio Recording