I need to do location recording with a recorder, and also overdub.

The reason is, i am a songwriter and I need to send the unit to musicians in different
cities so they can overdub their parts. So they need to listen to backing tracks while they sing in the mike I send.

I don't think the Sound Devices can do this, any ideas?

thanks!

The new H4n from Zoom is supposed to function as a 4-track, so I'd think it might do what you're looking for.

Alternately, a USB mic and any of the crazy variety of multitrack software out there would also work.

As far as I know, the Sound Device "7 Series" recorders can not play a track and simultaneously record another track.

The zoom h4 can be a usb interface and is a recorder.

Zoom H4 Handy Recorder | Sweetwater.com (http://www.sweetwater.com/store/detail/H4/)