Convert AAC 5.1 audio to AC3 Dolby Digital 5.1

Does anyone know how to convert 5.1 AAC to AC3?
If so, what software do I need and encoding/conversion settings.

I've tried numerous convertors like imTOO and even Maven3D Pro, but I cannot manage a successful conversion. Either end up with conversion error or file with no sound.

Thanks in advance for any help.

 

Somehow I managed to do it, see here for the tutorial:

http://forums.afterdawn.com/thread_view.cfm/312854