The A/D converter is going to convert what comes out of the preamp into something the computer can understand.
The only time you WOULDN'T need an A/D converter would be if you were running analog, like if you were going from the preamp to a tape machine.
If it doesn't have a converter built into the preamp, you can just feed the output of the preamp into the line input of an audio interface. The AI has converters in it, and that will get it to your computer.