ага, сложно вот в инструкции посмотреть
Monitor In - If you want to hear the signal from a line level source mixed with all the other audio coming from your computer, but do not want this audio recorded, then plug the source in here (this Monitor in signal is not fed to the Record Sends). Note that this is a stereo jack, so you should use a stereo 1/4-inch TRS audio cable for this connection.