To connect a turntable to the SP-404MK2, the audio signal must be converted to line level.
Many modern turntables have a built-in preamp that allows you to switch between phono and line levels. If your turntable does not have a built-in preamp, use an external phono preamp to boost the signal before connecting to the LINE IN jack.
You will also need an RCA to 1/4" adapter. Connect the RCA side to the turntable or phono preamp, and the 1/4" TS cables to the LINE IN jacks on the rear panel.
Once connected, press the [EXT SOURCE] button on the SP-404MK2 to enable external input.