Here's how to assign the built-in Rhythm Guide in the RC-505 to the headphones output only.
1. Choose a memory location to edit.
2. Press the RHYTHM EDIT button.
3. Press the parameter right button 3 times to choose "Rhythm: Line Out."
4. Turn the MEMORY / VALUE knob to select "OFF."
5. Press WRITE twice to save the settings to the memory location.
If desired, repeat the steps for any other memory locations.