Hosa CMS-105 3.5 mm TRS to 1/4 Inch TRS Stereo Interconnect Cable, 5 Feet

Although the other posters are correct, an audio interface would be ideal, you can hook up the RD9 to the Behringer monitors. Just have the volume turned all the way down on the drum machine and the monitors when turning them on and off or unplugging the any cable. Once they’re both powered on, turn the volume of the RD9 up about half way to 3/4 then gradually increase the gain (volume) of the monitors until you’re ok with level of sound. When powering off, turn down the RD9 and the monitors (any order is fine) then power things off. Use this cable to go from the headphone output of the RD9 to line one input of the MS20s. https://a.co/d/0hgnFfAy Save $ for an interface next. Hope this helps.