The TD-07 has a combined stereo phones/output jack.
In addition to connecting headphones, the TD-07 can be connected to an amplifier or mixer if desired.
For the best sound experience, it's recommended to connect to two channels on an external mixer (stereo), or connect to both the L and R inputs on a stereo amplifier.
A stereo 1/8" [3.5mm] to dual 1/4" cable can be used for this type of connection. One like this for example: