Volume problems

Hi there, I am completely new to this forum so I’m not too sure how I should post information so I do apologise if I don’t follow the normal procedures that you guys have.

I have a Tascam US-122mkii and have noticed that it only contains one knob for both the line out and phones outputs. I am a total beginner with home recording and I was wondering whether anyone could shed some light on how I may be able to adjust this as I only want playback from the phones when I am recording so my microphone won’t pic up the sound from the monitors. I would really appreciate some help with this.

Thanks

Richard