How do I use headphones to sample?

Hey guys!
I just got a first mix i/o and I am having trouble getting the headphones to sample the music I am playing. I bought a left/right cable to hook up to a speaker and that works.
Now I can hear the music through the headphones and the speaker but not sample anything.
If I click preview on the turntable it just plays the track on the main speaker.
If I click the headphones picture it just plays the music a little bit louder on the speaker.
Any help would be nice
THANKS!
I just got a first mix i/o and I am having trouble getting the headphones to sample the music I am playing. I bought a left/right cable to hook up to a speaker and that works.
Now I can hear the music through the headphones and the speaker but not sample anything.
If I click preview on the turntable it just plays the track on the main speaker.
If I click the headphones picture it just plays the music a little bit louder on the speaker.
Any help would be nice
THANKS!