external speaker imbalance sound output on macbook pro retina

my macbook pro retina display sound output its not imbalance, on the left side when it conected to headphone or other external speaker sounds so weak, and the right side sound normal. my slider sound balance already on the middle, can you guys help me solve this problem? sorry for my bad english..

How about the internal speakers in your MacBook Pro? Are they balanced so that the sound is the same from the left and right speakers when the balance control is in the middle?

the sound from internal speaker is normal and balance.. when i using HDMI to monitor which have 3.5 mm audio port and plug into it , the sound output is normal and balance from my external speaker. Do ihave to replace only my audio port or replacing entire logic board? the problem is my rMBP it's not under warranty anymore. I hope its only software problem

I don't believe there's anything wrong with the sound hardware. Have you tried another set of headphones and speakers to make sure they're OK? It's possible the audio output jack is dirty inside and may not be making good contact with the left channel. If the internal speakers are OK and HDMI output is OK, it almost has to be something to do with the audio output jack.

maybe you are right,i used contact cleaner to clean up the audio port but seems no good thing happen.. its still no loud output sound on left channel, i don,t know how to make the left channel making good contact to input jack. maybe i should replace the oudio output port. i've seen on ifixit, look so hard to replace it on rMBP and so risky.. thanks for your fast response chscag.. i'm new in this forum.