Center imaging problems

I am having problems getting an exact center image. It always seems that the vocals are slightly to the right instead of exact center in most of the music I play, which is very annoying. Does anyone have any good test tracks with the vocals at EXACT center? Also what is the best way to compensate for not getting an exact center image? Balance control? Move left speaker forward just a little?
Play something mono. Move the left speaker more toward the center or move your listening chair to the left. Also swap inputs to the amp back and forth to see if the preamp has a weak channel. If not swap left and right speaker cables to see if the amp has a weaker channel. Balance controls usually add distortion to the signal. So avoid if possible.
Two theories come to mind. The first is that one channel is louder than the other. You can test one speaker at a time with an SPL meter, and if it turns out one is louder, then you will have to track down which is the offending component. This can be done by swapping out components one at a time, assuming you have redundant components lying around.

The second theory is room interactions. If your setup is asymmetrical, or if large objects in the room are near one of the speakers, it can cause center imaging problems. I have experienced this in my system. It can usually be solved by addressing the asymmetries. Good luck.
Cable out of phase..( one side )???
Often it a room/set up issue. Sometimes it an imbalance in the electronis somewhere, or as likely as that, a hearing imbalance. If when you swap cables from one speaker to the other the voice doesn't shift to the other side you may just have a hearing problem in your left ear or an unbalanced set up for example which can be caused by an open area on the left side and a wall of the right side. Simply shifting speakers or your listening chair as suggested by Rwwear can be a good solution.
Not all recordings will center a vocalist.
True Shipo. That's why a mono recording makes it easier to tell if the image is out of balance.
First, you need to eliminate some variables, not counting the variable of your hearing, which might need to be checked.

As has been mentioned, play a known monaural signal and see if it is centered. It should be. If not:

If you know-that-you-know that you have the speakers at EXACTLY the same distance from your ear to the tweeter on each side (the tweeter is the primary source of directionality), then these steps should help:

The easist thing to do would be to do some cable swapping, channel-to-channel, until you've made sure that your source, your pre, your amp, etc. have all been swapped left for right (and of course, returned to their former position). Do this one component at a time and listen to the same recording each time. If th eproblem has changed channels. then that component might have an uneven output (or sloppy volume control).

If you still have a problem, and again you absolutely know that you're in the exact center of the speakers, and that they are the same distance from each ear, then you'll need to swap the speakers, being doubly careful to measure distances. An inexpensive laser measuring tool is really useful for this.

By now you've swapped everything, and something should should have shown up.

If it still favors one side, I'd check to see if the slightly louder side has less reflective surfaces on the side wall.
"If it still favors one side, I'd check to see if the slightly louder side has less reflective surfaces on the side wall."

Or if that side wall is nearer to its speaker.
This should be tested in mono. Can you select mono on your preamp? If not try a mono recording.

Try extending a length of string from the center of your listening seat to the front left speaker. Hold the string firmly and extend it to the right speaker to see if the speakers are the same distance from the listening position. Everyone that uses this method swears by it.

Measuring from the speaker to the front wall for distance is usually a waste of time since most rooms are not square.
There are many things that can cause a slight imbalance.

Are you using tube equipment? Tubes that are not properly matched can cause this problem.

When was the last time you cleaned your connections? Interconnect and speaker connections should be cleaned on a regular basis. Many audiophiles use Caig products. Caig cleaners leave a residue and I prefer a good cleaner such as Radio Shack cleaners that do not leave a film.

Is your system setup symetrical?

Are your speakers too close to side walls? Side wall reflections can cause all kinds of problems.

Do you play your speakers extremely loud for extended periods of time? This can cause perminent damage to some speakers.
This problem nearly drove me crazy. I did everything under the sun, only to find it was the room.