centering the human voice

If the room physically or acoustically is not symetric, you might end up setting speakers /pre settings differently to compensate and center voice..........your adcom pre has balance knob try it as Face suggests, tell us how did it go
Luis

i am now using the copland cta305 pre which does not have the balance control

Can we assume the voices you are trying to center are recorded that way?

You should be able to locate a voice in the soundstage with certain quality recordings but no guarantee voices are always mixed in so as to occur in the center.

Your room may well be a factor in an unbalanced soundstage left to right but I think that would probably affect more recording elements and not be isolated to voices only.

How far are the speakers from the side walls. What sort of surfaces are on the side walls? How far do you sit from the speakers?

Can you try the speakers along a different wall, even temporarily just to hear the effect?

Don't be afraid to sit a bit off axis if the soundstage sounds more balanced there.

I really don't think there should be this much of a problem unless one speaker is _very_ close to some very reflective surface (something like tile or glass). Our ears are actually pretty good at sorting out reflected sounds from direct sounds (Floyd Toole's book has a lot of info on this).

In the end, if you can't get stable imaging by moving the speakers around, trust your ears and contact your dealer about a possible problem with the speakers.

I think mapman has a valid point. I also find vocals on a number of recordings to be off to the right or left a bit, and I also get perfectly centered vocals, which makes me believe that this is just the nature of the recording. As he mentioned, the rest of the soundstage should shift also if it is a speaker placement/room issue.