This article is from the Audio Professional FAQ, by with numerous contributions by Gabe M. Wiener others.
You probably want a device called the Thompson Vocal Eliminator made
by LT Sound in Atlanta, Georgia. The device will cancel out any
vocal that is mono and panned dead-center. The unit works by
filtering out the low frequencies, phase cancelling the rest of the
signal, and then mixing the filtered bass back in. The result is a
signal with the center, common information cancelled out. Sometimes
it works well, other times it sounds awful. [Gabe]
 
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