Extract Vocals via Inverting Instrumental track, any gotcha?

I’ve read several tutorials on create an acapella by putting the regular track and instrumental version side by side, and then inverting the instrumentals to cancel out the non-vocal portion of the main track. The track in question is Nelly’s Air Force Ones, for which I can’t find that an acapella was ever released, that I ripped from vinyl to AIFF.
I can get the combined audio to sound out of phase and the vocals somewhat more prominent, but by no means isolated. I’m wondering if there’s any further audio engineer level gotchas that I might be missing. I ripped from my SL-1200 with the quartz lock on, but the main track was on B2 and the instrumental on B3, closer into the center. Looking at the waveform, it looks just ever so slightly different, perhaps the B3 instrumental is going at slightly faster/slower?

The global idea is you can’t separate instruments or singers into individual parts independent of the rest of the song. Almost under all circumstances.

In the case of having the instrumental and fully mixed versions of the same song and then extracting the pure vocal depends on so many things being perfect that it’s in practical terms impossible. If you had the Music CD version of the works you’d stand a fighting chance since a CD player changes its speed as it plays to always maintain the proper bitstream. Turntables – even the crystal controlled ones have manufacturing tolerances on the crystals and they use and maintaining strict sync lock is generally not possible.

Anything you download from the internet has been compressed and thus not in the running. Compression damage is legendary.

As far as trying to subtract two different vinyl tracks from each other, that’s doomed to failure too because vinyl lathes change the pitch of the grooves depending on loudness so the rotational accuracy depends on the whims of the cutter operator with a cup of coffee working in a basement somewhere. This is over and above a producer somewhere putting their personal stamp on the record by “customizing” the instrumental mix “to make it better.”

There are some software for hire packages that claim Vocal Isolation. You might try them.

–Voice Trap
–Extra Boy


Unfortunately Kn0ck0ut won’t get rid of the big Nelly, (I’ve tried), you’d be better off looping Nelly-free bits, e.g. attached …

If you want to avoid being sued for breach of copyright make a similar backing track from Freesounds.

OK, thank you very much, both of you, for the sanity check. I’ll stop banging my head against this one. I played around with this for a while, and got the to the point of dropping about 50% of the backing music by changing the speed of the instrumental by some crazy factor, like 0.03%. But still nowhere close to an acapella.
According to Discogs, the only combination of the “dirty” main track and instrumental is the record I have, the B-side of the Dilemma promo 12". The was a CD single released with the radio version and instrumental.