session null conundrum

[James Drake]

Hi,

I always print my FX returns in case i have to do a mix tweak without my outboard.

Tomorrow i have to do some tweaks in another studio, this is the first time I'm going to try this out.

I already printed a stereo mix, so I duplicate this track, put one to INPUT, and invert the other, and hit play. In theory I should hear nothing as the LIVE mix on the Pro Tools bus should be identical to the printed mix I did before.

But they don't null.

I notice that stuff changes in the delay compensation bit of the channel strip when I switch a track from INPUT to REPRO. So I fiddle with a sample delay to try and get to null. I hit 10 samples and it sort of sounds like the mix has nulled, but not quite. It sort of sounds to me like just the FX returns didn't null. So now I notice that changing the FX returns from INPUT to REPRO also changes the delay compensation on these tracks as well.

Where are delays happening, and where doesn't delay compensation work on all of this? I want to do a mix tweak here and there, but otherwise have this mix null with the old mix. But with printed reverb returns I can't seem to do it.

Any ideas?