is it possible that the processor has reversed the phase?
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I agree with GMC that one of the speakers may be out of phase with the other. On the other hand...
If I understand you correctly, you are saying that one of the speakers has 18ms of delay relative to the other speaker. If that is correct, then I think that could be causing both your imaging problem and the "reverb" effect you described. Why don't you try reducing the delay in increments of, say, 4ms and see if anything improves.
BTW, 18ms seems like a lot. Can you describe your room and speaker layout? Is it highly asymmetrical?
Most people will tell you that a fraction of an inch difference in the distance from each speaker cam affect imaging. An 18 mS delay is equivalent to moving the speaker about 18 feet further away than the other one so yes, that would easily be audible. Can't you just turn off or adjust the delay and see what happens?
An 18ms delay indicates something is very wrong. I consistently get 0.00-0.04ms delays between main speakers when measured with my TacT equipment. The variation is due to slight changes in the placement of the microphone including being slightly off axis (not pointing 100% straight ahead).
Whenever I set up to do new measurements I always do a quick measurement first (3-4 pulses) and calculate a correction just to check the time delay between the left and right channels. If it is too far off I adjust the microphone by rotating it slightly in the proper direction.
If your speaker placement is symmetrical and the microphone is placed properly you should get zero or minor delay. Check all wires to make sure you have not reversed any connection. If that does not help manually change the delay to zero on the unit using the programming features on the remote.
Thank you all for your comments.
The setup is the classic nearly equilateral triangle with less than an inch difference in path length from each speaker to my seat.
I checked polarity, covered up first reflections (there is a brick chimney on one side), and reran the Tact setup program. This time it yielded less than 1 ms delay and the black hole between the speakers vanished. I will chalk it up to a glitch of technology...or perhaps the user thereof:)
I have seen Tact's do this. A friend's setup would put 22 ms delay in one channel from measurements. We manually changed that ending up with something like 2 or 4 ms. Once we did that everything seemed right and fine. Soundstage was good at that point. Never figured out why it would do that. Thought it might be a glitch or bug in the Tact. Maybe some aspect of the room was fooling it somehow though it seems extreme. My suggestion would be set it to no delay, then maybe set it a couple or 4 ms either way to find what gives the most balanced, centered imaging and staging.
Yes, exactly this fix by ear approach is what Tact should make unnecessary. I find the units incredible in what they can do, but they do have a few quibbles now and again. IMO, anyone not using a Tact or similarly good room correction doesn't have a serious hifi rig in the modern world.