mirrored L212s

[MJC]

I seem to remember about a year ago a discussion about moving the L212 drivers about 2" over on the baffle, towards each other in a paired set. This was, I think, according to Greg Timbers, to improve the sound stage.
My questions are; what was the exact dimension, and are the drivers kept in perfect vertical alignment?

[boputnam]

Originally posted by MJC
My questions are; what was the exact dimension, and are the drivers kept in perfect vertical alignment?

I don't know anything about the original discussion, but I do know that there is a preference here to maintaining "perfect vertical alignment", for sure, if you are considering mirror perspective.

[4313B]

I think it might have come up during a conversation with Drew as opposed to Greg, but something on the order of 1-1/4" to 2" should be fine. Try to keep the left and right dimensions to the vertical axis of the array non-multiples. Reference the offset in the 240Ti. 18" wide baffle, vertical axis 2" off center. Notice the multiples of the values of those dimensions. If the vertical array is in the center of the baffle then any diffraction effects are reinforced. Technically no transducer should be equidistant from any edge. The concept should probably be treated as a "best practice" or something to that effect. The fact that the stock L212 is symetrical isn't a show stopper.

[MJC]

Originally posted by Giskard
[ The fact that the stock L212 is symetrical isn't a show stopper. [/B]

I would think that the main advantage of the 250s over the 212s is the fact that the 250s are mirrored. I think I remember the comment being that making the 212s symetrical was marketing.
It would not have stopped me from buying the 212s way back in '79.
I've been thinking of making new boxes for the two main L212s with them mirrored, but leaving the center and two surround L212s as is. I won't see that there would be any advantage to offsetting the surrounds, much less the center.
With the price of the Performance Series going from less than $7000, when they were reviewed 18 months ago, to over $10,000 now, make the upgrading of the 212s look better and better.
I noticed on the JBL web site that the Performance Series has been added to the Synthesis Four system, with the boxes being squared off and having THX certification, but the Performance system still having its original shape and non-THX. And they can only be bought thru a Synthesis contractor.

[4313B]

The baffle width for each transducer in the 250 is "optimized" and each transducer is also offset from centerline. I definitely prefer the 112 over the 108 for midrange. It might be fun to try the N250 filter with the 112, LE5, and 066 Basically it would simply require some minor changes to the capacitors in the conjugate filters. Anyway, something to ponder...

[MJC]

Originally posted by Giskard [/i]
I definitely prefer the 112 over the 108 for midrange. It might be fun to try the N250 filter with the 112, LE5, and 066 Basically it would simply require some minor changes to the capacitors in the conjugate filters. Anyway, something to ponder...

Yes I agree, the 112 is a great mid-bass driver. I can't understand why JBL didn't use it in other models. Which of coarse, makes it that much harder to find. And I'm still short one. I missed a pair last year, when I was out bid.

[4313B]

Well it's a pretty expensive driver to manufacture and it's pretty specialized. It is best suited for exactly the applications it was used in - the L212 and the 4315. For larger systems JBL cooked up the 10" 2121/2122/2123 and for the largest the 12" 2202. I'm not sure exactly why it wasn't used in the 250 other than that the 108 is cheaper; plus the 108 has a round frame and that kind of fits in with the visual scheme of the 250?

[DavidF]

Shifting the driver array over seems quite a bit of rework. Two suggestions before you try it. Try adding a strip along one side to effectively widen the baffle. This does not have to be permanent but could suffice to try before you cut up the existing baffles. Also, try foam strips down one side of the upper-range drivers, or both sides. This has the effect of breaking up some of the flow that becomes a problem at baffle edges, etc. The driver array should be vertical (and staggered vertically- now that is a rework project!) with the network design on the L212.

David F

[MJC]

[QUOTE]Originally posted by DavidF
[B]Shifting the driver array over seems quite a bit of rework. Two suggestions before you try it. Try adding a strip along one side to effectively widen the baffle. This does not have to be permanent but could suffice to try before you cut up the existing baffles.
--------------------------

I would just build two new boxes, I've already built 4, one of which was a horizontal center that had two 112As. That didn't fly though, too bass heavy. Now I have 5 identical boxes with the center box mounted on a ' tilted foot' instead of the 13" base(fits below the screen of the RPHDTV). The two originals are mounted upside down on the side walls of the HT.