Ok - here's the situation.
JBL designed the large studio monitors back when all that was available were:
(A) The small format ( 2420/21 and 2425 ) 1.75" coil diaphram that had a 3 bolt 1" exit.
(B) The large format ( 2440/41 and 2445 ) 4.0" coil diaphram that had a 4 bolt 2" exit.
However since then the newer 1.5" format drivers ( with both the 3" and 4" coils ) have been introduced and they are capable of *significant* advantages in response, distortion, power and of course, weight.
So it sounds like a good idea to stick them in the 4343 and 4345's to improve the critical upper midrange.
The additional benefits are:
(I) Much shorter length from phase plug to lens ( the 1.5" drivers have NO integral horn )
(II) Newer diaphram materials ( beryllium and Coated titanium ) along with newer geometry aluminum AND improved adhesives.
(III) Newer Magnetic material that reduce the weight from 1/3 to 1/10 of the original !!
(IV) Newer phase plug geometry that reduces cancellation and distortion - much smoother response
JBL did not make a compatible horn that fit these new drivers that could be retrofitted into the older monitors. The only way to do this was to get a pair of 2311's ( good luck ) AND a pair of 2330's ( good luck X5 ) and redrill the 2330 to accept the new driver. And even when this is done the horn length is still appox 7" ( see table below )
So, with a little determination, a pair of sacrificial 2307's and basic power tools, we created a new horn that fits the old monitors and is only 4.75" long.
The process was straight foward:
(1) Built a jig that allowed a standard bandsaw ( with a metal cutting blade ) to lop off the original flange and some of the horn.
(2) Cut the new flanges from 3/4 Baltic Birch plywood ( 5.75" dia ) with a router circle cutting jig.
(3) Bore a 1.5" hole in the center with a $$ forstner bit *** USE A DRILL PRESS ***
(4) Rabbet a recess to allow the horn to fit into - dimensions on drawing are appox. ( the castings were a little different )
(5) Drill mounting holes with forstner bit on drill press - Note that 2 radiuses are used ( 8 holes total )
(6) Clean surfaces and remove old paint from horn.
(7) Assemble the parts with fiberglas resin and put 1/8 nylon rope into the gaps.
(8) Sand, primer with rustoleum 7220 and paint with 7769 texture black paint.
Actual horn length table:
2307 / 2421 is 10 1/2 " ( 8 1/2 + 2 )
2311 / 2441 is 7 1/2" ( 4 3/4 + 2 3/4 )
2313 / 2451 or 2435 is only 4 3/4"
The final product worked out well and I hope to have them in the 4345's this weekend with the new 2435 drivers.
Next project will be routing a horn sub baffle into the 4345 to allow multiple horn combinations with individual panels ( based on the 2332 cutout )