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After further REW testing and listening tests I decided the OB should be removed. It's humped response was too extreme to be useful in filling the 150Hz trough. The pair of older drivers, borrowed temporarily from the IB, were to be returned to the 6' tall, eight driver manifold.
While I was about it decided to move the four newer drivers down to the bottom four positions. The older drivers were moved up to the top four positions. I haven't tried this layout before. Logic suggests that the lower positions will enjoy more bass reinforcement. The upper drivers less so. Though this may be pure speculation given the manifold's position: Almost reaching the narrow ceiling. Just under the 45 degree overhang of the sloping attic walls.The changeover of eight 15" drivers is not something to be undertaken lightly. Sixty four furniture screws and 64 T-nuts have to be removed and then refitted. One hundred and twenty eight loosening and fastening actions in all. Then there is the matter of the weather. If it is cold outside there is a hell of draught though each empty driver cut-out.
This time, I finally made a bit to drive the screws with a rechargeable drill. I simply cut off an Allen key close to the bend using a thin, diamond wheel in the angle grinder. Allen keys are too hard for hacksaws which just skate off.
I can't be both sides of the manifold at the same time. So I have developed a system. I leave a middle driver out on each side of the tall box. This allows me to reach through the empty cut-outs. So I can fit the T-nuts to the inner ends of the screws inside the box.
Even this is not without difficulty. Because the screws are loose in their holes. So they want to push straight back out as I apply the T-nuts to the bare threads inside. I usually hold the screw thread with a finger nail while gently starting the nut. Once the threading is started the nut can be spun on until the locking spikes meet the inside of the plywood box.
The first screw to fit is always the bottom one on a driver fitted from the outside of the box. The heavy magnet inside the box dictates this order. The magnet wants to pull the back of the driver downwards. So the rim pushes out at the bottom. This is the complete opposite to my fuzzy logic. The first screw is pushed through the driver rim. That driver must now be held firmly against the baffle until the first T-nut is safely fitted.
This is where the missing drivers are helpful. All it takes is the bottom screw and T-nut to hold the driver perfectly safely. The basket sits in the cut-out so cannot move sideways. Now one can relax and work on all the other drivers. The final driver to be fitted is the real problem. One can't reach through an empty cut-out to fit the bottom T-nut.
My method is to push as many screws through the last driver as possible. To hold the driver in place by friction alone. I can then go around to the inside and fit as many T-nuts as possible. This ensures the driver won't suddenly slip. At least one screw will usually hold it safely in place. Once the first T-nut has a grip the battle is almost won.
Then I have only another 56 T-nuts left to fit. This usually leaves my finger tips rather sore. Even if I avoid the anchor spikes the stamped disks of the T-nuts are narrow and sharp. I never remember to have a tray to hold all the nuts and screws. Inevitably T- nuts drop as screws are removed or pushed through the holes. I always seem to end up with a nut or two missing.
Discipline demands that they must be found before they can wreck a driver! Usually they are hiding around the back of a driver magnet out of sight. The worst possible scenario is when a T-nut drops right inside the motor and coil area. There are small vent holes in the cast baskets which are just large enough for a T-nut to fall in and be held firmly by the magnetic field. Sometime a nut just sticks to the spider face by magnetism. These can be more easily rescued with extended fingers through the basket spokes. Those which make it inside the motor area need long nosed pliers, patience and some dexterity to get them out again.One thing I noticed this time is that the older AEIB15 drivers are much heavier than the new ones. The older magnet back plates are much larger in diameter. As are the ferrite magnets. The baskets look almost identical.
The newer cones are flared, reinforced paper. The older ones are straight tapered and vinyl or polypropylene. Or whatever plastic was used in their manufacture. The older drivers have screw terminals. The newer ones are sprung. The sprung terminals will not take such large wire as the screw type. This isn't a problem with a single wire. Only when two wires must be twisted together in one terminal does it become a big problem. Unfortunately most terminals will have two wires inserted.
Now all I have to do is remember how to wire up the drivers again. The older drivers are 4 Ohms each. These will be wired in series-parallel for a total load of 4 Ohms on one channel of the EP2500 power amp. The newer drivers are 8 Ohms each. So will be wired all in parallel for a combined load of only 2 Ohms on the other channel. As usual, I will apply a 9 Volt battery to the amplifier end of the bared cables on each set. Just to ensure all eight cones jump outwards at the same time. I usually test the DC resistance as well with a DMM to ensure the combined loads are correct.
It will be interesting to see how moving the drivers around will affect the response and thus the BFD filters required. It could be a total disaster but was still worth trying. I hate "if-only I had tried that" situations. Usually the thought of removing eight heavy drivers and all those screws and nuts leads to sudden and permanent inertia! :-)
Click on any image for an enlargement.
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