The construction of the Duetta
So that nobody can use the excuse, that he does not know how a Duetta is built, we have prepared some instructions which we will show in the well-known manner of single sections with pictures and explicatory texts.
If one is lucky enough to be having a good hardware store or woodcutter close by, then the whole process from a couple of boards to a complete casing for the speakers is easy. The only thing left then is some measure of glue and a little time.
The Duetta is made up of as many boards as can be seen on the picture on the left. If the boards are cut to size and glue is at hand quickly the form will change drastically. But first things first.
The glue shall be applied on one of the long edges of the board that will be the lid.
The lid is glued to the board for the side that is layed down beforehand. For this sensitive fingers are quite handy because that way the flush fitting is easily achieved. The immediately afterwards applied front board helps keeping the right angle. While the glue is curing we repeat the process of the the first casing. This method will be applied throughout the remaining instruction.
A crossbar is applied to further support the lid and also connects to all adjacent walls. The 2 outer pieces of wood were not glued on and were removed later.
Afterwards the read wall was glued on and the assistant of the constructor checked the construction thoroughly. Special attention was payed to the corners. Especially here one needs to pay attention because there may not be any air leak in the corners.
After all the boards are glued on. glue is applied onto the other edge of the boards and the second side will be applied. 30 minutes after the start both bottom sections werde done.
Since we now explained the principle of the construction we will in less detail show the construction of the upper section.
The order of the construction is already known. Lid onto the side, then the front. Mark the positions of the inner boards.
The cover of the ER4 will have glue applied to the edges and the front. The upper reinforcer, the base and the rear reinforcer and at the end the side finish the whole process.
How easy the process of properly veneering and thusly enhancing the optical appeal of the speakers is is what the following instructions are about.
After the casing has been grinded flush we need simple wood glue, a plastic roll, a metal rasp in triangular form and a sanding block. With the roll we apply a layer of glue to one side of the box as a thin whitish film.
While the glue on the case is curing a little, cut the veneer to size with a carpet cutter with about 2cm additional border. During this process the veneer frequently rolls up because the surface of the thin wood swells and thusly enlargens. After the curing is done the buckling is all but completly gone and the glue is glazed. This is the moment when the electric iron takes over. The control should be set between wool and cotton or alternatively a little above 2nd degree. Push the iron slowly and with pressure from both of your arms over the surface of the veneer. About the edges tilt the iron a little so that the edges are firmly glued as well.
Start with the edges that are lying in the direction of the fibre while taking care to only use a downward motion. Don't mind if there are some little edges, they will be gone after sanding with the sanding block.
As a best practice solution time has shown that one should mind an order. Start with both sides, followed by rear, front, base (if wanted) and lastly the lid. To prevent that the already applied veneer gets glued on we advise to tilt the roll at the edges to an angle of about 45°. After all the surfaces have been veneered the edges get smoothed with the sanding block. But take care not to sand through the material.
Milling with a variable positioning device
We always get the same reactions when people see the MPX18. "How is this variable?" is what everyone asks while trying to move the glued together ledgers. The explanation is so easy, that anybody understands very quickly afterwards. In the following an explanation with some pictures.
The name was made up for the inside dimensions. That is why at first we drew a square with an edge length of 18cm onto the veneer.
Connecting boxes for biwiring are almost always rectangular and thusly difficult to cut cleanly with a jig saw. That doesn't really matter, because the hole will be covered by the connecting box afterwards. What now if the connecting box should be level with the front as well? well, take the MPX18!
Now we know exactly where the positioning device should be located. To prevent that the device moves undesiredly, we hold it in place with 2 clamps for as long as we need to.
Now we'll get to the variability. Just add 4 pieces of 12mm MDF into the frame. What we want to create is a hole for the magnet of the ER4 which has to be cut out with 94 x 84 mm. To make things a little more complicated, we need to calculate a little. The hole is 94 X 84 mm, the frame is 180mm , the milling head 10mm and the milling collet 30mm.
So we get for the wider side:
180-94-30+10 = 66mm
Since we will add onepiece on each side, divide by 2 and we get 33mm width and 180mm length.
Respectively we get for the smaller side:
The length is the result out of the width of the frame minus 2 x 33mm = 114mm.
After we drilled the magnet chamber with the necessary depth of 22mm there was some more calculating to do. Frame still was 180mm, the hole 110 x 110, head 10mm and collet 30mm. So we get:
(180-110-30+10)/2=35mm x 180mm für the long ledge and 180 - 50 = 130mm x 25mm for the short ones. We set the milling depth at the collet.
For the round milling we dind't have to reinvent the wheel, since there are lots of good tools avaible on the internet. The one we used is the GEAT-SDS 3 which was discontinued, but similar should be available aplenty.
Cross-over and isnulating material
In order for the diffrent frequencies to travel to the right chassis we need a cross-over. This is made up of 3 parts, which filters the harmfull signals from the good ones.
Two parallel capacitors before a small coil parallely to the tweeter make up the highpass.
The midranger is made up out of a capacitor and a coild before as well as a notch filter of R, L and C connected parallely.
The least parts are used by the bass which needs a coil and a big elko. Yes, an elko!
A good place for the bandpass filter is the stabilizer on the rear board.
The cross-overs for the highpass and the midranges i glued onto small boards and soldered red and black cables onto them.
The bigger of the two boards with the cross-overs has been glued to the back, the smaller one on the stabilizer. That way, all parts are sufficiently far apart.
Two mattings of insulating material fit into each top compartment. They got cut in the mid and stuffed into all compartments.
One thing that is of special importance is that the blue connector is the positive pole and the red one is minus. This is of special importance if the ER4 should be connected via a luster terminal. A little foam rubber is filling the compartment, the hole for the cable is sealed with some repair wax.
For the bass casing we will need three mattings of 40x50cm. They as well get cut in the middle. One is put into the forward chamber, the second above the cross-over and the last one behind the bass.
Since the woofer has to be reversed against the other speakers the small connection flag has to be soldered with the red and the broad one with the black cable. Take a note, that prior to screwing the woofer onto the casing, predrilling is advised!