The Standing Duetta Top by Matthias
What’s interesting is that I was completely satisfied with my 7.1 computer box system for years. But after the subwoofer, which was also the amplifier, suffered a defect, I went on the search for new loudspeakers. Sure, a 7.1 home theater is a great thing, but on the whole I definitely spend more time listening to music. So I was looking for good stereo speakers that could also be used as front loudspeakers. One thing was clear right away: I didn’t want squallboxes, I wanted standing speakers that could get by without a subwoofer.
Since my standards for loudspeakers had been pretty modest before, the first place I looked was an electronics store. Unfortunately, all of their speakers sounded tinny and pathetic, probably because of the distributor technology. So I went to the nearest hi-fi store. Since I was only looking for standing boxes and didn’t have a price range in mind yet, I was fairly open to suggestions. The salesperson suggested Danish speakers from a less well-known manufacturer because of their good price-performance ratio. I was excited about the sound right away. I had never heard anything like it before. For the first time I could hear what a sound stage meant, and experienced a truly deep note. All I could think about was how much listening time I had already wasted with my previous “loudspeakers.” The initial euphoria, though, was followed by a more analytical approach. The tweeters were fairly harsh and aggressive for my taste, and the bass had a kind of volume boost. I soon realized that I found rumbling deep notes and metallic high notes unpleasant. So a few new items were added to my loudspeaker wish list: standing boxes, a non-aggressive, non-metallic high range, and a nice dry bass.
It soon occurred to me that I could build my dream loudspeakers myself. Out of all the assembly reports, I especially liked the rounded structures. But I still wasn’t sure whether do-it-yourself loudspeakers offered a convincing sound. So I drove over to the Intertechnik listening studio to hear the BlueNote and the Duetta, both loudspeakers that feature the highly acclaimed ER4. All of the positive sound descriptions had made me very curious to hear this particular tweeter. And yes, it was a bull’s-eye. I definitely needed loudspeakers with the ER4 – that was one item checked off the wish list. But unfortunately the deep tone wasn’t quite right yet. Then came the Queen – a truly amazing loudspeaker, but unfortunately the bass wasn’t dry enough for me. Meanwhile, the listening room had filled up with visitors, some even standing outside the door, and someone had the idea that you could just listen to the Duetta Top by itself. So the crossover was quickly rewired, and I couldn’t believe how well the “little” guys played by themselves. The deep tone in particular convinced me right away. So the listening session was a complete success, and it really was the only way to pick out the right loudspeakers. In terms of the sound, I had found my ideal loudspeakers. When I got back home, I listened to my old loudspeakers again, and the difference could not have been more obvious. I couldn’t believe it when I read online that you could also build the Duetta Top as a standing speaker.
When I then also learned by email that the large model goes 3 Hz deeper, and that it is 0.5 dB quieter below 80 Hz but even tighter in the bass, it was clear: the Duetta Top, as a standing model, was my project. When I told my father about the loudspeaker-building idea, he was just as excited about the project as I was, so we both started building it together.
The assembly report
The planning phase was a bit drawn-out. I wanted to build something special, so I spent a long time tinkering with the shape and thinking about the best way to implement my ideas. I also wanted to make sure the loudspeakers were completely curved. Just rounding off the side pieces a little wasn’t an option for me. After countless sketches, calculations and multiple paper models, I ended up with the final 3-D rendering, and my plan was complete. The speakers would use two colors. I ultimately decided that the fronts would be painted black and the rest of the body would be laminated in macassar. Because of the small radius of the back wall, I decided to use the slat technique. I also thought building it with several layers of bent plywood would be more complicated. In addition, bent plywood is lighter than MDF. In order to avoid visible joints, the slats had to be cut to size according to the construction data. A cabinetmaker took care of cutting the precise angles in the 22-mm-thick MDF board.
Finally, the day had arrived. After all of my planning, I finally had the wood in my hands. I started by building the skeleton, which fulfills two functions at once. During the assembly phases, it serves as a support structure for building everything around it. At first, the primary function of the skeleton was as a base for the slats on the back wall when everything is pulled tight with tension belts. Finally, the skeleton also provides the reinforcement matrix. For the elliptical curve, I created a template in advance, which I transferred to the MDF boards using the router and a copying ring. Once everything had been cut out and adjusted, the individual pieces could be put together for the initial test. Gradually, the final form started to take shape. Once the slats had been placed as a test, I was very happy that the planning phase had been so extensive and precise after all. At this point, nothing was glued down yet, but we quickly took care of that. To make sure the skeleton was glued together straight, the construction was painstakingly adjusted. In addition, we screwed guides made of wood scraps into the table in order to hold the skeleton to the side pieces during the drying phase. Once everything was completely dry, the slats could be glued on one piece at a time. We quickly found that the slats are pressed on with more force if you use screw clamps. Still, we also held everything together with tension belts. The tensioning works especially well if you put on all of the slats, even the ones that are not yet glued down. Like an archway, the tension passes all of the force on to the slats being glued. All of the slats were glued down one by one, and the cabinets were put together for the first time. It was a great moment, especially since it was very easy to see the progress.
Now the back wall was finished, and it was time to move on to the fronts. The round shape of the back wall made it impossible to place the cabinet on the work surface in a stable way with the front facing up. So we cut a negative form out of MDF scraps and cushioned it with some pieces of carpet. The negative pieces were stuck to the back wall to use as feet. As you can see from the picture, this construction makes it convenient to work on the front. I was also surprised by how stable the cabinet was with the feet resting on the work surface. We put all of the front components in again to test them, and noticed that the large front panel was too big.A small measurement error had occurred during the cutting phase, so the panel was still 10 cm too long. But we quickly fixed that. Still, it was a little tricky to glue the front side together. After all, the front was going to be painted and the back would be veneered, so we didn’t want to glue the two pieces together until right at the end. Still, we used the back wall as a fit piece for gluing the front. In order to keep the two pieces of the cabinet from being glued together, we stuck a sheet of film in between them. Now gluing the slats for the front curve was no problem. For the drying phase, we tightened everything up with tension belts. After gluing, it was easy to remove the front pieces from the back wall. The photo shows the front curve attached to the lid with just a strip of tape, and simply resting on the back wall.
To make the sound as tight and dry as possible, I spared no effort in insulating the cabinet. In the next step, I used contact adhesive to attach 4-mm-thick bitumen panels, in order to reduce cabinet vibrations and to dampen the emanating sound. The next layer was supposed to be 15 mm of felt. Unfortunately, the felt hadn’t arrived yet, so I moved on to the other steps for the time being.
In this photo, all of the front pieces are already glued together; the curve has been sanded, and everything is ready for the upcoming experiment: gluing a 3-mm-thick HDF board to the curved front panel, with the HDF board 44 mm wider than the MDF substructure so that the edges overlap and create a nice finish. To make sure the front and back walls didn’t get glued together, film was placed between them again. This step was very tricky. The HDF board was mistreated with a hot iron to bend it around the curve, since wood can be bent with heat. People often assume that you need water to bend wood, but the water is just more efficient for conducting heat. In fact, applying heat does ensure that the wood stays bent. I also remembered that fiberboard expands when it comes in contact with water. It turned out that the steam function on the iron really does make the surface expand, which was actually helpful for bending the wood. After the first round of ironing, glue was applied to the entire surface of the MDF substructure. During tensioning with the belts, the curve was continuously re-ironed until the HDF board fit against the curve. During the drying phase, tension belts were attached all the way around the cabinet. Now the moment had come to undo the belts and see whether the bending experiment had been successful. Lo and behold, everything held its shape, and even the gap dimensions were correct.
Inspired by this success, we quickly rounded off the back walls with a belt sander. The photos once again clearly show the successful construction from the bending experiment. Now the square HDF board needed to be rounded off. To do so, we stacked the front and back walls together. Using a jigsaw, we first roughly cut the HDF board to match the curve, and the belt sander did the rest. Just to be safe, we stuck some cloth tape to the back wall. Now that all of the woodwork was finished, we temporarily assembled the loudspeakers in their unfinished state, ignoring all of Udo’s warnings. After all, I wanted to see whether all my hard work had paid off and whether the soldered crossovers worked.
It didn’t stay in that state long, because the roll of felt had finally arrived. Now the insulation inside the back walls could be finished. The photos show a small glued-on HDF board, which reduces the cabinet volume of the ER4 and balances out the additional thickness of the HDF board on the front wall.
It’s almost impossible to achieve high-gloss paint results, for instance, without a dust-free painting chamber. So we sent the front pieces to a professional painter. I also decided to have a cabinetmaker do the veneer work, since it was my first time building hi-fi speakers and I had never done a veneer before. For a flat shape I would have done the veneer myself, but after all my work on the round shape I didn’t want to take any risks. In addition, I had chosen a high-quality veneer (macassar), so I wasn’t interested in any new experiments. But that’s just where things started to go wrong. The cabinetmaker works with a vacuum membrane press, which uses vacuum pressure to attach the veneer to the round shape. Originally I thought the precise woodwork would let me avoid puttying the slats on the back wall. But after the first dry pressing attempt without the veneer, the lines between the slats were obvious. So I did end up puttying. But that wasn’t the problem. After the second dry pressing attempt, the problem was clear. Apparently, the vacuum membrane press builds up too much pressure for the slat construction, so the cabinet and the slats shifted and warped. The result was an ugly stair-step effect, along with some protrusions that would have been visible even through the veneer. So I attached all of the slats individually with screws and puttied everything one more time. I also changed the veneering method. The cabinetmaker used a multi-component adhesive, which takes less pressure than the glue method. Finally, the veneering step was a success.
Here you can see the veneered back walls and the painted fronts waiting for their final union. The painted front walls were quickly given the same insulation as the back walls, and the rest of the work steps also went quickly: Installing the wood panels that have been molded to the round shape of the back wall along with their soldered crossover components. Filling the cabinet loosely with the insulating wool, keeping the area behind the reflex channel free. Attaching the threaded sleeves for the rubber feet and placing all of the wiring. The last picture captures the moment just before the wedding, the final gluing of the front and back walls.
The standing version of the Duetta Top is very similar to the compact version. The deep tone in particular always surprises me. I wouldn’t have thought that the 7” chassis could have such a powerful, deep bass that is still very dry and crisp. Currently, the two speakers have taken over the role of the nonexistent subwoofer in my home theater system – and they do it really well. But playing music is more important, and that’s where the speakers show off what they can do. The reproduction of voices and singing is amazing. You just hear much more detail without having to concentrate on picking out certain fine points. What I especially like is the sound that is disembodied from the speakers, and which keeps getting better over time. After all, the E4 needs a decent amount of time before it is broken in. It mercilessly reveals bad recordings, while good recordings benefit from these speakers. I am completely satisfied with the sound, and the appearance is a bonus. A few months have passed now, and I have had a chance to test the speakers extensively. The tone has gotten even rounder, and my musical tastes have evolved, too. My original music collection, which was more in the electronic range, was usually mixed for subwoofers and a lot of bass – no sign of a true deep tone. I could always tell that the boxes had more in them, and that the songs just weren’t taking advantage of it. So I finally came around to nu jazz and electroswing. Parov Stelar, for instance, sounds great through these loudspeakers. Ultimately, I just have to say that in my opinion, the Duetta Tops are ideal for this kind of music. The taut, crisp deep notes and the ER4 simply take your listening pleasure to a new level. Live recordings sound especially good. You really have the feeling that you’re there. Longer comparisons with other hi-fi loudspeakers are a constant reminder that the standing Duetta Tops model was absolutely the right decision.