Jean Paul’s Duetta
How it came about
Now that I have reached the age when many of life’s decisions have already been made, I wanted to fulfill my long-held dream of having a nice “hobby room.” The hobby in question is music, with a little home theater show every so often. The room itself was renovated with a good deal of work on my part – we removed a wall, redid the electrical wiring, and put in oak flooring. The project gradually took shape. A system made with Rotel components was put together, and I pulled my old boxes from more than 20 years ago out of the basement and connected them.
It was clear from the start that the old boxes would just be a temporary solution. Like all of these projects, there was an important hurdle to overcome: approval from my dear wife. Up to then, she had believed that the most beautiful boxes were the ones you couldn’t see. That’s why we have a subwoofer satellite system set up in our dining room. But in the new room meant for relaxing, I was playing in a different league. I started by making my way to the local hi-fi shop. Unfortunately I, and above all my wife, was not satisfied with the solutions on offer, or else the sound didn’t meet my expectations. I don’t want to talk about the specific boxes here; I’ll just say that there were a variety of units in the €2000 + X range.
Somewhat sobered, I sat there at home with a wobbly sound system. The next day, I stopped at a magazine kiosk and came across one for DIY boxes. Great, I thought, I’ll get this issue. Maybe I’ll find some inspiration. I devoured the magazine and was intrigued by the possibilities, high-end projects flying around in my head. After two months the next issue landed on my desk, with more great projects. Well, let’s see what the next one has to offer…
At the same time, of course, I started reading things online and found out that there was a real DIY scene. I also realized that I had far too little experience with the material to come up with a decent result, and I wasn’t going to gain any. I’m not an untalented craftsman, but I’m no engineer. By now a good year had passed. I had read a couple more magazines, poked around online (my wife asked me if I had finished reading the whole internet yet), and naturally I ended up at Loudspeakerbuilding.com.
I had an idea: on the one hand, I think it’s hard to build something you haven’t listened to before. Sound descriptions can be very subjective. On the other hand, I wonder how it can be that new “super-projects” are introduced every two months. If a project is good, there can’t always be new and better ones coming out. So after long deliberation, I decided to visit these people. If there are that many people who are happy with the Duetta, Minetta or a representative from the SB series, I might as well take a look for myself.
The drive was a quick one. I was a couple of minutes early, and I stood in front of the listening studio feeling a little disappointed. Somehow I had imagined the shop looking a little more glamorous. Another customer was waiting too, and he greeted me with some concern: “Hey, do you know if the master is coming in today?” The store wasn’t open for another five minutes, so no worries, I’m sure the “master” is coming. And he did come. We soon started chatting, and I liked Udo’s friendly attitude right away. First he asked about my requirements (room size, system, usage, etc.). Then we came to the usual suspects: Duetta, SB. We started feeding the loudspeakers with music. A little of Bach’s organ concerto, then “Child in Time,” “Deep Purple,” “Basin Street Blues” from jazz guitarist Phillip van Endert, and finally Johnny Cash arranged by Rick Rubin. What can I say? I felt at home with the Duetta right away. A clear sound, nothing exaggerated, love at first listen.
But how would I resolve the WAF issue? After all, the Duetta is not exactly a small box, more like an overgrown boulder. As I explained above, the most important thing for me is music in stereo, then the home theater experience. My plan was to start with the Center. If it met my tonal standards, I could start thinking about the main boxes. I also decided to hide the Center in a piece of furniture I built myself, in keeping with the request for invisible boxes. So the design of the Center didn’t need to be perfect, and it was also an ideal test for the larger Duetta stereo box project. I took home the components for the Center – the project had begun.
The Center, test run and lessons learned
What was it going to look like? I hadn’t decided on the final design yet. But I knew I didn’t want to deal with veneer, since that takes a bit of experience and because the Duetta would require large sheets of veneer. Besides, there is plenty of potential for frustration, which I wanted to avoid. So I decided to use multiplex and finish the surface accordingly. I wanted it to be obvious that it was wood, so high-gloss paint was out. The surface needed to have some visible wood grain and a tactile structure. When I held the chassis in my proud hands at home, it was clear that a light shade, white, would look great with our dominant floors.
Armed with the list of wood pieces, I went to my local hardware store. I ordered the baffle board from the appropriate online retailer to make things easier for me with the router. I was sure there would still be plenty of work left. The hardware store even cut the wood fairly precisely, although the material (21-mm birch multiplex) was somewhat poor quality. The wood itself had large gaps along the cut edges, along with pressed-in gravel and discolorations. I started by refining the surface using wood putty (glue mixed with sanding dust and sawdust). I took care of larger flaws by gluing in toothpicks. All in all not bad, but work that I would like to avoid with the larger Duettas.
Meanwhile the baffle board had arrived, so I had all the materials. I compared the baffle board with the other vertical pieces and used a router to even out any differences. The next step was SANDING. To make sure the surface was living-room ready, I found the following procedure to be useful: sand the material roughly, first with a #60 grain, then #80 and #120. Then apply the first layer of stain (Clou XXX). Use a rag to apply it to the wood and polish it until all of the streaks have disappeared. Now the wood needs to dry in your work room for five to ten hours, depending on the temperature. Then sand the surfaces again, this time with a #80 grain, then #120 and finally #240. Repeat the process until the wood fibers are no longer standing up. After each sanding step, work in another layer of stain; each step will require less stain. I also made an important discovery: buying cheap sandpaper is not worth it. Bad sandpaper can turn the surface of your wood red, for instance, while I don’t have that problem with a brand-name product.
The rest of the Center assembly is actually predictably easy: the reflex channels are attached to the baffle board and the whole thing is glued to the base plate; then come the sides and the rear wall, and finally the lid – done. Since my Center was going to be mounted low, almost on the floor, and after some consultation, I planned in two bars to serve as feet so that the ER4 could “see” the ear. Otherwise I basically followed the original assembly plan. The channels were glued onto the baffle board using Lamello pieces. As a test, I proudly set the chassis elements into the openings. Horror! The opening for the ER4 was rotated by 90°. I checked my drawing right away, and it was correct. I tried to make the opening a little bigger so that the ER4 would fit. Unfortunately there wasn’t enough material left to screw it in – nothing worked. The online wood retailer handled my complaint quickly and with no difficulties, and a few days later I had a new baffle board. And I had learned another lesson: you need to check the fit of the materials and the parts before you do the next steps of the project. Let me just expressly state here that I am extremely happy with the CNC router.
Now I attached the crossover to a little piece of scrap wood, screwed it onto the rear reinforcement, filled the middle chambers with insulation and glued on the lid. All of the glued parts are connected with Lamellos. Then I laid the insulation in the outer chambers and put in the chassis elements. For aesthetic reasons, I didn’t use the screws that came with it to screw in the chassis, but rather drive-in nuts with Allen screws – more on that later. In any case, my pride and joy now stood before me: the Center! I couldn’t wait to connect it to the system to see how they would get along. And they get along wonderfully: its precise sound outperforms all of the other boxes in the room! In 5-channel stereo mode, Johnny Cash (Mercy Seat) is suddenly singing right there in the room, not spread out diffusely. My wife was impressed: “It sounds better than anything else we’ve ever had in the house.” Now if I could just come up with a good design for the main loudspeakers, nothing would stand in the way of “Project Big Duetta.”
Stripes are slimming
The main loudspeakers had a couple of tasks: they needed to be one piece, since the two-piece design looks too much like student housing to me (nothing against students or their housing!). They needed to be mitered. And they needed to have something that would integrate them into our living space without competing with the floor. I came up with various attempts using SketchUp; I think I spent a good 20 hours experimenting. The Duetta thread by Matthias (DA) in this forum was also very helpful, and he gave me his SketchUp file and an Excel table full of formulas – thank you for those! Anyone who wants to build it can PM Matthias; he’ll answer as best he can, and he’s very willing to helpt.
Finally, I came up with the idea of building two different boxes: one would have piano keys on the side made of leftover materials from the flooring project, and the other would have six strings across the front. I told a cabinetmaker friend about my idea. His response: “That’s going to be complicated! When you cut grooves into multiplex, you always need to pay attention to the grain, or the material tends to crack! The strings go from top to bottom, and the keys from right to left. That means you have different grain directions, which will probably look awkward.” So it was either the strings or the keys. In the end I decided on the strings because I thought they made the silhouette look narrower. Then it was time for another visit to the store, where I picked up the components. As I mentioned earlier, I wanted to avoid the problems with the Center and the bad wood, so I had the whole box made by the CNC service once my construction had been approved. The delivery time was accurate, but it still took a good four weeks. Then three nice heavy packages arrived. One thing was clear right away: our basement (just under 10 m2) is too small for the assembly work, so I decided to use the garden shed. It’s not any bigger, but at least you don’t track any dust through the house (consider the WAF!). The baffle boards have six grooves cut into them, each five mm wide and five mm apart, centered.
Pieces of the oak floorboards will be inlaid into these grooves. That means the surface treatment has to happen first, because it would no longer be possible after adding the inlay. So, as I described above, first comes SANDING and staining. My tip: If you want to be able to enjoy your loudspeakers later, make sure you use hearing protection – and wear a dust mask during the sanding, because there’s going to be a lot of noise and plenty of fine dust. In addition, the oak floorboards needed to be trimmed to the right shape, so I cut the pieces into five-mm strips using a circular saw. At 20 mm, the floorboards were good and thick, so I also had to trim the depth of the thin strips.
I screwed the slats onto a small piece of wood and trimmed off another 8 mm with the handheld circular saw. My initial attempts with a trimming cutter were pitiful failures – the material splintered. Next I placed the oak pieces evenly into the grooves, carefully tapped them in a millimeter at a time, and glued them with Ponal joint glue.
Once a piece of oak cracked, which raised my blood pressure considerably. But I was able to take the piece out and save the baffle board, so all was well for the time being. I trimmed the projecting ends with a round Dremel saw tip, and sanded everything smooth with a sanding attachment.
I made the holes for the Allen screws for the ER4 and the Hex 7-360/ 37 for an M4 screw with a 3.2-mm groove cutter on the Dremel, because it is essential for the holes to be even. Then I inserted the drive-in nut from the other side. Rather than hammering it in, I “pulled” it into the wood with a screw and a washer. That way the drive-in nut is guided by the screw, and it sits straight in the baffle board when you’re finished. I used a similar process for the 11-581-50 Hex, although I used M5 screws; I “pre-drilled” the holes with the 3.2-mm groove cutter, then enlarged them with a conventional drill. In order to avoid potential leaks, I doubled the baffle board in the upper section from the opposite side, and used an MDF ring for the bass. I used a Forstner bit to drill corresponding niches for the places with the drive-in nuts, which I then sealed with hot glue. I attached the screws with PVC washers, as shown. They protect the chassis and add an extra seal.
In order to assemble the boxes themselves, I “rented” the dining room for two days. Once again I realized that my options were limited without the right equipment and the right space, especially as a layman. That made me all the more pleased with the results. I had a healthy respect for the process of gluing the boxes together. I started with a side piece and placed the rear wall and lid on it.
Then I attached the separator for the upper section, next the other side piece, and finally the baffle board. In order to make sure the box was air-tight, I attached the Lamellos with joint glue and glued the inside edges with PU glue so that there was an extra strip of glue exposed on the inside.
In any case, it all worked, even if it was very difficult. For the second box, I put the second side piece in last instead of the baffle board, which was much easier. I tightened the boxes with tension belts and blocks, and forced them into shape with screw clamps.
It probably would have been better and simpler to work with more screw clamps, but I only had two in the right size, and I’m no cabinetmaker. No matter, I still ended up with two beautiful boxes. I built the frequency crossover for bi-amping and then screwed it onto a reinforcement (I glued on two felt gliders first to avoid vibrations). Then I added the insulating wool and attached the chassis and the feet. I gave the lid a striped pattern with multiplex slats, and sanded it for a long time to create a mirror-smooth finish. As a finishing touch, I applied wax oil, after sanding the white-stained surfaces by hand one more time with #600 sandpaper. Udo had recommended some rubber feet, which I set into the base plate by a couple of mm so the box looks like it’s floating 2 mm above the floor.
And there it is
Naturally I could hardly wait to hear the Duetta in my own home. So I set it up, connected it and listened. The first disappointment came right away: the right-hand mid-range speaker wasn’t doing its duty. I quickly unscrewed it to take a look, but it wasn’t the wiring. I took out the bass again and checked the crossover. The problem was quickly solved: a soldered wire had come loose when I put in the insulation. I screwed everything back together and connected it – the mid-range speaker worked! Now it was time for a little patience: I hadn’t used bi-amping for the boxes in the first step, I just bridged the bases to them. But somehow I had expected more bass, and a rounder tone. A quick look online told me that some people had to switch the poles in the bass to get the right results. But I had checked the crossover several times, and even had it checked by a colleague who is an expert in the field (thanks, Bernd!). The polarity wasn’t the problem, though – the amplifier had set the lower frequency limit at 120 Hz, a value that doesn’t even really make sense for a subwoofer. So I corrected it to 40 Hz, and after two hours it was finally there, that Duetta feeling. First I listened to the same pieces I had heard in the listening session (Phillip van Endert and his jazz guitar, and the album “The Time We Spend,” are highly recommended for anyone who gets excited about jazz guitar.) Then I was sucked in further and further into the quicksand that comes from pride in the project and fascination with the sound, and I had a hard time breaking free: Pink Floyd “One of these days,” Cat Stevens “Father and Son,” Jaques Loussier “Toccata and Fuge,” Frank Sinatra “Fly me to the Moon,” Beatles “Girl,” Dave Brubeck “Take Five,” and so on. The hours pass quickly when you are enthusiastically rediscovering your music collection. Interestingly, this is the first box where I can hear the differences between an mp3 and a CD or LP. So we rediscovered our good old vinyl, and we are enjoying many relaxing hours with it. The Duetta really is the Queen of Blues.
Now all that remains is to say thank you: thank you for your support and tips – it’s a level of service above and beyond normal customer support. Thank you to Matthias (DA) from the forum. Thank you to my buddy Bernd for checking the frequency crossovers.
I’m sure that I will be building two more Duetta Tops as rear boxes in the foreseeable future, but first I’m going to build a sideboard to serve as a home for my hi-fi devices and the television.