“Duetta Top” designed as a slim pedestal loudspeaker, by Andreas
Just to get it out of the way, let me admit: there are definitely much more space-saving ways to package 20 liters of breathing room for a delicate bass chassis. And there’s probably a much less complicated way to place a tweeter at ear level. Sure. In addition, six or seven boards glued together to make a nice-looking box would probably sound just as good, and it would be finished much faster than this project. It’s all true.
Nothing against two well-made square boxes with nice proportions and a slick finish – that’s perfect! But this time the project could afford to take a little longer and be more complicated, and it would definitely look completely different. A project just for the fun of it, to blow off some creative steam. After all, the nice thing about do-it-yourself projects is that we DIYers have plenty of the one resource that all the professional manufacturers are trying to save: time. For us, that comes free. We may not know how to combine a pair of chassis elements with a handful of coils and capacitors to come up with a dream couple. But as long as there are people like Udo, who can do that just as well as all of the manufacturers’ developers, we don’t need to worry. And the things we build don’t need to be profitable; they don’t need to appeal to a magazine editor, and they don’t need to ship in a compact format. They just need to make us happy – both acoustically and visually.
For 30 years, my companions have been homemade Dynaudio mammoths from the era when nothing was better than big and heavy – unless it was even bigger and even heavier. Not too bad, but now it was time for something new. But what? I start reading through the trade literature and doing some online research, but unfortunately I don’t have the time to drive around the country and listen to samples. Still, I keep coming across the name “Duetta” and hearing about all the positive experiences people have had with this magazine and the forum. So I write an initial email “to whom it may concern” and immediately receive a friendly, informal answer. That makes me happy, and the decision is easy: I’m going to make a couple of “Duetta Tops,” to be complemented by a suitable subwoofer later on.
First, though, a little about the design, completely based on my whims: loudspeakers make music, so why not have a structure inspired by the body of an instrument? With curved sides and graceful contours, along with a silhouette that feels light and dynamic, like a sail stretched by the wind. Organic shapes and techno-friendly surfaces – playful, exotic, but not illogical. Somewhere between a tonal sculpture and a sound machine.
To build something like that, I need to create “tensioned” side surfaces, in other words surfaces that were curved in three dimensions. The idea: thin multiplex wood is glued to a correspondingly shaped, stable frame. The curves of the wooden frame interact with the material tension of the multiplex to create an even arc. The problem: it only works with thin wood – and I’m not trying to make a resonant body, but a loudspeaker. So I have to create stability with double walls and filling in between them. Easier said than done – what that means in practice is a lot of parts, complicated cuts, a difficult assembly, and hoping that nothing breaks (or if it does, then hopefully right away, not at the first clash of the kettledrum!)
I start with extensive bending tests using 3-mm birch multiplex. How does the layered wood respond to tension? How tightly can you bend it before it breaks apart? After a few days of trial and error, I begin to trust the material. I create a 1:1-scale drawing of the cabinet, specifying only the main dimensions (e.g. the chassis installation mass and the exact shape of the frame). All of the details will be improvised later, as the construction progresses.
In this design, the necessary volume for the bass is created by a pointed, hollow body, whose opposing sides are either curved or at diverging angles from one another. No parallel interior walls, no standing waves – that can’t be bad, in any case. The necessary net volume of about 20 liters needs to be roughly estimated in advance by calculating the area of many different cabinet shapes. A lengthy drawing phase follows, until the cabinet proportions look balanced in the drawing and the volume is right, too. Not a trivial process for a hollow space that is about as regular as a cough drop. But I want to do the whole project in my workshop at home, without any CAD support: pencil, compass, ruler, drill, router, miter saw and jigsaw. Nothing more. Back to the roots!
And off we go.
Around Easter 2012, I order the Eton chassis and the crossover components from Intertechnik. I knew it would take a while before they could make music together. But I had no idea it would take more than a year…,
But let me start from the beginning. The first step is creating a large number of curve templates that can be used as drawing and router jigs for the rounded cabinet elements. For these, I use two-millimeter-thick aluminum profiles, bent “free-form” into the desired shapes until they match the scale drawing. The material tension alone creates perfect, even curves. These are then transferred to multiplex wood, cut out, sanded and then later used as drawing templates for the contour router.
The surrounding cabinet frame, consisting of the baffle board, lid, floor and the very narrow rear wall, is made of MDF sections, which are first cut out roughly using a jigsaw and then rounded off with a router using the abovementioned curve templates. Each component already has all of the necessary grooves, cutouts and holes that it will need later. Reinforcements are placed in the four corners, defining the precise assembly angles of 94.5°, 62°, 113.5° and 90°. The inside of the baffle board already has a generous phase for the bass. This large chassis opening cannot be completely cut out yet, because the very narrow baffle board would break right away. But there is already a corresponding groove in the front, so that only the remaining 3mm of wall needs to be cut through later on.
To make sure the frame isn’t distorted when the multiplex surfaces are attached, you need a solid but temporary reinforcement. This consists of a 19-mm-thick MDF board, which connects the entire surface of the front and rear wall during the construction phase. The board is exactly the same height as the bass cutout, and is firmly glued onto it. So that they can both be removed together later on, the board is only connected to the narrow rear wall with screws. The plan: after finishing the body, the bass cutout will be completely separated along the circular groove; the screws on the back will be removed, and the reinforcement will be pulled out. If the cabinet still warps after that, it will need to meet its maker as firewood. Hot air instead of inspiring sounds in the living room?
Now things get tense – in the truest sense of the word. The first of the four 3-mm multiplex sheets, cut slightly larger than needed, is affixed using assembly glue. Before that, 3 20-mm wood screws were installed along the 2-meter-long glued joint, very close together. Apply the glue, position the board, correct it, press it down, tighten the screws, take it back apart, correct it again, screw it in … good thing I have eight hands – six of them borrowed from the best wife and the sweetest children in the world. The screws hold down the freshly glued board, since the round shapes make it very hard to use clamps. Once the glue has hardened, the overhanging boards are evened out with the router and the screws are countersunk. It worked. Once, at least.
A quick side note: All of the necessary “Damping 10” cutouts are made right away for the interior wall insulation, since this is the only time – with the cabinet still half open – that they can be precisely measured. Then I glue on three more multiplex sides. Does it stick? It sticks. The glue sticks, the screws stick, and the multiplex sticks, too. Two rough bodies are finished, nothing broken or warped: no firewood … yet.
To the right and left of the baffle board, I now glue on three layers of wooden slats along the full height – each one 5mm x 30 mm thick, with countless horizontal cuts so that they fit perfectly along the round baffle board shape. The exterior multiplex wall will be attached to these slats later on, and then the resulting hollow space will be filled in. That will create the three-dimensional curved sandwich walls. Assuming everything goes well, that is …
At the pointed lower end of the baffle board, where the base will be attached later, the body is given a precise cut with the miter saw. To do so, the cabinet is attached to the saw table with millimeter precision. The cut is at an exact right angle to the center axis of the baffle board, and angled 4.5° backward. This cut surface, only 15 square centimeters – together with the center axis of the baffle board – is now the only reliable reference point for all of the angle and length measurements in the body. Since there are almost no right angles, and most of the edges and contours are actually round, it is a constant challenge to measure and position the cabinet and its components. Accordingly, there is quite a bit of time between the subsequent work steps because of creating overlays, screens and templates.
Now it’s time to separate the bass cutout and take out the interior reinforcement. If the multiplex boards on the left and right sides pull with different degrees of tension, the cabinet will be distorted and end up as firewood after all … but everything stays balanced. With two screw clamps serving as the base, the delicate body now faintly resembles a combination of a rough cello and a harp.
Finally, we can determine the actual net air volume. Was it calculated correctly? What happens if not? If it’s too big, you can fill it in, but if it’s too small? Sort of like Karl Valentin’s saying, “I cut it off three times and it’s still too short…” Then comes the confirmation: 21 liters of granulate can fit inside. Once it drains back out, that means 21 liters of workshop air pour into the cabinet. Done! Belated thanks are given to my math teacher and his famous geometry lessons; don’t tell me you never learn anything useful in school !
In the next assembly step, the cabinet gets its exterior sandwich walls, and thus its final exterior shape. To this end, the top and bottom of the frame are first covered with a second layer of shaped MDF boards, and additional solid slats are glued onto the narrow rear wall. The frame now has a 15-mm-tall edge all the way around, onto which the exterior multiplex walls can be set.
The final shape of the exterior walls is now developed in detail using a 1:1 cardboard cutout. This in turn is transferred to a wooden template, and the now-proven 3-mm birch multiplex can now be used to cut out four precisely identical side walls with the profile router. They really do look like windswept sails, and their wide, curved edges make the loudspeaker cabinet look like an open, almost floating sculpture.
Now there are four more rounds of the same: apply glue, position the board, correct it, tighten the screws, correct it again, screw it in again … and hope that nothing cracks this time, either. But Murphy’s Law takes pity on this project, and everything works. In the connection area for the still-missing base, I will set up a separate “office” for the frequency crossover later on; that’s why there is already a large opening for the cables. There are also two slit-shaped openings on the bottom, to provide access to the hollow wall areas. The casting material will be poured in here later, giving the side walls more mass and stability.
The structure is still light and easy to handle, though, so before the baffle board is installed, I create a removable puzzle of eight interlocking components, made of MDF, multiplex and plastic. Once again, they’re all curved: build the templates, measure, dowels, screw the pieces in, smooth the edges, adjust … for many weeks, the shriek of the router and the roar of the jigsaw are the only music I hear. I start to regret making the design a little on the complicated side. I’m never building loudspeakers again! And if I do, they’re only going to use six or seven boards …
Measurement purists might want to skip a couple of lines here. The series-built, somewhat rustic Eton AMT will be visually completely integrated into the baffle board. To this end, it is given an MDF frame with a cover of whisper-thin black nylon fabric, as well as two flat polycarbonate covers. They are angled in the direction of the sound output, and rounded. Proponents of the high-end purity law will object that the corners near the sound output influence omni-directional behavior and the amplitude frequency curve. But still. The Design department pushes them through, concerns from Development are ignored, and the Marketing strategist has already gone home for the day.
Meanwhile, the building sketches for the base are ready. It supports the body from the back on four vertical pillars, which give it a light and floating feel. The structure is made of MDF, solid wood slats, bent multiplex, stainless-steel pipe and aluminum profiles – a total of 42 components each – plus screws, dowels and pegs. After all, why make things simple when they could be complicated? Bent supports extending far out the back guarantee stability. They are shaped on a wooden bending template made of 5x20-mm-thick aluminum profile, using raw muscle power, then cut, filed and drilled.
Time to screw/stick/clamp everything together and subject it to a critical test. Does everything fit? Are the proportions right? Is the structure stable? At the back end of the supports, transparent yellow inline skate wheels act as the base and a visual accent. I like those kinds of found objects – there’s always room for a little humor …
Before the casting phase gets underway, the body is spackled, sanded, primed with black paint and given additional interior cross-bars made of beech dowels. These provide maximum stability while taking up the least possible space. Just in time for winter, there is also a cozy felt layer for the interior walls, using “Damping 10” from Intertechnik. Fortunately, all of the felt pieces are already cut to size.
Christmas is just around the corner.
And Santa … in the form of the UPS courier … brings me some acrylic resin. The two layers of multiplex on each side of the cabinet surround a 15 to 18-mm-tall hollow space with exactly 2.5 liters of volume – measured by filling it with sand. This hollow space will now be cast with a compound of river rocks and acrylic resin. That should keep sound permeation and natural frequencies from being an issue; besides, the high mass of this composite will provide more than enough inertia to counteract the bass chassis.
In order to fill in the gaps between the 2.5 liters of gravel, you need exactly one liter of acrylic resin, according to a simulation with water – done in a measuring cup, of course, not in the wooden cabinet. 2.5 liters of rocks plus 1 liter of resin equals 2.5 liters of total volume – that’s logical. But what filler gives the resin the ideal consistency to support the gravel really well? How hot will it get while it’s curing? Sample castings are evaluated in transparent CD-ROM cases, and the precise ratios are defined. But it’s impossible to predict how the glued wood joints and the tensioned multiplex will really act when heat is applied. If something goes wrong now, the junk won’t even be good for firewood – rocks and artificial resin aren’t very good fuel.
2.5 liters of gravel will completely fill all of the hollow wall spaces. But I only mix the filler in small quarter-liter batches – “shaken, not stirred” – and pour it in carefully. The chemical curing reaction releases so much heat energy that you can’t even touch the surface anymore. The structure is now subjected to extremely high temperatures. As it cures, the temperature in the hollow wall spaces sinks, and the next batch of resin is mixed and poured in. In this way, the cabinet wall is gradually filled with resin, one layer at a time, and only a small area is heated up each time.
Everything goes well. The assembly glue holds, the multiplex doesn’t split, and after that intense heat treatment, it is probably very much looking forward to its future “non-tense” life as a loudspeaker body. A whitish resin-gravel mixture now fills the double cabinet walls. There’s relief, even though the total weight of the body has just increased tenfold, because the experimental part of the project – the tensioned multiplex sandwich with artificial resin filler – seems to have been successful.
Breathe a sigh of relief.
From this point on, I am making all of the assembly parts that will complete the design of the loudspeaker: baffle-board and cabinet covers made of lacquered MDF, support legs wrapped in foam rubber, and the brackets for the protective front cover with plastic strings. The design sketches for these parts are transferred directly to the MDF using a pencil, ruler and compass, and are then cut out with a saw and router. A “low-tech” approach rather than CNC – it doesn’t always work on the first try, but that makes it all the more enjoyable when you’re holding the finished product in your hands.
Between the two chassis elements, an unusually shaped bridge will later keep the strings away from the bass membrane – a “citation” from the field of instrument construction. The bridge itself is cut out with the routing template from two-layered, glued MDF, and then mounted on the lower section with 2-component glue – another found object: a metal furniture pull with two handles. Painted matte black, the whole thing looks like it was made of a single piece.
Time to focus on the cabinet’s finish. The surfaces are supposed to look a little “high-tech,” as I mentioned earlier, and my favorite in these cases is a combination of titanium-gray and carbon-black. I use stainless-steel pigment paint from Duplicolor – which creates a very nice-looking metallic finish – and Di_NOC from 3M – a decorative carbon film that is often used in vehicle construction. It’s ridiculously expensive, but amazingly beautiful and fantastic to work with.
Like the gluing of the multiplex sides, working with the film is a family event. The real challenge is not so much gluing it onto the curved surfaces without bubbles as it is creating clean edges and corners. Eight hands are barely enough: they can hold the fan (since the film is applied hot), remove the protective film, unroll the film, press it on firmly, and wipe the sweat from my brow – my body temperature shoots up not just because of the hot air, but also because of the adrenaline-pumping idea that one of the big film sheets, the same price per square meter as Italian marble, could be stretched too far or get wrinkles. Not for the faint of heart. The reward for my anxiety: a surface that looks as beautiful and lively as silk. As a nice contrast to the titanium-gray and matte-black painted surfaces, the Di_NOC surface changes depending on the direction of the fibers and how the light hits it, and the fine lines wonderfully emphasize the sculptural cabinet shape. Great stuff.
First I apply the stainless-steel paint over all of the primed surfaces, then sand it with #800 sandpaper while it is still wet. Then comes another “liquid” coat and the intermediate drying. To create a truly metallic-looking surface, the final coat is applied as a very fine spray. To do that, I heat up the paint can in a water bath to about 60°C. (Attempt at your own risk – I assume no liability for the explosive accidental painting of a workshop or kitchen …). The excess pressure produces a very fine mist, which I apply once or twice from a great distance, very evenly and so thinly that it does not run. That creates a silky, shimmering coat of stainless-steel pigment, which very closely resembles the dark matte shine of titaniumt.
Sanding, spackling, sanding, priming, sanding, painting, cutting film to size and applying it – that all takes time. When I asked for advice a year ago on Easter because of a small technical detail, I said I was expecting to be done at the end of the summer and would be happy to write a construction report for this forum. The end of the summer might be about right – good thing I didn’t say what year …
But now, after more than a year, I’m on the home stretch: soldering the frequency crossovers. Components along the signal path are directly connected using silver solder, without any cable bridges, and the coils are at opposite corners of the circuit board. The finished crossovers are screwed vertically onto the support feet, where they can do their work without any interruption from bass-related attacks on the air pressure. Between the two rear stainless-steel supports, there is an opening in the cabinet box to provide the circuit board with fresh air so that the resistors never get too hot. The cover for this opening is another found object: the cover of a cosmetics package, coated in stainless-steel paint.
The home stretch after a marathon, you might say. Let’s not get over-confident! 25 kilograms of dry weight are no problem as long as the cabinet openings can still be used as handles, before the chassis and reflex tubes get in the way. First, though, the base with the pre-wired frequency crossover is installed. Thirteen screws and dowels, together with the four rear supports, connect the base plate and the body so securely that the significant leverage forces from the heavy cabinet are safely transmitted to the ground. From now on, the loudspeaker stands on its own feet.
The tweeter is squeezed into its MDF frame, soldered in and mounted. Behind the dark nylon cover, all that can be seen are its distinctive sound slits, emphasized by two reddish lines and framed by two covers, which visually blend in with the cabinet thanks to their carbon covering. To ensure that this covering does not produce high-frequency vibrations, a soluble glue binds it to the AMT’s assembly panel.
Time to glue on the painted baffle-board cover. Overall, the baffle board now consists of three layers of MDF with a total thickness of 30 mm. The top, rear wall and bottom of the cabinet are also given their painted covers; they are glued in three layers and measure up to 32 mm thick; then come the solid wooden slats and horizontal reinforcements. Can’t hurt.
The reflex tube at the top is given a felt sweater on the outside, and the inside end is covered with a very wide-meshed, fine nylon fabric. That keeps the loudspeaker from being used as a piggy bank, but it offers almost no resistance to air movement. Before the tube is pushed into the cabinet, it receives an arched collar made of foam rubber at the outlet – a nice contrast with the matte black paint and the shimmering carbon surface. Incidentally, foam rubber – sometimes also known as “sponge rubber” – is a great sealing material, but it is also very decorative and amazingly resistant for surface applications. It is easy to cut, and once it has been glued onto wood, it can even be sawn and drilled. Circular cuts are easy to accomplish with a compass, scalpel and a little bit of skill.
The attachment holes for the bass are pre-drilled using a stable wooden template, whose long guides keep the drill at a perfect right angle in the correct places. Solder the chassis, screw it in, done. Almost.
High-end purists should skip a couple more paragraphs here. The beautiful greenish glow of the Eton Hexacone membranes simply didn’t get along with titanium-gray and carbon-black. Can you paint something like that? I admit that I was seriously worried about the idea of unleashing a catastrophic effect on two brand-new loudspeaker chassis elements because of uncontrollable solvent reactions. But there was no choice – the membranes were painted. Or rather, “dusted.” I used a mask to cover the surround and the edge of the basket, and then applied a very thin layer of matte black paint from a large distance – so thin that the original gray-green color of the membrane actually shows through in the sunlight. I was in luck: there were no chemical reactions from the solvent, no catastrophe, everything “peaches and paint,” you might say. The Design department won out once again, the concerns from the Development lab were waved away, and the Marketing guy was off early again.
Even if the sound specialists still have their doubts, I want to claim with blissful ignorance that the increased weight of the membrane is still within the natural manufacturing tolerances because of the micrometer-thin layer of pigment; I will be happy to forward any discussions about its influence on partial vibrations and sound distribution in the membrane to the specialists in the “High-End Esoterics” department. For my part, I am happy about the two wonderfully silky, shimmering black, unique Eton pieces. And the chassis elements also get a flat ring covered in Di_NOC film, glued on over the edge of the basket and the surround base, so that they are visually better integrated into the baffle board.
The front cover made of round plastic cords protects the bass membrane from curious fingers, but because of its low initial load it does not vibrate – it really doesn’t, I promise. The small logo adds a colorful accent to the front, to go with the tweeter covering. And because the evening sun happens to be spreading a warm light and good cheer throughout the living room, let me take a couple more pictures. You never know, the exterior walls could still crack with the first kettledrum …
Finished. It is now the summer of 2013.
It took a long time, but it was a lot of fun. For the statistics: I created 150 components made of wood, plastic, rubber and metal for each of the two loudspeakers, and they are held together with more than 200 screws, pegs and dowels. Then there are the electronics and drivers, glue, resin and gravel. That comes out to 25 kg of inert mass, distributed throughout 970 mm of height and 450 mm of depth. The body is 220 mm wide, and balances on a 350-mm-wide base.
As I said in the beginning, there’s definitely a way to enclose 20 liters of air in a less bulky design. You could also easily set a traditional 70-liter box on the surface of these loudspeakers. So it’s pretty big for a little 20-liter loudspeaker. And it’s almost too small for an almost one-meter-tall loudspeaker. That’s true. Still, the point wasn’t to make the best possible use of the living room, but to create an exotic shape using an “experimental” construction approach.
That went really well. A lot of it was easier than I expected, for instance making the curved sides. And of course the devil was in the details. The biggest challenge turned out not to be the construction itself, but the limits of my own patience. Still, the hobby is called “loudspeaker building,” not “loudspeaker owning,” and in fact building it was as much fun as listening to it is now. And I did end up with a little firewood after all: the cutouts and wood scraps made for at least one well-tempered fall music evening …
And how does the construction sound? Does the bass sound a little “blacker” thanks to the dark membrane paint? Does the tone sound “rounder” because of the curved cabinet? All joking aside, the shape of the loudspeaker, that wild mélange of machine and instrument, tells you a lot about how this legendary Eton chassis combination sounds: as dynamic as a Lamborghini and as elegant as a Stradivarius. Phenomenal. How much the tone differs form an original Duetta Top is hard for me to say. I run this “special model” from a good old Luxman amplifier with a Class A pre-amp, whose silky tone harmonizes wonderfully with the lively AMT tweeter accordion. After a few weeks of breaking them in – and the Eton drivers really do need it – I can understand and confirm everything I’ve heard and read about the fantastic sound of this assembly kit. Once more in the words of Karl Valentin, “Everything’s been said about it already – just not by everyone yet.“
Thank you to the loudspeaker-building team at Intertechnik for their wonderful “Duetta Top” and the great forum! I would be very honored if anything in this project serves as inspiration for your own constructions.