When we started building our own loudspeaker boxes several decades ago, SEAS was already a big name in the industry; every serious retailer had a whole collection of structures using their chassis components, given that they were the best thing yet invented for enjoying music. This time, with MS Micro and D’Appo 4, very small boxes provide huge listening pleasure.

Taking the ferry from Denmark to Norway is a wonderful trip. If you have good luck and good weather, you’ll remember the experience fondly for a long time. After a five-hour trip through the expansive Oslo fjord, the little town of Moss pops up on your right – a place that the loudspeaker world has long known about. There, almost hidden, is where curious visitors will find one of the best-known chassis manufacturers in Europe: Scandinavian Electro Acoustic Systems, better known to all of us as SEAS. Tradition and innovation, technology and environmental protection are not in conflict for the company’s employees, who are also the owners. For many years, they produced the same sound converters, with consistently perfect quality, that were also responsible for properly reproducing music in many ready-made boxes. For some time now, though, we have seen an ongoing change in the chassis constructions and the disappearance of old familiar membrane materials like polypropylene and XT; even the old paper components look very different today. Naturally, all of the goods are still manufactured in Moss, where counter to all expectations for a major loudspeaker factory there are just two assembly lines ending in a large warehouse, its shelves filled to the ceilings with many hobbyists’ objects of desire. Wherever possible, the products avoid using environmentally harmful components. In fact, that’s almost not worth mentioning – any place where the company owners and their children live themselves, and where their daily food is grown in the rural surroundings, is not going to be the target of Greenpeace protests anytime soon.

The shift toward new manufacturing technologies some years ago is not surprising. Like every serious chassis manufacturer, SEAS now has better control over defects in its goods thanks to laser interferometers and Klippel equipment. That means they can make cone and calotte speakers with all of the appropriate sophisticated touches.

Now, dear reader, you are probably asking yourself what all this has to do with the assembly kit we are describing here. Well, nothing really, but it does tell you a lot about the Norwegians’ company philosophy. Instead of setting up new factories in low-wage countries, like most of the industry, and flooding the market with mass-produced goods, they are betting on local production with the usual high quality standards and series consistency. In contrast to many of their competitors, the Scandinavian chassis elements not only look the same, but they are also the same on the inside. At this point it’s worth mentioning that the prices for SEAS products are in a much higher price range. But as they say (with good reason) when money is tight: If you want a good product and buy a cheap one, you’ll end up paying twice.

Equipment

After our long but necessary preface, we now come to the reason for this article: the small boxes featuring SEAS equipment. The model for the Micro is of course the popular assembly kit of the same name, which was equipped with the distinctive square W111 and the 19-mm K 21 FD dome tweeter with an aluminum membrane. Despite – or maybe even because of – its size, the first SEAS MS Micro was one of the company’s best-selling constructions.

What people liked less, though, was the basket shape of the mini-bass, which often drove hobbyists to despair when they had to cut its contours out of the baffle board. Those baskets are history now, and their successors – the CA 12 RCYs – are a convenient round shape. The coated paper membrane cannot be unwound, and it has a sturdy dust cap made of permeated woven fabric that moves like a rigid piston when it is pushed down from the front. But if the membrane is pushed from the back between two of the six flow-optimized ledges, it bends very slightly at the edge; you can almost see the perpetrator’s fingerprint on the other side. The magnet system is the same, with a #90 ferrite ring and #6 pole plates. The large openings under the flat centering spider give you a glimpse of the vibration unit, with its 22-mm diameter and 12-mm coil height, which allows a linear lift of 6 and a maximum displacement of 9 mm. The deviations for the TSP parameters were completely within the range of the measurement tolerance compared to the SEAS data sheet. Various measurement flows and environmental conditions are responsible for the differences here, while all of the chassis elements measured were almost equal in value.

WAs the name indicates, the NoFerro 800 TV tweeter belongs to the category of ferrofluid-free calottes with magnetic shielding and about 800 Hz of natural resonance. Its membrane, made of coated silk, is centered in the 2-mm-tall air gap using a wide polymer surround to prevent wobbling. This is especially important because there is no oily liquid to prevent friction losses at the pole plates. Since the reintroduction of this lubrication-free method, which was previously the only possible construction, SEAS has continued to optimize the necessary membrane-guiding technology. Thankfully, they have also put a huge selection of non-cooled calottes on the market. The main advantage of giving up even higher durability is the lower weight that the voice coil has to move through the air gap as an adherent mass. That means the membrane is able to reproduce even the smallest signals, which would have long since disappeared into the magnetic soup with ferrofluid calottes. At the same time, though, the improved ventilation of the surround material should not be forgotten; without the unnecessary hindrance in the air gap, the air behind it can escape into the magnet chamber.

This is also where the pole-piece hole, filled with a foam rubber stopper, comes to an end; it prevents the second impedance peak that is often seen as a reaction to the hole length. The voice coil is glued to an aluminum carrier with slits at irregular intervals. The plastic front plate, onto which the entire vibration unit is cleanly glued, is forcibly centered using four screws, so even a layman without any engineering background can put in a cost-effective replacement part in the event of a defect. Only a few manufacturers offer this service for their tweeters, even when they cost several times more than the SEAS calottes.

Cabinet and insulation

It’s almost traditional not to limit ourselves to one kind of chassis combination; we like to give do-it-yourselfers alternatives or additions at the same time. That means they don’t have to follow the saying, “Out with the old, in with the new” when changing their living room or stereo constellations – for the most part, they can keep the same familiar sound. That’s why once again, we’re offering two variations on a theme: MS Micro and D’Appo 4. Both have a volume of 8 liters per CA 12 RCY, the same reflex tuning and the same baffle-board width. Mathematically speaking, according to LspCAD, 2 liters would have been enough for each chassis, but the resulting reflex channel lengths would have meant winding them around the cabinet several times. Instead of a tube, we glued a matching board onto the floor of the boxes. To make sure they would fit on any shelf, we cut every extra centimeter we could from the height of the Micro. That meant it needed to be correspondingly deeper so that the enclosed space wasn’t too small. The construction consists of exactly seven boards, and there’s no need for internal reinforcements, which make the assembly harder without providing any benefits. The crossover is glued to the rear wall behind the tweeter, and the interior is filled with two Sonofil mats (1 bag). The only concession to the visual aspect are the small grooves around the side boards, which make the little box seem even more delicate than it already is.

The structure of the D’Appo 4, which as a free-standing box naturally needed to be built upward rather than outward, is not much more complicated. In order to put the tweeter at ear height, we set it on a crossover chamber. Although do-it-yourselfers often say that putting the components on the outside of the box improves the sound, we can’t confirm that here. We have yet to see a significant difference when we glued the crossovers into the cabinet after they were developed outside, thus exposing them to opposing pressure conditions 60 to 400 times a second. Because of the narrow walls, we also skipped the additional reinforcements and stabilizers here. The slim silhouette also reinforces the groove running around the side, and two bags of Sonofil fill up the box evenly.

Crossover network

There are different ways to develop frequency crossovers. One person might take the manufacturer’s measurements and plug them into his preferred simulation program. He puts together the pieces based on the resulting values and is happy that the tweeter plays high notes and the woofer plays low notes. That’s all this approach is good for. Another useful method is to put the self-created measurement data into the simulation, then to disconnect as many circuits as possible and set the program to “optimize.”  That at least creates usable results, with a fairly smooth frequency curve. During later measurements, you usually only need to make a few adjustments for the simulation and the reality to match up. Unfortunately, this process often tempts people to remove all of the peaks from the curve, even the small ones. The signal can often get lost among all the components, and then it barely makes it out of the intended membrane. So this kind of crossover development also requires a lot of experience to achieve good results. The great advantage here is in using fewer components whose legs tend to break off just at the wrong moment after you’ve used them a couple of times. Still, we should mention that even this observation, while plausible at first glance, is only half true. In order to gather experience, there’s really no way to avoid building large numbers of crossovers, and that means having all kinds of hardware in the form of coils, capacitors and resistors. Since you also have to take measurements after the simulation, you can also – again assuming plenty of experience – start with the measurements right away and save yourself the trouble of entering the data into the computer.

Even while measuring the frequency curve of the CA 12 RCY, we noticed the distinctive hump caused by the baffle step around 800 Hz, followed immediately by a serious dip at 1.8 Hz (green). We have noticed this quirk for some time now in all of the new SEAS paper membranes.

Now, it’s not a crime to put an anti-resonant circuit in the signal path in order to bring the mountain down to the valley, and then to force a smooth frequency curve with the remaining crossover at the expense of the volume. But we should remember that we’re dealing with a dwarf loudspeaker, one that will sound much too thin if it tries to come up with the whole mid-range. It was by no means the chassis developer’s goal to create a mid-range speaker with the CA12 RCY. The little guy should be able to provide a little bass pressure and undertone warmth, so the loss in volume can actually be helpful. So we didn’t even try to fill in the middle, but filtered it right away with 2.2 mH (red) and 4.7 µF (blue). That also dropped the resonances at 7 and 9 kHz down low enough that they couldn’t disrupt the other tones.

At 3.5 kHz, the narrow baffle board creates a deep indentation in the amplitude of the NoFerro 800 TV, which we think is one of the best calotte-based tweeters (purple). That didn’t worry us, though, because it had already disappeared in the 15-degree curve. First we lowered its volume by about 7 dB, using an L regulator at 3.3 and 4.7 ohms (green). At 4.7 µF (orange) and 0.33 mH (red), I thought the curve dropped off a little too soon at lower frequencies. So we replaced the capacitor with 5.6 µF and were happy with the results (blue).

At the same polarity, the total of the two filters produced a mid-range trough almost an octave wide, forcing the voices back a little and creating the illusion of spatial depth. At the same time, tonal errors caused by exaggerations – which can be especially disruptive in this area – don’t stand a chance of scraping your ears. We can live with the justified complaint that we haven’t built honest loudspeakers this time. Because why in the world should every box be used to analyze the faults of the recordings? Small boxes can quickly become annoying because they’re missing the foundation “down below”; the MS-Micro intentionally has more of that than you would expect.

The history of the MS-D’Appo crossover is easy to tell, because there isn’t anything too special about it. The parallel CA 12s initially provided a prettier frequency curve than the lone warrior in the Micro. The goal here was to keep the sound level at the 88 dB already available at 300 Hz. To do that, we needed a 1.2-mH coil, which naturally had too little effect on too-high frequencies (red). 10 µF helped energetically here, even giving us a volume gain up to a good 3 kHz (blue).

The taller baffle board largely saved the tweeter from the hole at 3.5 kHz, but two resistors with stole about two dB of its sound pressure. Unlike its sister, we connected it a good bit lower, with 2 inductors, which strongly compressed the dip in the middle

Measured values

Even though the Micro was not designed to fill large living rooms with sound, its 84 dB of mid-range sound pressure and its -3dB point of 70 Hz mean that it no longer belongs to the class of quiet speakers with a 12-cm bass. Even in the 90-dB noise (Klirr) measurement, it has impressively low distortion values. Only below 300 Hz does the K2 increase above 1%, and K3 through K5 are still far from that. We performed the angle measurements at 5-degree intervals from 0 to 90 degrees on a turntable. Unfortunately we couldn’t place the rotation center on the baffle board, so the volume decreases as the angle increases, due to the change in distance from the microphone. However, it does have the advantage of keeping the curves from turning into an ocean of colors; instead they are almost all visible individually. The second waterfall, which in reality shows the clustering behavior, is made up of all the measurements, while the angle diagram only includes the 15-degree intervals between 0 and 90. We will leave the interpretation of the remaining results up to the expert reader. The minimum impedance, at 5.4 ohms, is 6.5 kHz; the maximum, at 33 ohms, is 1220 Hz. Using 33 µF _ 0.49 mH _ 6.8 ohms, it is filtered out for tube operations.

As some of you might expect, the volume level of the D’Appo 4 is not 90 dB, which theoretically should have been reached by doubling the CA12. Still, an average of 88 dB isn’t too bad for such a skinny little guy. The 90-dB noise values are correspondingly lower for this box. At 4.1 ohms, the lowest impedance for the D’Appo is 470 Hz; the maximum of 18 ohms at 1350 Hz is smoothed out by the absorption circuit consisting of 33µF _ 0.39mH _ 5.6 ohms

Sound
The listening test started with the SEAS Micros, which we set up next to the wall and ran off of the SAC system made up of an Epsilon-VV, La Forza monoblocks and a tuned Teac CD player. Their offerings weren’t bad at all. The resolution of the fine structure in the high range of Berlioz’ “Symphonie fantastique,” performed entirely by percussionists, as well as the unexpected precision of the loud sounds all the way to the upper bass, left quite an impression. The only thing that bothered us was the proximity to the wall; since the eyes could see it as a limit, it was hard for the ears to imagine a stage behind it. So we set up the little boxes on stands farther into the room, and the recording space sounded completely different under the new conditions. Left in that position, Deborah Henson-Conant was able to coax a “Baroque Flamenco” from her harp. What was surprising was the size of the instrument, which we wouldn’t have expected from such small boxes. The excitement in Deborah’s voice when she asked us to “Dance With Me” spontaneously elicited some hefty foot-tapping. By now it was clear that the little Micro, despite its low efficiency factor, was good for more than just dissecting music. We needed the tube! We quickly put together an impedance correction made of a capacitor and resistor, and Ray Charles landed in the CD player. Now he wasn’t just singing “Fever” from tiny boxes; he was accompanied by an acoustic bass at almost its original size. The lightweight membrane had its share in the wonderful clarity of the musical event. When there isn’t too much mass in motion, both the rises and the decays are short. The little box with the sympathetic voice does find its limitations when more volume is required. For instance, Deep Purple’s “Child in Time” was tonally perfect, but just too quiet for rocking out.

So it was only natural for the D’Appo 4 to come marching to the rescue, although it let us keep heating up the glowing pistons. The boxes harmonized with the good dominant tone and gave a brilliant debut with Patricia Barber’s Hammond organ, voice and Michael Arnopol’s bass. The title couldn’t have been any more apt: “Use Me!” Hugh Masekela gave us musical fireworks with “Lady.” The percussion, bass drum and electric bass were on a stage wide open to the back, with perfect resolution and impulses. It was helpful here to have the slight dip around 1.5 kHz, which prevented the voices from being obtrusive and the instruments from being foregrounded. Some of you might say that wasn’t free from sound effects. Our response: “That’s fine, as long as you like it!” How else are tiny loudspeakers supposed to give listeners the illusion of being much larger than their dimensions would allow? Unlike the well-known speakers that whitewash sound, we didn’t erase the attacks entirely; we just made the overall tone a bit warmer.

Memories of a bygone era, now twenty to thirty years in the past, came back when we finally put on the CD “Joe Cocker Live at Rockpalast.” This present from a sound-engineer friend, remixed from the original radio recordings of the Lorelei performance in 1983, let a much younger version of this rock-music hero stride into the room, filling the space with music. At the end, when he intoned his beloved hymn “With a Little Help From My Friends” with his characteristic emotionality, the audience roared and demanded an encore. Will we see you for a listening session?

Udo Wohlgemuth