Anyone who takes the effort to write a special-interest magazine, whether in print or online, gets most of his motivation from the readers’ responses to his work. Creating a positive response, not just in the sense of entertaining reading, is the be-all and end-all for all “self-promoters” – it is their way of creating a small legacy for themselves. We readily admit that, in the guild of scribblers, we are no exception to this rule – the applause from the audience is our bread and butter, too. Our readers’ desire to actively participate in the magazine has grown significantly. The reader response column is not the only active area; the Questions & Answers in the forum and the reports constantly provide us with new suggestions, some of which have developed into actual assembly suggestions solely because of the corresponding inquiries from readers. It’s not that we no longer have any original ideas for packaging a chassis in a marketable container. But our goal of building boxes that the world actually needs is much more accessible if the future users define them for us. It was this kind of close collaboration that brought us our latest baby. As is often the case with unexpected adoptions, it was already fairly big by the time it came to us. To distinguish it from all the others, we technocratically named it the SB 240.
As had often happened in the past, someone asked about an expansion; this time it had to do with the SB 18, the first Blues Class box with a unit price under 150 euros. That gave us the first two chassis elements in the box, namely the SB17NRXC35-8 and the SB26STC-C4, which can also be purchased separately from Intertechnik. The expansion actually existed already, because the single-bass box can easily be made into an SB 36 with twice the membrane area – what Boris had once requested as a more powerful version of the single-bass box. But we needed a little bit more, according to the feedback from more than three people in less than one day, and they even had a common suggestion: two SB23NRXS45-8 units. These are familiar from the SB 23/ 3 assembly kit, which took up quite a bit of physical space and was also conceptually impossible to overlook. Instead of taking up just as much space to describe the visual and technical advantages of the SBAcoustics chassis, let us refer you to the measurement presentations in past editions, under the chassis test.
One piece or two was the next question before we started drawing the SB 240. Certainly some people would be happy if they could keep using their SB 18 by setting it on a newly constructed bottom piece. They would just have to cram a total of 100 liters into a little cabinet about 75 cm tall. We won’t do that simple calculation for them – after all, it’s called DIY for a reason. Now, even if it’s a bit out of place here, we need to make a side observation. Clearly it is the norm these days to expect everything to be neatly laid out for you instead of racking your own brain or at least the internet for information. Not that we have anything against asking questions, but when it becomes clear that our magazine is only being read as meaningless entertainment, it is a little annoying. Enough scolding – we were young once too, and we had better things to do than taking all that black-and-white information, in those days still printed on paper (and known as a “book” when the pages were bound together) home with us. Still, there was the priceless advantage that we could place them under our pillows, which was enormously helpful when it came to making ourselves smarter during sleepless nights.
Back to our topic: two pieces would have suited our natural laziness if we had still had the SB 18 available for a demonstration. But whenever “if” and “but” are on the agenda, reality steps in and says, “no way!” So we ended up with a one-piece box after all, and the 7-inch speaker is more than happy with just 11 enclosed liters. The right place for the tweeter is at ear height, so we positioned it between the basses and the mid-range speaker. That way no one has to climb a ladder to listen to music. The 100-liter volume for the basses simply requires a height of 120 cm to create a halfway decent width and depth.
Assembly plan for the SB 240 kit as als downloadable Sketchup-file
And with that, we are once again well off topic, because a demonstrable cabinet assembly takes time, which is always in short supply for us despite our strong organizational skills. The assembly itself is child’s play, but creating a clean surface takes time. Good advice turned out to be valuable here, because the hardware store had some very tempting laminated furniture boards, measuring 120 x 40 cm, that plausibly represented beechwood. We got five of the boards for just under 70 euros; the man working the saw cut off half a centimeter along the longer side of each. That gave us some unfinished edges, which are easier to put together with joint glue. He was happy to cut up one of the boards into four sections for us, measuring 39 x 26.2 cm, which created the lids and floors for our boxes. To take up even more of his time, we had him cut four black-dyed MDF boards to 120 x 30 cm. He cut the reinforcements, measuring 26.2 x 10 cm, and the small boards for the mid-range chambers out of a scrap of particleboard, which made our wallet a total of 60 euros skinnier. From the resulting mish-mash of wood, we managed to put together two fairly similar-looking boxes at home. As you might expect, we even have photographic documentation. In order to avoid taking up too much space on the web, we kept the pictures small. Simply click to enlarge them.
Cross over network
The next fork in the road appeared in the measurement room. Two different crossover topologies, both with good sound results, were competing for our affections. While we had followed the SBAcoustics suggestion pretty closely for the SB 18, we forged our own path with the SB 36. We could get by with fewer components here, without making any sacrifices in the sound. That ultimately tipped the scales in favor of this version. For the expansion model with two basses, the SB 18 crossover would have needed to be changed anyway. Besides, it was unclear whether we would be able to keep using the components from the “old” crossover. So we cheerily got to work teaching the SB 240 to make the right fluting tones.
The new kid, or rather both of them, led the way. The basses were connected in parallel and measured in the box, with a microphone at the height of the tweeter. That produced a decent frequency curve up to 600 Hz, which disappeared into a serious dip at 1.2 kHz, then reconsidered and decided to play along again after that. The reason for this temporary failure is the differing distance from the microphone, which transformed the addition of the individual sound pressures into a subtraction. The simplest way to straighten it out would be to move the microphone exactly between the basses, but crossovers are designed to be at the height of the tweeter, not at knee level. So we simply inserted a medium-sized Audyn coil from the Ferrobar HQ 56 series ahead of the basses and a fat electrolytic capacitor parallel to them, which gave us the blue line on the monitor. We didn’t worry about the seemingly early end of the bass reproduction, with the -3dB point at 45Hz. At 1.5 meters tall, the box was well off the floor, which gives it a bit more around the bottom; but early reflections would create faulty results for measurements taken with the box positioned at the bottom.
Liberated from the bass, the 7-inch speaker (frequency curve in the box: red) no longer needed to reach so far down; all it had to do was fill in the area between the bass and the tweeter with the right volume. For the right adjustment with an ideal addition toward the bass, a large Audyn Q4 was sufficient. Toward the top, its limits were a little more complicated to set because the mountainous peaks at the end of the red line needed some serious erosion in order to become unnoticeable lows. Our experience with the SB 36 was helpful here, in which a small Q4 placed over the low-pass coil and a capacitor parallel to the mid-range speaker were enough to cut the peak down to size. The same approach worked here, too.
We don’t need to say too much about the tweeter crossover, whose measurement is shown in the traditional red in this box. A slight dip in the volume through a pre-resistor and a parallel resistor, made of 4-watt Moxes and a traditional 12 dB filter with a seemingly very large Audyn Q4 for the impedance of 4 ohms, as well as a relatively small parallel coil for the same reason, provide the desired amplitude (blue). All that is really left to say is that for an ideal addition of the branches, the bass needs to be connected with reverse polarity, and that almost all of the waves in the frequency line disappear at the 30° curve. It goes without saying that we also offer an impedance smoother for tube owners.
The following photos show how to assemble and install the crossover:
The components were glued onto small boards with hot glue, separated according to bass and mid-range, and carefully soldered. You don’t need to worry about getting them too hot and ruining them. The soldering iron should have a chisel tip set to 320-360 degrees, and the solder should be nice and liquid. The crossover boards were positioned on the left and right side walls, and the wires were fed through the cutouts to their connections. We drilled two holes for wire feed-throughs in the floor of the mid-range chamber, which were then filled in with hot glue. One Sonofil mat, folded once, fills the upper chamber; another two mats, rolled up loosely, go into the bass section behind it and under the reflex tube; and one mat is placed behind each of the two basses. The impedance correction only makes sense with tubes, so it is screwed directly onto the terminals on the outside.
As most of you probably know, for several years now we have been using our boxes with tube amplifiers not just for our own enjoyment, but also as demos for our visitors. As careless as we can sometimes be, we had forgotten to take the last CD “That’s Live” by Eric Burdon out of the player before turning it on, so the volume was naturally still very high. So we were immediately blasted with the old classic “Don’t let me be misunderstood,” at the usual volume for live concerts. There was no misunderstanding that! Unlike many of our other boxes, the singer was positioned a bit further forward, in line with the boxes, and the musicians were lined up behind him in a room whose depth could also be judged by the clapping of the enthusiastic audience. The music was open, airy and dynamic. The clean bass lines, distinct cymbals and powerful, physically palpable bass drumbeats – anywhere else, you would say “never heard anything like it, simply sensational” – could only belong to the Blues Class; there is no higher distinction we can give. To be honest, we hadn’t expected anything different, so we were not especially surprised that the old rock music sounded like good old rock – snappy, sometimes bright and always live. We had no complaints about the bass, but that had been clear from the start, too.
The next section continued in a classical vein: Horowitz in Moscow, the famous piano concert with the unusually fidgety audience and the coughing gentleman in the sixth row. The subtle playing of the maestro – who was 83 at the time – is fantastic, with the youthful flexibility of his fingers, the springiness of the piano notes and the wonderful echo in the room. Not even the abovementioned gentleman’s cold can disrupt the performance. After all, who has ever been to a concert hall where the entire audience sat still without a peep? Nor would Mozart, Scarlatti, Chopin or Liszt have had any complaints about their works.
As long as we’re talking about live recordings: we were overjoyed when Eckart, the first person to build the Symphony 285 at home, sent us the e.s.t. CD “Live in Hamburg” with the Swedish rock trio performing its more than 17-minute rendition of “Dolores in a Shoestand.” Why only three of the clearly 12 musicians are mentioned by name on the cover remains a mystery to us. Maybe it has something to do with the fact that they were very good at sharing the piano, bass and drums. We had actually wanted to go see the group live two years ago, but as usual, as we mentioned above, we were a little short on time. No one could have foreseen that a short time later it would no longer be possible to see them on stage. The listening test made up for it to some extent – it was breathtaking. Anyone who can sit still during that piece and doesn’t start moving his feet, head or entire body must be as deaf as the speakers themselves, which were still standing where we had left them when we opened our eyes nearly 20 minutes later – indifferent, not even a little bit exhausted from the great job they had done projecting the music into the room. We immediately wondered whether they had even been involved. Even a close inspection with our eyes open didn’t produce any clear results; after the third encore, there was still no noticeable connection between the music and the boxes standing in the middle of the room. We could understand the perspective of the audience members, who sounded like they never wanted to leave the concert hall again.
If you want to enjoy this loudspeaker by yourself, the assembling kit is available at the Intertechnik webshop.