The questions people ask – it’s almost unbelievable! “Can I get the FT 12 with a better chassis?” was one of them. When most stores would be happy if they could offer an FT 12 at all, even with the “worse” chassis. Of course, it’s our own fault – after all, we have been raising our readers’ expectations quite a bit with our assembly plans. But enough of this embarrassing patting ourselves on the back, which isn’t really our style. “Nothing ventured, nothing gained,” or so they say, and we were interested in the project anyway. So we quickly built a cabinet where an SB 17 with four ohms could hang out, and an SB 12 with SB 26 STC would provide the aesthetic element. First of all, in the self-satisfied knowledge that we were building something good, we covered the little MDF cabinet in 3-mm plywood, making sure to take plenty of photos of the process, and screwed the chassis into it.
“Poof!” went our dream of a new Blues-Class assembly kit when it came to the crossover. Even the bare measurements for the interior bass in the bandpass showed no similarities with the simulation, which we had naturally done in advance to determine the right volumes. We couldn’t even sugarcoat the results, because no one can persistently ignore a remainder of 78 dB in a four-ohm box at 2.83 V after the connection. The fact that an almost immeasurable number of components also had to fit in the box stopped the proceedings before we ever got to the listening test. There was one good thing about the abrupt end of this as-yet nameless box: no one could accuse us of being infallible.
But what were we supposed to do with that beautiful chassis, with solder already stuck to its connecting lugs? And there you have it again: the questions people ask! “I wish I could have a Blues-Class Needle for my bedroom,” wrote espressogeek in response to our community survey. Slim and tall, a small bass mid-range speaker and the tweeter that is so essential for the Blues Class; and of course we already had the chassis. The #17 was left in the dust, but that was its own fault. It could have just worked the way our theories said it would. On the other hand, the SB12NRXF25-4, which we previously only used in the SB 23/ 3 as a mid-range speaker, and its intended partner the SB 26 STC were absolutely on our list. So we quickly entered the parameters for the mini bass into a TQWT calculation according to the Needle requirements. Lo and behold, it said that a path measuring roughly 80 cm would be enough for just under 60 Hz at half volume. Longer lines would go deeper, but with 70 Hz of air resonance frequency it gives us a bit of a bellyache – even though most of the Needle equipment works that way with up to 150 Hz fres. For a one-meter line, it showed us the F3 with 53 Hz, too deep for our taste. So the TQWT was a no-go, too.
Narrow and tall? Just for fun, let’s do the calculations for a deep-tuned reflex system. With an interior that was 97 cm tall and 13 cm each for the width and depth, it came out to about 16 liters; and with a 6-cm-long HP 50 it gave us the desired bass to 60 Hz, with a steep rise below that. So we went ahead, bought the wood and started building. Even though we had already put a fair amount of money into this project because of the elaborate cabinet for the nameless traitor, we went with the 15-mm multiplex at the hardware store instead of MDF, which would have been three times cheaper. That way at least the surface was finished, and we saved a good deal of time, which is very valuable to us. The front was once again cut out in advance and fixed in place with dowels, and the corner brackets also helped hold it. Anyone who wants to read about the use of these two devices can look through the captions for the Axis 220 Neo construction photos. There are just a couple of pictures without commentary, and they can be enlarged by clicking on them.
The pictures were based on the assembly plan drawn with SketchUp, which can be downloaded as a Zip-file.
There are also a couple of things to say about the construction of the box. Tall and narrow always creates problems if the entire mid-range comes into the box from the back side of the membrane. At the one-meter height marker, waves over 300 Hz fit completely into the box; the insulation material has to keep them from reflecting against each other inside the box. But we can only see a tiny scrap of Sonofil stuck into the top part of the pedestal. Upon closer inspection, though, we also see the reinforcement boards that keep the lid from interacting with the base. They divide the interior into different sized chambers, which effectively weakens standing waves through the chaotic spacing arrangement. For the benefit of later DIYers, we gave this assembly principle the self-explanatory name “Asymmetric Chamber Line.” We will determine later on, using other constructions as appropriate, whether it’s a name that is really worth remembering. The initial measurements of the SB 12 RNXF in its cabinet told us that our reflex tube needed to be cut off at 10 cm, possibly also due to the structure. Next thing we knew, we were back in the thick of crossover development.
You are probably familiar with the method that we use to “build” our crossovers. First the bass is measured in the box; the narrow baffle board means it has only a small peak at 1 kHz (red). You might almost be tempted to just set up a wave trap, bend it straight at the top using a suction cup, and skip the tweeter. Of course we didn’t do that, because as the angle increases, you lose all of the high notes and thus also the resolution outside the sweet spot. So we went back to the SB 12 RNXF, almost as a matter of course, and gave it an air coil with 1.4-mm wire and a parallel electrolytic capacitor, which almost perfectly staved off the high notes. The only disturbance was the increase to 10 kHz, which was taken care of with a tiny little Q4-MKP soldered on over the coil (blue).
The tweeter also responds to the baffle board; its curve shows a sharp bend where the early reflections are missing on the front. That should be taken into account when building crossovers, because the drop is invisible below the angle. Anyone looking for linearity on the axis will quickly create an exaggeration in the diffuse sound, which the ear initially perceives as incredible detail, but soon becomes an annoyance. Since we generally design our boxes for parallel connections, it’s fine for the tweeter to be a little stronger around the top, where it sends out a narrower band of sound due to its membrane size. See, for instance, the 30-degree measurement for the SB 12 ACL. The SB 26 STC was satisfied with a third-order filter and a voltage divider. At 2800 Hz, the interface with the SB 12 was almost exactly 6 dB under the total curve, which for its part never dips below the branches. After connection, there was still about 84 dB of sound pressure remaining at 2.83 V (with four ohms, naturally 2 watts); the -3 dB point was actually at 52 Hz, which we did find a little unsettling in advance of the listening test. But boxes have been shelved even after that point, and no one was forcing us to unload the box on the public if it didn’t work out. Still, it’s safe to reveal that if it hadn’t worked out, we wouldn’t be writing this text. We even measured out an impedance correction for tube listeners. But before making any formal apologies, we have an obligation to show you the crossover construction, in five steps plus a labeled wiring photo.
Now all we had to do was put all of the parts together to make a whole before doing the listening test.
Oh, we almost forgot – the measurement diagrams are an important part of every report about new boxes.
|Frequency and phase||
Impedance and linearization
|Frequency under 0/ 30/ 60°|
|Distortion for 90 dB||Step response||Waterfall|
Even if it’s starting to seem like the bedtime ritual of small children who always need to do one more important thing before they can kiss their favorite stuffed animal good night: now it’s finally time to give the sound description for a noteworthy newcomer, one that did not give in to fate but made the best of it. So we started out easy, with a man and a guitar, a woman on piano – in other words, small lineups that didn’t demand anything too huge from the boxes. The detail and the resolution were fantastic – there was nothing annoying to clog up the ears. We had to suppress a small grin – we hadn’t expected the reproduction to be so good. Anyway, then came the classical department. We had chosen an LP with Vivaldi flute concertos just to be safe; no fat kettledrums, not too many instruments, but some that would take up the whole space behind the boxes nicely. And that’s what they did. It was wonderful to hear the precision of these narrow boxes in reproducing the flutes, the various string instruments and the continuo. Since it was a fairly old record, there was a note right at the bottom of the cover: “The stereo effect can only be heard when using a stereo playback device.” We enjoyed it all the way through, for both sides of the record. Even though we could only listen to it at low volume because of the late hour, we didn’t miss any of the dynamics, which are part of even the smallest chamber orchestra.
The next day, Zeppi was the first one to lay eyes on our new beanpoles. Then he insisted on experiencing them with his sense of hearing, too. He didn’t want to hear any of our flute concertos, he wanted something lively. As the musical child of the ’80s, he was happy with Yello’s album Baby. We intentionally chose “Blender,” and the first notes before the motorcycle rendered him speechless. We weren’t listening at low volume anymore – that doesn’t work with Yello. When the pounding bass notes started, he was forced to get up off the famous sofa to look at the speakers. He was trying to find out how the sound from these one-meter dwarves could be so dry and deep. We threw on Robby Williams, “Swing When You’re Winning.” The box acoustically conjured the 1950s American revue movies into the room, including their fantastic arrangements, without any effort at all. What was amazing was the ease with which the box made its natural size seem like no big deal – as tall as a Needle and Blues Class! That’s what we had wanted from the band-pass box, and now the castrated version could do it alone. “Don’t tell me those were the small boxes you have set up in front of the big ones!” were Christoph’s first words. He had walked into the shop during the presentation and hadn’t dared to disrupt it with a friendly “good morning,” which by now he had forgotten all about anyway. His face spoke volumes, though. Just in case anyone believes we think the SB 12 ACL is the best box in the world – we know plenty of better ones. But even when we think about it carefully, there are very, very few that can measure up to it in the same price class and shape. How the innovative Asymmetric Chamber Line impacts the surprising bass performance of the SB 12 ACL can only be determined once other shapes have been built for the same equipment. However, a glance at the impedance curve is interesting because it is not too different from that of a TQWT. Well, maybe in reality it is just a very big reflex box.
SB 12 ACL
|Chassis||SB12NRXF25-4||Wood list in 15 mm per box:|
|100,0 x 16,0 (2x) sides |
|Sales/construction||Intertechnik||100,0 x 13,0 (2x) front/ backwall|
|13,0 x 13,0 (2x) lid/floor|
|Function principle||Bass-reflex||7,0 x 13,0 (3x) reinforcement|
|Nominal impedance||4 Ohm|
|Terminal||T105 MS/ AU||Milling depth:|
|Damping/insulation||1 bag Sonofil|
|Reflex port||HP 50, 10 cm length||Mid-woofer: 6,5 mm|
|Tweeter: 3,5 mm|
The assembling kit include all speaker drivers, capacitors, inductors, damping, reflex port, terminal, cable as well as scews and can be ordered here.