When it comes down to it, the world of domestic loudspeakers is really fairly simple: a multi-channel home theater and stereo music are the only two ends of the scale that are relevant here. A car moving from left to right, a plane overhead and a train from the front to the back of the room are easy to reproduce when you have five to seven sound sources with a low-frequency foundation and an explanatory image. It’s rare to hear complaints that the specially purchased loudspeaker package doesn’t meet even fairly high standards. By contrast, even when the music has an outstanding tonal quality and wonderfully locatable instruments, listeners with just two ears love to fight over banal things like the structure of the stage, which one person thinks is too deep, another finds too wide and a third considers to be too far forward or back. There’s no such thing as perfection, but it should be close to the original – that’s not asking too much. These kinds of music listeners, a group we freely admit includes us, smile indulgently at the adrenaline junkies who listen to super-compressed chart toppers along with the movies from their home theaters – you go right ahead, it’s more than enough for that. We recommend the entry-level class for home theaters and save the Blues Class for ourselves. So it was a shock to us when, in recent years, more and more serious-seeming people started ordering our SB 18 and SB 36 in sets of five for their home theaters so that they could enjoy classical music and jazz in stereo with their AV amps. It was time to get off our lovingly tended high horse – counter to our perhaps somewhat arrogant estimation, the world of AV amplifiers hasn’t stood still after all! They apparently do exist, the connections between a feast for the eyes and the ears. It really is high time to establish the Blues Class in this area, too.
A home theater consists of a center, front boxes, rears and the essential active subwoofer, which frees the satellites from the bass under 80 Hz. The challenge was building an audiophile-friendly but still affordable whole, even with just two loudspeakers, due to the huge number of chassis elements. In order to make things even harder, we also decided to hold every component to the same standards. The best-suited chassis elements are of course the ones from SBAcoustics. The basis is formed by the SB15NRXC30-8 and the small SB29RDCN-000-4 tweeter. Both have already been introduced in individual tests.
The price for the #15 offers an outstanding price-performance ratio, even from a purely visual standpoint. Its tall exterior cast metal basket has a black powder coating. Its four separate spiders are wider in the lower section in order to give the raised centering device enough space. It is attached to the giant magnet system, measuring 100 mm across and with 5-mm-tall pole plates, using both glue and rivets. The voice-coil bracket made of Kapton measures more than 30 mm, and the coil height allows a 10-mm linear lift. The 8.1-gram paper membrane in the front has now been dyed black; the characteristic light speckles can still be seen on the reverse. It is held in the basket with a soft rubber seal, which is glued in cleanly. In order to ventilate the dust protector, a one-centimeter hole was drilled through the pole core, rounded at the bottom. Two gold-plated flat pins are used to connect it to the crossover; the wider one is the positive pole.
Like all of the chassis descriptions at SB Acoustics, the appearance of the SB 29 RDCN-C000-4 is also unusual; its membrane seems strangely pressed in at first glance. This is because of a little glue that fixes its center to the pole core, naturally not without good reason. Ulrik Schmidt, the developer of the tweeter, explained to us that this takes care of some of the usual disadvantages of the vibration properties in the soft textile membrane. If you are wondering where he picked up his skills, a glance at the chassis market will quickly tell you the answer. At 29 mm, the voice coil is huge despite the small dimensions of the tweeter. At 2 mm of coil height and with a 2.5 mm air vent, it can deflect sound in a linear direction by 0.5 mm. It has not been drowned in the commonly praised miracle drug ferrofluid. Ulrik almost popped a vein when we asked him about it – and why should he think any differently than we do in this respect? Also larger than usual is the outer bracket for the membrane, which only partially deserves its designation as a seal. In the ring radiator, it is also largely responsible for radiating sound. The calotte is driven by a neodymium magnet that is embedded in a small chamber. It also explains the very low resonant frequency of the SB 29 RDCN-C000-4, which allows coupling even at 2 kHz. With 94 dB/ 2.83 V/ 1m, the small tweeter is one of the loudest in its class – but at 48 euros, not at all one of the most expensive.
Chapter 1: the bass reflex box
The most unassuming loudspeaker in any home theater is of course the rear speaker, whose only task is to create the illusion of a room surrounding the listeners and well behind them. Unlike the early days of surround sound, however, today it is no longer enough for speakers to produce a diffuse frequency range from 400 to 4000 Hz. Nowadays, the little boxes practically have to present ears with the full spectrum of perceptible sound. In order for a #15 bass mid-range speaker to accomplish that, it makes sense to screw it into a bass reflex chamber in order to give this range between 60 and 100 Hz more volume. LSPCad calculated just under 8 liters with f3 at 63 Hz and a 21-cm-deep channel (blue); we preferred 11 liters (black), tuned only slightly lower with a 15.9-cm-long reflex tube and promising more volume below the -3dB level in exchange. The green line represents the simulated frequency curve with 11 enclosed liters.
The assembly is simple; the lid is glued onto the first side panel with joint glue, then the front panel. The rear wall and the floor follow, and then the reflex board takes its place. Now the only thing missing is the second side, and the rough structure is finished. To make sure the whole thing is appropriately dimensioned, we used SketchUp to draw an assembly plan.
Naturally you can also click on the individual assembly plan views; they will open in larger windows, and then you can print them out.
Cross over network
After building the rear boxes, all that’s missing is the crossover, whose structure was the reason for all the other considerations. In addition to using the same chassis for all of the boxes, it is also important for them to have almost identical phasing so that they are perfectly coordinated. This is determined largely by the components of the crossover. Thus it was important here not just to find the right part for the rear boxes, but also to determine whether the topology of the crossover could be used here. Since every combination also needed to produce a nice-sounding stereo box on its own, there was a great deal of measurement work, which delayed the completion of this item by a few days, into the next decade.
After measuring the SB15NRXC30-8 in its cabinet (red), the next step was to force its naturally occurring highs and lows into a shape that would work for a bass mid-range speaker. First we focused on the peak at 8 kHz, which had already been robbed of a good deal of its cheeky behavior by connecting a medium air coil and a very small capacitor in parallel in the signal path. Another capacitor parallel to the bass mid-range speaker did the rest. Unfortunately there was still a hump at 1 kHz caused by the width of the baffle board, which was vanquished by a suction circuit. We were satisfied with the results once we had connected a total of six components (blue).
The tweeter made things much easier for us; its unfiltered frequency curve in the box is represented by the red line. All it took was a second-order crossover and a volume corrector, consisting of an L-regulator, for it to match up perfectly with the SB 15 (blue). This can be seen from the overall curve, in which the bass is red, the filtered tweeter is green and the addition of the two is converted to blue. The separation frequency is about 3 kHz, which does not pose any problems for the two chassis elements.
Measurements SB 15 BR
|Amplitude||Impedance||Distortion at 90 dB|
|Angle 0/30/60°||Step response||Waterfall|
1x SB 15 NRXC 30-8
|Wood list in 19 mm MDF|
1x SB 29 RDCN-000-4
|Sales||Intertechnik||16,4 x 29,0 (1x) front|
|Construction||16,4 x 28,0 (2x) lid/ floor|
|16,4 x 30,0 (1x) back wall|
|Function principle||bass reflex||16,4 x 28,0 (2x) sides/ inside|
|Nominal impedance||8 Ohm||16,4 x 14,0 (1x) reflex board|
|Insulation, damping||Sonofil||bass: 7,0 mm|
|tweeter: 3,0 mm|
First listening test
Then came the first listening test – a negative outcome would have saved us further work. Now, the first chapter is already published, so it can’t have been all that bad. We hadn’t bought ourselves one of the better surround amplifiers ahead of time; in this case we preferred to use our familiar SAC combination of an Epsilon-VV and LaForza amplifiers. That kept us from being fooled by new effects, which often seem like revelations in the beginning and later turn out to be annoying. Anyone who is testing loudspeakers cannot simultaneously evaluate an unfamiliar amplifier. We didn’t want to spend too long on the sound test, since there were still a few other things to do. So we threw the familiar test pieces into the CD player and could hardly believe that we were only going to use these boxes as background pieces in a home theater. Nils Lofgren’s guitar had body; the strings were taut, and the details that are so often described in “Keith don’t go” were nothing spectacular – they were just there, as a matter of course. The tweeter played along, but without any clear signs like an unpleasant hissing or sharpness. That’s what we call harmonic interaction, and we consider it one of the most important criteria for the Blues Class. There was nothing to complain about in terms of tonality and clear speech, nor in terms of dynamics. The smallest background noises could be clearly identified, even though the remaining volume was miles higher. Even the bass test, which we did with the popular “Hotel California” from the Eagles CD “Hell freezes over,” did not sound like a small speaker, loud or quiet. It made us hungry for more, so we went right back to work – now it’s time to build the center speaker.
The SB 15 BR is best for a stand alone front stereo system and for a rear speaker combination.
The SB 15 BR loudspeaker kit with SB Acoustics drivers is available at Intertechnik.