Inspired by the satisfactory sound of the rear speakers, we went ahead and built two centers. How else were they supposed to work as a stereo box? We doubled the volume of the SB 15 Rear and the number of basses, and that gave us the name: SB 30. We put the chassis elements in the standard order for center boxes: bass – tweeter – bass.
Of course that isn’t without its problems, because as the angle increases, there is a bump in the frequency curve from 2 to 4 kHz. But it causes more problems for the microphone than the ear, since the microphone cannot measure any of the reflections from the room along with it. For stereo listeners, this “fault” disappears anyway because the speakers are upright. This is where the positive effects of the D’Appolito appearance come into play.
We once again used dyed MDF as the construction material, which we have long preferred over the natural color because of its heavier weight and its practical surface, which only needs a coat of clear lacquer. Two small support boards made of 10-mm MDF helped us construct the two reflex channels in the appropriate width of 1 cm. They were clamped between the lid and the front, and were then removed later before they became stuck. Naturally we also created a SketchUp assembly plan for the Center or the SB 30.
The crossover for the SB 30 was not too much trouble; the topology had already been determined by its little sister, so we just needed to convert the component values for the lower impedance of 4 ohms. Still, it wasn’t exactly a textbook case – the chassis arrangement still needed to be taken into account. Cutting the number of coils and resistors in half and doubling the capacitors didn’t work, but that’s why we used a measuring system rather than a simulation program to develop the crossover.
We could have copied and pasted the explanations for the diagrams directly from the AB 15, but we decided not to. With 92 dB of sound pressure for 2.83 volts, the bass component of the SB 30 is fairly loud for a small chassis – but the tweeter can get even louder. We let the lower flank drop off more steeply with our choice of resistors for the voltage divider. This measure shifts the separation frequency to just under 3.5 kHz, and creates a small dip in the transition area. This is especially good for movies, and in stereo operation it moves the stage back a little bit. As the following measurements under the angle show, it fills in slightly under 15 degrees and almost completely under 30 degrees. A smooth curve on the axis would thus have caused an exaggeration there, which would have seemed detailed at the beginning but quickly become shrill.
With the high efficiency factor and the enthusiasm already shown by the SB 15, it made sense to build an impedance correction for the SB 30 and tube listeners. It consists of a series-connected capacitor, coil and resistor, and is wedged in front of the crossover between the positive and negative poles of the terminal. It is invisible to transistors because the voltage doesn’t drop off in front of it.
The circuit plan and crossover photo are fundamentally identical to those for the SB 15 Rear, so we won’t include them heren.
Measurements for the SB 30
|Amplitude||Impedance||Distortion at 90 dB|
|Angle 0/30/60°°||Step response||Waterfall|
Loudspeaker SB30 with SB Acoustics speakers
|Chassis||2x SB15NRXC30-8||Wood list in 19 mm MDF|
|1x SB29RDCN000-4||or multiplex per box:|
|Sales and||Intertechnik||120,0 x 29,0 (2x) sides|
|Construction:||58,0 x 29,0 (2x) lid/ floor|
|Function principle||Bass reflex||58,0 x 16,4 (1x) back wall|
|Nominal impedance||4 Ohm||56,0 x 16,4 (1x) front|
|Terminal||T 105 MSAU||14,0 x 16,4 (2x) reflex channel|
|Damping/insulation||2 bags Sonofil|
Mid/Bass: 7 mm
Tweeter: 3 mm
Second listening test
Naturally we didn’t want to do the second listening test with the same equipment; this time our EL 34 provided the two SB 30s with the necessary signals. In addition, they didn’t come from a CD, but from my TD 320 and the corresponding black discs with precisely one groove on each side. Just in time for the test, a Benz ACE sound pickup made its way to us as a special highlight – our friend Andreas in Innsbruck brought it over directly from the Benz-Micro boss Albert Lukaschek. In addition to the last name of “H,” its nameplate also includes the hand-written addition “S,” which refers to its unique feature of a special needle grind. We are especially proud that the system was not only specifically chosen for me, but was even personally manufactured. Can there be any greater honor for a DIY loudspeaker magazine? Our warmest thanks go to both of them.
Classical music was called for. Coincidentally, we had recently bought a large selection of recordings from the 1960s to 1980s for very little money. It’s actually a shame how little those older cultural goods are valued today. Händel’s Concerto for Organ and Orchestra in B major, op. no. 1 is really no light fare for two #15 basses, but even at a decently high volume – not deafening, but still pretty loud – the eight-inch pipes of the Mössingen organ were not too big for our little chassis. The church nave was integrated realistically into our listening room, down to the furthest corners. The organ and orchestra were right there in our little room, and the musicians were so clearly present we felt we could have shaken hands with every one of them. Then we heard Mahler’s 4th Symphony, ranging from sensitive and solemn to lively and cheerful, without noticing any speakers at all that would limit the size of the room. It was simply pure music, from the triangle to the kettledrum. The director, in this case Bernhard Haitink, could hardly have been better positioned 40 years earlier than where we were sitting at that moment. Then it was time for a little pounding rock music with some volume, and it was no problem at all. Compared to the SB 15, everything was a little more powerful and dynamic, which was to be expected with double the membrane area. The second listening test didn’t have any more big surprises up its sleeve – it was the same chassis, the same crossover structure, the same high-quality sound level: just a straightforward Blues Class.
Now all we were missing now is the front loudspeaker in which we will add the subwoofer report – the third act of the story.
The speaker assembly kit includes all speakers, inductors, capacitors, Sonofil, terminal and cable. The kit is available from Intertechnik.