, 05-25-2008 at 02:26 AM (3119 Views)
“The Scandivifias” by Ben “Wolf” Shaffer; finished 9-19-07.
It all started with the first annual Parts Express Tent Sale (2005). It was towards the end of the sale day, I had already found some bargains, and I took a last scouring of the shelves. Then I found it. It was in a dusty worn box, marked $60, and faintly labeled, “SCAN-SPEAK”! With excitement I cleared off the merchandise on top of it, and opened the flaps to reveal a 15W/8530K00 5.5” midbass driver. This is known as a “Revelator” style product, or Scan-Speak’s top-of-the-line drivers. I was shaking mildly with adrenaline, and screamed, “REVELATOR” quite loudly. The discount store-wide was 20% on top of marked prices that day. I had thought about selling it upon purchase, as I only had one, and could regain my money spent quite easily. I found another said driver from “Atalio” on the Madisound forum for $110 shipped. Since the midbasses cost an almost unattainable amount of money at $209 each, I had achieved quite a bargain, and a project had started to brew. The 15W has a reputation for “the best bass in 15 liters”, so I had to go that route with a fairly basic two-way, and justify or condemn the reputation.
As stated above the Scan-Speak midbass was chosen due to cost, and the Vifa XT25-TG30-04 ring-radiator was chosen for the tweeter element. I bought the tweeters at the second annual tent sale for about $45 each, when list is closer to $55 each- Another bargain! I then had a nice quartet of low-distortion drivers that would supposedly sound very nice indeed. There is a ring-radiator Scan-Speak tweeter, but it was unfortunately far out of my price range. These drivers are manufactured by the same parent company in the country of Denmark. Since Denmark is considered Scandinavia, I swapped in Vifa, and it came out “Scandivifia”. A rather cool sounding name, pun intended.
Due to the extravagant nature of the drivers in this project, I wanted to create a somewhat extravagant enclosure. This project was going to be my first attempt at a time-aligned baffle, which either tilts back with tweeter above mid, or uses a stepped-baffle to physically place the acoustic centers of the 2 drive units in the same vertical plane. This helps to minimize driver offsets in the horizontal plane, but leaves the angle of emitted sound from each object at different angles to the listener. This is common, as coaxial time-aligned drivers are not commonplace. In stepped baffles, the tweeter is still on-axis to the listener. In slanted baffles it is not. I chose to use the slanted baffle, as I might be listening above or below the main tweeter axis in many different circumstances. It is a good compromise. Then I thought- slant them ALL, and drew up plans for the truncated pyramids presented herein. I measured the T/S parameters using the WT2 driver analyzer, and modeled an enclosure volume using Unibox, being careful to watch Xmax, port mach, and resulting box transfer function. I found what I wanted at about 18 liters. It’s a little larger than the rep’d 15 liters, but not much. It is about the same in response profile in either vented or sealed applications. The vented style cab took a 2” diameter by 9” long outer-end-flared port with lined cabinet walls. I chose to build this version, and use some $1 yellow foam “audiophile-approved-port-plugger” ***** to facilitate a sealed alignment later.
What a massive undertaking this part was! I used ¾” MDF for the majority of the pyramids’ construction. I planned on using a few pieces of my slowly deteriorating 1896 erected barn, which is a part of me. The boards were still nice on the inside, but the edges bowed towards the outside edges. I cut a few pieces off of a loose board, and went to work. I let them set for about 6 months to dry out, which was the majority of the time spent on the design stages. I had to draw up a sketch to get approximate angles for the perceived construction. This came out to about 77 degrees from bottom to side wall. Since the cabinet top’s angles were the complementary angle within 180 degrees, and the top and bottom were parallel, the saw blade could remain in the same angle for most all of the cuts. All the chamfers and angles were cut before construction started. I had to figure out the best way to get all the pieces to fit snugly, and at the same angle, whilst clamping and gluing commenced. Since the boards from the barn are straight, I had to add triangles to the font and rear of the cabs to complete the baffles. This took about twice as many pieces cut to fit the best. I also had to angle the top and bottom edges of said 3 pieces to align with the bottom and top pieces. The side pieces were cut at this same angle, which is the 77 degrees. The side pieces should be cut a little wider than anticipated, and flush-trimmed with a router. For the main-connection of all the pieces in a cab, I cut 2 pieces of ¼” hardboard with an additional ¼” in dimension on all edges. This would give me an edge to insert in a dado that runs the perimeter of the entire front and rear baffles. The slots are then sawn into the bottom, top, and side pieces, ¾” from the corresponding edges of these pieces, being sure to watch the angle of the sawn dado for insertion of the hardboard upon completion. The top and bottom dados are angled, the sides remain straight. A little wider dado is okay, as the polyurethane glue will expand and fill the small gaps. The dado distance from the edge is the thickness of the baffles, as the 3 baffle pieces are then secured to the hardboard pieces during construction. I did a dry-fit once the hardboards, sides, top, bottom, and 6 baffle pieces were cut. The top actually will slide on from the side, as it will not allow the hardboard to be inserted vertically. Since the pieces all fit, I screwed the barn-wood to the hardboard while in this assembly. This means the central baffle pieces will no longer need moved. Before the final assembly, I found out the easiest way to mount the port, was to glue it in place on the back side of the back baffle, opposing one of the 2 screws securing the barn-wood. This way I had a locator to find where it was positioned after construction, and the screw-hole would no longer require filling. The other screw was in place of the Neutrik 4-pole input jack; more on that later. I also embedded 2 Neodymium-Iron-Boron (Neo/NIB) magnets from the inside of the cab into each of the front edges of the side panels to facilitate a grill that wraps from side to side. This was before final assembly. I then proceeded to cut for 20 biscuits per cabinet. Two in each side of the 3 baffle pieces (both front and back), and 2 in the bottom of each of the side pieces. I then removed the barn-wood, only to reattach after laying down some glue. The screws/biscuits gave a tight-grip while the glue adhered to the slightly bowed barn-wood. This is another reason I used poly-glue for this project, as it fills-in and expands as it cures, not to mention forming a pretty tight bond with the materials used. The last piece adhered before side assembly was a 12” square piece of ceramic tile that I glued into the bottom of the cabs. This was for rigidity, damping, and a small counterweight. I kept the glue/biscuit method going until the entire cab was done. Due to the complexity, it took me a half hour or so to glue, assemble, and clamp a single cab. However- it was well designed enough that the cab didn’t need further positioning. It all fit like it was designed. To route the port hole after glue is cured, remove the screws, drill a pilot for the router bit, and flush-trim to the inside of the port tube. Round over the outside as desired with a round over bit. Remove excess poly-glue with sandpaper or a chisel. Now that the cab was constructed, I had to sand the barn-wood to a flat panel, and then route the recesses and driver holes, as well as rounding over the vertical edges. The sanding took quite a while with some 60g sand-paper. All the mating edges were sanded smooth with 120g, and then the open-grain edges were saturated with Elmer’s glue and once dry, sanded smooth again. The rest of the port-tube was then installed through the mid’s hole, with an elbow on the mounted end. This allowed the length to fit inside the enclosure. I left it unglued for testing, and the later addition of damping material. Remeasuring with the WT2 verified the Fb to be at 35Hz as intended. Final dimensions are 13.25” square on the bottom, 15.125” high, and the top is 7”W x 5.5”D.
After finishing (next section), I then added the damping material in the cabs. I applied truck-bed-liner to all internal surfaces to add mass, and then applied some sport-flooring to the rear and bottom panels with an adhesive. I followed up with a lining of round convoluted egg-crate mattress pad on the back panel, and ridged-type on the inner sides and bottoms. I cut a hole in the back-piece to go around the port-tube. The last piece was a small sheet of 5%/95% wool/polyfil batting over the bottom and back, underneath the port attachment. I finally reattached the port-tube for a last time and it also holds the lining in place. For humidity changes, and to reduce moisture problems, I snuck a small package of desiccant in between the damping foam.
Notes- wipe cabinets down between all steps to remove dust and debris, and wait at least 24 hrs before attempting the next step. Low humidity is a blessing if weather permits. Patience is a virtue with paint, as you have to let it dry. If you reapply before sanding, wait until the paint is dry to sand. It does not have to be set and cured to keep building layers. It does have to dry at least 24 hrs to sand. I use an old T-shirt to wipe off the dust, so to not marre the finish. After the last paint-layer is applied, whether clear or paint, wait a week before further finishing or handling the painted item. You don't want fingerprints.
After assembly of boxes, use either a sealer or Elmer's glue, and apply to raw-MDF-edges. This will avoid paint sinking into the MDF, and will allow a faster build-up of primer and paint.
Use a filler if you have gaps at this point. I have used Dap's Plastic-Wood with good results. Sand all surfaces smooth with 160g or 220g before priming. Some people use Bondo to cover the whole-cab/fill-gaps at this point instead.
I use a high-build primer, not Krylon, usually the type for automotive purposes. Duplicolor is my pick. Color depends on final paint applied, as lighter for white is needed, and Black doesn't mind which one. The primer being a different color allows you to see if you sand too far- be careful to not allow that, but easy to fix with more paint. It generally takes about 3-4 good coats before the thickness is built up, and let it dry before sanding with 160g/250g/400g in succession. You don't want any bumps in this step, as it is the prep for paint, and must be smooth. I have skipped the 250g, but it requires more elbow grease with the 400g. A non-clogging sandpaper is preferable here, as the primer always clogs the paper, but less with the non-clogging type.
If you need to mask- do this before priming or after priming, depending on your project's other finish. Post-paint; when you remove the masking tape, even though the paint may be dried or cured, you might need to lightly score the paint at the tape's edges to ensure a smoothly removed edge. The paint can pull and tear and not give as expected. Use either a razor blade or exacto knife, and possibly a straight edge if needed. You don't want the patiently laid paint-job peeling up on you. If the paint is of another thickness than the rest of the unfinished cab, you can sand the edges of the paint to blend them together. Be patient, and sand carefully with 400g, and then 1000g. You don't want to sand through at this point.
If it's smooth, you are ready to apply paint. Usually, I alternate 2/1-2/1 with color of choice, and a clear-coat to build layers, and fill in any small voids. I have used Krylon Crystal-Clear-Gloss for the clear, and it works well. I usually don't like Rustoleum paint, and use Colorplace rust-preventative for white/black, or Krylon for other colors. I usually sand w/ 400g between the grouped coats. Usually it's only 2-3 groups of 2/1 for a good finish. After you build up enough paint, that you don't sand through with 400g, you basically have to almost sand off the last layer of clear-coat. This seems strange, but as it's removed, it's buttery smooth. At this point, there shouldn't be anything other than a smooth finish of the paint you chose. After you get butter-smoothness all over the box, step to 1000g or so. You can wet-sand, but dry has worked for me.
Now it is ready for finishing. You should also consider alternatives to the end-result at this time. You could use a glitter, or metallic finish, or anything else really. Or just continue.
Normally, I use enamels for paint, so I can use a number of things for finishing. Polyurethane spray or brush-on(does yellow slightly), Polycrylic, Clear-Enamel/clear-coat, Lacquer, etc. If you clear-coat, sand with 1000g as the directions state. You usually have to wait a few days after the (eg: "2 light/one med-wet") application to apply again. Sand before reapplying.
On the Scandivifias, I used a clear-coat for the painted-section to encapsulate the alternative finish, and to match the solution it was immersed in. The alternative is a glitter clear coat, to make the white paint simulate a freshly fallen snow. The whole Scandinavian theme again surfaces. After I removed the masking, I then stained the wood-portion with a Minwax pickle-stain (a form of white-wash), and lacquered the entire cabinet with Minwax gloss-lacquer. Lacquer dries fast, and should be used with a respirator. I sanded again with 1000g after about 2 coats of lacquer. I then finished with about 5 more coats of lacquer. It is now nice and shiny!
Crossover Case Construction:
I actually completed the Crossover construction before commencing on the cabinets. I know this is backwards, but it came out okay. I used a ¾” piece of MDF, 6 3/8” x 9 3/8” with rounded long edges. The rounds are the bottom face. I then cut a 9 3/8” long piece of 6” PVC pipe in half lengthways. Measuring this can be critical, or use a sander for the best fit of the ½” thick MDF semicircular end caps. There is also a 1” dowel glued in place between forstner-drilled inlets in the end caps. In the center of this dowel, is a dowel-nut. The PVC cover has a single hole in its top-center, in which the ¼-20 bolt will thread through and into the nut. I biscuited and glued the end caps and dowel in place after I made sure the PVC cover fit well to the base, along with everything else. I cut a piece of acrylic for the terminal plate in one end. It has an icy look to it. The other end has a hole for a wire exit, which is terminated by the mating Neutrik connection. The crossover board was then screwed into the flat-area of the case. The cover received a few coats of Krylon Fusion gloss-white paint, as it chemically bonds to plastic. I then gave it the same glitter/clear-coat/lacquer as the cabs. The base does not have a glitter coat, but is finished with the same Krylon paint and a clear-coat. I used a brass bolt for aesthetics in securing the PVC cover. If you want spikes, I recommend tapping in a quartet of field points for arrows. Drill a few holes and insert. They are inexpensive, and provide the effect desired.
Due to the XT25 having a very strong following in the Acoustic Reality (AR) series crossover topology, I strongly considered this option first. There was much discussion on the Madisound forum about 5 years ago about this crossover style, and what its pros and cons were. Peter/”friend” was the instigator of these discussions. The main point made quite frequently, was that the XT25 worked well with this circuit. A little background…
Normally series crossovers require a smooth response driver, with bandwidth to spare across the crossover point. Smoother impedance drivers are also a plus, as compensation is not required. Larger gauge inductors with low DCR, and poly caps are preferred, as well as keeping added resistances to a minimum. Everything is in series with everything, and therefore affects everything.
The added benefit of the AR topology was the main padding resistor in between C1 and L1, or the main crossover parts. As long as it was above 5 ohms or so, the impedance seen by the amplifier was a very flat nominal load above woofer’s Fs. The resulting impedance-phase, or electrical phase, is also easily made to be fairly flat. Peter also stated that the main cap, C1, is optional, and that the woofer’s impedance peak is minimized if it is removed. One caveat with that arrangement; it now requires a very high-power resistor to dissipate enough heat from the amplifier (pretty much) directly driving it. Peter even externally mounted his resistors on the back of his cabinets for better air flow and heat dissipation.
With these statements in hand, I designed an AR crossover, with a little mentoring from Ken Perkins. He used this type in his “Nebula” project, also with the XT25. For initial layout, I used a .15mH 16 AWG foil inductor, a 7.3 ohm 100W Arcol power resistor, and a 25uF poly capacitor. I further protected the XT with a series capacitor, valued 10uF, and accomplished a second order crossover for it. The 15W has a very flat impedance curve, so a zobel was omitted. I also inserted an 18AWG 1.5mH inductor for Baffle Step Compensation (BSC) before the woofer. This is in essence a 12dB crossover slope as well, so I also have the tweeter polarity reversed. The XT25 has a horrendous impedance peak, so it needs to be dealt with to avoid exciting the resonance. I measured the tweeter and a varying range of attached components until the WT2 measured a flat impedance curve. The result is within 1 ohm variance from 1 cycle to 20 kHz, and consists of a 1mH coil, a 4 ohm total resistance, and a capacitance between 150 and 160uF. The DCR of the coil must be factored into the resistance, and since the DCR was .7 ohms, I used a 3.3 ohm resistor. These parts are in series, shunting across the tweeter connection. Order doesn’t matter. When I removed the LCR circuit in testing, the tweeter had a distinguished buzz sound in the lower treble, so I left it connected. I then connected the drivers to the crossover as stated, and the electrical phase was between +3 and +20 degrees from 330 Hz up. The measured crossover point appears to 2.5 kHz. There is a little hump in the tweeters impedance before flattening out a little lower in amplitude at higher frequencies. It also sounded a little hot on top, in reference to the 15W. I then added a 6.8uF Jantzen Z-Silver and a .1uF Mundorf Mcap Supreme in parallel with the 10uF part. I tried more and less capacitance, but this total value of 16.9uF yielded the flattest tweeter impedance curve, and the flattest electrical phase curve. In this layout, the electrical phase is now +3 to +11 degrees, without changing the crossover frequency. That’s a range of less than 9 degrees from 330Hz up to 20 kHz. Impeccable!! It also sounded a little hot across the treble now, instead of on top. At this point, I swapped out the 18AWG coil for the 14AWG Jantzen C-coil, and the bass is a hair tighter due to less DCR. The last change was to up the main resistor to an 8.5 ohm value. This is to my taste, so between 7.5-9 ohms is the range, with 9 being dull, and 7.5 being hot, or at least to me. I tried the crossover without the 25uF cap, and the Fs was greatly reduced, however, the output of the 15W also was greatly reduced. I opted to leave the 25uF cap in the circuit. I do believe that everything sounds balanced and clear in this crossover circuit. I have not measured the frequency response, but hope to do so soon. Since this is an outboard crossover, and parts are easily accessible, I may tweak for some time to come.
This is indeed the best bass I have heard from this small of a driver and cabinet. I have checked to see if the sub was engaged. Highs are crystal, not sibilant, and vocals are layered and distinct. Bass is also articulate and fairly extended most likely to the mid 30’s. The vented alignment is more extended than the sealed cab, but power handling is better when using the foam *****. I listen just a hair above tweeter level, so maybe the angle of tilt could be around 60deg or so for a higher placement in your room. The Scandivifias are also picky about source material at times, as they are such a revealing loudspeaker. Most of the time this is easily forgiven. Classical, orchestral, and acoustic music are incredibly reproduced with a very spacious reproduction. I have but one caveat- they need a decent amount of power to reach a moderate level, as they are only about 83-84dB sensitivity. This is also livable. They are not the cheapest of speakers to build, but are worth every penny I’ve spent on them.
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