Vifa Shielded MTM's

This Section Last Updated: 9-30-2005

This article was originally published at "Secrets of Home Theater and High Fidelity"
http://www.hometheaterhifi.com/volume_5_1/diy70398.html

THE MTMs

The Drivers

Front Left/Right Speakers and Center Speaker

My brother wanted the front left/right speakers and center speakers to be shielded. That way he could set the speakers close to the television without worrying about video distortion. This really limited my choices in drivers. There just aren't that many quality shielded drivers available for DIYers. After exploring almost all the shielded drivers on the market, and running them through my Bass Box speaker design program, I decided on the Vifa P17SJ-00 for the mid-woofer, and Vifa D25ASG-05 for the tweeter.

The Vifa P17SJ-00 is a mineral-filled polycone 6.5" shielded driver with a magnesium cast frame. Two of these drivers in a vented enclosure require a box volume of 1.1 ft3. With this alignment, Bass Box indicated a Fb of 44hz, and a F3 of 53 Hz. This was not a problem since the subwoofer would be handling the 20 Hz - 60 Hz region. The Vifa D25ASG-05 is a 1" shielded ferrofluid-cooled aluminum driver. I was hesitant about using an aluminum tweeter because I have heard some really bad aluminum tweeters that made me want to cover my ears. However, this combination of mid-woofer and tweeter is an excellent choice. I was very impressed by the way the tweeter and mid-woofers blended together in music and movies. The final speaker has a sensitivity of 93 dB/w/m and a nominal impedance of 4 Ohms.

The Enclosures

I have always liked a speaker that is narrow and deep. The MTMs follow that pattern. I decided to put the vent on the rear baffle because it makes the front of the speaker look cleaner, and I have never liked the 'whooshing' sound a vent makes. I was somewhat hesitant to do this since a significant portion of the sound the mid-woofer produces is a result of the vent. This is represented by the speakers Fb of 44Hz.

The enclosures for the center and fronts are identical. For the center, however, I positioned the terminal cup and the tweeter so they were facing upright when the enclosure was laid on its side. The boxes are made of 3/4" MDF. They have two shelf type braces, one above and one below the tweeter. All sides on the shelf braces are 1.5". The drivers are flush mounted in the front baffle. This is a simple thing that really improves the sound of the drivers and the custom look of the speaker. The box has a 3" diameter port that is 5.5" long. The diagrams on the left indicate the enclosure dimensions.

The Assembly

Below are the board dimensions and quantity needed to make two front speakers and one center channel speaker:

6 (21.5" x 8.5") These are the fronts and backs.
6 (15.5" x 8.5") These are the tops and bottoms.
6 (15.5" x 20") These are the sides.
6 (7" x 15.5") These are the shelf braces.

Making the boxes is pretty straightforward, and I built them with square edges. Measuring and planning the cuts is one of the most important steps in making the enclosure. I spent a lot of time planning the cuts ahead of actual construction. One wrong cut can just ruin your day! I used a table saw to make all the major cuts on the speaker walls and shelf braces. Then I used a jigsaw to cut out the holes in the shelf braces. I used a router with a circle-cutting attachment and a 1/2" straight cut carbide bit to cut the speaker holes. Once you have the router dialed in to the right diameter, it is just a matter of making the same cut on all the speaker front baffles. First I routed a 1/4" deep shelf. This cut enables the drivers to be flush mounted to the baffle. I made this cut on all the front baffles, then switched to a 1/4" straight cut bit, and used it to cut the hole out for the drivers leaving the 1/4" deep x 1/2" wide shelf for the driver to sit on. I made this cut on all the front baffles.

Once all the walls, and shelves were cut and ready to assemble, it was a matter of putting it all together with LOTS of screws and glue. I am fond of #6 x 1 5/8" drywall screws when working with MDF. These screws have a great bite to them. One thing that you must do before you put in the screws is to drill a pilot hole and counter-sink hole. If you do not drill a pilot hole, the MDF will split. I used a counter sink bit. It is a drill bit with a cutter that does the counter sink in the same stroke as the pilot hole. You can use about any type of wood glue. There may be better, fancier glues, but good old fashion wood glue does a great job.

To attach the drivers to the shelf, I used T-Nuts and machine screws with hex nut heads. This is a preferred way to attach a driver to the shelf. Using wood screws has the potential to strip out the MDF, resulting in a loosely secured driver. You really dont want that after all the work you put into the baffles. I used a rubber weather stripping on the shelf to seal the driver. Once everything was assembled except for the top or bottom piece, I sealed all the inside joints with silicone sealant. This is important to avoid any air leaks. Next I stuffed the enclosure with damping material. I always use a product called Acousta Stuf, which can be purchased from Parts Express. I used about 1/2 pound for each box. I used 3M Spray Adhesive to attach the stuffing to the box walls. I always "tease" the stuffing to give it more volume. I then mounted the crossover in the box. It is always best to get the crossover as far away from the drivers as possible to avoid interaction of the speaker magnets with the inductor coils. I mounted mine on the bottom panel with 1/2" wood screws.

Now it is time to layout the wiring. I used 12 gauge Monster Cable wire. I recommend that you solder the speaker wire to the speaker and crossover connections. You would think that after soldering all these speakers, I would get better at soldering (nope). But dont solder anything yet. You need to do the following step first. Once you have test fitted all your drivers, terminal cups and ports, and the box is air tight, it is time to apply the veneer. I used a contact cement to stick the veneer to the MDF. First make sure all your faces are well sanded and smooth. If a screw is sticking up just a little bit, it will be noticeable through the veneer. Make sure you apply contact cement thoroughly. MDF likes to soak this stuff up, so several coats may be necessary. Continue applying coats of adhesive, waiting for each coat to dry, until you have a shiny coating on the surface. Apply the cement to both the veneer, and MDF, wait 10 minutes or until the glue is dry to the touch, and stick it. (WARNING: you have one try at this. Once this stuff touches . . . there is NO moving it. You may be able to rotate it a bit if only a corner is stuck). Then I used a rolling pin to smooth and tack down the veneer permanently.

Once one side of veneer was on, I used a Veneer/Formica trim bit in the router to trim off the excess veneer from the edges. It has a bearing on it, and puts a nice bevel on the veneer while cutting away the excess. Once the veneer was applied, I put on the stain. The choice of stain and topcoat is completely up to you. My brother wanted a deep, reddish color with a glossy finish (personally I like a pickle finish like Ralph Calabria did in DIY project #1 :-) Once the cabinets were finished, I mounted the drivers in the holes, and then I soldered the speaker wires to the drivers, and bolted down the drivers using the T-Nuts.

This crossover network is a 3rd order (18 dB/octave) Butterworth design. The crossover point is at 2.9 kHz. Seeing as how I am solder-impaired, and I dont have the faintest idea how to build a good crossover, I left that portion of the design up to Madisound. They use a test program called LEAP, and they can customize a crossover to any speaker combination that they sell. The finished crossover is an incredible work of art. I have always been impressed with Madisounds craftsmanship in this department. I told them to use the absolute best parts they could. I provided them with the information on the types of drivers I was using in the MTM, as well as some additional information, and Madisound did the rest.