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Budget Bookshelf Speaker Project

Below are the plans for a pair of sealed "bookshelf" speakers for use as satellites for an existing subwoofer. They are housed in a sealed MDF enclosure, and a 2nd order Linkwitz-Reilly crossover at 3.85kHz. The woofer is Parts Express # 269-469, a 4 in. extended range driver, and the tweeter is Parts Express #279-255, a Pioneer Mylar dome tweeter. Both elements are rated at 15W, both have 91dB sensitivity, and both are shielded. The woofer is a whopping $0.95 each, while the tweeter is $2.95.

Project Table of Contents
1. Initial Design and Box Assembly
2. Machining and Baffles
3. Building the Crossover
4. Finishing and Final Assembly

The objectives of this project are legion:

Based on the output of Abacus, I will get a very flat response with a sealed 7 liter box. The predicted f3dB is 133 Hz, sufficient for use with a subwoofer.

1. Initial Design and Box Assembly

I used Microsoft Works Spreadsheet to do the volume calculations. Using the 0.6 x 1.0 x 1.6 golden rule dimensions yields a box size of 4.62 x 7.7 x 12.32 inches. I did not want to use the weird fractions, so I adjusted to 5 x 7.5 x 12 inches, close enough for me. This gives a box volume of 0.260 ft.^3, compared to the ideal value 0f 0.247 ft. ^3. I figure the slight rounding up will help accommodate the internal speaker volume, which is very minimal.

The cabinet is made with 1/2 in. MDF with no internal bracing. All the joints are butt-type and glued with polyurethane glue. Prior to beginning assembly, lay the parts out on the bench and label them for east identification. I used "A" for the baffle and back, "B" for the small top and bottom pieces, and "C" for the two sides. See Figure 1.

 


Figure 1. All Parts Laid out on Bench

Figure 2. Dry run, checking for dimensions

Before putting glue on anything, double-check the dimensions of the pieces. In Figure 2 note the length of "C" plus the two thicknesses of "B" equals the length of "A". Also, the width of "B" is equal to the width of "C". 

 
To avoid any nails which can nick router bits I chose to simply use masking tape as the preliminary clamps. I placed the baffle piece "A" down on the bench, and glued up the two "B" pieces and the two "C" pieces. By pulling very tight on the masking tape I was able to pull all the joints into alignment. Be very careful that the two pieces "C" are aligned flush with edges of "A" and "B". There are no dados or internal guides to prevent "C" from slipping inward. Make sure all the joints are well aligned and tightly taped as shown in Figure 3 prior to attaching the final piece "A" on top of the assembly.


Figure 3. Box Glue up before adding top


Figure 4. Box Glue up Complete

Note that enough glue has been used such that it slightly oozes out of each joint as seen on the inside of the box in Figure 3. I slightly bias the amount of glue I put on each edge such that there is more glue on the inside. This minimizes the amount of sanding to the finished box. If in doubt extra glue can be added to the inside edges just in case.

Once you are satisfied that the base is aligned and adequately glued, attach the backside "A" of to the cabinet. Because you can’t inspect the inside of this joint use a liberal amount of glue. Then tape the whole assembly, making sure to pull the masking tape tight across each length. Remember; do not push against either "C" piece because it is unsupported inside the box. If this happens you must partially disassemble the box to realign the side. The finished taped assembly is shown in figure 4.

Set the first box aside and repeat the above procedures for the second box. Once this box is done both boxes can be clamped as one unit. Be careful not to exert any sideways force with the clamps. See Figure 5. Now is a good time to go watch the "Battle Bots" marathon on Comedy Central. The boxes will require at least a few hours to dry. If possible leave them alone overnight.


Figure 5. Both Boxes Taped and Clamped

2. Machining the Baffles


Figure 6. Measuring the Speaker Diameters

After adequate time has elapsed for the glue to dry (cure) the machining starts. First establish the correct diameter holes for the woofer, tweeter and wiring cup. To measure this yourself, see Figure 6. Take two pieces of wood with straight sides across two pieces of the circular object you are measuring. Then simply measure the distance between the two sticks for an accurate diameter measurement.

The best tool for cutting smaller diameter holes of arbitrary diameter is a fly cutter, as shown in Figure 7. An Allen setscrew allows adjustment of the outside cutting spur. A fly cutter MUST be used in a drill press. Make sure to have a damp sponge or rag handy to cool the spur. I cut through the ½ inch MDF in four passes. After cutting for 10 seconds remove the bit from the hole and turn off the drill press. Press the sponge against the spur. Yeah, sizzle baby! This small precaution keeps the spur sharp and properly tempered.


Figure 7. Cutting the woofer hole in the baffle

Now it is time to route the recesses for the woofer and tweeter. This next step can be considered optional, but lends a professional look and better sonic quality to the speaker. A tweeter which is not flush mounted can have horrendous refraction problems. Routing can be done freehand our by using a guide bushing and a template. The woofer I used has rounded corners and the tweeter has truncated corners. The templates I made have square corners. This results in a rounded corner with a ¼ inch radius. This will yield a bigger hole than required, but I figure a template routed hole which is slightly too big will look better than the irregular edges achieved by routing each hole by hand. Furthermore, I think the time to construct a 100% accurate template exceeds the cost of the components in this project.


Figure 8. Router template assembled on baffle

I constructed the template out of ½ inch MDF scraps and some ¾ inch pine scraps. First clamp the two pines strips to the side of the enclosure. I cut two scraps 4 1/8 inch wide to allow for the extra 1/8 inch from my guide bushing in the router. Then two other scraps are cut long enough to cross the baffle and the pine strips. The four pieces of ½ inch MDF are centered around the marked outline of the woofer. Then the four pieces of MDF are nailed to the pine strips. I used a pneumatic nail gun to drive 1 inch 18g brads through the MDF into the pine. This can be done by hand too. The finished template is shown in Figure 8.


Figure 9. Setting router depth

Set the router bit’s depth placing a scrap which is the same thickness as the template on the router base. Place the speaker next to router bit and adjust the depth such that the speaker will be flush mounted. See Figure 9.

Route the speaker recess by slowly running the router around the speaker circle until the bit is no longer cutting. Remove the router, turn it off, and set it aside. Before removing the template clamps verify that all the area underneath the speaker is planar. Also check the corners. Sometime sawdust builds up and prevents the bushing from following the edge of the template. The finished routed recess is shown in Figure 10.


Figure 10. Baffle Routed for Woofer

3. Building the Crossover
  


PE 4" Extended Range Speaker
269-469

The first step in selecting, designing and building a crossover is to know your drivers. The woofer (and I use the term loosely) for this project is the Parts Express model #269-469. This is very close to a full range driver, with a frequency response of 118Hz to 18kHz. However, I have heard from other sources on the net that there is a notch around 5kHz. But at $0.95 each who can complain.


Pioneer Mylar Dome Tweeter
279-255

The tweeter is the Parts Express model # 279-255, a Pioneer Mylar dome tweeter. This too is available at a bargain price of $2.95. It has a resonant frequency of 2.6kHz. Using a common rule of thumb says that with a first order crossover the frequency should be set at twice the resonant frequency, or 5.2kHz.

Unfortunately the optimum crossover for the tweeter overlaps the alleged dip in woofer’s response. Therefore I will choose a 2nd order crossover, set to 4.25kHz. I chose to use a Linkwitz-Reilly algorithm, well, just because. You could choose Butterworth or Bessel too.

I clicked on over to Steven Reid’s online Java calculators at:
http://www.globalnode.com/users/stevenr/spkrs/ 

Given the above criteria and the 8W impedance of my two speakers, the crossover calculator indicated I needed 2.34µF capacitors and 600µH inductors.

Here is where I went VERY unconventional: my component selection. Because this was such a cheap project I didn’t want to spend $30 on crossover components for a $5 speaker. So I used what I had access to.

The inductors are from Gowanda. They are 100µH each, toroid inductors with an iron powder core. They are good up to 100kHz, and have about 25mW of equivalent series resistance. They are rated for a DC saturation current of 2A. All in all, vary marginal, but I did have 24 pieces for free. I guess they’ll do. Each leg of the crossover used 6 of these in series.

The capacitors are unconventional too. I used Taiyo-Yuden ceramic multi-layer SMT capacitors. The only cheesy thing about them (for this application) is they are rated for only 25V. This is incredibly low compared to most crossover capacitors, but remember these are only 15W speakers. The reason most people use 250V caps is for linear performance, not because they need the breakdown performance. I used part # TMK316BJ474, which is a 25V, 1206 size, X7R dielectric 0.47µF capacitor with 20% tolerance. These are rated for about 500mA ripple current each and have an ESR in the low milliohm range. I used five in parallel on each leg of the crossover.

The crossover boards were made with single-sided FR4 with 2oz copper. I did a simple layout in my PCB software and printed it out in a 1x scale. The boards were made by first glue-sticking a copy of the layout on top of the copper, and then grinding out the isolation with a Dremel tool. The finished boards with the capacitors mounted are shown in Figure 11.


Figure 11. Crossover boards with SMT caps mounted


Figure 12. Crossover boards with components and wires

Next the inductors and the input and output wires were mounted, shown finished in Figure 12. Again, I used what I had on hand, which was 18g solid wire I used for the sprinklers. But it did have all those neat colors. Colors are cool. The red wire is input plus, black is input minus. Yellow is woofer plus, white is woofer minus. Orange is tweeter plus, green is tweeter minus.

Next I used a hot glue gun to firmly fasten down the inductors to keep them from rattling. The completed crossover boards are shown in Figure 13, ready to install.

Next I used a hot glue gun to firmly fasten down the inductors to keep them from rattling. The completed crossover boards are shown in Figure 13, ready to install.


Figure 13. Crossover boards ready to install

4. Finishing and Final Assembly

Finishing. Sigh. I have some nice veneer. I have a paint sprayer and a compressor. But I am not going to cast pearls to swine. I went to Home Depot and bought some black marble contact paper.

Contact paper is a roll of self-adhesive vinyl covering in a variety of colors and styles. I wanted to do the paisley … not! Actually there was an oak and a pine style which could be used in the future. The great thing about contact paper is if I decide to paint this in the future I can peel the paper right off!

To prepare for the paper the boxes were sanded smooth. My original crude method of using masking tape for clamps yielded pretty good results. I mostly sanded to remove the glue which oozed out of some of the joints. Figure 15 shows a corner after sanding. I used a belt sander to make the job go quick.

The final assembly begins with installing some internal damping to prevent any possible resonances. I had some ¾ inch egg-crate foam I scavenged at work. This was consistent with the budget so far. I used this to line the top and bottom, back and sides. The piece for the back and sides is made from a single piece, with the back side lightly scored to provide a sharp bend inside the box. This piece also holds the top and bottom in place.

I then took the boxes and a leaf blower outside and removed virtually all the dust, inside and out.


Figure 15. Corner Detail after Sanding

Apply the paper to the front and back first, allowing an inch of overlap to the sides, top and bottom. Then cut a single piece to wrap around from the bottom, up one side, across the top, and then back down the other side to the bottom. The seam is out of site. Also, this "strap" around the case overlaps the front and back edges, where they have been folded over. This will prevent peeling.


Figure 16. Boxes covered awaiting drivers

The wires from the crossover to the rear terminal cup were poked through the egg-crate foam, then soldered to the connectors on the terminal cup. Figure 16 shows the two enclosures, front and back, prior to mounting the drivers.

The Woofer and the terminal cup were mounted using #6 x 1/2 inch sheet metal screws and the tweeter used #4 x 1/2 inch sheet metal screws. I was going to but T-nuts, but again the cost of the T-nuts would exceed the cost of the speaker.

Figure 17 shows the speakers where I finsished them, surround by the debris from their construction.

The contact paper was cut to match the through hole diameter, not the recess routing. That way the pattern from the contact paper extends around the edges of the speaker for a very smooth look.

Figure 18 shows the speakers installed in my system. There is a Sony D790 Pro-Logic Receiver with a Sony CX220 200 disc CD changer. Behind the right speaker is an 8 inch DVC subwoofer with a passive crossover. The sub is painted the same hunter green as the closet for stealth and SAF puposes. It is a single-reflex bandpass enclosure.


Figure 17. Boxes covered and drivers mounted


Figure 18. Speakers Installed, listening to "Who's Next"
 

Special thanks to "TheSpeakerGuy" (name withheld to protect the innocent) for saving us all a bunch of money on this project. We appreciate your contributions both here and on the technical discussion board!!

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