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Get big sound from your computer with this . . . Multimedia Sound System Last month, we introduced this high performance Multime­dia Sound System and gave circuit and constructional details of the sound card which fits into a slot in your PC. This month, we complete the story with the constructional details of the speaker boxes. Design by RICK WALTERS Not only does this system deliver much higher sound quality than the vast majority of Multimedia speakers with their tinny little drivers but the all-up cost is quite a bargain. In fact, this month we present the details of not one but two different versions of the speaker boxes. Both employ fully magnetically shielded drivers and both have been designed with the help of BassBox CAD software. The first system to be featured uses magnetically shielded drivers from Jaycar and has a speaker box with external dimen­sions of 224mm high, 160mm wide and 212mm deep. A perspective diagram of the enclosure is shown in Fig.2. It is made of 12mm thick MDF (medium density fibreboard). Two of these boxes can be cut from one sheet of 12mm MDF, measuring 1200 x 450mm. Fig.1 shows a proposed cutting plan which provides all sides of both enclosures, with a little offcut left over. If you are cutting this with a circular saw, don’t just mark up the sheet and slice away otherwise you will end up with wrong-sized sheets. You must allow for the kerf (width of the saw cut) otherwise you will end up with unusable pieces. Better still, if you are not an expert November 1996  59 Fig.1: two boxes to house the Jaycar drivers can be cut from one sheet of 12mm MDF, measuring 1200 x 450mm. This diagram shows a proposed cutting plan which provides all sides of both enclosures, with a little offcut left over. PARTS LIST 2 5-inch magnetically shielded woofers, Jaycar CW-2102 2 magnetically shielded dome tweeters, Jaycar CT-2006 2 35mm speaker ports, Jaycar CX-2678 or equivalent 2 9-pin “D” plugs 2 9-pin “D” backshells 1 short cable with 3.5mm miniature stereo plugs 1 1200 x 450 x 12mm sheet of MDF 2 pieces of Innerbond, 220mm x 160mm 14 3mm x 20mm screws 4m heavy duty figure-8 cable (woofers) 4m light duty figure-8 cable (tweeters) 8 12mm square self adhesive feet grille cloth PVA wood glue speaker sealant (mastic) carpenter, take your sheet of MDF to your local cabinet maker and ask him to cut it for you. Mine only charged me $5, which seemed a very reasonable price to get all the panels square and the exact size. Actually, while you are there, you might ask the cabinetmaker if he will make the boxes up for you, together with the holes cut in the front baffle. You will save quite a bit of work. The two front panels (one for each box) each measure 160 x 224mm. When you are marking the positions of the tweeter, port and woofer holes, mark one for a left channel speaker and one for a right channel speaker. Fig.2 shows the hole positions for the right channel speaker and these should be flipped over for the left channel speaker, to keep the tweeter at the outside edge of the enclosure. Cut the large holes with a jigsaw and sand the edges with a medium grade of emery paper. Temporarily sit the speakers and the plastic port in their respective holes and mark the mounting screw positions. Then drill each hole with a 3mm drill. Once this is done the box can be assembled. You may or may not decide to assemble the box with timber cleats but either way, the back panel fits between the sides; it should hold the box square as you assemble it. We suggest that you place the back panel on a flat surface and then “wrap the two sides and top and bottom panel around the rear panel. Use plenty of glue and panel pins to hold the panels while the glue dries. Ideally, you should use clamps to hold the panels together while they are assembled as this makes it easier to hold everything square. Make sure that the all edges of the side panels are flush with each other at the front, so that they make a good join with the front panel when it is attached. It is the last panel to be screwed into place. When all the glue joints are dry, drill a 6mm hole for the speaker leads in the rear panel of both boxes. The next stage it to finish the boxes either by painting, veneering or possibly even by covering them with 60  Silicon Chip Fig.2: this perspective diagram shows how the enclosure for the Jaycar drivers goes together. 224 wall paper. If you paint, you will first need to fill any holes and any surface imperfections with a product like Spak-Filla and then sand smooth. We suggest a satin or low gloss paint for a washable finish. Gloss paints show every surface imperfection while flat finishes tend to show dirt. We painted ours with a water based driftwood colour which matches the computer reasonably well. You may wish to leave the speakers exposed as they look quite impressive but they are then prone to damage. Many commer­cial units use aluminium mesh for protection but this is not ideal for the best sound. We bent and soldered lengths of November 1996  61 The drivers can be left exposed but since they are going to be used on your desktop we suggest that they be covered with a grille cloth frame (as shown at left) to avoid accidental damage. This grilled cloth frame can be bent up from coat hanger wire and then covered with a light material. coat hanger wire to make a speaker cloth frame the size of the front of the box and support­ed it at the top on two flathead nails 10mm in from each edge. Two round head nails were used on each side to locate the frame, so it would not move sideways. A piece of suitably coloured light material was made into a miniature pillow case and slid over the wire frame. The open end was tucked under the front edge of the box and held with a couple of staples. Then the rubber feet were stuck on. Final assembly Once the box is finished to your satisfaction you can begin the final assembly. The tweeter can be mounted directly on the front panel; its large flange gives a good seal. The same applies to the woofer flange. However, before both woofer and tweeter are mounted you will need to connect their respective wires. Use thin figure-8 flex with a stripe to connect to the tweeter, wiring the 62  Silicon Chip stripe to the negative lug. The woofer has two pairs of connections; connect the negative terminals together and the positive terminals together and then wire the paired terminals with thick figure-8 striped wire; the stripe goes to the paired negative terminals. Feed both woofer and tweeter leads through the hole you previously drilled in the rear panel. Apply a large dob of mastic around the hole, both inside and out, pushing it in as far as you can, to prevent air leaks. Remove and discard the outer sleeve from the plastic port. Cut the remaining port to a length of 20mm, measured from the face, and mount it on the front panel using mastic seal around it. Fig.3: the pinouts for a DB9 plug. Wire the speaker leads to it as described in the text. Place one piece of Innerbond, measuring 220 x 160mm, loosely in each enclosure. This is used to help damp any inter­nal reflections. Cut the speaker leads to a suitable length for your comput­er setup and then solder them to the DB9 plug. Fig.3 shows the pinouts on the plug. Solder the woofer positive leads to pins 1 & 6 and the negative leads to pins 3 & 8. This leaves plenty of room to solder the two thin figure-8 leads; positive to pin 4 and negative to pin 5. You can now test the speakers by using a multimeter switched to the x1 Ohms range. A click should be heard from each speak­er as the prods are applied to the appropriate plug pins. Equalisation resistors Before you install the amplifier card, you will need to install the equalisation resistors R1-R8. As it happens, the values are the same for both versions of the speaker box presented in this article. R1 & R3 are 1.5kΩ, R2 & R4 are 10kΩ, R5 & R7 are links, and R6 & R8 are omitted. To install the amplifier, first remove the mains lead from your computer, open the cover and remove the blanking Fig.4: this is the cutting plan for the enclosure to house the alternative Altronics drivers. This version of the Multimedia Speakers is taller but not as deep as the other version and does not use a port tube. The hole in the baffle is sufficient to tune it. Version 2: The Altronics Alternative This second version of the speakers uses magnetically shielded drivers supplied by Altronics in Perth. These speakers are a little larger than the other version presented in this issue so the speaker box has different dimensions and its volume is six litres (versus five litre for the other version). It is taller but not as deep. Fig.5 shows the perspective view. The arrangement of the front panel is different from the first version and the front panel is now glued in place, while the back panel is screwed. It was made this way to enable all panels to be cut easily from the same 1200 x 450mm sheet of MDF – see Fig.4. No plastic port is required for this second design. The box is tuned to the correct frequency with just the front panel hole and does not need any additional porting. Both speakers have spade terminals and if you don’t wish to solder the wires to these you could fit spade lugs to the ends of the wires. The woofer only has one 8Ω voice coil and you will not have to paral­lel the voice coils as detailed in the text. PARTS LIST (Ver. 2) 2 5-inch magnetically shielded 8Ω woofers, Altronics C-3085 2 magnetically shielded 8Ω dome tweeters, Altronics C-3005 2 9-pin “D” plugs 2 9-pin “D” backshells 1 short cable with 3.5mm miniature stereo plugs 1 1200 x 450 x 12mm sheet of MDF 2 pieces of Innerbond, 270mm x 190mm 20 3mm x 20mm screws 4m heavy duty figure-8 cable (woofers) 4m light duty figure-8 cable (tweeters) 8 12mm square self-adhesive feet grille cloth PVA wood glue speaker sealant (mastic) November 1996  63 Fig.5: this perspective diagram shows the enclosure for the Altronics drivers. 64  Silicon Chip Fig.6: this printout shows the BassBox parameters for the Jaycar version of the MultiMedia Speakers. Fig.7: the BassBox parameters for the Altronics version of the Multi-Media Speakers. The two versions sound very similar to each other. plate from an empty slot, preferably one closest to the power supply connectors on the main board. Plug the amplifier card into the slot and secure it in place with the retaining screw. Insert the speaker plugs in the sockets and tighten the retaining screws. The left speaker is fed from the top connector. Close the comput­er and reconnect the mains lead. You will have to make, or buy, a connecting cable to link your sound card to the amplifier card. The 3.5mm stereo socket we used on the PC card seems to be becoming the new standard so you will need a cable with a stereo 3.5mm socket at each end. The presets on the amplifier card should be adjusted to give a suitable level when your sound card volume control is at its normal setting. Finally, please note that the 22kΩ bass boost resistors on pins 2 & 6 of IC1a & IC1b on the amplifier card should be changed to 10kΩ, while the associated 150kΩ resistors on pins 3 & 5 should be changed to 68kΩ. Also, note that the 0.1µF monolithic capacitors specified in the parts list are used as supply bypasses. Their positions can be clearly seen as small blue capacitors on the colour photo on SC page 67 of last month’s issue. November 1996  65