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By JULIAN EDGAR Big Blaster Subwoofer Capable of thunderous bass, this easy-tobuild subwoofer can handle up to 250 watts RMS and uses a compact 37-litre enclosure. 56  Silicon iliconCChip hip www.siliconchip.com.au www.siliconchip.com.au The driver used in the subwoofer is 10 inches (25.4cm) in diameter, is rated at 125W RMS and uses a voice coil that’s 50mm in diameter. It costs $99 (you need two for this design) and is available from Jaycar. I N MARCH 2003, we presented the “Little Dynamite” subwoofer – an easy-to-build design that used a single 10-inch driver in a 25-litre ported enclosure. At the time, we said that we’d later be describing a larger, higher-powered subwoofer and this is it. This new subwoofer is a flow-on from the previous design, where many different enclosure variations were modelled using BassBox speaker design software. It uses not one but two 10-inch Jaycar drivers, two 25-litre pre-built sealed enclosures and two ports. The two 25-litre enclosures are combined to make one unit with a capacity of 37 litres and we’ll look at just how this is done shortly. The resulting enclosure is longer than before and this has allowed us to use longer ports. This, in turn, has allowed the box to be tuned to a lower frequency which benefits the bottom-end response. In addition, the use of two drivers in­creases the sensitivity and power handling of the finished sub­woofer. Another benefit of the new design is that if you built the previous unit and want to upgrade, you can do so without starting all over again. The enclosure is constructed by marrying two of these pre-made Jaycar boxes together. Each box has an internal volume of 23 litres and is supplied fully carpeted, with the speaker hole precut and speaker terminals fitted. because of the cheapness and ready availability of the pre-built Jaycar subwoofer enclosures (they’re even carpeted!). These en­ c losures are priced at just $59.50 each and at that price, they’re hard to go past – just assembling the materials to build one would cost you more than that. As mentioned, we combined two such enclosures for a usable internal volume of 37 litres. Apart from the low cost of the two boxes, this approach has a number of advantages over building something from scratch: (1) the panels are all small in area and so the stiffness of the finished enclosure is quite high; (2) very little woodworking needs to be done and any that is required isn’t critical in nature (ever tried to cut out the hole for a loudspeaker? – it’s harder than it looks if you want to do a good job!); and (3) the Main Features • • • • • Easy to build 250 watts power handling Suitable for both home and car Versatile wiring allows both 4-ohm and 2-ohm connections Excellent frequency response long, thin design that results is – while unconventional – very suitable for car and home use. On the latter subject, the overall dimensions of 900 x 340 x 250mm allow the subwoofer to fit up against the back seat in most sedans and hatches (we measured a WRX, a 200SX and a Commo­dore – plus several other cars – and the 200SX was the only Design details Despite considering fancy isobaric bandpass designs and all sorts of other exotic types, we eventually came back to a simple bass reflex enclosure for this subwoofer. This was primarily www.siliconchip.com.au The response of the subwoofer, as predicted by the BassBox speak­er design software program. The yellow line shows the response for an in-car environment, while the red line shows the modelled response within a room. May 2003  57 The end panels of each enclosure are marked and then cut out as described in the text. This opening allows air to freely pass along the length of the enclosure and gives room for the long ports. tight one). In a home application, the long, thin design can be easily slipped behind a chair or it can be fitted with feet and placed upright in a corner (the feet are required to give port clearance). As before, the drivers used in the design are the Jaycar CS-2274. These are 10-inch units with 125 watts RMS power han­ dling, a maximum cone movement of 9mm, a voice coil dia­meter of 50mm and a resonant frequency of 33Hz. They cost $99 each. The boxes that are “siamesed” together are the 25-litre (actually 23 litres) CS-2520 sealed subwoofer enclosures. Note that although ported versions of these enclosures are available, it’s better to start with the sealed boxes and cut the port holes as required, to suit the special ports used. The enclosures are first modified by cutting a panel out of the end of each, then joining them together with sealant and nuts and bolts. This makes a strong, airtight enclosure. The ports are the Jaycar CX-2688 flared ports, which can be easily adjusted in length by adding 65mm-diameter plastic pipe. The flared section is used at both ends of each port, reducing the chance of port noise that could otherwise occur as air flows around the sharp inner edge. Optimising the bass The bass response is optimised by tuning the enclosure to 26.5Hz using two 600mm-long, 63mm internal dia­meter ports. Modelled using the Bass­Box software, this combination of tuned box frequency, 37-litre box volume and specified 10-inch drivers, gives an in-car frequency response that is quite strong down to 20Hz. Inside a home, the modelled bass Once the opening has been cut out, align the two enclosures and drill four holes for the attachment bolts (yes, I did drill one hole in slightly the wrong place!). That done, use a sharp knife to cut away a piece of carpet all around the opening. 58  Silicon Chip Using curved corners in the cut-out gives room for the nuts and bolts which will later join the two boxes – and also makes it easier to use the jigsaw. A food can that’s just the right size makes a convenient hole marker. response rolls off by 3dB at 40Hz. However, there is sufficient cone excursion left that this can be boosted to give bass that is audible down to 30Hz. Even more importantly, the use of the twin drivers and long ports allows much louder bass for the same input power: at 100W input power and at 20Hz, the modelled output is 5dB greater than the previous single-driver design. However, as noted before, these drivers are not very sensitive units – so you’ll still want an amplifier capable of at least 100W RMS per channel (more on this later). Building it The first step is to line up the two enclosures end to end. To do this, place the boxes so that their terminal strips are uppermost, then move them apart again. The sides that were touch­ing are the ones that have to be cut open Once the carpet has been cut away, apply water clean-up Liquid Nails (or a similar building adhe­sive) around the opening. As the two box halves are forced to­gether, this adhesive will seal the gap, in addition to providing more strength for the join. www.siliconchip.com.au This view shows one of the four nuts and bolts that hold the two halves of the enclosure together. Notice how the Liquid Nails that has squeezed from the join has been spread along the internal ribs to ensure an airtight seal. so that when they are later joined together, one large enclosure is formed. On each of these sides mark a line 50mm in from the top, back and bottom and 65mm in from the front edge. That done, drill a hole in each panel to take a jigsaw blade and then cut out the panels, following the lines that you have marked. Note that this will leave a “rib” around each of the openings. This rib not only helps strengthen the final assembly but also accepts the connecting bolts. To join the boxes, first hold them in perfect alignment, then drill four holes – one through each corner of the ribs. That done, good-quality nuts, bolts and washers can be used to rigidly fasten the two enclosures together. However, before you do bolt them together, remove a strip of carpet from around the opening and then run a bead of water clean-up Liquid Nails (or some other similar sealant/ adhesive) around the join. Make sure that you don’t use too much or it will squeeze out from the outside of the join and look ugly. The carpet that’s still present between the surfaces will compress as you tighten the bolts, so go right around them three times, tightening them up. After that, you have to let the adhesive set – preferably overnight. The ports are positioned towards the back of the enclosure with their openings at either end (one at the top and one at the bottom). They require an 85mm-diameter hole and this should be positioned 40mm in from the box edges. The flared plastic vents are connected together using cheap 65mm-diameter plastic pipe (see text). This makes it easy to construct ports of the required size and flow characteristics. that they don’t interfere with each other). The first step is to use a round or half-round file to remove the sharp inside edge from the preformed flared ports (ie, at the non-curved ends). This is done to eliminate any sharp steps between the flared vents and the Cutting the ports The next step is to cut the holes for the ports. Two are used – one at each end of the enclosure. Note that these ports must be at the back of the box so that they clear the speaker magnet assemblies, one positioned at the top and one positioned at the bottom (so www.siliconchip.com.au The sharp “steps” that would otherwise occur in the transition from the flared port to the plastic pipe are smoothed using a half-round file and some fine sandpaper. plastic pipes when they are later joined together. Finish off the job using some fine sandpaper. With these edges smoothed, cut each plastic pipe to the correct length (about 520mm) so that when both flared ends are pushed firmly into it, the total length of each port is 600mm. Don’t be tempted to glue the flared vents to the plastic pipe at this stage, though – that step comes later. Once the ports have been temporarily assembled, spray some black paint inside them to hide any scratches that you have made and to hide the white plastic. The next step is to cut the holes for the ports. An 85mm diameter hole is ideal – we drew the two cutouts with the help of a can of food that conveniently had the right diameter. The holes should be positioned with their edges about 40mm in from the edges of the box. If you place the port opening furMay 2003  59 How To Make The Brackets To Hold The Ports In Place (1) Start by cutting off a surplus length of the 65mm plastic pipe. It should be about 30mm wide. (2) Use a hacksaw to make a long­ itudinal cut along the 30mm pipe section. (3) Use a heat-gun to soften a little less than half the diameter. Flatten this piece out (careful – it’s hot!). ther in from the edge, you will find it easier to miss the internal rib with the long ports – but you’ll also be getting closer to the magnet assemblies of the woofers. A trade-off is to mark where the plastic pipe touches the internal rib and then soften this area on the pipe with a heat gun (done with the port out of the box!). It will then be easy to compress the port pipe a smidgin at this spot to give better rib clearance. The port volume and flow changes will be only tiny but the tweak makes it all a bit easier to fit everything in. Gluing these long ports into place is not sufficient to secure them – you will also require brackets to hold them rigidly inside the box. An effective bracket can be easily made by first cutting off a 30mm surplus length of the 65mm-diameter plastic pipe. That done, square the ends and then make a single cut longitudinally along the section. Next, using a heatgun, soften the pipe to one side of the cut and then bend that section outwards (use oven mitts as the pipe is hot!). With a bit more heatgun work, you should end up with a bracket which wraps itself at least halfway around the port. The other end of the bracket is attached to the inside of the enclosure using short self-tapping screws. You will need one bracket for each port and they can be glued to the port tubes using Liquid Nails (or similar) building adhesive. In addition, a generous amount of adhesive should be placed around the back of each flare that faces out of the enclosure, while additional adhesive is placed on the ports where they sit on the internal rib. Connecting The Drivers The two woofers can be driven in parallel from one amplifier or they can be driven separately by a stereo power amplifier but there are a number of traps here. If you get it wrong, you could blow your amplifier. If you want to drive the woofers in parallel, they will constitute a 2Ω load. Many car subwoofer amplifiers will happily drive a 2Ω load, so that is one option. A second option is to use the two channels of a stereo amplifier to drive each woofer separately (4Ω loads). Or, if you have a 4-channel car amplifier which can be bridged to drive 4Ω loads, then you can use that to again drive each woofer separately. What you must not do is connect a stereo amplifier in bridge mode to drive the two 4Ω woofers in parallel; ie, a 2Ω load. In this case, the separate “bridged” amplifiers will each “see” a 1Ω load – most amplifiers cannot drive a 1Ω load and will blow fuses or be seriously damaged. When running the subwoofer in 2Ω mode, simply connect the two drivers in parallel. To do this, connect the power amplifier to one set of the 60  Silicon Chip speaker terminals and then run more cables to the other speaker terminals, making sure that you connect posi­tive to positive and negative to negative. An amplifier driving a 2Ω load will deliver more power than it does into a 4Ω load. 4-ohm load If you want to configure the sub­ woofer as a 4Ω design, you’ll need a 2-channel amplifier. One channel connects to one set of terminals on the subwoofer, while the second channel connects to the other set. Note, however, that you must feed a mono signal to the subwoofer amplifier; eg, by using a Y-connector lead on the input. In other words, both channels of the amplifier must be driven by the same signal. If you have a single subwoofer output from a head unit or other source, this should be fed into one end of the “Y” cable which then connects to each amplifier channel. Make sure that you get the phasing right – ie, connect positive to positive and negative to negative. If you have the phasing to one of the drivers reversed, there will be a distinct lack of bass. Finishing off Once the ports are in place, acrylic speaker damping material can be cut to size and stuck to the inner walls of the box. We suggest 350 grams/square metre material (Jaycar AX-3690) but any similar material is fine – eg, acrylic quilt wadding. Be careful that you don’t block the entrances to the ports www.siliconchip.com.au (4) Twist the tail to form a mounting foot for the clamp. Each port is held in place by attaching one of these clamps to the centre rib of the enclosure. – in fact, it is wise to be quite sparing in your use of the material around the port entrances. Next, solder some heavy-duty speaker cable to the terminals and attach the other ends to the screw terminals on the drivers. Keep this wiring completely separate – each pair of terminals connects to its nearest driver. Be sure to connect the positive terminals to the positive terminals on the drivers; similarly, the negative terminals go to the negative speaker terminals. Once the wiring has been completed, the drivers can be slipped into their precut holes and the locations marked for their mounting screws. That done, remove the drivers and drill small diameter pilot holes for the screws. If you’re fitting metal grilles, you should also drill the holes for The ports are sealed to the panels by applying Liquid Nails from inside the box. It’s a lot easier if you have small hands, so at this point a helper may need to be press-ganged into action. their mounting lugs and attach the T-nuts under the front panels at this point. Finally, reinstall the drivers and fasten each into place using eight coarse-thread MDF screws. As before, the carpet will compress as you tighten the screws, so go around each driver and re-tighten it at least three times. Phasing This step is very important in this It is important that there are no leaks around the ports, so make sure that the sealing is well done. Any leaks here can cause whistles. Similarly, there must be no leaks around the edges of the drivers. www.siliconchip.com.au design – apply a 1.5V battery across each set of external terminals in turn (positive to positive and negative to negative) and check that the corresponding woofer cone moves forwards in each case. If a cone moves backwards when the battery is applied, open up the enclosure and swap the wiring connections to the relevant driver. The next step is to connect the subwoofer to an amplifier (see panel). This view shows the two ports in place inside the enclosure. Note the supporting brackets – these are in addition to adhesive which is placed directly on the ports to secure them to the internal rib. May 2003  61 The final steps before screwing the drivers and their grilles into place are to solder the cable to the terminals and then place the acrylic speaker filling along the internal walls. Make sure that the port entrances can’t be blocked – hold the acrylic filling in place with a few dobs of adhesive. Parts List As can be seen in this overall view, the ports extend into the opposite ends of the enclosure. The clearance between the ports and the magnets of the drivers is quite tight – make sure that they don’t touch. Once it’s connected, begin by driving the unit quite gently. Moisten a finger and move it around the edge of each driver, to check for any air leaks around the frames. Now do the same around the edge of each port – there will be air movement within the ports but there shouldn’t be any around the edge of the flares. Next, listen carefully for any buzzes, rattles or whistles. If everything is OK, wind up the wick a bit more. Naturally, during this test procedure, all other speakers should be disconnected so that you’re just listening to the subwoofer. This will allow you to easily identify any problems. A good test is to drive the subwoofer from the soundcard in your PC and download some free audio frequency generator software from the Internet – eg, the NCH Tone Generator from www.nch.com.au/tonegen/index.html 62  Silicon Chip 2 10-inch Response Subwoofers (Jaycar Cat. CS2274) 2 25-litre sealed subwoofer enclosures (Jaycar Cat. CS-2520) 1 acrylic speaker damping material (Jaycar Cat. AX-3690) 2 10-inch protective grilles (Jaycar Cat. AX-3522) 4 flared speaker ports (Jaycar Cat. CX-2688) 4 2-inch x 0.25-inch bolts, nuts and washers 1 150cm (approx.) length 65mm-dia. plastic pipe 1 0.5m-length of heavy-duty speaker wire 1 tube building adhesive; eg, Liquid Nails 16 speaker attachment screws Alternatively, you can just download the software and burn some test tones onto a CD so that the subwoofer can be checked in a car. Using the software, you can generate sinewave signals at all sorts of audio frequencies. The first use for this is to determine the range of frequencies that are audible. In the case of the prototype (tested in a 5 x 4-metre room), there was strong bass down to 40Hz and audible bass at 30Hz. It was also clear that the cones became dramatically unloaded at about 25Hz – the exact frequency depending on the power being fed to the sub-woofer. That means that a subsonic filter should be used if the subwoofer is going to be driven hard. The other use of the software is to check for peaks and troughs in the frequency response. This can done by doing a slow sweep across a range of frequencies – eg, from 150Hz down to 25Hz. However, these peaks and troughs will also be affected by the listening environment. In the case of the prototype, there were minor peaks at 67Hz and 100Hz. Note, however, that high SPLs (sound pressure levels) shouldn’t be maintained when using sinewave signals. In other words, be careful that you don’t wreck the drivers by driving them or the amplifier into distortion while doing this testing. Conclusion The use of the pre-built boxes really does make this design dead-easy to make – you should be able to put it together in just a few hours. The end result is an impressive subwoofer, especially considering its cost and overall size. SC www.siliconchip.com.au