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Opus One 4  Silicon Chip A superb 2-way loudspeaker system to build By LEO SIMPSON Designed by well-known audio consultant Richard Priddle, Opus One is a fine 2-way loudspeaker system which really packs a punch. It has a very good 1-inch dome tweeter teamed up with two 6-inch bass drivers via a carefully designed crossover network. Virtually anyone can put these speakers together, whether they have woodworking skills or not. No special tools are re­quired either, although you will need a bottle of PVA glue and a Philips screwdriver. When you have put them together, you will have a set of speakers that you can be really proud of. These speakers have a number of features which will make them really attractive to the do-it-yourself audio enthusiast. First, they have up-to-theminute styling, with rounded edges on the top and bottom plates and a full-width snap-in grille cloth frame which sets off the side panels which are finished in black bean veneer. Second, they are very easy to put together, by virtue of the precision machining of the various panels. These have mating grooves so that the panels slide and lock into each other, pro­ducing a very rigid assembly. This new machining method is a big advance over previous knock-down cabinets whereby the sides wrapped around the front and back panels and the corners of the wraparound section depended on the flexible veneer to hold them in one piece. If you were a bit cack-handed and tore the veneer, you were in serious trouble. To make it dead easy to put these speakers together we’ve gone mad with the camera and produced a step-by-step pictorial. This was useful for us as well because it showed how easy it was to make a mistake. We made the mistakes just so you would be warned. Well, that’s our brief article. After all, you need to know what speakers have been used and how they’ve been connected. Drivers & crossover Two 175mm woofers are used in parallel in the Opus One. These are 8Ω speakers with a large ferrite magnet, a neoprene rubber roll surround and a curvilinear cone. Their Thiele/ Small parameters are: Qts 0.49; Vas 21.13 litres and Fs 46Hz (free air resonance). They have a power rating of 40 watts and an efficien­cy of 88dB/W at 1 metre. The 25mm soft dome tweeter is a beauty. It is also an 8Ω unit, with a power rating of 50 watts and an effi- ell w y r e v e r a they comp ilt systems u with fully b 0 or more 00 1 $ g n i t s o c “ story, anyway. We know you will want to rush out and buy the kit and start assembling it but before you do that why not just humour us by reading the rest of this ” Fig.1: the crossover is a Linkwitz-Riley configuration with a very low Q lowpass network for the woofers and a higher-Q high-pass network for the tweeter. This arrangement creates a time delay for the tweeter at the nominal crossover frequency of 2850Hz. August 1998  5 Fig.2: if you intend building the cabinets yourself, you will need internal cleats to hold the panels together. Use screws and PVA glue to hold everything together. 6  Silicon Chip ciency of 92dB/W at 1 metre. The tweeter and woofers are coupled together with a 2-way crossover network which is shown in Fig.1. This uses a Linkwitz-Riley configuration. This has a very low Q lowpass network for the woofers and a higher-Q high-pass network for the tweeter. This arrangement creates a time delay for the tweeter at the nominal crossover frequency of 2850Hz. The tower-style cabinet is a conventional bass reflex design with a short flared vent for low air velocity. The cabinet dimensions are shown in Fig.2. Listening tests What do they sound like? We listened to them on a wide range of music and they do give a very good account of them­selves. In particular, the tweeter is a fine unit which could be found in systems costing many times the price. The sound quality is slightly “forward” giving a very good presence on voice but they are equally good on classical music or rock. They compare very well with fully built systems costing $1000 or more. Fig.3: the finished design has a smooth frequency response from around 40Hz to above 18kHz, as shown on this diagram. The overall efficiency of the system is about 92dB/W at 1 metre. Cost & availability This loudspeaker system is available in kit form or fully built from Altronics in Perth – phone 1 800 999 007 to order. The price for a complete kit for a pair of Opus One loudspeakers is $599. If you want to build your own cabinets, you can save money and just buy the short-form kit which contains the loudspeakers, the fully assembled cross­over networks and the flared vent ports. This short-form kit costs $339. Alternatively, you can read this article and then take the lazy man’s approach and order a pair of finished loudspeakers priced at $895. Delivery charges are applicable. Before we conclude, we should note two changes to the Opus Ones which are not shown in the photos. First, the Innerbond filling material has been changed to a grey open-cell foam which is claimed to have superior damping. Second, the port is a larger diameter flared tube which is also shorter than that shown in the photos. It also looks better. When you sit down to listen to them, you will have a fine pair of speakers which look and sound great. Fig.4: the Opus One has a minimum impedance of 4Ω so it will cause no problems with your amplifier system. Now, to find out how to build the Opus Ones, just look at the pictures and read the captions on the following pages. August 1998  7 STEP 1: It’s always a good idea to do a “dummy run”. No, this doesn’t refer to the person doing the assembly – it ensures you have all the components and also helps you understand how they all fit together. STEP 7: Connect the wires from the crossover network to the appropriate spade lugs on the input terminals. The green wire goes to the red terminal and the black wire goes to the black terminal. Push completely home and check by gently tugging the wires. STEP 13: If you did the “dummy run” , about now you’d be starting to think it’s just a bit harder with glue in place. Don’t force it: use a bigger hammer (and a block of four-be-two) for a bit more gentle persuasion. The tops of all four baffles must line up, which also ensures that the whole shebang is square and true. 8  Silicon Chip STEP 2: Insert the female plastic grille retaining clips into the front baffle. The inset shows a simple wooden jig we made to prevent the clips from being damaged during insertion. This device is not patented. STEP 3: Similarly, insert the male plastic grille retainers into the grille surround. Again, the jig makes this a lot easier! Put the grille to one side – you won’t need it for some time. STEP 8: Now comes the fun part - where you get to make a real mess with the glue. Oh, did we forget to mention that you should spread an old sheet out to protect the carpet in the lounge room? Having done that, run a bead of PVA glue right the way around the channel in the bottom baffle. STEP 9: Similarly, run a bead of glue in the channels on both side baffles. Too little glue could mean air leaks conversely too much glue will not only ooze everywhere but could stop the panels completely mating. And that, no doubt, will upset the panels almost as much as you. STEP 14: Remem­b er what we said about too much glue? Here’s a perfect example but, of course, we didn’t make this mistake; we just dummied the pic to show you what not to do! Just wipe it off with a rag and remove the remnants with a damp sponge. While you’re about it, go over the whole box and remove any other gloozes. STEP 15: Once happy (truly, he was happy a minute ago. . .) you can put the top baffle on to hold everything square. BUT DON’T GLUE IT ON YET- you need access to the inside of your speaker box once all the other glue has dried. (It’s dry when the PVA glue is near clear instead of milky). STEP 4: Insert the port tube from the front of the baffle and screw it into place. To ensure an airtight seal around the tube, run a thin bead of glue right around the join between tube and the rear of the baffle. We used PVA glue but silicone sealant is also OK STEP 5: Place the speaker connection terminal plate onto the outside of the rear baffle and screw it into place. A piece of timber under the baffle will stop the terminals from being bent. STEP 6: Screw the preassembled crossover network to the inside of the rear baffle, somewhere around the centre. Orientation is not important. A dollop of glue under the crossover before screwing will ensure it won’t work its way loose later. STEP 10: Slide the side baffles into their mating channels on the bottom baffle. Their edges should line up perfectly. Beware of the trap for young players: there is a front and a back to the side baffles - the front edge has a woodgrain finish while the back edge is plain. STEP 11: Once you are happy that the side baffles are properly in position, slide the back baffle into place. Do we have to mention that the connection terminal plate goes to the bottom? No, we didn’t think so either. STEP 12: If you guessed that the front baffle was next to be placed in position, go to the top of the class. Ah, but did you have the port at the bottom? Well done! This panel may need some forcing to get it into place – grunting and puffing appears to help. STEP 16: While the method of construction will ensure the whole thing stays together while the glue dries, it’s important to force the panels together so that the glue is in turn forced right into the joints. Here we use a clamp to achieve the result - just make sure you protect the surfaces from damage. STEP 17: Curses! We only had one clamp. Here’s the alternative method of ensuring all the glue is forced into the joints: gravity! Again, you should remove any oozing glue from the outside of the cabinet before it dries. Speaking of drying, put the cabinet aside for a good 24 hours to ensure the glue dries completely. STEP 18: When completely dry remove the top baffle. Cut a small rectangle of double thickness (ie, folded over) acoustic wadding and fasten it to the inside of the top baffle, taking care to clear the glue channels. We used a staple gun but drawing pins, tacks or even a conventional desk stapler “opened out” could be used. August 1998  9 STEP 19: Similarly, cut a double thickness of acoustic wadding and fasten it to the bottom baffle. This requires some manual dexterity - it’s easiest to work through the bottom speaker cutout and around the port tube. By the way, don’t increase or decrease the amount of wadding recommended: over-damping will result and you won’t know about it until your speakers are finished! Note that the wadding supplied in the kit may have a different colour and texture than that shown. STEP 25: Both rebates for the woofers must be lined with a sealer to ensure air-tightness. Adhesive-backed foam is used - simply peel off the backing paper and stick the foam around the rebate. This stuff is not designed to go around tight radii so it’s not the prettiest job in the world. But it is all hidden by the woofers anyway, so what’s it matter? 10  Silicon Chip STEP 20: Working through the (still open) top and also the speaker cutouts, position single thickness pieces of acoustic wadding on all vertical surfaces and fasten them in place. Once again, you will need to work through the speaker cutouts in some places. STEP 21: It’s quite OK to cover the crossover network with wadding if you wish but you still need to get at the wiring. Either cut out around the crossover or simply cut a slot in the wadding and pull the wiring through. Count the loose wires - there should be six of ‘em. Pull the loose wire through the speaker cutouts. STEP 26: Connect the wires from the crossover network to both woofers. It doesn’t matter which pair of wires go to which woofer; it does matter which way around they go. Connect the red wires to the “+” terminals and the black wires to the “–” termin­als, then temporarily sit the woofers into their cut-outs. Now you’re going to check that you haven’t crossed over any connections from the crossover to the speaker drivers. STEP 27: The trick is to briefly touch a nine volt battery to the rear panel speaker terminals and check that both woofers move in the same direction. Connect the battery + to the red terminal and touch the battery – briefly to the black terminal. There should be a “thump” and you should see both woofer cones move outwards. If one moves the other way, its connections are reversed. So reverse them! STEP 22: It’s time to start putting the speaker drivers in. Connect the tweeter wires: black to the “+” (red) terminal, white to the “–” terminal. Make sure the connections are secure by gently pulling on the wires. If necessary, squeeze the connectors gently with a pair of pliers to make them a firm fit. STEP 23: Push the tweeter into its rebated cutout so that the holes in the tweeter line up with the holes in the baffle. It should be quite a firm fit, so try to get the alignment right first time! There is no “up” or “down” side but it makes sense to have the terminals on the speaker closest to the crossover network. STEP 24: Screw the tweeter into place with the screws provided: they should be nice and tight but not overtightened because it’s easy to strip the chipboard. If you do strip the hole, you could try the old “broken matchstick in the hole” routine. STEP 28: Screw the two woofers into their respective rebates, first lining up their mounting holes with the holes in the baffle. Whatever you do, do not let the screwdriver slip and punch a hole through the cone or surround! STEP 29: If all is OK, it’s time to finish off the enclosures. Run a bead of PVA glue around the channel in the top baffle, taking care not to get any on the acoustic wadding. STEP 30: Now push the top baffle all the way home onto the enclosure. It must go all the way down; you may need to give it some gentle persuasion (a fist, for example). Chances are some glue will ooze from the joints – if so, you know what to do! STEP PENULTIMATE (don’t you just love those technical terms?): While the glue dries, some weight on the top will hold everything in place. If you don’t have Sydney White and Yellow Pages, you could move interstate STEP LAST (but not least): Clip the grille onto its retaining clips on the front baffle . . . and you’ve finished. Now connect up your stereo and . . . stereo? Uh, Oh! Did you forget to build the other speaker? August 1998  11