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The JV80 Speaker System Design by PHIL ROUTLEY Words by LEO SIMPSON Assembled by MICHELLE ONEILE ... all you need is an hour or so and a screwdriver! 8  Silicon Chip www.siliconchip.com.au The JV80 is a fitting successor to the very popular JV60 system described back in August 1995. The new design uses bigger Vifa woofers (8-inch) and a Vifa D26 ferro-fluid cooled dome tweeter. It is a bass reflex tower design with two ports and it includes overdrive protection. O to make the speaker cabi­nets though; they are available ver the years, we have published a number of dofully assembled, with the plastic ports, rear terminal panels it-your­self speakers but the JV60 has easily been and some the BAF (bonded acetate fibre) Innerbond lining the most popular and long-lived. already installed. Now there is this new system, still using Vifa speak­ers Better still, the cabinet finish is a simulated light timber but with bigger (8-inch) woofers and a teensy little Vifa veneer rather than boring old black. Mind you, the grille D26 dome tweeter. In fact the tweeter looks so small and cloth is black but that could easily be changed to a scrim insubstan­tial that you’d wonder how it could possibly (open weave) fabric to match or complement your room stay with the pace set by the two big woofers. Yet it does decor, if you wish (or your partner dictates!). it easily, due to some fancy technology which we’ll get to The cabinets are made of MDF (medium density fibrein a moment. board) and have a volAnd while home theatre Specifications ume of about 73 litres systems seem to be all the go Power rating.............100 (not including internal at the moment, these JV80s W (typical program) bracing). The two 66mm are equally suitable for a Type.........................2-way, Bass Reflex with two po rts flared ports are 140mm high-quality stereo system Impedance...............8Ω Frequency range...... 30 long. or as the front speakers in Hz to 20kHz Nor do you have to a high-quality home theatre Crossover................3.5kHz Linkwitz-Riley Sensitivity................91d assemble and solder the system. In fact, the bass B/1W at one metre (on tweeter axis) crossover net­ w ork. It response of the JV80s is so Protection................Polysw itch PTC thermistor is supplied hard-wired good that you can dispense onto a piece of MDF, with the subwoofer in a home ready to be installed into theatre system. the enclosure. What’s more, we would go so far as to suggest that you In essence, what you have to do is to install the crossdispense with any centre speaker as well. But we’re getting over network, make the various internal connections way ahead of ourselves . . . and install the speakers. If you are reasonably handy, OK, so we’re looking at a pair of good-sized tower speakyou could do the whole job in an evening. You will only ers which stand 950mm tall, 277mm wide and 350mm need a screwdriver to assemble the speakers – not even deep, not including the grille cloth frame. You don’t have The “works”, as they come out of the box. There is a pair of crossovers, four Vifa 8-inch woofers, two tiny dome tweeters (also shown enlarged at right) plus two packs of screws and some sealing compound. We suggest throwing the sealing compound away and using draft-excluding foam! Not shown are two large pieces of acoustic wadding. www.siliconchip.com.au November 2003  9 larger conventional ceram­ic magnet. In fact, the magnet structure is so small that it has been fitted with a heatsink, to better dissipate the heat produced in the voice coil which is also ferro-fluid cooled. The voice coil diameter of the dome tweeter is 25mm. In other words, it is a standard 1-inch fabric dome tweeter but until you hear it, you are not going to believe that such a tiny assembly can deliver so much high-quality sound. Crossover network And here are the boxes, again as they come . . . the boxes are complete, the grilles are fitted, the driver holes are cut out and rebated, the input connector is screwed to the back panel and there is even a layer of acoustic wadding on the back. There’s not much more to assembly than fitting the drivers and crossover. The crossover network is quite complex and provides a third order filter for the tweeter (nominal attenuation slope of 18dB/octave) and a second order filter (12dB/octave) for the woofers which are connected in series, to give a nominal im­pedance of 8Ω. Crossover frequency is 3.5kHz. A feature of the woofer attenuation network is impedance equalisation, as provided by R1 and C2. In effect, impedance equalisation cancels out the rising impedance of the woofers, due to their voice coil inductance (3mH), so that the crossover network “sees” a resistive load of close to 8Ω rather than an impedance which rises linearly as the frequency increases. What happens is that impedance equalisation is a special case of a parallel resonant network which results in a constant resistance, ie, all reactive components due to capacitance a soldering iron is required. In fact, if you have a power screwdriver, so much the better – it is a lot quicker and easier. Speaker line-up Two 200mm (8-inch) Vifa P22WP01 4Ω woofers provide the “muscles” in the JV80 enclosures. They are fully characterised with Thiele-Small parameters (Vas 82l, Fs 31Hz & Qts 0.27) and each has a nominal power rating of 100W. They have a cast magnesium chassis (basket), polycarbonate cones and synthetic rubber roll surrounds. As already noted, the tweeter is very tiny and seems espe­cially so since it has such a small magnet structure. But first impressions are deceiving here because the magnet is an alloy of the rare earth Neodymium which means that it can deliver a voice coil flux density equivalent to a much 10  Silicon Chip The crossover network incorporates impedance equalisation for the seriesconnected woofers. Crossover frequency is 3.5kHz. Note the phase reversal of the tweeter, with respect to the woofers. The Polyswitch PTC thermistor provides protection against over-drive. www.siliconchip.com.au These curves show the action of the crossover network in attenu­ating the signal to the woofers and tweeters. The attenuation slopes are close to 12dB/octave. or inductance are cancelled out. In this particular case, we have R1 and C2 as one leg of the parallel resonant network and the resistance and voice coil inductance as the other leg. The resistance of R1, 8.2Ω, in one leg, is close to the total DC resistance of the two Vifa woofers, in the other leg. While the calculation is not simple, the result is that the capacitive reactance of C2, a 10µF capacitor, cancels out the 3mH inductance of the two woofer voice coils. Both the inductors are air-cored, avoiding any distor­tion effects which result from saturation and other nonlineari­ties in ferrite or iron-cored inductors. And the crossover ca­pa-citors The overall frequency response of the JV80s – and as you can see, the bass response is well maintained to below 30Hz. The modest peak at around 150Hz is due to room effects and should be ignored. are all high-quality polypro-pylene types for low distor­tion. Before we leave the crossover network, note that the phasing of the tweeter is reversed with respect to the woofers. This is common in complex crossover networks where the crossover slopes are 12dB/octave or greater and which often have rapid phase rotation in the vicinity of the crossover frequency. The decision whether or not to reverse the phase of the tweeter (or midrange in a 3-way system) is made on the basis of which results in the smoothest frequency and phase response. So now you know. Don’t forget to make the correct Here’s a close-up of one of the crossovers. Input is on the left, output to the woofers is centre bottom and to the tweeter right bottom. We suggest marking the input and outputs with a felt-tip pen to save any confusion later on. www.siliconchip.com.au tweeter connection - positive terminal to the negative speaker line. Over-drive protection One of the difficulties designers face is setting a nominal power rating for a loudspeaker system. As already noted, the woofers each have a nominal rating of 100W while the tweeter has a nominal rating of 50W. However, this is a “music program” rating – a pretty vague term. Without any doubt, if you fed a constant tone at 100W into the woofers they would ultimately destroy themselves, while a 50W constant tone to the tweeter would probably burn it out in short order. In fact, the tweeter has a continuous rating (operating power) of only 5W. Faced with this dilemma, the designer can only specify a nominal overall power rating for a speaker system and then hope that users will not get over-enthusiastic (or stupid) with the volume control. After all, a 100W amplifier driven hard into clipping will deliver far more power, perhaps 200W or more, which can easily destroy a 100W speaker. So what to do? The designer of the JV80 speaker has taken the same approach as in the earlier JV60s; use a PTC (positive temperature coefficient) Polyswitch thermistor in series with the crossover network. Normally, these devices have a very low re­sistance and thus have a minimal effect on the signal to the drivers. But when the signal current exceeds a critical (RMS) level, the Polyswitch suddenly goes to a high resistance state November 2003  11 This is the impedance curve for the JV80. Notice the double hump at low frequencies which is typical of a bass reflex enclosure. The enclosure is tuned to 35Hz (Mk1). Minimum impedance is 3.62Ω at close to 6kHz (Mk3). and effectively remove the drive signal and thereby protects the speakers from damage. After a short cool-down period which depends on the initial overload, they revert to their low resistance state and the signal can pass once more. Listening tests After we assembled a pair of these speakers (see the step-by-step photos), we had a long listening test with the JV80s, comparing them with a similarly-sized tower system which costs about three times the price. The results? Surprisingly good. The JV80s have generous power handling and quite good efficiency so they can really deliver a punch. If you have a 50W/channel ampli­fier, they will be more than adequate in average -sized living rooms. In larger rooms, go for a 100W/channel amplifier. Overall frequency balance is very The polarity markings on the dome tweeters are not easy to see, especially in dim light. This close-up highlights the +ve marking (no, you won’t find the red ring on yours!) 12  Silicon Chip These curves show the “off axis” response of the JV80s and illus­trate the good treble dispersion of the Vifa 1-inch dome tweeter. good, with smooth extend­ ed bass down to below 35Hz (subwoofer territory) while the tweet­er is smooth right up to the limits of audibility, although tapering off slightly above 10kHz. The tweeter has a modest peak at around 5kHz which does give a touch of emphasis to sibilants but also gives a slight prominence to voice and brass. Overall, we think the result is really very good, especially when the cost is factored in. By the way, for clean, unmuddied bass, the JV80s should be installed at least one metre away from walls and room corners. Do not place them close to TVs either, as the woofers are not mag­ netically shielded. Step-by-step photos The photos on the following pages show the procedure for assembling the speakers. In brief, they are: (1) Drill holes and mount crossover network inside enclosure, adjacent to rear terminals. (2) Connect two wires to rear terminals (red to positive, black to negative). Mark tweeter (T) and woofer (W) wires on crossover board with black felt-tip pen. (3) Run sealant around rebated woofer holes. Throw away the sealant supplied in the kit. Use Raven RP14 self-adhesive draft exclusion foam tape instead (available from hardware stores). (4) Solder push-on connectors onto crossover wires, if these are not supplied already fitted. Do not solder wires directly to tweeter – you run the risk of melting off the lugs. IMPORTANT: connect the black wire to tweeter positive; the red wire to negative. Install tweeter. (5) Install roll of Innerbond filler – don’t obstruct the port tubes. (6) Connect red crossover wire to positive terminal of one woof­er. Then run another wire from its negative terminal to the positive terminal of the other woofer. Then connect the black crossover wire to the remaining woofer terminal. Install the woofers but only with a couple of screws each. (7) Do woofer phasing check: connect a 1.5V battery across rear speaker terminals – both woofers should move in or out together. (8) Fit the remaining screws to woofers – do not over-tighten the screws. If using a power screwdriver, set it to the lowest clutch setting. (9) Clip on grille clothe frames. Connect amplifier and CD play­er. Enjoy! Where from; how much? The JV80s are available only from Jaycar Electronics stores (and their on-line or mail order “Techstore”). The complete kit – enclosures, speakers, crossovers, ports, terminals, wadding and even the screws – retails for $939 per pair (Cat. AA-0124). To be frank, we don’t think that building your own cabinets will save you a lot of money – but if you must build your own, you can buy the rest of the kit, comprising four woofers, two tweeters, two crossover net­ works, rear terminal panels, flared ports, sealant and Innerbond, for $589 (Cat. CS-2580). www.siliconchip.com.au STEP-BY-STEP: Putting the JV-80s together 1: We started construction by soldering mini spade connectors onto the appropriate leads. Jaycar have assured us that this will be done already in their kits so you may not have to worry about this step. At this stage, we also made a connecting lead for the two woofers – again, this should be supplied with the kit. 2: If there aren’t already holes in it, drill two mounting holes (say, 3mm) right through the MDF boards which hold the crossovers. Hole position is unimportant but it’s probably best to avoid drilling through a component... By the way, mark which pair of wires are which (input, woofers, tweeter) with a felt-tip pen. Saves a lot of confusion later! 5: Push a couple of the large screws through the holes in the crossovers and locate the pilot holes in the back panels of the boxes. Screw the crossovers down tight – you don’t want them rattling around when music is playing! 6: Turn the box over and unscrew the input terminal plate. Note how one of the terminals has a red ring and one has a black ring? That fact becomes important in just a moment . . . 3: Feed the crossovers through the centre speaker cutout and place on the inside rear of the box. Each crossover should be situated very close to the hole for the input terminal, with the input leads towards the terminals (the leads are pretty short!). www.siliconchip.com.au 4: Using the holes drilled through the crossovers as templates, drill a couple of smaller, shallow “pilot holes” (about 2mm) in the inside back of the box. Take care that the acoustic wadding doesn’t try to wind itself up on the drill bit! 7: We had to solder the input wires to the crossovers direct to the input terminals. Your crossovers may come with quickconnect spade lugs so that they simply push on. Connect the red input wire to the terminal with the red ring and the black input wire to the terminal with the black ring. Check twice! ctober 2003  13 NO ovember 8: “Fish out” the red and black cables for the woofers – here’s where you’ll be thankful you marked which wires were which on the crossover. You’ll note we kept them from falling back in by temporarily sticking them to the front of the box with a piece of insulation tape. Tape colour is optional. 12: You’ll be much better off using some of this self-adhesive, draft-excluding foam (Raven RP14, which you can buy at any hardware store). This is actually the second box, which we did after having so much trouble with the gunk on the first box! 13: Take the RED woofer wire and push its spade lug onto the “+” (or red) speaker terminal. 9: Similarly, find the two wires for the tweeter and bring them through the tweeter hole. Another piece of insulation tape will keep them captive. Darn! You can never find a bit when you want it . . . 14: Attach the woofer connecting wire to the “–” (or black) terminal of the same woofer. Our connecting wire was red, just to confuse you. Let the other end of this wire fall into the hole. 10: There are two pieces of acoustic wadding, one for each box. You can feed the wadding through either hole. The idea is to cover as much of the inside of the box as is currently not covered (remember there is one piece supplied already fixed to the back of the box). 11: Before placing the speakers, you need to ensure no air can escape around them. Some caulking material is supplied with the kit – we tried to use it but found it stuck much better to our fingers than to the wood. Take a tip: dice it. 14  Silicon Chip 15: We are about to place the first woofer in its rebated hole. Can you spot our deliberate mistake? Yes, of course the black wire has to be fed through to the OTHER woofer hole. But you knew that already, didn’t you? www.siliconchip.com.au 16: Place the woofer in the hole and carefully push down on the speaker edges until it is seated properly. You could put the screws in now but it’s probably best to do it all at once – after a final check! 17: Next comes the second woofer. The wire from the first woofer connects to the “+” (or red) terminal; the black wire from the crossover connects to the “–” (or black) terminal. 18: You know what to do next – you've done this before, haven’t you? Watch that cone and edge! 20: Here’s that ’orrible sticky sealing stuff again. You don’t need it! We found that the tweeter is such a tight fit in its hole that you don’t need anything to seal it. But seeing Michelle had painstakingly posed for this picture we didn’t have the heart to leave it out . . . 21: V-e-r-y carefully push the tweeter into its rebated hole. Handle only by the edges and for heaven’s sake, don’t slip and put your finger through the speaker cone! 22: Before the final step, let’s check the connections. Grab a 1.5V or 9V battery and briefly touch it across the input terminals while you watch the woofer cones. It doesn’t matter which way around you connect it – all you are looking for is both woofer cones moving in the same direction. If the cones move in opposite directions, you have reversed the connections to one of the woofers. 19: Two woofers down, one tweeter to go. Now here’s the trap for young players: the RED wire from the crossover connects to the “–” terminal, while the BLACK wire goes (of course) to the “+” terminal. Yes, it sounds wrong – but it’s right (check the circuit diagram out if you don’t believe us!). 23: Finally, screw in all three speakers. Again, we cannot emphasise how careful you need to be here: one slip and the cone is history. Once completed, all that’s left is to snap the grilles in place, connect the speakers to your amplifier . . . and settle back with your favourite piece of music and beverage, basking in the listening pleasure of your new JV80 speakers (not to mention basking in the glory that will be yours when your friends find out that you built these SC speakers yourself!). www.siliconchip.com.au NO ovember ctober 2003  15