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Build the JV60 Here’s your chance to build a modern high performance loudspeaker using high quality drivers made by Vifa of Denmark. This is a tower design producing lots of bass. It uses two 170mm woofers and a 25mm aluminium dome tweeter with a ferrofluid cooled voice coil. By LEO SIMPSON Handsome and well finished, the JV60 cabinets have a capacity of 50 litres and only take up a modest amount of space in your listening room. T HESE DAYS, you have two main choices as to the type of loudspeak­er system to buy from your hifi retailer. The first is a compact bookshelf style system and the second is a tall narrow cabinet generally referred to as a “tower” style using two small woofers. Now it might seem that the compact bookshelf system is the one to go for if you don’t have a lot of space or you are on a budget. But you will always find that the manufacturers recommend that their compact systems be placed in floor stands to give the best overall performance. Typical floor stands are about 400 to 500mm high and have a “foot print” 18  Silicon Chip which is about 350mm square. So while the loudspeak­er cabinet might be quite compact, its effective bulk and the floor space it takes up are much greater. If you can afford it, the tower option is always the better choice. You get a bigger cabinet and this almost always means better bass (cleaner and more extended). And this is the benefit provided by the tower speakers presented here. By building the kits you save money and thereby you can afford a system that otherwise could be out of reach. And if you can make your own cabinets, you can save more money into the bargain. On the other hand, a big bonus of the kit presented here is that the cabinets themselves are supplied fully assembled. There is absolutely no carpentry work to be done and the cabinets are very professional in appearance. Virtually all you need to assem­ble these fine loudspeakers is a Phillips head screwdriver and a soldering iron. Nor do you have to put the crossover network together since it is also supplied fully assembled. Dimensions of the JV60 system are 895mm high, 260mm wide and 315mm deep, which includes the thickness of the grille cloth frame. Made of 16mm particle board and internally braced, the cabinet has a volume of close to 50 litres. By the way, this design has been produced exclusively for Jaycar Electronics by Australian Audio Consultants, PO Box 11, Southport, SA 5410. The loudspeaker line-up is two 170mm woofers and a 25mm dome tweeter but the system is not strictly two-way. A glance at the circuit of Fig.1 shows that the crossover is a modified twoway system with one of the woofers (W2) effectively handling bass and midrange frequencies while the other (W1) handles bass frequencies below 200Hz. This has been done to achieve a strong and extended bass, as we shall see. This system is geared par­ticularly to those people who love plenty of bass, without the need for any boost from the amplifier. Ferrofluid cooled tweeter The Vifa tweeter featured in this system is the D25AG-35-06. It has a couple of unusual features, not the least being the fact that it has an aluminium dome tweeter instead of the more usual Mylar or synthetic fabric dome. The 25mm aluminium dome is protected from prying fingers by a plastic shield which is de­ signed to avoid phasing and beaming effects which can occur with any sort of obstruction in the beam of a tweeter. Also unusual is the ferrofluid cooling of the tweeter’s voice coil. This has been a feature of high-quality tweeters for quite a few years now but it has seldom, if ever, been featured in a kit-built system such as this. Ferrofluid is a patented synthetic oil mixture with sus­pended iron powder. Loudspeaker System These are the Vifa drivers, crossover network and rear terminal panel provided for each speaker system. Also included in the kit are Innerbond filling and mounting screws. The suspended iron means that the oil has no effect on the magnetic circuit of the tweeter. It has two bene­fits for a tweeter. First, it helps cool the tweeter voice coil which can otherwise become very hot when operating at high pow­ers. This can be easily understood since a tweeter voice coil is a very light assembly and it is suspended in the magnetic gap where air flow is very slight. With the ferrofluid, the heat in voice coil is conducted away to the magnet and frame of the speaker where it can be dissipated harmlessly. The other benefit of ferrofluid is that it applies a degree of damping to the suspension of the tweeter and thereby can help smooth the overall response. Other specifications of the tweeter include a nominal impedance of 6Ω, a free air resonance of 850Hz and a nominal power handling (IEC­268-5) of 100 watts. Woofers For the bass and midrange there are two 170mm woofers, type P17WJ-00-08. These units feature a cast magnesium basket with a synthetic rubber surround. The cone material is mineral filled polycarbonate. It has a sensitivity of 88dB and a frequency response usable to 4kHz. One of the best aspects of using Vifa drivers is their consistency. The drivers were measured using the Loudspeaker Measurement System (LMS) and these measurements were compared with the manufacturer’s published data. To some extent, driver parameters will vary on the production line, even within a batch. A manufacturer who is consistent manages to maintain these varia­tions in such a way that the repercussions August 1995  19 RED P2 RDE245A C5 0.1 POLYESTER P1 RDE070A C3 6.8 BP INPUT FROM AMPLIFIER YELLOW TWEETER D25AG L3 0.22mH BLACK L2 0.39mH BLUE R2 5. 6  5W C6 0.1 POLYESTER W2 WOOFER P17W1 C2 10 BP L1 4mH RED R1 5. 6 5W C7 0.1 POLYESTER C1 33 BP W1 WOOFER P17W1 JV60 SPEAKER SYSTEM Fig.1: the JV60 is a modified two-way bass reflex system with one of the woofers (W1) only handling bass frequencies below 200Hz. are negligible. All driver parameters are related and if one group are a little high then another grouping should be a little low, counteracting any change. Vifa seem to manage this effortlessly. The 50-litre enclosure has two 66mm ports 197mm long. The internal brace is an essential feature of the cabinet and is placed underneath the topmost woofer. A shelf brace should never be placed in the centre of an enclosure. This method of bracing carries out several functions, the first of which is to connect adjoining panels and help to dissipate vibrations. A shelf brace also divides panels into smaller segments, thus moving reson­anc­es to higher frequencies and Solder the wires to the rear terminal panel, before fitting it into place. 20  Silicon Chip lowering vibrational energy. Constructors who wish to build their own cabinets may use the drawing of Fig.2 as a guide. Increasing wall thickness to 18mm will have very little effect, although increasing it to 25 or 32mm will be a considerable advantage (but make it much heavi­ er). Ensure that the internal volume remains the same, even allowing for the increased brace thickness. The cabinet is tuned for a corner frequency of about 35Hz (-3dB point) and, as such, it produces copious amounts of bass. Before we look at the crossover network, let’s take a quick look at contemporary design techniques in this area. In the past crosso­vers were designed by placing textbook components into the cir­ cuit and assuming that they would do the job. However, this does not take into consideration several factors, the two principal ones being (a) the drivers’ natural roll off slopes and (b) the interaction between the drivers’ motor system (ie, magnet, voice coil and suspension) and other components. Nowadays, CAD packages such as the Loudspeaker Enclosure Analysis Program (LEAP) allow a designer to check and recheck systems at every stage of development. Computer optimization allows one to consider all variables when designing crossovers. Crossover design The crossover design is unusual, as can be seen from the diagram of Fig.1. It is based on a second order (12dB slope/octave) Linkwitz-Riley filter. The tweeter section uses a 0.22mH inductor and a 6.8µF capacitor operating at nominally 3.5kHz, well above the free air resonance of 850Hz. As noted above, the two woofers have separate crossover networks. Woofer W2 can be regarded as the main woofer as it handles the mid­ range frequencies as well. Its associated inductor L2, 0.39mH, provides a roll-off of 12dB octave above 3kHz by virtue of the inductor’s impedance and the driver’s natural roll-off characteristics. R2 and C2 provide impedance equalisation so that the woofer “looks” like a resistor as far as the inductor is concerned. L1 is a 4mH inductor and rolls off the second bass driver W1 at 6dB per octave above 200Hz. In effect, the second woofer is there to provide a Fig.2: use this diagram as a guide if you are building the cabi­nets yourself. The dimensions may be varied slightly but the capacity should still be close to 50 litres and the shelf brace must be included. 808 A B 253 655 507.5 895 (863) C BRACE MOUNTED 9 BELOW THE BOTTOM OF HOLE B 360 D 227 INTERNAL BRACE 4 HOLES 80 x 80 SPACED 23 APART ABOUT BRACE CENTRE 207 E MATERIAL: 16 PARTICLE BOARD CL HOLE SIZES: A AND E : 76 DIA. B AND D : 146 DIA. C : 74 DIA. DIMENSIONS IN BRACKETS ARE INTERNAL * ENCLOSURE BACK INSET 11 FROM REAR EDGE 296 * (253) 259 (227) DIMENSIONS IN MILLIMETRES JV60 SPEAKER ENCLOSURE August 1995  21 The crossover is mounted on the rear panel of the cabinet, beneath the terminal panel. Identify all the wires first before installing the crosso­ver. 3dB boost to frequencies below 200Hz. R1 and C1 again provide impedance equalisation for the woofer. The only capacitors not mentioned so far, C6 & C7, are included to improve the power factor of the bipolar electrolytic cap­ acit­ors and thereby improve the sound quality. Two levels of overdrive protection are provided by Poly­switches. Poly­ switches are special low resistance thermistors with a positive temperature coefficient. Normally they have a very low resistance and thus have minimal effect on the signal fed to the drivers. But when the signal current exceeds a critical level, the Poly­ switches suddenly switch to a high resistance state which effectively removes the drive signal. After a period which depends on the initial overload, they revert to their low resistance state and the signal can pass once more. The important aspect of Poly­ switch­ es is that they are in­ tended as insurance against damage. The speakers should not be repeatedly overdriven otherwise the characteristics of the Poly­switches will alter and thus their future performance can be prejudiced. Two polyswitches are included in this design, one to pro­tect the whole system and the other to protect the tweeter which is the driver most likely to be damaged if an amplifier is driven heavily into clipping. This JV60 system can be used with amplifiers capable of 20-100 watts. Assembly As already noted, the JV60 cabinets are supplied ready-built so there is no carpentry required. The first task is to fit the crossover network inside the enclosure. As can be seen from the photos this is hand-wired on a piece of medium density fibre board. It should be attached to the rear panel Take care when fitting the drivers not to damage the cones. They are well made but if you are ham-fisted you could damage them. 22  Silicon Chip AUDIO PRECISION 50 K ALEX IMPEDANCE (OHMS) vs FREQUENCY (Hz) The UV People ETCH TANKS ● Bubble Etch ● Circulating LIGHT BOXES 10 ● Portuvee 4 ● Portuvee 6 ● Dual Level TRIMMER ● Ideal PCB DRILL ● Toyo HiSpeed MATERIALS 1 10 ● PC Board: Riston, Dynachem ● 3M Label/Panel Stock ● Dynamark: Metal, Plastic 100 1k 10k 20k Fig.3: this is the impedance plot for the JV60 loudspeakers. with a couple of screws, just above the terminal panel. Before you do mount the crossover network, you need to identify all the wires on it so that you can make the correct connections to the various drivers. First, identify the two input wires. The red wire connected via the large yellow Polyswitch is the hot (+) input wire. All black wires go to the negative terminals of the drivers. The positive terminal of each loudspeaker driver is marked with an adjacent spot of red paint. The red wire connected via the small yellow Polyswitch goes to the tweeter and the blue wire goes to the main woofer (W2). The remaining red wire goes to the second woofer (W1). Having mounted the crossover on the rear panel, connect and solder the two input wires to the rear terminal panel, then screw it into place. Install the two plastic port tubes and screw them into place. Connect and solder the two wires to the tweeter and then screw it into its central position on the baffle. Next, connect the two wires to each of the two woofers. The main woofer (W2) mounts at the top of the cabinet while the second woofer is mounted at the bottom. Before fixing the woofers into place on the baffle, you need to insert the Innerbond wad­ding into the enclosure. For two cabinets you will be supplied with a little over a metre of 900mm wide Innerbond. Half this should be placed in each enclosure. You will need to place about a third of it in the top section and the other two thirds in the bottom section. Just pack it in loosely and then place the woof­ers in position on the baffle and screw them down. Note that it is important not to over-tighten the screws otherwise they will strip their holes. If this happens, drill pilot holes in a slightly different position and re-fasten the screws. Listening tests When you have finished one loudspeaker system, hook it up to your amplifier and have a listen. If all is well, go ahead and assemble the other loudspeaker. If the sound is not quite right, make sure that you have connected all the speakers correctly. If the phasing is wrong, the speakers can sound quite strange. If the woofers are out of phase with each other, the bass will be practically nonexistent. Kits for the JV60 loudspeakers are available from all Jaycar Electronics stores and their dealers. Prices are as fol­lows: (1) Speaker kit – includes four woofers, two tweeters, two crossover networks, two rear terminal panels, Innerbond and mounting screws, $579.00; (2) Cabinet kit – includes a pair of cabi­nets finished in blackwood veneer and two grille cloth frames with SC grille cloth fitted, $299.00. ✸ AUSTRALIA’S NO.1 STOCKIST ✸ K ALEX 40 Wallis Ave, East Ivanhoe 3079. Phone (03) 9497 3422, Fax (03) 9499 2381 TRANSFORMERS • TOROIDAL • CONVENTIONAL • POWER • OUTPUT • CURRENT • INVERTER • PLUGPACKS • CHOKES STOCK RANGE TOROIDALS BEST PRICES APPROVED TO AS 3108-1990 SPECIALS DESIGNED & MADE 15VA to 7.5kVA Tortech Pty Ltd 24/31 Wentworth St, Greenacre 2190 Phone (02) 642 6003 Fax (02) 642 6127 August 1995  23