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Build these 3-way Bass Ref lex Loudspeakers 22  Silicon Chip Here is the loudspeaker for those who don’t want to be bothered with small unobtrusive boxes. This is a large tower design which can’t be hidden. You’ll need a large room for a pair of these speakers but the reward will be really great sound and lots of power handling, with a 10-inch driver providing the bass reproduction. By LEO SIMPSON F OR THE POWER HUNGRY au- dio enthusiast, a 10-inch woofer in a large cabinet is equivalent to a thumping great V8 to a lead-footed petrol head. This speaker really performs, with prodigious power handling and beautiful bass down to below 30Hz. But as well as power handling and extended bass response, this system is very satisfying in its handling of all types of music, with a smooth mid­ range and very clean treble reproduction. Lest you become overly concerned about how much space is taken up by two of these cabinets, they actually take up no more floor space than typical compact two-way loudspeakers mounted on stands. They are big but they are only marginally larger than the JV60 design we featured in the August 1995 issue. The dimensions are 320mm wide, 920mm high and 315mm deep, including the thick­ness of the grille cloth frame. The cabinets are made of 16mm veneered particle board, internally braced and with an enclosure volume of about 64 litres. As with the JV60’s previously featured, this design was produced exclusively for Jaycar Electronics by Australian Audio Consultants, PO Box 11, Southport, SA 5410. This is a 3-way design, featuring two ferrofluid-cooled tweeters, the same as the single tweeter in the JV60, together with two midrange drivers and a 10-inch woofer. The full circuit is shown in Fig.1. It shows a October 1995  23 repeatedly over-driven otherwise the performance of the Polyswitches will be prejudiced. Two Vifa D25AG-35-06 tweeters are connected in series to cover the treble range above 3kHz, as determined by L1 and C1. Two Vifa P13WG-00-08 drivers cover the midrange frequencies from 500Hz to 3kHz, as set by L2, C2 and L3. R1 & C3 provide im­pedance equalisation for the midrange drivers so that they present a more “resistive” impedance to the filter components and thus ensure that steeper attenuation slopes are achieved. A single Vifa M26WR 10-inch woofer is used for the bass frequencies and it is coupled via L4 and C4. Again, impedance equalisation is provided by R2 and C5. Nominal impedance for the complete system is 8Ω. The overall impedance characteristic is shown in Fig.3. This shows the classic double hump of a bass reflex design, with the minimum impedance of 7Ω occurring at about 120Hz. 10-inch woofer When you unpack the boxes, each enclosure will look like this. The moulded port tubes are hanging in the enclosure bracing panel. They need to be removed, the baffle glued in place and then the speakers can be installed. Not shown is the grille cloth frame which is supplied in finished form. conventional 3-way crossover net­ work with attenuation slopes of 12dB/ octave. Note that the treble, midrange and bass filter networks each have Poly­switch PTC ther­mistors, giving comprehensive protection against overdrive. Normally, these Polyswitch PTC thermistors have a very low resistance and therefore have a minimum effect on the signal fed to the drivers. However, when the signal current exceeds a criti­ cal threshold, the Poly­ switch 24  Silicon Chip suddenly goes virtually open cir­cuit and thus prevents the loudspeaker from overdrive. After a short period which depends on the initial overload, they revert to their low resistance state and the signal is once again con­nected to the drivers. As noted in the JV60 article, Poly­ switches are there solely to provide insurance against overdrive or as far as the woofer is concerned, against catastrophic DC faults in the power ampli­fier. The speakers should not be The heart of this design is the Vifa woofer. It has a large cast magnesium basket and the very stiff paper cone has a syn­thetic rubber roll surround. The voice coil diameter is 50mm and the effective cone area is 337 square centimetres. The free-air cone resonance is at 26Hz and the frequency coverage is up to 1kHz. Its sensitivity is 88.5dB and nominal power handling is 160 watts. Peak power handling is an impressive 500 watts. The 64-litre enclosure has two 66mm plastic ports 200mm long. The two midrange drivers are housed in their own sealed plastic enclosures which stop their cones from being pumped back and forth by the woofer. One of the big attractions in building this kit is that there is virtually no carpentry required. The pair of cabinets is supplied finished except for the front baffle which has to be glued into place. If you want to build your own cabinets, that is certainly an option and we have given full construction details in the diagram of Fig.2. Note that you can vary the cabinet dimensions slightly if you wish but the enclosure volume must still be close to the 64 litres. Assembly Assuming that you have acquired the complete kit, the first step in the These are the Vifa drivers, crossover network and plastic mid­range enclosures provided for each speaker system. Also included are the rear terminal panel, Innerbond filling and mounting screws. C1 3.3 POLYESTER P1 RXE075 RED T1 2x D25AG T2 L1 0.8mH INPUT assembly is to glue the baffles into each speaker box. We found that our sample cabinets had become slightly out of square while in transit and so they had to be carefully pushed back square while the baffles were pushed into place. PVA glue is supplied as part of the kit and it should be run all around the rebate for the baffle before it is pushed into place. Wipe any excess glue off the front of the baffle before it dries otherwise it will be difficult to remove. Allow a good half hour or more for the glue to dry. Then the two bass reflex ports can be screwed into place. The next task is to mount the cross­over network board onto the rear panel (inside the enclosure, of course). Before you do that, identify all the wires for the various drivers; their various colours are marked on Fig.1. Mount the crossover with four screws and termi­nate the two wires to the rear GREY BLACK P2 RXE160 L2 0.8mH L3 9mH P3 RXE300 BLACK C2 15 BP YELLOW R1 10 5W C3 3.3 POLYESTER 2x P13WG M2 BLACK L4 4mH C4 3.3 POLYESTER M1 BLUE R2 6. 8  10W C5 10 BP W1 M26WR BLACK JV100 SPEAKER SYSTEM Fig.1: the JV100 is a 3-way bass reflex system with the tweeters and midrange drivers both connected in series to their respective filter networks. Comprehensive overload protection is provide by the three Polyswitch PTC thermistors. October 1995  25 Fig.2: use this diagram if you intend building the cabinets yourself. The dimensions may be varied slightly but the enclosure volume should be close to 64 litres and the shelf brace must be included. CL 20 65 785 C 252 B 590 920 (888) A 288 20 INTERNAL BRACE 4 HOLES 100 x 80 SPACED 23 APART ABOUT BRACE CENTRE 375 B C HOLE SIZES: A = 234 DIA B = 118 DIA COUNTERBORED 139 DIA x 3 DEEP ON OUTSIDE C = 86 DIA D = 77 DIA 65 85 BRACE MOUNTED 50 BELOW THE BOTTOM OF HOLE B D * D DIMENSION IN BRACKETS ARE INTERNAL ENCLOSURE BACK INSET 11 FROM REAR EDGE MATERIAL: 16 PARTICLE BOARD DIMENSIONS IN MILLIMETRES 70 70 * 295 (252) 320 (288) JV100 SPEAKER ENCLOSURE panel connector which can then be screwed into place. This panel should be mounted with the termi­nals facing down. This makes it easier to secure 26  Silicon Chip the speaker wires from the amplifier. The terminals are quite large so you will have no trouble even if you are using very thick speaker cables. Connect and solder the two wires to the tweeters. Note that an intermediate wire runs between the positive terminal on one tweeter and the negative Kit Availability Kits for the JV100 loudspeakers are available from all Jaycar Electronics stores and their dealers. Prices are as fol­lows: (1) Speaker kit – includes two woofers, four midrange drivers, four tweeters, two crossover networks, two rear terminal panels, Innerbond and mounting screws: $1179.00. (2) Cabinet kit – includes a pair of cabinets finished in blackwood veneer, complete except for the pre-cut baffles which must be mounted in place: $298.00. (3) A pair of assembled and finished grille cloth frames: $80.00. Alternatively, you can purchase the com­ plete kit for a pair of speakers for $1499.00. The crossover network uses iron cores for the two larger induc­tors, while the other two are air-cored. midrange drivers to be reversed to that of the tweeter and woofer. This normally gives the best sound quality. Again, there is an intermediate wire between the positive terminal of one midrange and the negative terminal of the other. The wires for each midrange need to be passed through the small hole in the end of the plastic enclosure. After the wires are soldered to the drivers, the access hole in each plastic enclo­sure is plugged with the supplied sealant, to make it airtight. Each midrange and its plastic enclosure is then dropped into position in the baffle and secured with four screws. Next, insert half the supplied terminal on the other. This is most Innerbond filling material into each important for phasing. enclosure. This can be loosely tacked Next to be fitted are the midrange into place. drivers. Note that while the midrange Finally, solder the remaining two drivers may appear to be incorrectly wires to the woofer and mount it on phased in Fig.1, the diagram is correct. the baffle with four screws. Do not It is conventional for the phase of the over-tighten any of the screws because it is fairly easy to strip the holes. If this happens, rotate the speaker, drill pilot holes AUDIO PRECISION IMPEDANCE (OHMS) vs FREQUENCY (Hz) in a differ­ent position and 50 re-fasten all the screws. Be very carefull when you are wielding your screwdriver during this assembly procedure. If you are careless, you could slip and damage one of the driver’s cones and that could mean an 10 expensive repair. When you have completed 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, check that you have connected all the speakers correctly. If the phasing is wrong, the sound can be 1 quite strange and may even 10 100 1k 10k 20k have a disembodied quality. That won’t happen to you, provided you have been Fig.3: this is the impedance plot for the JV100 speakers. We plotted a curve for each of SC the prototypes and they were virtual­ly identical. very careful. October 1995  27