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If you can wield a screwdriver & follow a simple wiring diagram, you can build this exciting full-size loudspeaker system. It uses three drivers which you install in a readymade enclosure for excellent sound quality. By GREG SWAIN Studio 3-55L 3-way loudspeaker system W E HAVE been keen to describe a full-size loudspeaker system for some time now. Basically, we wanted a system that produced good quality sound when driven by ·amplifiers rated at up to SOW or more, but which didn't cost the earth. Good quality commercial loudsp eaker systems are quite expensive, so the do-it-yourself enthusiast can save quite a few dollars by building his own. In deciding on the type of system we wanted , it didn't take long for the basic design parameters to emerge. The first thing we decided was that 16 S1ucoN CHr P the system had to have three drivers, to ensure a smooth frequency response right across the range. This is in keeping with most other full-size loudspeaker systems currently on the market. Next, we wanted a 12-inch (30cm) woofer to ensure reasonable efficiency and good power handling. We also specified a good quality tweeter, since the top end response is a very important parameter in any loudspeaker design. And finally, we needed a good match between the tweeter, woofer and midrange drivers. Unfortunately, not all of those aims are easily reconciled. In the first place, the enclosure cannot be too small if we want sustained bass response down to about 50Hz, or so. If the enclosure is made too small , the woofer resonance shoots up from around 30Hz to 80-lOOHz. We eventually settled on an enclosure volume of 55 litres as the ideal size for the selected woofer. Selecting the drivers OK, so much for the basic design parameters. The n ext task was to select suitable drivers. Initially, we investigated the possibility of using one of those fancy polypropylene cone woofers in a sealed enclosure designed to Thiele/Small parameters. We tried a number of combinations of various drivers but the results were always disappointing. Most of the woofers that we tried looked good on paper but when installed in a cabinet, they lacked bass response. We also had trouble matching the woofer sensitivity to the midrange and tweeter. So they just didn't stack up. Finally, we decided to try a locallymade woofer -the Magnavox 12W099. We teamed it with a Philips AD1160/ T8 tweeter and a 5-inch midrange unit selected from the Jaycar catalog, and the results were irpmediately encouraging. Unlike the earlier polypropylene units that we tested, the Magnavox woofer gave good, tight bass response which was well sustained down to below 50Hz. Its sensitivity was also a good match to that of the midrange unit, although the tweeter clearly needed backing off. In terms of construction, the Magnavox 12W099 woofer uses a large pressed steel chassis, a ceramic magnet and a heavy paper cone. Paper cones are now regarded by some as being old-fashioned but correctly designed paper cones are still far superior to polypropylene cones that don't happen to perform. The 12W099 is fitted with a synthetic rubber roll surround for the outer suspension. This is now a common form of construction since it helps maintain linear operation at high cone excursions and is also useful for damping cone induced resonances. The free-air cone resonance for the 12W099 is quoted at 30Hz ±5Hz. Other specifications include a 25mm hightemperature voice coil, a nominal power handling capacity of 80W (on progra..'11 signals), a frequency response up to 4kHz and a sensitivity of 95dB at 1Wand 0.5 metres (89dB at 1 metre). The 5-inch midrange unit we eventually settled on is of Taiwanese origin and has a quoted frequency response of 350Hz to 5.5kHz. It features a white paper curvilinear cone and closed back construction. This latter feature is important since we don't want the necessity of a separate enclosure for the midrange to stop it being pumped by the woofer. By contrast, we didn't have any The crossover network used in the Studio 3-55L loudspeaker system is a 3-way LC unit with crossover frequencies at 800Hz & 5kHz. It is supplied ready assembled & features clip-on spade terminals. trouble deciding on the tweeter. The Philips AD11610/T8 dome tweeter was the natural choice, both in terms of price and performance. It features a 1-inch textile dome and is specified to operate over the range from 750Hz to 22kHz with good off-axis dispersion. By the way, both the tweeter and midrange units are listed in the Jaycar catalog. The tweeter (Cat. CT-2012) is prir.P.rl at $28.50 while the midrange unit (Cat. CM-2080) normally sells for $26.50. The 12W099 woofer is avail- able separately for $59.95. (Cat. CW2022). To keep costs to a minimum and to make construction as easy as possible, it was desirable to use a commercial crossover network. But while this is a good idea in theory, it's not always possible to use an off-the-shelf crossover network without any need for modifications. Fortunately, we were able to pick a crossover that worked although we did have to provide some attenuation for the tweeter, as already noted. The unit selected is Tr-ansfer Function Mag - dB volts/volts (0.10 oct) M .o L 5.0 A 10 s s 0.0 -5 . 0 -10 .0 -15.0 -20.0 -25.0 -30.0 -35.0 -40.0 I 100 .0 1000.0 10000 .0 log Frequenc~ - Hz Fig.1: the frequency response of the completed system (level controls set to zero). As can be seen, the frequency response is fairly flat (within ±5dB) from about 45Hz up to around 15kHz or so. SEPTEMBER1991 17 This photograph shows the main components of the Studio 3-55L loudspeaker system. They are (clockwise from top right): the Magnavox 12W099 12-inch woofer, the 5-inch midrange, the Philips AD11610/f8 tweeter, the crossover network, the recessed input terminal panel & the level attenuators. again straight out of the Jaycar catalog (Cat. CX-2616) and is listed at $22.95. It's a fairly simple LC unit with crossover frequencies at 800Hz and 5k.Hz. Having decided on the crossover network, we also decided to add a couple of level controls (or faders, as they are commonly called) for the midrange and tweeter. These level controls are basically constant impedance attenuators and they enable the user to independently adjust the levels of the midrange and tweeter. Many hifi enthusiasts want the facility to adjust their loudspeakers in this fashion, either to compensate for room acoustics or to better tailor their loudspeakers to suit particular kinds of music. Testing the design At this stage, we decided to seek Magnavox's cooperation to check the 18 SILICON CHIP design validity and to fine tune the system. In particular, we wanted to check the system on Magnavox's computer-controlled acoustical analyser. This system goes under the fancy name of"Maximum-Length Sequence System Analyser" (or MLSSA for short). Among other things, MLSSA plots impedance, frequency response and crossover response curves, as well as 3D cumulative spectral decay plots based on impulse response measurements. These tests soon revealed that the tweeter was somewhat more sensitive than the other two drivers, as we had already found from initial listening tests. The solution was simple - a resistive attenuator to reduce the tweeter's level by about 3dB. Apart from that, the acoustic tests confirmed that the design concept was virtually spot on. Fig.1 plots the frequency response of the completed system (level controls set to zero). As can be seen, the frequency response is fairly flat (within ±SdB) from about 45Hz up to around 15kHz or so. This is quite a good result, with good overall balance between bass and treble response. In practice, we felt that the system was slightly on the bright side with the level controls set for a flat response (ie, to zero). We thought that the system sounded best with both level controls set to -3dB but that will depend on your personal preference and your listening room's acoustics. Fig.2 plots the impedance curve for the Studio 3-55L. It is fairly constant between l00Hz and 20kHz but below lO0Hz, it rises steeply to a maximum of 18Q at 65Hz. This corresponds to the low frequency resonance of the system and results from intera_ction between the enclosure and the woofer cone. Note the dip in system impedance 20 to a minimum of about 4.5Q at 5kHz. This dip occurs at the midrange/ tweeter crossover frequency but should not cause any problems with today's solid state amplifiers. ' .;, 15 ::E :c !:!. J w u z cc Where to buy the kits fz 10 Although we haven't mentioned it yet, Jaycar were keenly involved with this project right from the beginning. Most of the parts were already in the Jaycar catalog and the company has been able to produce a complete kit priced at just $499 for the pair, including cabinets and all hardware (but not including the optional spade connectors). Considering the performance of this loudspeaker system and the cost of comparable comni~rcial systems, $499 represents excellent value for money, in terms of overall sound quality and power handling. We've also gone to considerable trouble to make sure that these loudspeakers look the part. Unlike earlier kit loudspeakers, the cabinets are supplied fully assembled to ensure a high standard of finish . You don't need any messy PVC glue or woodworking tools for this loudspeaker project unless you elect to make the cabinets yourself, that is. · As supplied, the cabinets are cut from 18mm-thick particleboard and are professionally finished in black simulated-woodgrain veneer. They come complete with clip-on speaker grilles and with the holes already cut in the baffle. All you have to do is mount the crossover, install the Innerbond filling and wiring, and screw the drivers, faders and input terminal panel into position. Alternatively, you can make the cabinets yourself and buy the various components separately from Jaycar. The loudspeaker cabinets are available for $250 a pair while the hardware pack costs $279. This hardware pack includes the loudspeakers, crossovers, terminal panels, level controls , resistors, innerbond filling material, cable and screws. ~ Crossover design Fig.3 shows the circuit of the crossover network, together with the additional components that have been added to produce a working design. For those who are unfamiliar with crossover networks, their job is to divide the audio spectrum so that each a. - _/ ffi ,. \ \I---,-- -- ' 'r--r-- ~ a. "' _/ V 5 0 20 I\ r--... - ,.Y vv I I I 10k 1k 100 20k FREQUENCY (HERTZ) Fig.2: the impedance curve for the Studio 3-55L loudspeaker system. The 18Q peak at 65Hz corresponds to the low frequency resonance of the system, while the dip at about 5kHz corresponds to the midrange/tweeter crossover frequency. driver is fed with its optimum range of frequencies. Because we are using three drivers here, we need a 3-way crossover network. As already mentioned, the network used has crossover frequencies at 800Hz and 5kHz. ThiJ, means that the woofer handles frequencies below 800Hz, the midrange frequencies from 800Hz to 5kHz, and the tweeter frequencies above 5kHz. As shown in Fig.3, the crossover network uses 1C filter sections to rolloff the response at the designated frequencies at 12dB/octave. For the woofer, 11 & Cl form a low pass filter with a -3dB point at 800Hz. For the tweeter, the equivalent components are 14 & C4 but, in this case, these form a high pass filter with a -3dB point at 5kHz. The midrange filter network looks somewhat more complicated but is · simply two LC filter sections connected in cascade. C2 & 12 form the high pass filter section, while L3 & C3 form the low pass filter. In reality, of course, it's not quite as simple as that since there is some interaction between the two filter stages and the load. The end result, however, is a bandpass filter which channels the middle frequencies to the midrange driver. Fig.4 plots the response of the crossover filter sections while Fig.5 shows the low frequency response of the woofer. Note the peak in the response in the region of 65-70Hz which corresponds to the system resonance. Below this figure, the woofer output drops away steeply but there is still plenty of bass down to about 45Hz. The low output from the crossover network is coupled directly to the woofer, while the middle & high out- ,- - - - - - - - - - - - 7 j Fig.3: the crossover network uses LC filter sections to roll off the response at the designated frequencies at 12dB/octave. Note the resistive attenuator fitted to the tweeter. I C4 I I I L4 I I TWEETER I I I INPUT C2 L3 I MIDRANGE I I I I I I WOOFER I I L. _ _ _ CROSSOVER BOARD _ _ _ I _j SEPTEMBER 1991 19 Transfer Function 11ag - dB volts/volts (0,10 oct) -4.0 M L -5.0 A s s -6.0 -7.0 -8 .0 -9, 0 -10.0 -11.0 -12.0 -13.0 -14.0 10000 .0 1000.0 100.0 log Frequency - Hz Fig.4: this graph plots the response of the crossover filter sections. The woofer handles frequencies below 800Hz, the midrange frequencies from 800Hz to 5kHz, and the.,tweeter frequencies above 5kHz. Transfer Function 11ag - dB volts/volts (0 .10 oct) 29.0 M L 27.0 A s s 25.0 23.0 21.0 19.0 17.0 15.0 13.0 11.0 9.0 10.0 100.0 log Frequency - Hz Fig.5: the low frequency response of the woofer. Note the peak in the response in the region of 65-70Hz which corresponds to the system resonance. Below this figure, the woofer output drops away steeply but there is still plenty of bass down to about 45Hz. puts are fed to their respective drivers via the level controls. Although they might look like it, these level controls are not simple potentiometers but are constant impedance pads. These vary the signal fed to the drivers while maintaining a constant input impedance as seen by the crossover network. As mentioned earlier, the tweeter output is also attenuated by -3dB to improve the overall balance of the system. This attenuation is provided by the 4. 7Q and lOQ 10 watt wirewound resistors associated with the tweeter. Construction We'll assume here that you've purchased the ready-made cabinets but, if you have the necessary woodwork20 SILICON CHIP ing skills, there's nothing to stop you from making your own cabinets from the dimensions shown in Fig.6. That way, you can save even more money and you can choose a veneer to suit your existing furnishings. The first step is to make up the wiring looms using the 2-way cable supplied. Fig. 7 shows the details. Cut each loom to the length indicated, then strip about 6mm of insulation from the ends and tin them using your soldering iron. Alternatively, the leads can be fitted with spade connectors, as shown in Fig.7. Note, however, that Jaycar will not be supplying the spade connectors as part of the loudspeaker kit, since most constructors will prefer to save money be soldering the various connections. On the other hand, the spade connec- tars do make the wiring looms somewhat easier to install and you may consider them worthwhile for this reason, despite their extra cost. Make sure that the spade connectors are crimped tightly to the leads if used. Also, be sure to fit the correct size of connector to ·each wiring lead - see Fig.7. Note that the wiring looms to the midrange and tweeter consist of two sections. Twist the ends of the common (black) leads together before soldering them, or terminate them in a common spade connector. The 4.7Q and lOQ resistors are soldered to the tweeter wiring loom to ensure troublefree connections. Bend the leads of the lOQ resistor so that it can be soldered directly to the tweeter terminals. Alternatively, you can fit the lOQ resistor leads with spade connectors. After crimping, these spade connectors should also be soldered to ensure reliability. The next step is to drill the mounting holes for the loudspeakers, level controls and input terminal panel. This isn't a difficult task but take care when handling the woofer and midrange drivers, otherwise you could end up with a screwdriver through one of the cones. The woofer and the twe~ter are secured using 10-gauge x 20mm selftapping roundhead screws, while the midrange is secured using 10-gauge x 12mm self tapping screws. The level controls, crossover and input terminal panel are all fastened using the smaller 12-gauge x 12mm screws. Use the drivers and the various other components as templates for marking out the positions of the screw holes. These holes must be slightly smaller than the core of the screw threads, to avoid the risk of stripping out the holes when the screws are tightened. Before drilling, check that the marked hole positions are not too close to the rims and that, for the sake of appearance, they are all symmetrical in relation to the baffle centre line. It's also a good idea to centre-punch each hole position before drilling, so that you don't accidentally scratch the veneer with the drill bit. The crossover network is mounted on the inside rear panel, just above the input terminal panel. Don't try drilling the holes with any of the components in position. PARTS LIST \ ~ REAR MOUNTED IN 18 x 5 GROOVE IN TOP, BOTTOM AND SIDES .., "' "' 115 115 «> «> "' A = N N A' 390 .I BAFFLE MOUNTED IN 18 x 4 REBATE IN TOP, BOTTOM ANO SIDES 10 280 DIMENSIO NS IN MILLIMETRES HOLES:- A 282 DIA. SECTION A'·A' B 110 DIA. C 84 DIA D 52 DIA . CHAMFERED 3 x 3 ON FRONT E 50 DIA. IN BACK PANEL INSIDE DIMENSIONS: - 650W x 354W x 2340 INSIDE VOLUME MUST NOT BE REDUCED MATER IAL:· BAFFLE : 658 x 362 x 18 PLAIN PARTICLE BOARD BACK : 660 x 364 x 18 PLAIN PARTICLE BOARD TOP, BOTTOM ANO SIDES : 18 VENEERED PARTICLE BOARD Fig.6: if you have the necessary tools & woodworking skills, you can probably save quite a few dollars by building your own cabinets. You can vary the assembly details to suit yourself but don't alter the internal dimensions. That's just asking for trouble. When the pilot holes have been drilled, mount the input terminal panel on the rear panel and secure it using four 12-gauge x 12mm screws. This done, cut the Innerbond mate- The 5-inch midrange driver features closed back construction so that it cannot be "pumped" by the woofer. rial into two equal pieces and use one piece to line the back, sides, top and bottom of the cabinet. There's no need to secure it - when the crossover network is screwed into position , it will sit in place quite naturally. The crossover network can now be installed. It sits on top of the Innerbond material, which means that a certain amount of patience is necessary to align the mounting screws with the pilot holes. However, once the first screw is in, the rest should be easy. Next, use a sharp utility knife to cut a slit in the Innerbond material to give access to the input panel terminals. This done, connect (or solder) the input wiring loom between the input terminals and the crossover. Make sure that the positive lead (red) goes between the"+" connection of the input terminal panel and the "IN" terminal of the crossover network. The tweeter, midrange and woofer wiring looms are now connected to 2 pre-built 55-litre cabinets plus clip-on covers 2 12W099 Magnavox 12-inch (30cm) woofers (Jaycar Cat. CW-2122) 2 5-inch midrange loudspeakers with sealed backs (Jaycar Cat. CM-2080) 2 Philips AD1161 0ff8 dome tweeters (Jaycar Cat. CT2012) 2 tweeter level controls (Jaycar Cat. AC-1682) 2 midrange level controls (Jaycar Cat. AC1680) 2 3-way crossover networks (Jaycar Cat. CX-2616) 2 recessed input terminal panels (Jaycar Cat. PT3004) 1 1.5-metre length of lnnerbond 1 ?-metre length of colour-coded medium-duty figure-8 speaker cable (eg, Jaycar Cat. WB1706) 2 10Q 1OW wirewound resistors 2 4.7Q 10W wirewound resistors 8 10-gauge x 20mm roundhead self-tapping screws 4 10-gauge x 12mm roundhead self-tapping screws 14 12-gauge x 12mm roundhead self-tapping screws 36 mini female spade connectors (Jaycar Cat. PT4520); optional , see text 8 medium female spade connectors (Jaycar Cat. PT4522); optional, see text Where to buy the kit Complete kits for the Studio 3-55L loudspeaker system are available from Jaycar Pty Ltd , PO Box 185, Concord 2137. The kit comes complete with fully assembled cabinets (with all holes cut), loudspeakers, lnnerbond material and all hardware but does not include the optional spade connectors. The price is $499 for the pair. The loudspeaker cabinets are also available separately for $250 a pair, while the hardware pack costs $279. the crossover network and the far ends of the looms pulled through the holes in the baffle. Fig.8 shows the wiring details. Pull the tweeter leads through SEPTEMBER199 1 21 The woofer should be installed last so that you have access to the crossover network while installing the other parts. * TO CROSSOVER (W)C>-.__ RED ~(+) BLACK "------<:::J (-) 700mm LONG * (CI r::::>-'"" TO WOOFER WOOFER WIRING LOOM * TO CROSSOVER 700mm LONG (MID---._ RED * --<](3) (Cl&--- BLACK~ l·Ii::::>---.. TO MIDRANGE LOUDSPEAKER (+) • * BLACK (1) TO MIDRANGE LEVEL CONTROL (2) RED 500mm LONG MIDRANGE WIRING LOOM TO CROSSOVER * (T) ,......._ (Cl 700mm LONG RED ~--=======================:;;;;;_BLACK ~(3) BLACK 500mm LONG TO TWEETER RED TWEETER WIRING LOOM *MINI SPAOE LUG • MEDIUM SPAOE LUG TO CROSSOVER * * * 240mm LONG (IN)~(+) * (C)~(-) TO INPUT TERMINALS INPUT WIRING LOOM Fig.7: the wiring looms can either be fitted with the optional spade connectors or soldered directly to the various components. The numbers on the level controls are stamped into the terminals. 22 SILICON CHIP the tweeter .hole, the tweeter level control leads through the HIGH level control hole, the midrange leads through the midrange hole, and so on. It's now simply a matter of clipping the various connectors to the drivers and level controls, then securing each component to the baffle with its mounting screws. The logical approach is to install the tweeter first, then the level controls and the midrange driver. Leave the woofer until last, so that you can check the various wiring connections back to the crossover network. Exercise great care when connecting the leads to the level controls, since it's all too easy to transpose the two red leads that run to terminals 2 and 3. These numbers, by the way, are stamped into the terminals of the level controls, just below the eyelets. You can use your multimeter to assist you in identifying the leads, if necessary. Be sure to match the level controls with the tweeter and midrange. The HIGH level control (for the tweeter) is mounted on the righthand side of the baffle, while the MID level control is mounted on the lefthand side. Sealing the components The crossover network sits on top of the Innerbond material & is screwed to the back of the cabinet. Note the slit cut in the Innerbond to the left of the crossover network to give access to the lugs on the input terminal panel. Fig.8: follow this diagram carefully when installing the wiring & pay particular attention when connecting the level controls as it's easy to make a mistake here. The 10Q & 4. 7Q 10W resistors can be mounted directly on the back of the tweeter. Solder the resistor leads to ensure reliable connections. We didn't bother to use gaskets or sealing compound around the drivers or level controls, since these proved totally unnecessary. All you have to do is do the screws up firmly to get a good airtight seal to the baffle. Don't overtighten the screws though, otherwise you'll strip the hole. The only component which might be a bit suspect in this regard is the woofer, because it has such a large diameter rim and because its mounting holes are so far apart. If you're worried about sealing, then we suggest that you run a strip of thin draught excluding tape around the bottom rim of the woofer before screwing it down. Do the same for the midrange and fader controls if you wish. The tweeter comes with its own gasket, so sealing is not a problem. After you've assembled one cabinet, the second cabinet can be assembled in exactly the same fashion. You can then connect them to your amplifier, select some music and sit back and enjoy the sound. We're sure you'll be impressed. Finally, although we've specified the power handling capacity of the Studio 3-55L's as BOW program, they can be used with amplifiers rated up to 100W or more - provided that you're sensible. They can produce quite prodigious sound levels, so lack of power handling capacity will not be a problem. SC Constant Impedance Attenuators - How They Work As noted in the circuit description, the 3-55L loudspeaker system uses constant impedance attenuators which have the advantage of not altering the action of the crossover network. If conventional potentiometers were used, the crossover frequency to the tweeter or midrange would alter according to the setting and this could have undesirable effects on the sound quality. These "constant impedance" attenuators have two concentric wirewound elements which are internally connected to the common wiper. For proper action, an 8Q attenuator must be used in conjunc- INo--- an LOUDSPEAKER COMo----.....__ _ ___:;;_, Fig.9: the circuit of the constant impedance attenuators. tion with an 8Q driver. To understand the control action, consider what happens when the attenuator is wound fully up so that the tweeter (or midrange) gets the full signal. In this case, the driving source "sees" just the voice coil of the driver, in parallel with the lower 36Q resistive element which has a negligible effect on the loading. At the other extreme, for maximum attenuation, the driving source "sees" just the 8Q resistive element of the attenuator, in series with a small portion of the lower resistive element which is itself in parallel with the· speaker driver; thus the signal to the driver is heavily attenuated. For in-between settings, the driving source "sees" a series/parallel combination, of the attenuator's resistive elements and the tweeter's voice coil - all adding up to close to the nominal figure of 8Q. SEPTEMBER1991 23