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By LEO SIMPSON Build the Minstrel 2-30 Loudspeakers Hands up all those who want a new set of loudspeakers but can't afford the prices for imported models. Well you can all put your hands down now because this 2-way system is a real bargain. continued next page The key components of the Minstrel 2-30 system are the 200mm woofer (left), a 25mm aluminium dome tweeter (right) and a 2-way crossover network. Let's face it, many people would like to update their speakers to something newer but price is a big consideration. This is especially the case if you purchased a rack or midi system and now want to upgrade the speakers. Some of, the speakers supplied with the cheaper rack systems really do let the side down as far as sound quality is concerned. So now we are able to present a decent sized 2-way loudspeaker system with an all-up price of $269 for a pair. That price includes all the parts you need to build two complete loudspeaker enclosures. Not bad,huh? We've called the new loudspeaker system the "Minstrel 2-30" - "Minstrel" because it makes music and "2-30" because it is a 2-way system with an enclosure volume of just over 30 litres. The enclosure is a completely 38 SILICON CHIP sealed unit which means there is no messing about with vents or ports - just two loudspeakers on the front baffle and that is that. The enclosure is very well finished too, and is every bit as good as you would expect from commercial loudspeakers purchased from a hifi dealer. You can obtain this high standard of finish easily because the cabinets are so easy to assemble - no woodworking tools are required. The' cabinets are finished in black with a simulated wood grain. The matching grille cloth frame is covered in black fabric and is held securely in place with plastic clips. At the rear of the cabinet a recessed panel has two spring loaded terminals for easy connection to the loudspeaker leads. Two loudspeaker drive units in each enclosure cover the full audio spectrum; hence the term " 2-way". The low frequencies, up to 3kHz, are covered by the woofer. This has a nominal diameter of 200mm (8 inches) and is conventional in construction. It has a pressed steel chassis and uses a large ceramic magnet. The heavy paper cone has a generous synthetic rubber roll surround and a large centre dust cover. The voice coil diameter is 25mm. Free-air cone resonance is close to 45Hz. The matching tweeter is a 25mm aluminium dome unit with a protective grille over the front. It handles all the audio frequencies from 3kHz up to the limit of audibility. Crossover network To couple the woofer and tweeter together we have designed a fairly complex crossover network. Or, putting it in perspective, it's a fot more complicated than the crossover networks found in many comme::cial loudspeakers costing a great deal more than the Minstrel 2-30. We didn't do this because we're keen on complex networks but because the ultimate sound quality of any loudspeaker probably depends more on the crossover network than the basic quality of the drivers. Our crossover network is shown in Fig, 1. It rolls off the signal to the woofer for frequencies above 3kHz at a rate of 12dB/octave. Similarly, it rolls off the signal to the tweeter for frequencies below 3kHz. To get a slope of 12dB/octave, you need an LC section for both the tweeter and the woofer. For the woofer the relevant inductor is Ll and the relevant capacitor is Cl. For the tweeter, the equivalent components are LZ and C2. If we ignore the other components for the moment we can easily see how the crossover network functions. In a very simple crossover network a single inductor feeds the woofer and a capacitor feeds the tweeter. As the signal frequency rises, the impedance of the inductor starts becoming significant until, at the nominal crossover frequency, its value is equal to the impedance of the woofer. So the power level in the woof er is reduced by half and this is known as the - 3dB point. Below the crossover frequency virtually all the audio power is fed to the woofer. The opposite happens with the tweeter and associated capacitor. As the frequency rises the impedance of the capacitor reduces until, at the crossover frequency, it is equal to the impedance of the tweeter. Again, this gives "half power" into the tweeter. Above the crossover frequency, virtually all the power is fed to the tweeter. With this simple type of crossover network the attenuation slope (ie, the rate of signal rolloff) is - 6dB per octave. The crossover used in the Minstrel can't simply be regarded as two 6dB/octave networks combined for each driver. It's more +0---------------, C2 L1 540µH 4.7 L2 600µH 7.2fl 5W SEE TEXT Fig.1: the Minstrel 2-30 crossover network. L1 and the 5.2µF capacitor roll off the signal to the woofer at 12dB per octave for frequencies above 3kHz. Similarly, L2 and the 4.7µF capacitor roll off signals to the tweeter at a 12dB rate for frequencies below 3kHz. ~ 30 s I I 20 10 0 I - 20 J/ ' --- - ... 100 lk 10k complex than that and takes the form known as a constant resistance parallel network. The term "constant resistance" refers to the fact that the crossover network presents a virtually constant impedance to the amplifier over the whole range of frequencies. This is desirable for the amplifier as it means that its performance will not vary according to the load impedance. The term "parallel" refers to the fact that the woofer and tweeter networks are effectively in parallel. Both the tweeter and woofer networks are matched so that Ro2 = (L/C)/2 In the case of the woofer, Ro is 7.20 and for the tweeter Ro is 80. Impedance equalisation 40 1 The tweeter handles all the audio frequencies above 3kHz. It is secured to the baffle using four small wood screws or self tappers. 20 FREQUENCY (Hz) Fig.2: the impedance of the Minstrel 2·30s is virtually constant between 100Hz and 20kHz. Below 100Hz it rises steeply to a peak of 3812 at 65Hz which corresponds to the low frequency resonance of the system. k To make sure that the crossover network works as it should, it is necessary to have "impedance equalisation" for the woofer. This is provided by the 7. 20 resistor and 16.BµF capacitor connected in parallel with the woofer. What these components do is to effectively cancel out the inductance of the woofer's voice coil so that it does indeed present a resistance of 7.20 to the crossover network. If impedance equalisation was not included the gradual rise in impedance with frequency, due to the voice coil inductance, would degrade the attenuation slope of the crossover. The net result of this is that the woofer would be getting more high frequency signal than it should. Because all woofers tend to FEBRUARY1989 39 A . : ---e I IPI SECTION ON AA HOLES : I : 71 DIA C: 111 DIA COUNTER IORED 208 DIA 4 DEEP ON OUTIIDE become quite peaky above their rated frequency range the sound quality would thereby be degraded. Impedance equalisation is not necessary for the tweeter but because it is more efficient than the woofer, some signal attenuation is required. In fact, we provided about 3.6dB of attenuation which is more than the nominal efficiencies would seem to require but which we felt gave best overall balance between tweeter and woofer. The attenuation is provided by the 2.70 and 150 resistors associated with the tweeter. Fig.2 shows the result of the crossover network in the Minstrel 2-30. As can be seen the impedance is virtually constant between 100Hz and 20kHz. Below 100Hz the impedance rises steeply to a max40 SILICON CHIP ALL PANELi 1U TIICK PARTICLE BOARD ALL DIMENSIONS IN MILLIMETRES Flg.3: for those who want to make their own cabinets, this diagram shows all the relevant dimensions. These dimensions can be varied somewhat, provided that the enclosure volume is not varied by more than ± 10%. Don't use material less than 16,5mm thick. imum of 380 at 65Hz. This corresponds to the low frequency resonance of the system which is a result of the interaction of the enclosure with the woofer cone. We designed a printed circuit board for the crossover network, as shown in Fig.5 but as it happens, a fully assembled crossover network will be available by the time this issue goes on sale. The prototypes of this assembled network arrived too late to be photographed for this issue but they are well made units with large air-cored inductors, bipolar capacitors and wirewound resistors. The sealed loudspeaker enclos- ure has been designed according to Thiele-Small principles to give a smooth bass response down to below 60Hz. This is a good compromise between bass response and box volume. Combine this with the relatively high efficiency for the system (95dB at 1 watt at 0.5 metre) and you have the makings of quite a potent little system. When we finally had a pair of the final prototypes going, the overall sound quality was quite gratifying. The overall balance between bass and treble is very good and the bass response will be more than adequate for most people's tastes. The treble is smooth and extended without any tendency to "spit" as sometimes happens with inferior tweeters. Power handling is good too and the Minstrel 2-30 will comfortably handle the full power output of stereo amplifiers rated up to 50 or 60 watts per channel, on normal program material. Combine that with the efficiency noted above and a pair of Minstrel 2-30s have the capacity to produce lots of good quality sound in average to large listening rooms. (That's a polite way of saying that they're deafening when you turn the wick up). BAFFLE Where to buy the kits The Minstrel 2-30s have been a cooperative project between Jaycar Electronics and SILICON CHIP. Jaycar have been able to produce a keenly priced kit which will appeal to many buyers. As noted above, the total price is $269 for a pair, with all parts included. If you wish to build the enclosures, the price for the four drivers, two crossovers and all other necessary bits and pieces is $149. The cabinets are also available separately at $129 for a pair in kit form. They will be available from all Jaycar stores. Building the Minstrel 2-30s For those people who used to make their own speaker cabinets (read: your's truly), building the Minstrels just seems too easy. No woodworking skills are required and the tools needed are very basic - a screwdriver is about all you'll need. Of course that doesn't mean that you can't make your own cabinets from scratch. The diagram of Fig.3 gives the major dimensions of the ea binet. Both the baffle and the rear panel are rebated into the wraparound sides, top and bottom panels. If you like, the need for routing can be eliminated by the use of cleats. The enclosure dimensions have been selected partly for efficient cutting from large sheets. These dimensions may be varied though, provided the internal volume does not change by more than ± 10%. We have specified 16.5mm particle SIDE I SIDE GLUE ALL MITRED EDGES AND BAFFLE GROOVE. INSERT BAFFLE IN GROOVE AND WRAP SIIES AND BOTTOM AROUND BAFFLE Fig.4: cabinet assembly details. Make sure that the woofer hole is at the bottom and that the T-nuts (used to secure the woofer screws) are inside the enclosure when it is folded up. board as a mm1mum thickness material but there is no reason why thicker material could not be used to advantage. For most people though, the idea of building cabinets will be a no-no. Unless you are skilled in carpentry, obtaining an airtight cabinet with a good looking finish is no easy task. If you fall into this large majority of people, buying the cabinet kits is a good proposition. Out with the Aquadhere Virtually all you need to put the cabinets together is a bottle of PVA (polyvinyl acetate) glue such as Selleys Aquadhere. You don't need large sash clamps although there is nothing to stop you using them if you have access to such handy tools. Instead of clamps you will need a roll of masking tape or packaging tape which can be purchased from your local hardware or stationery outlet. The cabinet kits are supplied with all holes cut. All you need to do is to assemble and glue them together - a one step process. Fig.4 illustrates how the cabinet goes together. The base, sides and top are supplied as one long strip of timber which has been routed so that it will "hinge" at three points, corresponding to the two top corners of the cabinet and one of the base corners. The "hinge" is the simulated veneer and so you have to handle it carefully to avoid tearing. In effect, the long hinged section is wrapped around the baffle and rear panels. The glue is run into all the joint sections before assembly so that assembly is just one step. FEBRUARY1989 41 • L1 0 • .....,=- C1 I I C1 4.7µF ~~ ,:g, C: . u.. • :~ . J FROM AMPLIFIER --- + • I--- ---- WOOFER • C2 4.7~F + • 150 5W -------- TWEETER ____, ' *MOUNTED ON COPPER SIDE OF BOARD Fig.5: parts layout on the printed circuit board for the SILICON CHIP version of the crossover network. If you want to make your own crossover, you can order the coils from Nelson Components, 66 Blackbutt Avenue, Lugarno, NSW 2210. Phone (02) 53 9684. Note the 560 resistor which is mounted on the copper side of the board. Inductor L1 was secured to the hoard using super glue, while L2 is secured by a thick wire strap which runs through the centre of the plastic former and has both ends soldered to the board. The commercial version looks quite different although the circuit is the same. 42 SILICON CHIP Before beginning assembly, there are a few holes to be drilled in the baffle and rear panel. It is more convenient to drill these now rather than have to do it afterwards. The recessed terminal panel is secured using four small roundhead woodscrews (or self tappers} so 2mm holes should be drilled for these. You can use one of the terminal panels as a drilling guide. Don't drill the holes all the way through - 10mm deep will be sufficient. The crossover network is screwed to the inside of the rear panel, just below the terminal panel. It is secured using two 15mm-long 10-gauge self tappers. Again, don't drill the holes all the way through the panel. On the baffle the tweeter is secured using four small wood screws or self tappers. The woofer uses four 25mm 3/16-inch Whitworth roundhead screws secured into T-nuts (which are on the inside of the baffle}. The four T nuts require 1/4-inch holes to be drilled. (At the time of writing we could not be sure whether the baffles will be supplied drilled with these holes}. The T-nuts should be driven into the rear baffle before the enclosure is assembled. With all the drilling completed, you can proceed to glue the enclosures together. The procedure is as follows. Lay out the continuous side piece on a flat surface such as a large table or floor (preferably carpeted}. The three fold joints should be flexed as little as possible. Now run a fillet of PVA glue into each of the three V-cuts for the corners and into the rebated channels for the baffle and the rear panel. The baffle (front panel} is now fitted into the channel of what will become the base panel (of the hinged side piece}. Make sure that the woofer hole is at the bottom and that the T-nuts will be inside the enclosure when folded up. Now, while holding the baffle vertical with respect to the base panel, insert the rear panel into its channel in the base panel. Again, make sure that the rear panel is in the right way, with the holes for the crossover network at the bottom. The next step is to carefully wrap the sides around the two vertical panels, making sure that no stress is placed on the three corner joints. With the enclosure now folded up completely, the final corner is held in place with strips of masking tape or packaging tape. Pull these strips on as tight as you can, so that the final corner joint is as tight as possible. Don't worry if some of the glue oozes out of the various joints. It can be wiped off easily. If it gets on the vinyl simulated grain it can be easily peeled off when it is dry. Leave the assembly for at least an hour to make sure it is really dry. If the humidity is low the glue will dry much quicker than this but if the weather is wet and humid, it can take a lot longer to dry. In any case, you can leave the masking tape on the enclosure while further assembly is taking place. Before the crossover network is screwed into place three pairs of wires need to be soldered to it. This can be standard figure-8 flex. Use Close-up view of the recessed terminal panel. To ensure an airtight seal, run a bead of silicone sealer around the mounting flange. about 200mm for the connections from the crossover to the rear panel connector and about 400mm each for the woofer and tweeter connections. Incidentally, the crossover network diagram of Fig.1 shows a 7.20 resistor in the impedance equaliser, a value which is not available. The value fitted in the crossover will be 8.20. To bring it close to the correct SC01102891 .:.I Fig.6: this full-size reproduction of the PC artwork is for those readers who wish to make their own crossover networks. value, solder a 560 1W resistor across it. Now screw the crossover network to the inside of the rear panel. Dangle the wires for the rear terminals out the back and solder them to the terminals, then screw the the terminal panel into place. Similiarly, feed the tweeter leads out through the small hole in the baffle and solder them to the tweeter lugs. Make sure that the polarity of the tweeter and woofer connections are correct. Fig.1 shows the tweeter reversed in polarity with respect to the woofer. This is as it should be. Before mounting the woof er, place the BAF (bonded acetate fibre) filling inside the enclosure. A piece 1 metre x 1 metre will be sufficient for two enclosures. All you have to do is cut the supplied piece in half and place it loosely inside each enclosure. This done, solder the crossover wires to the woofer and screw it into place. Each of the four roundhead screws for the woofer should have a washer under the head. This is to stop the screw heads from pulling through the woofer frame. Do not over-tighten the screws otherwise the frame could be distorted. To ensure that the woofer frame· seals properly to the baffle opening, it is a good idea to run a thin bead of silicone sealer around the baffle rebate. Do the same for the tweeter. You are now ready to do a quick listening test. Connect the speaker system to your stereo system and listen to a music signal. Make sure that both the tweeter and woofer are working. Turn up the volume and bass and then run the cupped palm of your hand around all the joins of the cabinet to test for leaks. None should be apparent. Finally, the grille cloth frame has to be fitted. We understand that this will be supplied with the grille cloth already fitted. It is then simply a matter of fitting the grille frame clips into the frame and the four matching sockets into the corners of the baffle. The grille cloth frame now clips firmly into place. And that' s it. For $269 you have a pair of bargain-priced loudspeakers. Connect them to your system and settle back to enjoy the music. ft FEBRUARY1989 43