This is only a preview of the February 2014 issue of Silicon Chip.
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100W Digital Amplifier, Li-Po Battery . . . . . . enough power to blow your soCKS off . . . PortaPAL-D by John Clarke Part III – Building the Box We’ve finished the electronics module – now it’s time to put together the cabinet which houses that module along with the two speakers to make LOTS of beautiful music. T he PortaPAL-D box is made from 16mm MDF (medium density fibreboard) and rectangular DAR (dressed all round) pine. Its overall dimensions are 332W x 600H x 318D mm, chosen to suit the standard MDF sheet sizes that are available. The box is covered in speaker carpet with corner protectors, a handle and a top hat for use with a speaker stand. The two speakers are each protected with a metal grille. Fig.15 show the speaker box construction. The speakers are within their own sealed box, while an “alcove” is made in the top rear of the box to house the PortaPAL-D chassis. The inside volume for the speakers is around 33 litres, however, by packing the space with fibreglass insulation or fibrous wadding, the effective 84 Silicon Chip volume can be increased by as much as 40%. This apparent increase is due to a reduction in the speed of sound in the box due to the packing. Fig.16 shows the typical bass end response with an unfilled box. This shows a 2.7dB peak at about 120Hz. With filling, this peak can be reduced to below 2dB so that the bass response becomes more damped. A further benefit of the wadding is in the reduction of internal reflections from the cabinet walls. Incidentally, we used WinISD 0.44 box modelling software by Juha Hartikainen (www.linearteam.dk). Thiele/ Small parameters for the specified Altronics C2005 speakers are: VAS= 27 litres, QES= 0.962, QMS= 4.172, QTS= 0.782 and Fo= 58.428Hz. The basic box shell comprising the top, bottom and two sides is made using two 600 x 300mm and two 300 x 300mm sheets of MDF. The front speaker baffle is 300 x 568mm and fits into these surrounds, as does the 300 x 365mm lower rear piece. Two more pieces form the “alcove”, one 300 x 157mm and one 300 x 201mm. These can be cut to size with a machine or hand saw and are assembled using PVA glue and nails or screws. If they offer a cutting service, you might find it better to have the store where you purchase the MDF to cut the pieces for you, as this will inevitably result in a squarer, more even finish. Internal cleats using 12 x 12mm DAR pine can be placed inside the edges of the box. You will need to keep the edges free of cleats where the box is siliconchip.com.au 300 300 25 A 90 205 (REBATE DIAMETER) 45mm LONG STRIPS OF 12 x 12 DAR PINE ATTACHED TO EACH SIDE 193 19 E 201 157 HOLE DIAMETER 182 300mm LONG CLEAT (12 x 12) C 600 D REBATES 6mm DEEP 365 193 HOLE DIAMETER 182 C L E SID DF) D M N HA 6mm OR T F H ,1 RIG 600 WN O 0 x SH ITY (30 OT LAR C N 35mm HOLE FOR TOP HAT (IF REQUIRED) 300 B 300 ALL MATERIAL 16mm THICK MDF UNLESS SPECIFIED A & B ARE 296mm LONG STRIPS OF 18 x 18 DAR PINE C & D ARE 600mm LONG STRIPS OF 18 x 18 DAR PINE ALL DIMENSIONS IN MILLIMETRES E IS 300mm LONG STRIP OF 18 x 18 DAR PINE Fig. 15: here’s a diagram of the complete PortaPAL-D Speaker Cabinet, albeit without one side (that’s so you can see how the electronics module housing is made). We’ve deliberately selected the sizes so that it can be made from standard sheets of MDF (medium density fibreboard). If you have the option, we’d suggest you get the MDF supplier to cut the panels to size for you – that way, you get nice, straight, clean cuts which make for a nice, airtight box. siliconchip.com.au February 2014 85 Fig.16 shows the typical bass end response with an unfilled 33 litre box. This shows a 2.7dB peak at about 120Hz. By packing the space with fibreglass insulation or bonded acetate (BAF) wadding, the effective volume can be increased by as much as 40%. This apparent increase is due to a reduction in the speed of sound in the box due to the packing. With this filling, the 2.7dB peak can be reduced to below 2dB so that the bass response becomes more damped. A further benefit of the wadding is a reduction of internal reflections from the cabinet walls. made to house the PortaPAL-D chassis. Once the glue has dried, mark out the 205mm and 182mm rebate diameters for the two loudspeakers to sit into on the front panel. Use a router to cut this rebate to a depth of 6mm. Now fully cut out the 182mm diameter holes with the router. If you intend to install the speaker stand top hat, the hole for this (located centrally in the base of the box) can be cut now using a 35mm diameter hole saw. Similarly, the holes for the handle that mounts on the top can be drilled. While we only used (and specified in the parts list) a single handle on top of the box, the finished PortaPAL-D is quite heavy (17.5kg) so can be quite tiring if carried any distance. You might prefer to place a handle, say, one third the way down each side of the box, for easier carrying by two people. Another refinement which you might consider is mounting four small furniture castors or wheels, one in each corner, to make the PortaPAL-D easier to move. That’s up to you. For the handle, we used two of the screws and captured nuts that are provided with the speaker grille clamps to mount the handle. This leaves just three screws and nuts for each speaker grille mounting using the clamps. That is sufficient for these grilles when spaced out 120° as shown in the photographs. Position 86 Silicon Chip the grilles over the speaker holes and mark out the hole positions for each clamp. Drill and attach each T-nut for the grilles and handle(s) by tightening up the screw to pull the nut into the MDF. Once these nuts are secured, remove the screws. The front surround is 18 x 18mm DAR pine and can be cut and glued to the front of the box. The purpose of this is to recess the speakers (even though protected by grilles) from the inevitable bumps and scrapes of a portable system. When the glue is dry, round off the eight corners of the box using a rasp or file to form the same curvature as the corner protectors. Electronics chassis housing The internal MDF boxed-in section for the PortaPAL-D chassis can now be made. Cut the sheets and DAR pine to size and glue these in place. Note that there is not much clearance between the back of the top speaker magnet and the internal box. There needs to be a gap between the speaker magnet and box otherwise resonances are likely, so check that there is at least a 1mm gap between the speaker and the MDF sheet before finally gluing in place. Note that when installing the speakers, there will be a nominal 1mm thickness of sealant around the rebate to seal the speaker from air gaps. This should be considered when checking the clearance gap. Two 12 x 12 x 45mm DAR pine pieces are set 19mm in from the rear and 25mm down as shown. These are for supporting the top of the PortaPAL-D chassis. The lower 18 x 19 x 300mm DAR pine piece supports the lower PortaPAL-D chassis. When all is complete, ensure that all the joints are airtight by running a bead of PVA glue around all internal joints. At this stage, test the PortaPAL-D chassis for fit into the sealed cavity. Hopefully you will not need to make any changes to the box so that the chassis will fit. The advantage of having the two handles on the front panel will be realised when trying to remove the chassis. Drill pilot holes for the 4g x 16mm panhead screws that secure the panel to the cabinet. You may wish to paint the inside of the PortaPAL-D chassis section of the box black so that any exposed MDF or pine that is not covered by carpet is not obvious. Carpet The speaker carpet is attached to the box using contact adhesive. The carpet can be cut into just three separate pieces. First is the surround piece that wraps around the entire sides of the box, second is the front baffle (296 x 564mm) and third is the rear panel at siliconchip.com.au These shots, front and rear, show the completed PortaPAL-D box, with carpet, handle and corner protectors fitted, immediately before installation of the electronics chassis (left) and the two speakers (right). The speaker wire is already in place, emerging from the hole drilled in the left photo for connection to the PortaPAL-D chassis. 300 x 401mm. You will need a long straight edge to cut the carpet along and a steel ruler to make the measurements. A ‘Stanley’ knife (or a larger hobby knife) can be used to cut the carpet against a cutting mat. Cut the front baffle carpet first. Lay it against the baffle as a sanity check and if it appears correct, remove and apply a smear of contact adhesive to the front baffle. Fix the carpet in place, smoothing out the carpet against the baffle (a small roller is ideal). Now for the side carpet piece: this needs to be wide enough to also wrap around the front 18mm DAR pine, folding at two 90° bends to reach the front baffle. It also has to fold around at the back edge and reach 19mm inside the box where the PortaPAL-D chassis fits. That means the carpet needs to be 389mm wide and 1864mm long. The amount of overhang at the front while siliconchip.com.au wrapping the length around the box sides will need to be 36mm and the amount at the rear at 35mm. Again, loosely wrap the carpet around the box to make sure it is going to fit properly and if all is well, remove and apply contact adhesive to the bottom of the box. Glue the beginning end of the carpet to this with the end of the carpet placed along the box edge. Then apply the adhesive to the next side and wrap the carpet around that side taking care to maintain the correct overhang front and back. Continue gluing the top and then the other side, affixing the carpet as you go. Rub your roller (or hand) over the carpet to smooth it out and to maintain contact with the box till the adhesive is dry. It’s probably best to leave the box until the adhesive is dry to prevent pulling away. Once you’re satisfied that the carpet won’t move, trim each corner with a sharp knife or scissors to allow the carpet to wrap around the front and back of the box. Test how each piece will wrap around the box before cutting off too much carpet and before gluing in place. Any removal of too much carpet can be covered over with a suitable shaped extra carpet piece carefully glued in to fill the hole. The fold-over at the rear needs to go down the sides into the recessed PortaPAL-D cavity by 19mm. The rear piece for the lower portion of the box can be cut to 300mm wide x 401mm and this needs to start by wrapping into the bottom edge of the PortaPAL-D cavity by 19mm and then glued down the 18mm DAR pine and then the back of the 300 x 365mm panel. The side wrap carpet can be cut to just 16mm for the lower part of the box allowing the 300mm width to fit. Fittings When the adhesive is dry, cut out the carpet about 3mm smaller than the 205mm perimeter of the rebate February 2014 87 Here’s what your finished PortaPAL-D should look like, from the front (speaker side) shown at left and the rear (control side) shown at right. With a maximum power of 100W and a continuous 50W, you’re not going to lack for volume – and the comprehensive range of mixer controls means it will handle just about any application. Add the Li-Po battery and inbuilt charger, it’s a real winner! With 20/20 hindsight, we would have replaced the single carry handle with a pair of more robust handles on the side – it does get a little heavy even after carrying it a short distance! (That does mean a two-person carry, though). And we’d also think about putting some small castors or wheels on the bottom to make it easier to cart around. hole for each speaker hole. Also find the T-nuts for the speaker grill clamps and handle and poke a hole through the carpet at each nut. A size 2 Philips screwdriver can do this. If using the top hat, carefully cut out a hole in the carpet, same diameter as the top hat stem, and insert that into the hole. By pressing the top hat down in place, and using the top hat flange as a cutting template, carefully cut the carpet around the perimeter of the top hat flange. Remove the circle of carpet and reinsert the top hat. Pilot-drill the mounting holes for this and screw in the screws. Attach the handle to the top of the box. The corner protectors can now be attached using 6g x 16mm bronzecoloured countersunk wood screws. Installing speakers The speakers are next and will require wiring up as they are installed. The specified speakers (Altronics C-2005) are 200mm, 8Ω coaxial models and connected in parallel, to present a 4Ω load to the amplifier. These speakers feature push-button terminals so no soldering is needed. However, they must be connected in phase; ie, plus to plus and minus to minus. The easiest way to do this is to cut the 1m length of 7.5A figure-8 cable in half, bare all ends to 1cm and tightly twist together one end of each (make 88 Silicon Chip sure the stripes or polarity markers are twisted together). Drill a small hole suitable for one of the figure-8 wires to pass through the rear of the PortaPAL-D box cavity, about 25mm down from the inside top. Hang the twisted-together pair of cables out of the top speaker hole and the other end of one cable out of the bottom speaker hole. From inside the case, push the other single figure-8 through the hole in the box cavity. We’ll mount the lower speaker first. Connect the figure-8 cable to the push terminals, with the stripe or marker on the figure-8 going to the red (+) terminal. To give an air-tight seal between speaker and box, we’re using Blu-Tack putty. Roll a long length so that you end up with a cylinder about 2mm in diameter and mould this all the way around the rebated section in the box for the lower speaker. Repeat until you have a solid run of Blu-Tack all the way around (ie, no gaps). Pack about 90% of the wadding in the volume behind the lower speaker hole and slide the speaker into the lower hole under the carpet lip that surrounds the rebated outer hole diameter. Press the speaker into the hole to compress the Blu-Tack. Now carefully (!) drill pilot holes into the rebate at the four mounting holes on the speaker and secure the speaker in place with 8g x 12mm panhead screws. Now the upper speaker. It has the twisted-together pairs of figure-8 connecting it but there is plenty of room in the push terminals. Once again, ensure the striped wires go to the red or + terminal. Insert the remaining wadding around the outside of the speaker hole (but not directly behind where the speaker goes) and install the speaker as before using Blu-Tack and screws. Now the grilles can be positioned over the speakers and held in place with the clamps. Where the speaker wire comes through from the speakers to the PortaPAL-D chassis, ensure that you have plenty of cable to work with and then seal the hole with Blu-Tack. This wire connects to the ‘to speakers’ terminals on the speaker protector. A 2-way 15A terminal strip is an option to allow the ‘to speakers’ output to be extended for an easier connection to the speaker wire. Insert the PortaPAL-D chassis into the box cavity and secure using 4g x 16mm panhead screws. Construction of the PortaPAL-D is now finished. Turn on and check that it works with the inbuilt battery, then connect power and check that it charges. SC siliconchip.com.au