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Sensa Design by Allan Linton-Smith 30  Silicon Chip We thought Allan Linton-Smith had designed the ultimate high fidelity build-it-yourself loudspeakers when he presented the “Majestics” back in June last year. But we were wrong! siliconchip.com.au ational! These look even better and sound amazing – but they’re smaller and even easier to build. They compare more than favourably with “big name” speakers costing many times the price. We call them The Senators. siliconchip.com.au Words by Leo Simpson SSeptember eptember 2015  31 2015  31 How do Senator and Majestic compare? “Senator” Speaker Frequency Response 10/06/2015 18:07:50 +25 +20 +15 +10 Relative Amplitude (dBr) We’ll let the graphs tell the story: At left is frequency response for the Senator (dark blue) compared to the Majestic (light blue). As you can see, the overall response is flatter however the bass -3dB point is higher at ~25Hz compared to around 15Hz for the Majestics. The centre graph shows the distortion plot at 1W. Midrange distortion is very low for the Senator speakers although the smaller bass drivers result in higher distortion at lower frequencies compared to the Majestic. No surprises that treble distortion is similar as they use the same tweeter and horn. And at right, like the Majestics, the Senator speakers have a similar but less peaked impedance curve. Note that impedance is above 8 ohms at all frequencies, although it may dip slightly below above 10kHz if the treble boost network is in-circuit. The upper bass impedance peak is around 80Hz, compared to 57Hz for the Majestic. +30 +5 Majestic +0 Senator -5 -10 -15 -20 -25 -30 -35 -40 10 20 50 100 200 500 1k 2k 5k 10k 20k Frequency (Hz) Did you lust after the Majestic loudspeakers described in our June 2014 issue? That massive 15-inch woofer and the Celestion horn tweeter gave very high efficiency, super power handling and staggering bass response down to below 20Hz. But now we present a somewhat less bulky loudspeaker system, still with high efficiency and high power handling but easier to build and even (gulp) better looking. T he Majestic loudspeaker certainly raised a lot of interest last year. Some readers simply did not believe that it has a linear response down to below 20Hz. “Not possible!”, they said. On the other hand, no-one who built these loudspeakers has written to us complaining that it doesn’t go down to 20Hz and below. If you assess it on sheer performance, it must be one of the greatest loudspeaker bargains ever produced over many decades. However, the Majestic design does have a number of drawbacks. First, there is no denying that it is B-I-G! And heavy; too big, too heavy and too imposing for most people’s homes. Nor is it particularly easy to build, even allowing for the fact that the cabinet is based on the readily available Kaboodle kitchen cabinets (from Bunnings hardware stores) which can provide a flawless finish. Another problem was that the originally specified Etone tweeter horn and also the Etone 15-inch woofer have become unavailable and the only suitable alternatives are presently the much more expensive Celestion tweeter horn and Celestion 15-inch woofer. That makes the Majestic much less attractive to any audio enthusiast with a tight budget. So a number of readers asked us for a somewhat slimmeddown version of the Majestic, incorporating all of its desirable characteristics but smaller, better looking, easier to build etc etc... Well, as much as we’d like to, the laws of physics simply do not allow us to produce a smaller loudspeaker system with the same high efficiency and frequency response down to below 20Hz. But we have been quietly (actually, not so quietly!) bea32  Silicon Chip vering away to produce a very attractive alternative to the Majestic which we have called the Senator. The woofer It is based on a Celestion NTR10-2520D 10-inch woofer with a stated efficiency of 96dB/1W<at>1m and a power rating of 250W (AES). By the way, that is not some “mickey mouse” rating that you see with some consumer audio equipment. This is the AES (Audio Engineering Society) rating which tests the speaker with continuous band-limited pink noise for two hours in free air (no baffle). That is a very stringent test and you can imagine how hot the voice coil and magnet must become during such a protracted test. And think of the long-term stresses on the cone and its suspension. Incidentally, the voice coil is 64mm (2.5 inches), wound with copper-coated aluminium ribbon on a fibreglass former. The magnet is not as massive as you might expect with such a high efficiency driver since it is a neodymium type and quite compact. In fact the overall mass of the Celestion woofer with its rugged cast alumininium basket is only 2.2kg. Interestingly, as you can see in our photos of the drivers (page 35) the voice coil is ventilated to improve heat dissipation and improve linearity (since the air otherwise trapped behind the cone’s large voice dust cap is not subject to large pressure variations by high amplitude signals). The curvilinear cone is made of Kevlar-loaded paper, with a fabric roll surround. The loudspeaker’s spider (which suspends and attaches the voice to the chassis) is particularly large at about 130mm in diameter, and together siliconchip.com.au 10 “Senator” Speaker Distortion 10/06/2015 19:27:03 70 “Senator” Speaker Impedance 11/06/2015 19:23:43 5 60 50 Senator 1 Impedance (Ohms) Total Harmonic Distortion (%) 2 0.5 0.2 Majestic 40 30 Majestic 0.1 Senator 20 0.05 10 0.02 0.01 Dotted line indicates effect of treble boost network 20 50 100 200 500 1k 2k 5k 10k 20k 0 10 20 50 with the specially treated roll surround, allows for large linear cone excursions. And the tweeter The specified tweeter is the same Celestion CDX1-1730 compression driver mated with a Celestion T1534 horn (as featured in the follow-up article on the Majestics in the September 2014 issue). The crossover network PCB is the same as the Majestic but with slight component changes, to be detailed next month. The specs If you take a look at the specification panel for the Senator and compare it to the spec panel for the Majestic (page 24, June 2015), you will see that it comes pretty close. Sure, it does not have a flat response all the way down to below 20Hz but the response is only 5dB down at 20Hz and less than -3dB at 30Hz. That is a very good result from a 10-inch driver in a considerably smaller enclosure. Note that the frequency response from 60Hz to 20kHz is Senator 100 200 500 1k 2k 5k 10k 20k Frequency (Hz) Frequency (Hz) very smooth and in fact, a little smoother than the original published response curves for the Majestic with the Etone horn (June 2014) or the Celestion horn (September 2014). Not only that, the Senator’s efficiency is only 2dB less than that of the Majestic and its power handling is only 1.6dB less, so this is capable of dangerously high sound levels – an estimated 119dB at a distance of 1 metre! Mind you, we don’t recommend anyone driving a finished Senator at such insane levels for any more than normal program peaks. Unless the cabinet is strongly reinforced, there is no guaranteeing its integrity if subjected to extreme and continuous power levels up to 250W. The 200W/channel ULD amplifier module in this issue is ideal for driving the Senator – providing, Speaker Specifications Power Handling: up to 250W RMS (AES standard, continuous pink noise) Sensitivity: 95dB / 1 watt <at> 1m Frequency Response: +5/-3dB 20Hz – 20kHz Harmonic Distortion: <2.4% 20-50Hz; (typically less than 0.5%) Cabinet size: 340 (w) x 740 (h) x 433 (d) (mm) Woofer: Celestion NTR10-2520D 10” Tweeter: Celestion CDX1-1730 compression driver, mated with a Celestion T1534 horn Crossover: 2-way first order (6dB/octave slope) siliconchip.com.au September 2015  33 that is, that you don’t try to drive it at maximum power for even relatively short lengths of time. If you did, you’d probably damage your hearing, if nothing else. What didya say? One the other hand, if you drove it with the modestlypowered Currawong valve amplifier, for example, it would still be capable of extremely loud volume levels in an average listening room. Apart from being able to deliver very high sound levels, there is another advantage of the Senator’s high efficiency of 95dB/1W<at>1m, compared to a typical high fidelity loudspeaker of maybe less than 90db/1W<at>1m (and sometimes less than 87dB/1W<at>1m) so that you are very unlikely to drive your amplifier into clipping on program peaks. There just isn’t the need to have the volume control turned well up. That means the high dynamic range of modern sound recordings will be fully preserved, even if you are listening to the most boisterous symphonies or operas. Gentle impedance characteristic Another worthwhile advantage of the Senator is that it has a “gentle” impedance curve over the whole audible spectrum from 20Hz to 20kHz and nowhere does the impedance drop below 8Ω. This in contrast to some loudspeakers which can have dips in the impedance curve well below 4Ω. The benefit of this gentle impedance curve is that it is very easy to drive for typical audio amplifiers and they will produce less distortion in the process, since distortion in all amplifiers increases as the loudspeaker impedance is reduced. Cabinet features As with the Majestic loudspeaker system, the Senator is based on the “Kaboodle” Flatpax Kitchen cabinet system, available from Bunnings Warehouses, so it will be locally available to everyone in Australia. This system, which usually allows you to create your dream kitchen cabinets, is very easy to construct so you will get a very professional finish with the basic cabinet together with high quality thermoformed doors ... er, speaker baffles ... and sides imported from Germany. So the speaker box is really a 450mm wide kitchen wall cabinet with the front door used for the two sides and a side panel for the baffle and top. Some simple modifications are required to put it all together. The specific cabinet we used normally sells for a shade under $75 so for $150 (pair) you have the basis of a great speaker system. And the beauty is that every piece is cut 100% square (you only need to make a couple of cuts yourself to make them into speakers). Most home-made speaker systems fail in the beauty department but you can build this one and get a perfect finish and a totally professional appearance and build it in a weekend…but if you want to keep your partner happy, you had better let the decorator of the house choose from the huge range of colours available. You could try a black piano finish or brilliant glossy First step is to assemble three sides of the carcase from the Kaboodle 450mm Kitchen Wall Cabinet. You’ll need an additional pair of hands to assist here – remember that the weight of the top and bottom panels can tear them loose from the side panel. All joins should be both glued’n’screwed (don’t rely on screws alone because the joins need to be airtight). Second step is to fit the front and rear panels – note that one side of the speaker carcase (aka cabinet!) is left open. Also note that we have not, as yet, cut any holes – these are best done with the panels attached so you have something to grip while cutting! 34  Silicon Chip siliconchip.com.au The Speaker Drivers We Used . . . As mentioned in the text, the Celestion NTR10-2520D woofer used in the Senator is quite different to that in the Majestic – apart from the obvious difference in size (10 inches vs 15 inches). While this size difference (and the smaller enclosure size) would normally be expected to have a major impact on bass response, we’ve gone to a lot of trouble to ensure this is not as significant as you might expect. In fact, we’re pretty happy with the performance of the Senators! Of particular interest – and very obvious in the photo above left – is the vented voice coil, which improves both heat dissipation and linearity. The Celestion CDX1-1730 compression tweeter mated with a Celestion T1534 horn, on the other hand, is exactly the same as that used in the revised Majestic speaker (September 2014). This change was forced upon us because of supply problems with the original tweeter but, as they say, every cloud has a silver lining: this driver/horn combination performs superbly in both the Majestic and Sentaor speakers. CL 300 TWEETER Third step, before you cut the holes in the front panel for the drivers, is to cut and fit the side support, which keeps the box rigid. It can be cut from a piece of hardwood, MDF, etc. Like all panels, glue’n’screw it in place. siliconchip.com.au Step four: speaking of the holes for the drivers, here’s the cutting detail for the woofer, tweeter and port to be cut in the cabinet front panel. The same holes need to be cut in the Kaboodle dress panel; obviously this panel is slightly larger so refer to the diagram overleaf to get the correct dimensions. We don’t fit the speakers until much later but the holes are best cut now. 120 100 140 720 233 DIAM WOOFER 330 90 DIAM 100 September 2015  35 SIDE VIEW CL 30 300 JAYCAR POCKET HANDLE 60 60 TWEETER 100 HOLE FOR HANDLE POCKET Step five: 720 similarly, the holes in the rear panel to house the handle and the plate-mounted speaker terminals should be cut now. Neither positions are particularly critical; however, the handle must have clearance from the top panel as it slopes upwards inside the box. SIDE SUPPORT 417mm WIDE (135 x 19mm DAR HARDWOOD OR 15mm MDF) WOOFER TERMINAL PLATE REFLECTOR 280 X 415mm 90 DIAM HOLE FOR SPEAKER TERMINALS 90 CROSSOVER PCB 200 white, or be really daring and go for “seduction red” in full gloss or the more conservative “Myrtle gloss”. If you want it all to match with some modern colour schemes you might even like to use “mocca Latte” and let the speakers blend into the background! Many Bunnings outlets have samples of their finishes on display so you get to look and touch them before you decide. If white is your colour, the Kaboodle in gloss white finish is immediately available at many Bunning’s outlets but we decided that we must build a pair in “seduction red”. These (as for some of the other colours) need to be ordered and will take about two weeks. Construction While the cabinet is based on a standard Kaboodle kitchen wall cabinet and dress panels, our method of construction is quite different from that normally used and as depicted in the instructions and the DVD that is available from Bunnings. By all means look at those instructions and DVD because they give useful background info but then you should largely ignore it and follow the assembly diagrams in this article. We are taking a kitchen wall cabinet which would normally be wide and shallow. We rotate it so that it is fairly narrow but deep, giving an enclosure which has a relatively small frontal area but is quite deep. That way, a pair of these speakers will appear to be less bulky and take less space in your listening room. So what we are basically doing is building a box within 36  Silicon Chip PORT (88mm ID) 150 150 Step six: the reflector board (at bottom) is made from the Kaboodle shelf, glued into position as shown. Wires from the terminal plate will be routed around this piece. a box. We assemble the standard cabinet “carcase” with one side missing. To take the place of the missing side, we put in a support panel. We then cut the holes in what will be the front panel for the tweeter, woofer and bass reflex port. We also cut the holes in what will be the rear panel for the terminal plate and the hand-hold (very handy for lifting the rather heavy finished enclosure). The full procedure is outlined in the accompanying diagrams. However, as you proceed, there are a few important things to keep in mind: (a) Leave the protective plastic coating on the Kaboodle gloss dress panels until the enclosures are complete and all the hardware (speakers, crossover PCB etc) are installed. They’ll minimise scratches and damage from any “oopses”. Be especially careful not to drop screwdrivers or other tools onto the dress panels. (b) All joins must include a bead of silicone sealant to ensure they are airtight. We used bathroom caulking compound which has very good adhesive properties. It is good because it allows you move panels slightly to get the position just right but once it has set, it is extremely strong.    After all, a similar method is used to assemble aquarium tanks.    Too much sealant is better than not enough – but be careful to clean up any excess as you go. The longer you leave it, the harder it will be remove. (c) Openings for the woofers and tweeters need to be cut in the ‘door’ panels. These are best cut from the non-glossy siliconchip.com.au CL 320 TWEETER 120 100 Speaker Parts List Timber requirements (per enclosure) Kaboodle 450mm Wall Cabinet (Bunnings Part No W-51623) (16mm HMR panels) 2 End Panels (Bunnings: Seduction Red part no D65744)* 1 Door (Bunnings Seduction Red part no D65699)* 1 Shelf (becomes angled ‘diffuser’ panel) 415 x 280mm 1 Side Support (113 x 417mm, 16mm MDF or DAR timber) * Other colours will have different part numbers 140 720 720 Cutting detail for the front dress panel. The holes are in the same positions as the box front panel but the dress panel is slightly larger than the inner box panel, hence the difference in dimensions. Dress panel fitting is shown overleaf. Senator 233 DIAM Other components required (per enclosure) WOOFER 90 DIAM PORT 1 10” Woofer (Electric Factory 28/NTR10-2520D) # 1 Compression Driver (Electric Factory 28/CDX1-1730) # 1 Horn (Electric Factory 28/H1-7050) # 1 150mm offcut of 90mm PVC stormwater pipe 1 Terminal Plate (Jaycar PT3012) 1 Handle Pocket (Jaycar HS8012) 330 4 Legs to suit (Bunnings 100mm chrome “Leggz” Pack of 4) 2 rolls acrylic fibre (acoustic wadding) 700 x 1000mm (Jaycar AX3694) 1 Crossover PCB assembly (see next month) 1 cartridge of neutral-cure silicone sealant 100 ~ 2m heavy-duty figure-8 polarised speaker wire 48 50mm 10g woodscrews (countersunk head) 16 10g 40mm stainless woodscrews Miscellaneous screws to suit terminal plates, crossover PCBs and hand-holds # (www.elfa.com.au) side with a jigsaw fitted with a fine-tooth blade. (d) You will need to cut holes in the cabinet carcase for the loudspeakers etc and you will also need to cut corresponding holes in the front panel for the loudspeakers and the bass reflex port. e) These holes are best cut with a jigsaw from the “back” side for best results (so that the gloss exterior finish is not likely to be scratched). The same comment applies when you need to cut a side panel slightly shorter to act as the top of the cabinet. (f) You will need some 16mm MDF or particle board cut to the dimensions given in the relevant diagram parts list to make up the side support panel. (g) The four adjustable feet supplied with the Kaboodle kit should be left in their packing until the enclosure is finished and the hardware mounted. (h) The Kaboodle shelf should not be discarded as it is used to make the angled reflector panel inside the enclosure. (i) The driver units are mounted using stainless steel screws 10G x 18-25mm, countersunk head. They are the last items fitted. The completed inner box, from the front, with all holes cut, ready for internal fitout and wiring and the installation of dress panels. siliconchip.com.au Next month We’ll conclude the assembly of The Senator speakers with details of the crossover, as well as tying up some loose ends (such as wiring, fitting feet, etc). September 2015  37 The front panel goes on, with holes cut out to exactly match those on the inner panel... poke the speaker wires through the appropriate holes , ready for the speakers to be mounted when the boxes are finished. Next, the two side panels are glued into place. Make sure you wipe up any excess sealant before it cures. By the way, it’s better to leave the protective plastic on the panels until the boxes are finished. (Left): the crossover (which we finally mounted on the bottom of the box) is hidden by the rolled-up acoustic wadding in this photo. The front dress panel is also glued in place, ready for the speakers to be connected. We painted the inside (and some of the outside!) of the 90mm PVC pipe to make it less obtrusive. This detail will be covered next month. (Right): from the opposite side (looking from rear to front), the completed inner box with the input terminal plate and “pocket” handle fitted, along with the acoustic dampening acrylic material. Note how we’ve dangled the speaker wires through their holes, ready for mounting. Prototype boxes shown: Final versions are not mirror-image and have rectangular tweeter holes. 38  Silicon Chip siliconchip.com.au 433mm The top panel as supplied is too long, so it needs to be (very carefully!) cut to size (433mm deep) and smoothed off. When this is done, the top panel can be glued into position, chamfered edges up. It sits flush with the front surface but indented slightly (about 8mm) on each side, as seen in the photo below. Here’s what your finished Senator speaker boxes should look like, immediately before mounting the speakers in the holes. The back and underneath are not covered by dress panels; they’re the only ones that aren’t. SC Radio, TV & Hobbies April 1939-March 1965 The complete archive on DVD: every article to enjoyonce again  Every issue individually archived by month and year  Complete with index for each year – a must-have for anyone interested in electronics. This remarkable archival collection spans nearly three decades of Australia’s own Radio & Hobbies and Radio, TV & Hobbies magazines,from April 1939 right through to the final issue in March 1965. Every article is scanned into PDF format ready to read and reread at your leisure on your home computer (obviously, a computer with a DVD-ROM is required, along with Acrobat Reader 6 or later (Acrobat Reader is a free download from Adobe). For history buffs, it’s worth its weight in gold. For anyone with even the vaguest interest in Australia’s radio and television history (and much more) what could be better? For students, this archive gives an extraordinary ILICON HIP insight into the amazing breakthroughs in radio NB: requires a computer and electronics following the war years (and with DVD reader to view speaking of the war, R&H had some of the best – will not play on a propaganda you’re ever likely to see!) standard audio/Video This is one DVD which you must have in your DVD player. collection! ONLY $ 00 62 plus P&P The rear panel of the Senators houses the “pocket” handle (top) and the input connector plate (bottom). This out-of-sight panel is not covered by a dress panel. siliconchip.com.au Only available from S C ORDER ONLINE NOW AT WWW.SILICONCHIP.COM.AU September 2015  39