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The budget Senator Loudspeaker System . . . . . . finishing them off By Leo Simpson In this second and final article on Budget Senator speakers using the Altronics C3026 10-inch woofer, we complete the assembly details, including the crossover network PCB and discuss hand-winding the 2.7 millihenry air-cored inductor. L ast month we described how to build the cabinets, either from scratch or based on the very attractive Bunnings Kaboodle modules. What remains to be discussed is mounting the drivers, assembly of the crossover network PCB and obtaining the 2.7mH air-cored inductors. You will need one for each Senator speaker. Let’s describe the inductors first. In the original Senator loudspeaker articles described in September & October 2015, we specified a 2.7mH air-cored inductor from Jaycar, Cat LF-1330. These were well made but unfortunately have now been discontinued by Jaycar. Most constructors will want to buy their inductors and the easiest approach is to buy them from Australian audio company, Soundlabs, at www. soundlabsgroup.com.au Soundlabs have three 2.7mH aircored chokes, wound with 18 gauge, 16 gauge and 12 gauge enamelled copper wire. Most people would be happy with the 18-gauge model at 72  Silicon Chip $20 each, plus packing & postage. See www.soundlabsgroup.com.au/p/MUAC2m7-1mm/2.7mH+-+0.90+DCR+1 mm+Copper+Air+Core+Coil+18AWG Buyers from overseas might want to consider a similar product from Jantzen Audio, available from Parts Express at www.parts-express.com/ jantzen-audio-27mh-18-awg-air-coreinductor-crossover-coil--255-272 or Amazon at www.amazon.com/ Jantzen-Audio-2-7mH-InductorCrossover/dp/B0002M736A And then there is the option to These 2.7mH chokes from Soundlabs are wound with different gauge wire and on different formers, hence the differences. The 18AWG types are $20 each +GST; the 12AWG are $65 + GST. wind your own and save some money. We wound prototype inductors from 18-gauge enamelled copper wire and we have to state that it is not an easy task. It would be easier to wind the inductors from 20-gauge wire since it is thinner and not so stiff but the resistance of the resulting coil would be a little higher: about 1.6Ω instead of close to 1.0Ω measured on our prototypes. While high fidelity purists will no doubt argue that minimal inductor resistance is very important, the audible difference between inductors wound with 18-gauge and 20-gauge will be undetectable. You can measure the very slight difference in bass response but you won’t hear it. But because many readers would probably take the purist approach and the cost difference between the required 20-gauge and 18-gauge wire is zero – you will need to buy a 1kg reel of wire in both cases – we plunked for the heavier gauge. siliconchip.com.au But that makes it harder to wind, unless of course, you have access to a coil winding machine! First make your bobbins Our first attempt to make a bobbin used a 25mm length of readily available PVC electrical conduit and two cheeks made with a hole saw from Masonite hardboard and then glued together. That was OK but did not look particularly professional and the small diameter former made it very difficult to wind, because of its small radius. In any case, when we wound on the calculated number of turns, the inductance was considerably less than the required 2.7mH. Hmm – that was annoying. Our second attempt, pictured in this article, used a 1-inch length of 25mm OD electrical conduit and 67mm diameter cheeks cut from Perspex using a hole saw. We then glued them together with Bostik PVC Pipe Cement (Blue type N). The larger diameter former made winding a little easier but it was still tricky. In fact, I aborted the second attempt which involved using a geared manual drill clamped in a vise. It was just too hard to maintain the required winding tension while keeping the wire layers neat and keeping count of the number of turns. The method I finally used was to stretch the required 40 metres or so of 18-gauge wire from the back end of my garage and up the driveway and then slowly walk “along the wire” while I wound it onto the bobbin – while trying to keep the layers neat, keeping count and maintaining tension. It took about half an hour. The finished result can be termed “workable” but is far less neat than an inductor produced on a coil winding machine. And note that no matter how hard you try to keep the layers neat, the finished inductor will be “jumble wound”, not “layer wound”! By the way, if you go on-line to find a calculator for an air-cored inductor and feed in the parameters for the inductor we describe here, you will get a result of 295 turns. For example, see http://www.diyaudioandvideo.com/Calculator/Airsiliconchip.com.au The completed crossover to suit the (recommended) Celestion CDX1-1730 tweeter and Altronics C-3026 Woofer. You can also use the Altronics C-3004 tweeter, with a simpler crossover (see Figs 3&4 overleaf). CoreInductorDesigner/ As already noted, we had to use more turns, specifically 325. Ideally, you need to measure the inductance although if you wind it using this method you should get a value within ±5%, which is close enough. Still interested in winding your own? If so, we have produced a limited quantity of Perspex discs which can be glued up using a solvent-based plastic adhesive such as Sci-Grip Weldon at Acrylics Online: www.acrylicsonline.com.au/shop-product/accesso- The inductor we hand-wound using 325 turns of 18 gauge wire, on a former cut from PVC conduit and perspex cheeks (using a hole saw). Ideally it should be “layer wound” ... but we found this almost impossible. ries--adhesives/scigrip-ips-weld-on16-clear-acrylic-cement The discs can be aligned and held in place by a 1/4-inch or M6 bolt and nut (as pictured) but it is most important that when the finished inductor is mounted on the PCB, it must be secured with a brass bolt and nut. Do not use a steel bolt otherwise the inductance will be substantially increase and the harmonic distortion will also increase due to the significant non-linearity of the B-H curve of steel. Brass is non-magnetic. There is a trap for young players here: some “brass” bolts and nuts are actually brass-plated steel. If in any doubt, check to make sure your “brass” bolt will not be attracted to a magnet. Just as an aside: some loudspeaker manufacturers use iron-cored inductors in their crossover networks. This is a second-rate option. Sure, it produces a more compact inductor with less turns of copper wire but the resulting inductor will be quite non-linear and can cause significant distortion. Crossover network There are two versions of the crossover network; which one you use depends on the speaker you choose. The circuit of Fig.1 is the same as we used for the original Senator sysJune 2016  73 3.3F 1 5W HF PROFILE S1 12 10W CON3 (R1) 12 10W (C1) + 3.3 5W 4.7F 3.3 5W L1 2.7mH CON1 CON4 (R2) CON5 INPUT – SC 2016 CON6 CON2 BUDGET SENATOR CROSSOVER NETWORK + CELESTION CDX1–1730 TWEETER – + ALTRONICS C3026 WOOFER – CELESTION TWEETER VERSION Fig.1: the crossover required for the Celestion tweeter and Altronics woofer. (R1) 3.3 5W (C1) + CON3 CON4 4.7F CON1 L1 2.7mH CON5 INPUT – SC 2016 CON2 CON6 BUDGET SENATOR CROSSOVER NETWORK + ALTRONICS C3004 TWEETER – + ALTRONICS C3026 WOOFER – ALTRONICS C3004 TWEETER VERSION Fig.3: the simpler crossover, suitable for the Altronics woofer and tweeter. Fig.2: use this PCB overlay to assemble the crossover shown above (Fig.1) for the Celestion/Altronics combination 74  Silicon Chip tem described in September & October 2015 and you should use this if you are using the Altronics woofer and Celestion horn tweeter. No changes are required, even though the Altronics woofer is slightly less efficient than the Celestion 10-inch woofer. The accompanying component overlay is shown in Fig.2. On the other hand, if you take the cheaper option and elect to use both the Altronics woofer and tweeter, most of the attenuation resistors are omitted and the resulting crossover network is shown in Fig.3, together with its component overlay in Fig.4. The most important aspect of assembling the crossover network PCB is to make sure you make good solder connections to the inductor. Make sure that you thoroughly remove the varnish from the ends of the wires and then tin them with solder. You will actually need to do this anyway, if you are going to check the inductance value. Poke the inductor wires through the two holes on the PCB and then secure the inductor with a brass bolt, nut and washer. Then solder the two connections on the PCB. The rest of the assembly is straightforward. Mounting the crossover PCB While the crossover PCBs in the prototype Senators were installed behind the internal sloping panel inside the cabinet, we do not recommend this position as it would be virtually im- Fig.4: the crossover for the Altronics C-3004 tweeter and C-3026 woofer (Fig.3 above) requires fewer components. siliconchip.com.au Here’s an alternative method of bobbin assembly: the two larger discs form the outer cheeks, while the nine smaller discs are stacked to form the bobbin core. Note the use of a BRASS bolt and nut – steel bolts are magnetic and will adversely affect the performance of your crossover. The SILICON CHIP on-line shop (www.siliconchip.com.au/shop) has a limited number of these accurately-machined bobbin assemblies (11 discs, no bolt/nut) available for $5 per pair plus p&p (currently $AU10). possible to remove the PCB if a fault subsequently developed. Instead, we recommend mounting the crossover PCB in front of the sloping panel, on the floor of the cabinet, using four self-tapping screws. To connect the PCB, you need to crimp 6.3mm yellow female spade connectors onto the ends of the wires from the woofer and tweeter and plug these into the appropriate connectors on the PCB. You also need some 400mm-long spade-lug to spade-lug cables using spare speaker wire off-cuts to connect the input terminals on the PCB to the binding posts mounted on the rear panel of the speaker. If using the treble peaking switch (only applicable with the Celestion/ Altronics combination), drill a hole through the rear panel of the speaker and wire the switch up to one of the pairs of terminals marked on the PCB (ie, the middle pin and one of the upper pins). Alternatively, use a jumper shunt instead, shorting out the indicated pins to enable the treble peaking or placing it across the lower pins to disable peaking. except to note that you should use black (or even later painted black!) screws to mount the speakers. Another important point is that it is vital that the speakers are air-tight when mounted. It’s not so much of a problem with the Celestion drivers but the Altronics will definitely need a layer of draftexclusion tape between the case and driver to seal them. Draft exclusion tape is available from hardware stores, etc. But ensure there are no breaks in the tape to allow air in/out. Finishing off Your Senator speaker box(es) are now complete and almost ready for use. However, we do not recommend using them “flat on the floor” as this will tend to make the bass “boomy”. Raising them by, say, 100mm or so will virtually eliminate this problem and as a bonus, will raise the tweeters up to a level which is more in line with a typical listening position. Fortunately, Bunnings have an ideal solution to the problem, again intended for kitchen cabinets. We bought sets of their “leggz” 100mm cabinet furniture legs, as seen below. Each pack contains four legs so is suitable for one speaker box. Once fitted, they have the added advantage of being height-adjustable so can help fix any minor discrepancies in floor levels. You simply screw the legs to the outer corners of your speakers, in (say) 100mm from the sides and front. Sit back, relax with your favourite music . . . and enjoy! Mounting the drivers This is relatively straightforward, PARTS LIST The complete parts list for the Budget Senator Speakers was published in Part One, last month siliconchip.com.au Bunnings’ “leggz” are intended for weighty furniture use so are ideal for the Senator speakers. June 2016  75 Queries on Celestion horn tweeter response Recently, one of our readers queried the low level frequency response of the Celestion tweeter I have been looking at the specs of the Celestion compression driver used in the Senator and Majestic speakers. Whilst the curves are excellent at high power levels, the lower levels don’t look so good. The compression driver is quite efficient and according to Celestion, intended for larger auditoriums where it can be driven at optimum power levels. Lounge rooms of the average family home being much smaller only need a fraction of these compression driver’s capability to achieve a balanced SPL and therefore they would appear to be working in the poorest part of their performance envelope. Perhaps an article on compression driver technology might be in order, as evidently their initial adoption by the US hifi market is spreading. Their use also puts the crossover frequency in the most psycho acoustic sensitive part of audio spectrum which is considered not best design practice. However if the performance figures of the Senator design using the Celestion drivers is true in audio listening then 3-way systems are a thing of the past. It also raises the question of driving woofer and tweeter from separate amplifier modules. This is not only energy efficient but with the crossover put into a small signal network I would think cost efficient too, especially if the two amps were on the same board. The power supply would just be the same rating. Kelvin Jones, Kingston, Tas. Comment: You raise an interesting question. Both our frequency response curves and those by Celestion have been done at the standard power level of one watt. However the published curves from the Celestion brochure (reprinted above) are labelled in a confusing way and it would be easy to misinterpret them. In each case, the left-hand axis pertains to the upper frequency response curve and it is labelled as “SPL (dB)”. SPL stands for “sound pressure level”. The right-hand axis pertains to the lower curve which is actually the tweeter’s impedance and it is labelled “ Z (dBo)”. We interpret this to mean the “absolute” value of the impedance but it really should have been scaled and labelled in Ohms. The lower curves do not refer to a frequency response at a lower power level. Our measurements and listening tests have shown that this tweeter is very smooth at all power levels and in the case of the Majestics, performs beautifully up to 300 Watts RMS (250 Watts for the Senator), by dint of the attenuation resistors in the crossover network. Also, the response will vary depending on the type of horn attached as you will note from the difference between the plane wave tube and the exponential horn. To obtain the most linear response and lowest harmonic distortion from any compression tweeter, it should fitted with an exponential horn, as we have specified. By the way, the term “compression driver” refers 76  Silicon Chip to the fact that the diaphragm is “pressure loaded” by the attached horn. You cannot operate a compression driver without a horn. You can see a short description of compression drivers at https://en.wikipedia.org/wiki/ Compression_driver Nor does the term “compression driver” necessarily suggest that there is significant dynamic range compression although all loudspeakers are subject to some degree of compression due to heating of their voice coils and the resultant increase in their resistance. Note that for home use and especially for home movies, the power handling ability of the Celestion horn tweeter is very useful because many DVDs and Blu-ray discs have program material with an enormous dynamic range, ie, a whisper is as silent as a whisper and a cannon can be as loud as a real cannon. This can distort and sometimes ruin tweeters with low power handling ability. We agree that there is no advantage in having a 3-way system compared with the 2-way Majestic and Senator designs. As far as driving the tweeter from a separate amplifier is concerned, it is true that you avoid the attenuation losses in the crossover network but they would be more than offset by the increased power consumption associated with having a separate Class-AB amplifier. And of course, you would also need an active crossover network. We don’t think it is worth the extra complication. siliconchip.com.au 4367 Studio Monitor For those who have admired the Majestic Loudspeaker (SILICON CHIP, June & September 2014) and the more recent Senator speakers (SILICON CHIP, September 2015) but who don’t have the inclination or time to build these speakers, you could always consider a top-of-the-line commercial loudspeaker system of similar specification. An outstanding example would be the newly released JBL 4367 Studio Monitor. It is a 2-way system similar in size, efficiency and power handling to the Majestic and also uses a 15-inch woofer and a large horn. That is where the broad similarity ends though, as both the JBL woofer and tweeter have some exceptional features. For example, the woofer has a large (1.5kg) neodymium magnet and two 3-inch voice coils on a common pole piece. Its free-air resonance is 28Hz. And interestingly, the JBL compression driver has two annular diaphragms and a special wave-guide horn. Both horn tweeter and woofer are coupled via a complex crossover circuit featuring 16 polypropylene capacitors, nine resistors and six air-cored inductors. All are mounted on a thick MDF board using point-to-point assembly. The crossover frequency is 700Hz.The enclosure itself is made from 1-inch thick MDF. Overall weight is 61.2kg. The rated frequency response is 30Hz to 40kHz at the -6dB points and -10dB at 26Hz. Convoy International, the national distributor for JBL, will be launching it at the upcoming International Hi-Fi show which will be held at the Pulman Hotel, Albert Park on July 1st–3rd. Recommended retail price is a cool $25,990 per pair. OOPS! Some of our dimensions didn’t add up! A reader queried the dimensions of the Budget Senators published in the May issue – according to him they didn’t quite add up. And he was right! For some reason, Mr Murphy changed the width of the base plate from 300 x 381mm to 320 x 381mm. “Only 20mm”, you say. But that 20mm not only threw other dimensions out (eg, front and rear baffles) but meant that the cutting diagram also didn’t make sense. The important diagrams are shown here. Note that the only dimension which has changed is that 300mm width; everything else adds up when this is changed. And best of all, the cutting NOTE: diagram, which Tweeter cutout dimensions are we knew should to suit work, now reCELESTION horn; ally does work. for Altronics tweeter cut You should be 73mm diam able to get all hole. the pieces (except the reflector boards, as we mentioned last month) out of a single sheet of 2400 x 1200 x 18mm MDF with quite a bit of “meat” left over to account for saw thickness etc. SIDE A SIDE A FRONT A 730 x 417mm 730 x 417mm 730 x 300mm SIDE B SIDE B FRONT B 730 x 417mm 730 x 417mm 730 x 300mm REAR A TOP A 730 x 300mm 417 x 336mm REAR B 730 x 300mm TOP B BASE A BASE B 300 x 381mm 300 x 381mm 417 x 336mm MATERIAL: 2400 x 1200 x 18mm MDF etc. siliconchip.com.au June 2016  77