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By Allan Linton-Smith Workman 1000W loudspeaker T his design follows on from our 1000W IRAUDAMP9-based amplifier, published in the October & November 2023 issues (siliconchip.au/Series/405). Finally, you can build a speaker that the amplifier can actually drive to its full potential! This quality loudspeaker can safely handle 1000W RMS for extended periods. The speaker is housed in a sturdy US-built DeWalt transportable 233L toolbox (DWST38000) that measures 99×59×62cm. That makes it light, portable, rugged and very easy to build, requiring only minor modifications to the toolbox/case as purchased. The DeWalt toolbox is available pretty much worldwide! The result is a portable but powerful speaker with many applications. As for the drivers, it uses the 8W, 15-inch (381mm) Celestion FTR154080FD (or FTR15-4080HDX) woofer rated at 1000W coupled with an 8W, 1-inch (25mm) Celestion compression tweeter (CDX1-1745) rated at 75W, attached to a Celestion “No Bell” horn. When set up correctly, these drivers can easily handle a combined 1000W for up to two hours. The only catch is that our 1kW IRAUDAMP9-based power amplifier can only deliver its full-rated power into 2W. Our Class-D amplifier will drive one of these Workman speakers at 400W before clipping. That might seem low, but the speaker is very efficient at 97dB at 1W/1m (96dB for the HDX driver), so it will still be incredibly loud at that power level! If you need to drive this loudspeaker at the full 1000W, you could build two of our Class-D 1kW amplifiers and drive it in bridge mode. Each amp will ‘see’ a ~4W load, and they can each deliver 575W into 4W (or 500W with lower distortion), so they achieve the full 1kW configured like that. We published an amplifier bridge adaptor in the May 2019 issue (siliconchip.au/ Article/11626). Design considerations 72 This seriously powerful and efficient full-range loudspeaker can deliver a tremendous amount of sound, and it doesn’t sound half bad, either. It can be used for public address, DJ and music applications (if you happen to own a stadium!). I decided that a PA speaker needed to be light, portable and ideally transportable by the average person. I have no trouble loading it into and out of my Hyundai hatchback by myself, so I consider that goal to be met. The DeWalt box is light, has built-in wheels, is very tough and is water resistant, with an IP65 rating. Australia's electronics magazine siliconchip.com.au Silicon Chip The prototype crossover was smaller than the final one. Either way, there is plenty of space left in the box. The IP65 rating is ruined by our installation of the speakers in the box but, with speaker drivers installed, if the box is covered by a large plastic bag (eg, a garbage bag), it should survive a shower during transportation. It could even be used with a bag over it, although the sound quality may suffer a bit! The box is rated for a maximum load of 70kg. The woofer, tweeter and crossover together weigh about 13kg, and there is still plenty of room inside, so you could even use the box to transport stuff like cables, microphones and so on (although you’d want to be careful they wouldn’t move during transport and possibly damage something inside). If you’re careful, you could fit a big amplifier inside the box (even our big 1000W amplifier would fit), and together with a preamplifier, it could become a very good mobile PA system. Just be careful you don’t move it in such a way that any large, heavy items inside will shift around! The finished speaker can sit on the siliconchip.com.au ground or be suspended via chains or wires through its two strong steel vertical handles or the telescopic carry handle at the top. That could be very useful at outdoor functions, theatres, discos, churches or other public areas. You can padlock the box shut at a venue so nobody is tempted to poke around inside. I designed the loudspeaker using the box as a sealed enclosure, mainly to simplify construction over a more complicated ported design. That also makes sense because the woofer has a VAS of 140L (111L for the HDX version), so it is not a problem to run it in a sealed 233L box. In a sealed box, the woofer had a measured resonance of 40Hz, only marginally higher than its 38Hz free-air resonance. The priority was to create a design that’s really easy to build, even if you only have rudimentary woodworking and soldering experience. You can make this over a weekend for around $1k (about $1 per watt)! That might seem expensive, but try pricing a commercial speaker that can actually handle 1kW RMS. Many claim “1000W” but would melt in short order at that power level! Much of the cost is in the case and the woofer, two areas where you can’t really cut corners. Performance The overall performance of this system relies on the incredible power-­ handling ability of the Celestion woofer combined with the superb quality of the Celestion tweeter. The tweeter is ‘only’ rated at 75W. However, it is incredibly efficient, so we can heavily attenuate the signal going to it and still get a good bass/treble balance while keeping it within its ratings. Not many single-speaker designs can handle this power level; remember that power-handling claims are commonly exaggerated. If you look at the Celestion woofer’s construction, it is a bit of a beast, with massive coils, magnets, and heatsinks that allow it to cope with that much power. You also have to consider efficiency – it’s no good having a really powerful speaker if you don’t get much sound out of it. This woofer’s 96-97dB <at> 1W/1m rating is excellent, and it means you will get a truly deafening sound level at 1000W (just what rock fans need!). Australia's electronics magazine Tweeter Specifications ● Diameter: 120mm ● Depth: 56mm ● Weight: 3kg ● Power rating: 75W RMS (tested for two hours) ● Nominal impedance: 8Ω ● Frequency range: 1.2-20kHz ● Efficiency: 110dB <at> 1W/1m ● Recommended minimum crossover frequency: 2.2kHz (12dB/octave) ● Voice coil: 44mm diameter edgewound copper-clad aluminium ● Magnet: ferrite ● Diaphragm: PETP film ● Throat exit: 25.4mm Woofer Specifications ● Diameter: 381mm ● Depth: 170mm ● Weight: 9.5kg ● Power rating: 1000W RMS (tested for two hours) ● Nominal impedance: 8Ω ● Frequency range: 35-2500Hz ● Efficiency: 97dB <at> 1W/1m ● Voice coil: 100mm diameter, 22mm wide round copper ● Magnet: ferrite (3.1kg) ● Chassis: cast aluminium ● Former: glass fibre ● Cone: glass-loaded paper with weather-resistant impregnation ● Surround: cloth-sealed ● Suspension: double ● Xmax: 6mm ● VAS: 140L July 2024  73 Fig.1: the overall frequency response of the loudspeaker (mauve) is reasonably flat, within about ±5.5dB of the average over the whole range. The cyan and red traces show the contributions from the tweeter and woofer separately. Fig.2: the distortion levels are better than expected for a loudspeaker of this type, remaining below 2% from 50Hz to 20kHz. The measurement bandwidth is 20kHz, so the low distortion from 1.5kHz to 20kHz mainly represents noise (most likely from cabinet resonances). Fig.3: the harmonic distortion at 1W without noise is much lower than the THD+N shown in Fig.2. Odd harmonics sound bad but are very low in comparison with even harmonics; the second and fourth harmonics are the same note as the fundamental but at higher octaves, so they are in tune with it. 74 Silicon Chip Australia's electronics magazine High power handling is also helpful for situations where a lot of bass, mid-range or treble boost is applied because the speaker will have a fair bit of ‘headroom’. In movies, for example, the sound can have a huge dynamic range; an explosion can follow a whisper. You don’t want your speakers clipping when that explosion happens. Frequency response The frequency response of a loudspeaker is important; it is arguably the single most important factor determining whether it sounds good or not. The response should be as flat as possible. It’s essential to avoid peaks that could exceed its maximum power limit when running near the limit. Peaks can also sound bad and possibly even damage ears at high sound pressure levels. Dips are also best avoided as they create ‘dead zones’ where specific frequencies seem missing from the sound. For example, notes running up and down a scale can seem to disappear at a particular point if there is a significant dip in the frequency response. In the past, many of our readers have used cheaper drivers than those we recommend. That can sometimes work well, but other times, the design really relies on a specific driver. In this case, the driver’s 1kW rating is quite unusual, so we have not tested any alternatives. We couldn’t find many that were genuinely capable of handling 1kW! A quick check on the internet showed that most 15-inch speaker drivers can only handle 100-300W maximum; even if you find one that claims to handle 1kW, you will need to check that it complies with the AES standards. The woofer has a really nice response from 50Hz to 1kHz and is excellent for the human voice and woodwind instruments. However, it really shines with guitars, especially in heavy metal music, which Celestion is famous for. Fig.1 shows the responses taken with a microphone placed in front of the woofer and tweeter and one between the two. The reference 0dB level is set to 100dB sound pressure level (SPL). The overall combined response is relatively flat, within ±5.5dB over most of the range, with no harsh peaks. The response around 300Hz is critical for siliconchip.com.au Fig.4: the spectral plot of a 1W 47Hz signal at an SPL of 92dB. The first harmonic at 94Hz is 48.7dB lower (0.32%) than the signal tone; the 2kHz peak represents a THD of 0.02%. Above 10kHz, the THD contribution drops to 0.002%. vocals, while the response around 2kHz is important for electric guitars. Distortion levels The measured total harmonic distortion plus noise (THD+N) levels were better than expected for a speaker housed in a plastic box, staying below 2% from 50Hz to 20kHz – see Fig.2. Note that the measurement bandwidth is 20kHz, which is why the distortion level drops so much above 1.5kHz, as many of the harmonics above that fall above the audible (and measured) frequency range. We also measured distortion only (total harmonic distortion minus the noise) and compared the contribution of the even and odd harmonics at 1W, as shown in Fig.3. Odd harmonics are generally considered to sound bad, so it’s good that they are pretty low compared to even harmonics in this design. THD by itself is always lower than THD+N. This speaker’s harmonics are quite low, indicating good overall clarity. High THD figures usually result in muddy sound. For completeness, we also plotted the spectrum of the distortion components for a 1W 47Hz signal at a sound pressure level of 92dB, shown in Fig.4. Impedance While the speaker’s nominal impedance is 8W, like both drivers, as with The crossover With one tweeter and one woofer (ie, a two-way design), we can get away with the simple first-order crossover circuit shown in Fig.6. You may think that the 2.2μF value of the series capacitor for the tweeter is low, but Celestion recommends a 12dB-per-octave Fig.5: the minimum loudspeaker impedance is 5.4W at 2.5kHz. Across the rest of the range, it stays above 8W except for a brief dip to 7.6W at 160Hz. As a result, virtually any power amplifier should be able to drive this speaker. The prototype had the crossover capacitor connected directly to the tweeter, along with an experimental inductor. Now the wiring connects the tweeter to the crossover PCB. siliconchip.com.au most speakers, it varies quite a bit with frequency, (see Fig.5). The measured impedance shows two peaks, one at 40Hz (the woofer resonance) and one at 1.1kHz (tweeter resonance). 2.5kHz is the crossover point, and the lowest impedance value measured was 5.4W, which should not be a problem for most amplifiers. Australia's electronics magazine July 2024  75 Fig.6: the crossover circuit is dead simple, using just a series inductor to cut off high frequencies to the woofer and a series capacitor so low frequencies do not reach the tweeter. The two 20W resistors account for the higher tweeter sensitivity compared to the woofer and also protect the tweeter from being over-driven. roll-off with a 2.2kHz cut-off frequency. A single-order crossover only rolls off at 6dB per octave, so our capacitor achieves the required low-frequency attenuation by having a higher cut-off frequency without the problems that come with a much more complicated and expensive crossover. The first-order crossover used is naturally designed to handle high power levels. Two high-power resistors in series with the tweeter reduces its level by around 15dB. The tweeter has a sensitivity of 110dB per watt at one metre, but the woofer is rated at 96-97dB/watt at one metre. So we need to attenuate the tweeter by 13-15dB to match the levels, depending on the exact sensitivities of your drivers. This is good because, as mentioned earlier, the tweeter can only handle 75W maximum. Simulation shows that for an average output power of 1000W, a 40W series resistance would dissipate 138.6W and deliver only 28.2W to the tweeter, ensuring it does not burn out. This may seem like overkill, but I see a lot of burnt-out tweeters in PA speakers. To build the crossover, we used the same two-way crossover PCB that we designed for the Majestic loudspeakers from June 2014 (coded 01205141), replacing the 4.7μF capacitor with a 2.2μF cap and replacing the 2.7mH choke with a 1.5mH coil with extra thick wire so it can handle the power. The Majestic used a few onboard 5W and 10W resistors for tweeter attenuation, but there’s no way they would handle over 100W. Instead, we make up a 40W 400W resistor from two 20W 200W ceramic ‘rheostat’ resistors mounted beside the PCB and wired to it. These are connected in series. They have a slider arrangement that allows you to vary the resistance. You can reduce it slightly if you want more treble. In our tests, we set the resistance in series with the tweeter to 32W to attenuate the tweeter by 14dB (27W gave 13dB attenuation and 37W gave 15dB). Setting it below 20W is not recommended, as you risk exceeding the tweeter’s maximum power rating. The shelving circuit included in the crossover for the Majestic speaker to boost high frequencies is unnecessary because our 1kW amplifier has a 20kHz ‘lift’ that is common with most Class-D amplifiers. We mounted the whole crossover assembly on a 420×320mm piece of plywood and connected the wires to the tweeter and woofer using springmounted connectors. You could use less expensive (and probably more reliable) eyelet lugs if you want to. The binding posts I used come standard with Celestion woofers and I really love them! My back gets stiff when I bend over to hook up everything in this deep cabinet and the spring posts save heaps of time fiddling around with nuts and bolts. I bought 25 pairs from AliExpress for around $50 and they simply bolt to the PCB pads. If using them, you will need six for this project; you can also get them from eBay for about 10$ per pair, including delivery (search for “spring loaded binding post” or try www.ebay.com. au/itm/134778989440). Construction This project requires minimal construction. All you need to do is cut holes for the drivers and connector socket, solder up the crossover, mount it, and wire it up. That’s it! You need to make a couple of modifications to the case first, shown in Fig.7. Start by cutting two small pieces of timber to block off the 24×15mm deep reinforcing channels in the plastic to ensure an airtight fit for the drivers before cutting the holes. These The final crossover arrangment. It mounts on to a piece of timber, which can then be secured to the interior of the enclosure. Wire up the resistors as per Fig.8, not the way shown in this photo 76 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.7: cut the holes in the plastic case as shown here. Use a jigsaw for the larger driver holes and a hole saw or stepped drill bit for the smaller hole in the side for the connector. The timber pieces shown are used for reinforcement and to help seal the enclosure. pieces can be cut from a length of 30×18mm pine using a plane or saw. Glue them into the case using contact cement and allow a few hours for it to set before cutting the holes. Once you have done that, mark out the circles using a compass with a light-­ coloured pencil or chinagraph pencil, then use a jigsaw and slowly cut the plastic and the timber to the specified diameter. Next, make a 24mm hole in one side of the box for the Speakon speaker socket. The side is good because the box will still lie on its back for transport or whenever the lid is opened, and the connectors remain protected. You also need to modify the clamps that secure the lid. These will rattle unless you glue some rubber to them, as shown in the photo below. The clamps should then clamp everything firmly into place. Mount the inductor on the PCB using a ~25mm M4 machine screw, washer and nut. Use Loctite so that vibration won’t shake it loose (do that for all the screws used in this project). Now solder the capacitor, inductor and resistor wires to the PCB, as shown in Fig.8, and attach sufficient lengths of wire to reach the woofer, tweeter and the terminals of the input socket. Make the wires long enough to allow the lid (with the drivers attached) to open while the crossover is still attached to the inside of the case. Once you have all the holes prepared, mount the crossover at the bottom of the box with tapped spacers, machine screws and washers, then prepare the drivers by sticking a felt Left: the holes for the woofer and tweeter don’t need to be the neatest cuts, as they are covered by the drivers. The timber fills the channels that runs behind it. Right: the clamps that secure the lid need to be modified by gluing some rubber to the top of the clamp. This stops the clamps from rattling. siliconchip.com.au Australia's electronics magazine July 2024  77 or rubber material around the edge so they will give an airtight seal between the plastic box and the speakers when mounted. Screw the speakers to the box with suitable wood screws. You can use machine screws, nuts and washers, but it’s a bit fiddly, and you will need some blue Loctite to prevent them from working loose from the enormous vibrations they will experience. Now take the wires for the woofer, strip the insulation off by about 1cm at the ends and insert them into the spring clips (or crimp them to the eyelets and attach them to the PCB). Attach crimp spade connectors to the tweeter wires and push them onto the tabs on the tweeter. Ensure you get the polarity correct; the positive wire goes to the red dot on the tweeter. Finally, solder the wires to the Speakon chassis socket by poking some wire from the crossover into the terminals, then solder them. Attach it to the case using 3mm machine screws, nuts and washers. Check all your wiring thoroughly, then attach a reasonable length of speaker wire to the Speakon plug, ensuring the numbers on the plug match those on the socket. Before connecting your speaker to an amplifier, check the DC resistance across those terminals to verify that there are no shorts. You should measure around 5W. If all is well, power up the amp, feed in a signal and slowly wind up the volume. Be aware that the woofer cone has a very stiff suspension to enable it to handle 1000W, so it may need a few Fig.8: only two components are mounted on the circuit board: the 1.5mH air-cored inductor and the 2.2μF capacitor. The two series 20W rheostats are wired between the 2.2μF capacitor and the tweeter’s positive terminal. The resistors are screwed together at both ends for physical stability, but on the left end, the two resistors are insulated from each other and the blue wire only connects to the one shown closer to the PCB. 78 Silicon Chip Australia's electronics magazine siliconchip.com.au hours to ‘break in’ before it reaches its desired bass response. You might notice that we have not mentioned or used acoustic wadding in this speaker. It probably would sound better with wadding, but we wanted to keep the cabinet empty to make working on it easier and so it can be used to store items like leads and microphones, as mentioned earlier. If you don’t plan to keep anything in the cabinet, it would be worthwhile loosely stuffing it with acoustic wadding, which is available inexpensively on eBay. It is easy to remove later if you need access to the wiring or crossover. Listening tests Too often, PA sound systems at music venues are sub-par. The Editor recently complained to me that he was at a music performance at a Sydney stadium and could hardly even figure out what song they were playing, despite being familiar with the performer’s work. It just sounded like square waves! That is usually the result of poorly set up audio systems that have been taken way past acceptable limits. So we wanted to make sure this speaker actually sounds decent. It’s no good if it can deliver lots of sound if it’s just noise. We tested the speaker with a few different genres of music and made some adjustments, reducing the treble a little as it seemed ‘too bright’. This can be adjusted using the slider on the ceramic resistor in series with the tweeter by loosening its nut and re-positioning it. It would be good to measure the resistance before and after adjustment so you know what you’ve done. Ours was initially set at 32W but later adjusted to about 35W. We used the FD driver, so if you’re using the HDX, it may need to be set a little higher. As mentioned earlier, don’t go below 20W. Vocals shone in our tests, and the sound was very clear for a PA speaker. Some heavy metal we tried sounded really good! Even though this is a mono arrangement, every instrument could be clearly identified. The lead guitar really ripped in typical legendary Celestion fashion. Running the speaker at 300W barely troubled it, and the office vibrated, literally rattling filing cabinets and anything else that wasn’t screwed down! So, if that’s your cup of tea, this speaker is for you! SC siliconchip.com.au Parts List – Workman Loudspeaker 1 DeWalt DWST38000 99×59×62cm (240L) tool chest [Bunnings 0154687] 1 Celestion FTR15-4080FD or FTR15-4080HDX 15-inch (381mm) 1kW 8W woofer [eBay 232329975153 or 144393382135] 1 Celestion CDX1-1745 120mm 75W 8W tweeter [eBay 234994171597] 1 Celestion T5134 “No Bell” horn [eBay 325534095052] 1 2-Way Passive Crossover PCB (01205141) [Silicon Chip SC2734] 1 1.5mH air-cored crossover inductor with 1.5mm diameter copper wire [eBay 386228967177] 1 2.2μF 250V metallised polypropylene crossover capacitor [Jaycar RY6952] 2 20W 200W variable ceramic resistors/rheostats [eBay 225854220831] 1 Speakon panel-mount socket [Jaycar PS1082, Altronics P0792] 1 Speakon line plug or Speakon cable [Altronics P0795] 3 pairs of spring-loaded binding posts (optional, to connect wires to crossover board) [eBay 392075305616 or AliExpress 4000282183682] Hardware & cable 1 4m length of ‘jumbo’ (~3mm2) figure-8 speaker cable [Jaycar WB1732] 1 pair of red/black gold spade crimp lugs [Jaycar PT4568] 10 yellow 5.3mm crimp eyelet lugs 2 M6 × 55-60mm external hex head machine screws and flat washers 12 M5 × 16mm panhead machine screws, flat washers and nuts 1 M4 × 25mm panhead machine screw, washer and nut 4 M3 × 16mm panhead machine screws, washers and nuts 2 M3 × 16mm countersunk head machine screws, washers and nuts 8 M3 × 6mm panhead machine screws 4 M3 × 10mm tapped spacers 18 8G × 18-25mm wood screws 1 1m length of 30 × 18mm pine 1 420 × 320mm sheet of thin plywood or MDF 1 5m length of 16×10mm D-shaped self-adhesive weather stripping [Bunnings 0011953] 1 5m length of 5mm-wide by 6mm-thick self-adhesive brush-type strip or equivalent (‘door/window seal’) [eBay 274371043462] 1 small tube of blue Loctite or equivalent thread locker With the drivers and crossover fitted, the box can still be opened easily and there is plenty of room for storage, or acoustic wadding if you want to improve the sound quality. Access is also good for maintenance. Australia's electronics magazine July 2024  79