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High-end Part 1: By Phil Prosser Active Monitor Speakers With Subwoofer This hifi system takes a ‘no compromise’ approach to build a truly superlative sound system that will be at home in a modern living room. We are utilising high-end components that are readily available while avoiding falling into the abyss of overly expensive, gold-plated parts. The parts used are not cheap but this system will still be within reach for many people. T his new high-end hifi system will be presented in three parts. This first part details the design and construction of the relatively compact main (left & right) speakers that provide excellent sound quality without being overly obtrusive. Because these are active speakers, they need an amplification solution that integrates an electronic crossover, which will be the focus of the second article. The electronics are housed in an attractive two rack unit (2RU) black rack-mount case, including all the amplification, signal conditioning and power supply circuitry. The third article will present a very high-quality Active Subwoofer to round out the system. It is ideal to combine with the speakers presented here, giving a full-range sound system. However, nothing locks the subwoofer into use with these particular speakers; it would be a fine addition to almost any sound system. While you could build these speakers without the sub, I reckon almost anyone going to the trouble of building these will want to extend the bass all the way down to 20Hz. So basically, all three parts are intended to be combined into one excellent sound system. Still, I have taken a modular approach, allowing you to choose which parts to build. I was a bit cheeky in the intro when I said there were no compromises. There are always compromises – in this case, one of them is that the system is a little on the expensive side. While most of our recent loudspeaker systems have aimed to be ‘good value’ systems with excellent sound quality, this one erred more on the side of ultimate fidelity without worrying too much about the bottom line. Still, we aren’t talking sheep stations. The four drivers for the Active Monitor speakers total just under $1000, while the subwoofer driver is another $339. Add in all the other bits and pieces and you can probably build the speakers for a little over $1500. Factor in the electronics, and you’re looking at perhaps a little over $2000; that’s far from outrageous for a highend speaker and amplifier system. Another compromise we often make in our loudspeaker systems is to prefer larger enclosures. That helps us achieve excellent sound quality at a reasonable price. But for this system, I decided that many people these days do not want huge loudspeakers in their living spaces. So I have tried to keep with the modern principle of keeping the speakers as small as possible without ruining the sound quality. That is part of the reason they are a bit more expensive. It does have the significant advantage that they are far less likely to be vetoed by any people who might have the power to say “no”! Photos 1 & 2: the SB Acoustics tweeter (left) and mid-bass woofer (right) used in the Active Monitor speakers. They are quality units with a broad range of frequencies at which they can both operate, giving us many choices for the crossover frequency and slope. Note that these photos are not to scale. 62 Silicon Chip Australia's electronics magazine siliconchip.com.au Features & Specifications ∎ Modestly-sized bi-amplified monitor speakers plus a subwoofer ∎ Frequency response: 25Hz-20kHz, ±3dB (20Hz-20kHz, +3,-12dB; see Fig.8 and the following article on the Active Subwoofer) ∎ Distortion: <1%, 20Hz-10kHz (typically <0.3% for normal levels; see Fig.9) ∎ Over 400W total power (2×50W tweeters + 2×50W woofers + 1×200W subwoofer) ∎ Active crossover <at> 2.7kHz (woofer/tweeter) & 90Hz (subwoofer/woofer) ∎ High-end Satori drivers used throughout ∎ Good time alignment between woofers and tweeters ∎ Excellent off-axis response Driver lineup For this system, I have chosen drivers from the SB Acoustics Satori series, their premium product line. The ‘Active Monitor’ speakers utilise: Satori MW16P-8 bass mid-range: a 165mm (6.5in) driver utilising papyrus fibres in the cone with a rubber surround and a copper sleeve on the pole piece. That sleeve helps to reduce inductance change with cone position and reduces flux modulation and distortion. In this design, it operates down to 90Hz in the active implementation and ~40Hz without the subwoofer. Satori TW29R-B tweeter: a 29mm ring radiator with a frequency response within ±2dB over 1-20kHz. It has a low resonant frequency of 600Hz and a very well-behaved impedance. Distortion over the frequency range of interest is very low indeed. Does that sound good? Wait a moment; a whole octave is missing from this equation, but it is delivered by the active subwoofer using the SB34SWNRX-S75-6. It is a 12-inch driver, although it is actually 346mm in diameter, with a 3in (75mm) voice coil, one of the real measures of continuous power handling of a driver. It has a resonant frequency of 19Hz and a 22mm peak-to-peak linear cone excursion. In an 80-litre enclosure tuned to 25Hz, this driver will deliver solid bass to 25Hz (-3dB) and operate in its linear region right down to 20Hz at up to 200W. The parts combine to form a stereo system of the highest quality. Both my measurements and listening tests reflect that. The electronic configuration of this system is shown in Fig.1. Once built, you just need to plug it into your preamp and away you go. Let’s now turn our attention to the Active Monitor speakers that are the subject of this article. They are a ‘pigeon pair’ with the Active Crossover Amplifier that will be presented next month. Being active speakers, we dispense with the cost and complexity of passive crossover components. We also benefit from an amplifier directly driving each driver and the control that provides. If you use the Active Monitor speakers without the intended subwoofer, you can achieve a low-frequency cutoff The finished speakers shown with subwoofer and Philips CD player. The speakers can be mounted on top of 800mm tall stands, as shown here, bringing them to about ear height when seated (see the parts list). siliconchip.com.au Australia's electronics magazine November 2022  63 Projects used in a stereo Active Monitor Speaker system 4 x Hummingbird Amplifier – December 2021; siliconchip.au/Article/15126 3-Way Active Crossover – October-November 2021; siliconchip.au/Series/371 Multi-Channel Speaker Protector (4-CH) – January 2022; siliconchip.au/Article/15171 Power Supply – to be described next month Projects used in the Optional Subwoofer Ultra-LD Amplifier Mk3 Amplifier – March-May 2012; siliconchip.au/Series/27 OR Ultra-LD Amplifier Mk4 – August-October 2015; siliconchip.au/Series/289 Multi-Channel Speaker Protector (4-CH) – January 2022 Power Supply – to be described in two months of about 45Hz, but note that you will have to adjust the active crossover for that. 45Hz is OK, but by adding the active subwoofer, you will experience the entire audible spectrum from about 20Hz to 20kHz. Few speakers can deliver that, especially with low distortion. The drivers are very high-quality units and, as you will see, their performance is outstanding. While their price is not stratospheric, the quality of these components does mean the cost of building this system is relatively high. The drivers used have excellent frequency responses and are well regarded for their subjective performance. See the panel titled “Subjective vs objective performance” for some insight into how I approached this design. Working on a speaker system that costs thousands of dollars to build, I feel that I am obliged to bring both a scientific and analytic approach. But I also need to use a somewhat subjective and emotional assessment to tune the final result. Both approaches have a place in this exercise. Photos 1 & 2 show the two drivers used in the Active Monitor speakers, while Figs.2 & 3 are plots of their individual frequency responses in a sealed box at 1m. Those figures show that, in terms of simple frequency response, there is a very wide crossover region throughout which both the woofer and tweeter have a flat response and do not exhibit unwanted behaviour such as breaking up, unmanageable resonances, glitches and the likes. From this measure, these drivers are a good match. The 30° off-axis measurement provided by the factory shows that for a 2-3kHz crossover point, both drivers remain well-matched and provide good off-axis coverage. Editor’s note: a good off-axis response is an essential feature of a hifi loudspeaker unless you only ever listen from a single point in a room! The ‘woofer’ actually performs extremely well out to 10kHz looking at amplitude alone, but does show signs of breaking up in the 4.5-5kHz region. Fig.1: this block diagram shows the configuration of the speaker system. A single rack-mount case houses the power supply, four amplifiers for the Active Monitors, the speaker protector and the active crossover. The line-level subwoofer output drives the active subwoofer, which has an internal power supply and amplifier. 64 Silicon Chip Australia's electronics magazine siliconchip.com.au These drivers are a paper cone type with Neodymium magnets and the build quality is excellent. That is evident in the comparison of two drivers shown opposite in Fig.4. Modelling this driver indicates that a 21L enclosure tuned between 29Hz and 35Hz covers Butterworth-Chebyshev alignments. (Butterworth is a response with a ripple-­free passband, while Chebyshev allows a little passband ripple for a faster roll-off). Reducing this volume a touch to 18L and stuffing it well allows us to keep the size of the enclosure under control at the cost of the -3dB point moving up a few hertz, to 44Hz. Given that we are designing the system to have a subwoofer, this is a moot point. This slight reduction in volume means we can keep the depth of the enclosure within reasonable limits. Driver alignment The relative placement of the tweeter and woofer is critical to the operation of the speaker, especially through the crossover region. Aside from the obvious function of filtering signals for each driver, the crossover needs to do this in a manner that results in a flat frequency response through the crossover region. There is a critical interplay between driver placement and the operation of the crossover, shown in Fig.5. The result of this misalignment of the acoustic centre of the tweeter and woofer is a skewing of the beam pattern of the speaker downwards (about 5° in our case) and a null at the crossover frequency about 5° above horizontal. These effects occur for signal frequencies in the crossover region and result in a dip in the speaker’s frequency response. If you are not aware siliconchip.com.au Amplitude (dB) -10 -20 -30 -40 100 200 500 1K 2K 5K Frequency (Hz) 10K 20K Fig.2: the SB Acoustics tweeter’s frequency response, measured 1m away. The top-end roll-off is almost entirely due to filtering on the front end of the ADC used to make these measurements. 0 -10 Amplitude (dB) The MW16P-8 woofer 0 -20 -30 -40 50 50 100 200 500 1K Frequency (Hz) 2K 5K 10K 20K Fig.3: the SB Acoustics mid-bass woofer’s frequency response in a sealed box, measured 1m away. The top end above about 5kHz looks good, but there is breakup occurring in this region. The roll-off below about 200Hz is due to the enclosure, while the ripple in the low end is due to room modes. No effort has been made to make the plot pretty or smooth. The response over the 200Hz3kHz region is excellent. 10 0 -10 Amplitude (dB) The tweeter response extends well beyond 20kHz. Cone breakup is not evident until above 20kHz, and is well-controlled. So crossing from the woofer to the tweeter in the 2-3kHz region will: 1 - Provide a fairly continuous horizontal coverage from the speaker 2 - Have the potential to have a very flat frequency response 3 - Not excite breakup modes in the woofer 4 - Operate the tweeter several octaves above its resonance (600Hz) -20 -30 -40 -50 -60 1K 2K 3K 4K 5K 6K 7K 8K 9K 10k 12K 14K 16K 18K 20K Frequency (Hz) Fig.4: a frequency response plot comparing two woofers. I suspect that some of that difference is me standing a little too close to the measurement system! It is also very pleasing to note that the response is extremely smooth and flat. The ripple visible below about 1kHz is entirely room modes. of this, it can make designing a crossover very confusing! There are a few ways designers tackle this problem. The most direct manner is to increase the output in Australia's electronics magazine this crossover region by playing with crossover frequencies and shifting the phase at crossover by pushing the tweeter crossover frequency down. That approach can work but has the November 2022  65 effect of putting more energy into the room at off-axis angles. Editor’s note: I reiterate my earlier point that little change in sound quality or frequency response off-axis is very desirable. Another approach is to offset the drivers. There are several ways to approach this; a famous example is the Duntech speakers from the nineties, which used stepped baffles and acoustic treatment to reduce diffraction from the edges. My solution is a bit of a mix of the two approaches. If this sounds like I am hedging my bets, I kind of am. I have opted for a gently-sloped baffle that somewhat offsets the tweeter back from the woofer (see Fig.6). I have also recessed the tweeter, which is good for avoiding diffraction around the transition of the tweeter face plate to the front panel. The woofer is not recessed; I simply applied felt around it to help reduce the visual and acoustic impact of this choice. It would have otherwise been Fig.5: the drivers have different depths, so their ‘acoustic centres’ are not aligned when installed on a flat panel. Around the crossover frequency, both drivers are producing signals, and the phase shift due to this misalignment causes undesirable reinforcement and cancellation at different locations in the room. necessary to slope the front panel back by 8° to get the offset between tweeter and woofer perfect. My tests show that the result is quite good. By recessing the tweeter, we get away with a modest 5° tilt on the front panel while keeping the drivers in time alignment. To my eye, the sloped front makes a pleasing change from a rectangular cube for a speaker. Still, the effect is subtle enough that you won’t even spot the front panel tilt in many of the photos. The crossover I have set the crossover frequency to 2.7kHz, implemented by an active crossover in the amplifier. I have also implemented ‘baffle step correction’ in the amplifier. This accounts for the effect of acoustic radiation from the speaker transitioning from omni-­ directional at low frequencies to directional at high frequencies. The transition frequency is a function of the driver and its location on the front panel. The step can be as much as 6dB, but in our case, a boost of 3dB at frequencies below 250Hz works well. As the speakers are bi-amplified, each driver is powered directly from its own amplifier. However, I have included a large 100μF DC blocking capacitor (Jaycar Cat RY6920) in series with the tweeters as the last line of defence against faults or crossed wiring. This capacitor has no effect in regular operation but will save your bacon should LF or DC signals somehow make their way to the tweeters. If substituting this part, make sure you use a high-voltage, high-current capacitor; many small 50V-rated bipolar capacitors do not have the ripple current rating for use in loudspeakers. Performance Fig.6: by tilting the front panel and using some other tricks, we bring the ‘acoustic centres’ into horizontal alignment, so the signals around the crossover frequency coincide, and we avoid constructive and destructive interference. 66 Silicon Chip Australia's electronics magazine Measuring the system’s overall frequency response was an exercise to minimise room reflections. The final plot (Fig.7) is a composite showing two measurements, one with the speaker on a stand and measured at 1m, the second plot with the speaker facing upwards. Both plots are smoothed one-third octave to eliminate the usual ‘fuzz’ you get on these plots. It might look a bit lumpy, but loudspeaker systems are notoriously difficult to characterise in this way. As speakers go, this is actually remarkably siliconchip.com.au flat when measured indoors, on average from around 25Hz up to 20kHz. Fig.8 is a frequency plot measurement made outside over grass near the shed. For a stand 80cm tall and 1m from the microphone (with the speaker cone at about 0.9m), the “bounce path” is 2.06m (2 × √0.9m2 + 0.5m2). The direct path is 1.0m, the difference being 1.05m. Because it’s measured outdoors the -3dB point is shown at 35Hz rather than 25Hz quoted above. Subjective vs objective performance The engineer in me always thinks, “if you can’t measure it, then you can’t hear it”. Ultimately, this is true. But the question is: what exactly do we need to measure to determine what makes one speaker sound better than another? I have run tests to see what I can hear compared to what I can measure. I have been surprised at the results, concluding that the ‘character’ of a speaker comprises not only the gross frequency response but is also influenced by less overt parameters such as the stored energy in the driver, its breakup modes and distortion profile. The lessons my experience brings to the Active Monitor speakers are: ■ We should operate the drivers well within their linear regions ■ I took note of trusted reviewers’ opinions of the drivers ■ I considered the stored energy (waterfall plots) of the drivers ■ I sought to match the beam patterns of the drivers via the crossover ■ I prefer higher crossover rates where practical ■ I stuck to drivers with few breakup modes and definitely avoided exciting them ■ After designing the system using proper engineering principles, I still needed to listen to the result and then tune or tweak it until I was happy with the sound ■ I shouldn’t be afraid to tune a speaker, but I should consider why I am making any given change siliconchip.com.au Amplitude (dB) The cabinet material you choose comes down to the finish you want, your skill at woodworking and cost. I recommend you use MDF, plywood or chipboard. All these materials will work fine for the Active Monitor Speakers. I prefer MDF over ply. MDF is denser and has a reputation as a “deader” material than chipboard. But be warned, it is also heavier and makes an extraordinary amount of dust when cut and routed. Wear a mask while working with it; breathing this dust can be harmful to your health. My woodworking skill is modest, and in building the prototypes, I have intentionally stuck to tools that most people would have. The tools I recommend using are: 1 – Circular saw with the cut angle adjustable to 5° I used a cordless circular saw and it worked great. My old mains-powered unit would also have been fine for the job. A younger or skilled person could possibly make these cuts with a hand saw, but they would be nowhere near as clean or accurate. 2 – Jigsaw Used for a couple of cuts, especially 180 120 0 60 -10 0 -20 -60 -30 -120 -40 20 50 100 200 500 1K Frequency (Hz) 2K 5K 10K -180 20K Fig.7: two frequency response plots of the overall system. Depending on where and how the speaker is located, we can move the low-frequency dip around and usually work out what is causing it. The only way to avoid it entirely would be to stick the speaker up a tall ladder, but that’s a bit awkward! So it’s best to ignore the very bottom of the frequency response as it was taken outdoors. 10 0 Amplitude (dB) Building the Active Monitor speakers 10 Phase (deg) For sound travelling at 343m/s or so, the half wavelength frequency is 163Hz, which is pretty close to the dip in that figure. That shows this is a test setup phenomenon, not the speakers. -10 -20 -30 -40 20 50 100 200 500 1K Frequency (Hz) 2K 5K 10K 20K Fig.8: a frequency response measurement of the speaker system in a relatively open grassy area. That dip at 159Hz is because the measurements were taken outside with the sub arbitrarily placed. Australia's electronics magazine November 2022  67 in the braces. You could use a small handsaw instead, but it would be a miserable task. 3 – Cordless drill/screwdriver You will also need various drill bits and a Philips No.2 driver bit for the screws. 4 – Router and bits plus a circle jig You will need a 10mm round over bit (or similar size), a 6mm round over bit (or similar size) and a 16mm straight cut bit. 5 – Sash clamps You’ll need at least two; many more if you choose to glue the enclosures only (not glue & screw). 6 – Sanding disc with 120 grit paper This will be used to smooth the edges before routing. It is possible to do this by hand if your assembly is clean. 7 – 120-400 grit sandpaper and block Buy lots of 120, 240 and 400 grit sandpaper (you can buy it as 5m rolls). Change paper frequently to reduce the amount of elbow grease required. 8 – Builders’ bog To smooth over gaps. 9 – 100mm roller, short nap For applying the acrylic primer. Assembly tips If possible, attach your vacuum cleaner to your router. If you don’t do this, don’t say I didn’t warn you! It’s also important to work in a well-­ ventilated area to help prevent inhalation of sawdust. Work out what final finish you are aiming for before you start. This will affect your construction method and Fig.9: a distortion plot for the overall system. These levels are very low for a loudspeaker system, where 0.5% is considered good. It is excellent between 50Hz and 1kHz, with the distortion generally below 0.33%. The distortion is primarily second harmonic; the third harmonic is very low, which is why these speakers sound so good. planning. I chose to go for a smooth, painted finish. This choice was driven by cost, my existing décor, and to allow me to demonstrate that you can produce a good speaker finish at home with no special tools. I have laid out the design with rebated joints, allowing you to glue and clamp or glue and screw the enclosure together. It requires some precision in routing, but once your jig has been set up, that is reasonably easy to achieve. If you do not have a router, fear not. Rejig the panel sizes to use butt joints and screw them on the end grain (with pre-drilled holes!). Fig.10 shows the cuts for a 2400mm x 1200mm sheet of 16mm-thick MDF (or two or three smaller sheets). I could have brought the sheet home in a ute and made all the cuts myself, but instead, I asked the nice people at my Photo 3: my simple jig allows quick and repeatable 90° (and angled) cuts and routed lines to be made. This saved hours of fiddling with clamps at the cost of a few timber off-cuts. You can use this with a saw or router but be prepared to throw away your ‘alignment edge’ every time you change the angle. 68 Silicon Chip Australia's electronics magazine local hardware store to cut two vertical strips 188mm wide, two 210mm wide and two 358mm wide. I took the extra as an off-cut. That lot slid easily into a VW golf hatchback, and everything, including cutting, was $50 – the cuts came free. There aren’t many free things in life, so you might as well take what you can get! Figs.11, 12 & 13 show the details of the various panels that make up the enclosures for the Active Monitor speakers. Cutting the panels from the strips is relatively straightforward; just note that accuracy here will pay dividends in final assembly. Some things to keep in mind are: • The front panel and the internal brace are sloped back by 5°. I set my circular saw to an angle of 5° and admit to using some bog to smooth these joins in my assembly. • All rebates are routed to a depth of 5mm. Do a test cut or two and get this right; do not cut too deep, or your panels will need trimming to fit. • If you need to trim a panel, plane or use a disc sander – you do not want to lop off large chunks of timber. • Keep track of your left and right panels as they have the routing on opposite sides! I used an extremely simple jig to allow simple right-angle and 5° cuts and routed lines to be made, as shown in Photo 3. Once you have cut the panels to size, mark and route the rebate for the tweeter, which is 104mm in diameter and 5mm deep. Check that the tweeter fits your circle by routing an off-cut. Next, cut the tweeter and woofer holes, noting that there are two notches siliconchip.com.au Making circular rebates for the tweeters You need a router circle jig to make neat rebates for the tweeters, as they will be visible. You can buy a circle jig for most brands of routers, but you can also make one. Get a flat sheet of aluminium about 100mm wide and 250mm long and drill a hole near one end that’s larger than your biggest router bit. Measure and drill mounting holes for the router relative to that and countersink them so the screws will not scratch the timber. Next, drill a series of 3mm holes to provide various radii from the router bit and countersink them from the top. Attach the jig to a centre hole on your workpiece using a countersunk M3 machine screw with an extra nut between the jig and the workpiece to keep it stable, and voila, you have a circle jig. Just make sure to test it on offcuts to select the ideal hole in the jig before doing the final rebate. Fig.10: here’s how to cut the full set of Active Monitor panels from one or two MDF sheets. Smaller sheets could be used, or as described in the text, take advantage of the free cutting service offered by many hardware stores. Cut these as accurately as possible, then route and cut the indicated holes in the panels for the drivers, ports and holes in the brace. siliconchip.com.au Australia's electronics magazine November 2022  69 in the tweeter cutout to accommodate the connection terminals. Once the front panel is finished, you can cut the holes in the brace. I marked these and then used a jigsaw. There is nothing terribly special about the dimensions on these cutouts, but you want to leave sufficient material to strengthen the enclosure. Finally, cut out the speaker terminal hole. Again, a jigsaw is handy but not essential. Also make the cutouts for the input terminals and speaker port on the rear panel. Now route all the rebates. I recommend setting up a jig as this will save you a lot of time and give you consistent route line locations. Cabinet assembly Whether you plan to simply glue and clamp or glue and screw the enclosures, do a dry assembly to check that all the joints are neat and fit correctly. Make any adjustments now, so that everything sits flush. If you are planning to screw the panels together, be prepared to drill 2.5mm pilot holes for your screws and countersink the heads. If your timberwork is neat and achieves a clean and tight fit together on dry assembly, simply gluing the enclosures together is something you could consider. Apply a generous layer of glue to both surfaces and use masking tape to hold panels in place if you find yourself looking for an extra hand. Wipe away excess glue as you assemble them, keeping everything relatively clean. With the rebates and internal brace, which sit horizontally between the front and rear panels, the boxes should fit nicely together. Jiggle the panels into their final places and add sash clamps to hold the lot together while the glue sets. To ‘clamp’ the top and Fig.11: details of the Active Monitor front and rear panels. Note the notches for the tweeter terminals, the rebate to recess the tweeter and the rectangular areas routed out of the inside for the internal brace. 70 Silicon Chip Australia's electronics magazine siliconchip.com.au bottom panels down, I sat the assembled speaker on the floor and put some bricks and a hefty box of transformers on top of it, as shown in Photo 4. Once clamped, take a damp cloth and wipe all excess glue from the joints. This is an essential step as sanding PVA glue is extremely difficult. Applying the finish No matter how you choose to finish your speakers, the most important thing is preparation. Once the glue had set, I used a sanding disc to sand back all external joints to flush. I then used ‘bog’ to fill any gaps between joints, being extremely careful not to overfill. The sand-and-fill process was Fig.12: details of the top and bottom panels, and the internal brace. The brace fits horizontally between the drivers and ensures good rigidity. The bottom panel is larger than the top due to the angled front. Fig.13: details of the side panels, which are trapezoidal to fit the tilted front panel. The routed areas are where the front and rear panels attach as well as the horizontal internal brace. siliconchip.com.au Australia's electronics magazine November 2022  71 Photo 4: one Active Monitor speaker box, glued and clamped, waiting to dry. This sat for 24 hours before further work was undertaken. repeated, moving through 120 grit to 240 grit sandpaper until the boxes were totally smooth. You will see in the photos that I rounded the speakers’ edges. The front edges have a 10mm radius curve to the edge, while the sides have a 6mm radius. The front edges are rounded to reduce diffraction, although if your finish demands a square edge, that will be OK. My earlier prototypes had square edges and did not show significant refraction-related problems. Next, I used a roller and acrylic primer to seal the timber, paying particular attention to the end grain. We need this sealed; otherwise, it will absorb paint and be visible through the top coat. The primer had an orange-peel effect which required yet more sanding with 240 grit paper to make the finish smooth, as can be seen in Photo 5. After the acrylic primer had been sanded totally smooth, I applied a spray primer. This sealed any patches of MDF showing through. I sanded that again using 400 grit paper and then applied two top coats with a light sanding using 400 grit paper in between. I used “satin black”, which is more matte than gloss. Gloss is the worst case for showing any flaws. Luckily, the results were excellent. Photo 6 shows the least flattering aspect of the speaker, with a slight imperfection along the top rebate joint at the rear. I almost filled and re-coated this to make them perfect, but the reality is this is only visible from this angle, so I considered the extra effort unwarranted. Installing the drivers Set the speaker port to 100-110mm in length. This does not need to be super exact; a 90 to 115mm range is acceptable. The specified Altronics port is 110mm long with the adjustable extension removed. Install and screw the port in, as shown in Photo 7. The connector is the ideal place to mount the protection capacitor for the tweeter. The 100μF capacitor will protect against the application of DC and provide a small measure of protection against modest levels of fullrange signal. However, it will not protect against prolonged high-level low-­ frequency material. To prepare the connector, you must take the bridging straps off before soldering anything to the tabs. I hot-melt glued the capacitor to the input connector and wired it directly in series with the tweeter wire. I used 1mm2 (17AWG) heavy-duty speaker cable for the tweeter and woofer connection. Make sure you cut these long enough that you can solder them to the drivers when you install them. I started with 600mm lengths, soldered them to the input terminals and trimmed them to length when I connected the drivers – see Photo 8. Mark the woofer and tweeter connections so Photo 5: the primed Active Monitor speakers drying in front of the air conditioner. These were later sanded smooth before applying spray primer, then sanded again before the top coat. Photo 6: the ultimate finish, shown at the least flattering angle (see the top rear). A final smear of filler and a couple of top coats would resolve this, but I figured it wouldn’t be visible in the listening room. 72 Australia's electronics magazine you do not get them confused. I also suggest labelling the connectors on the exterior of the box so that in the future, when you have forgotten how you built these, the connections are clear. Poke the speaker wires into the enclosure, put foam tape around the terminal cutout, then screw it down with four wood screws. Next, cut two strips of thick wadding about 1m long, fold one and stuff it above the brace, then the second one below it. The aim is to loosely fill the enclosure with wadding to damp rear radiation into the enclosure. The drivers have pre-installed foam gaskets. Solder the tweeter wires to the tweeter, being careful to get the phasing correct. Now screw the tweeter in and follow by installing the woofer. Secure the drivers with 16mm screws. Finally, I installed the felt. It is best to cut the circles after it has been stuck to the front panel but cut the straight lines using a ruler and sharp knife before installing. The pattern I used is shown in Fig.14. This is required as we are mounting the woofer flush to the front panel; the felt neatens the appearance and allows the woofer to be moved forward, improving time alignment and reducing diffraction. Depending on the felt you can source, you might need two layers. Ultimately, you want the felt flush with the woofer frame. Parts List – Active Monitor Speakers (per pair) 1 Amplifier/Crossover (to be described next month) 1 active subwoofer (optional; but recommended; described next month) 2 SB Acoustics Satori MW16P-8 165mm mid-woofers [Wagner Electronics – siliconchip.au/link/abfi] 2 SB Acoustics Satori TW29R-B 29mm ring tweeters ➊ [Wagner Electronics – siliconchip.au/link/abfj] 2 100μF 100V bipolar crossover capacitors [Jaycar RY6920] ➋ 2 35mm adjustable speaker ports [Altronics C3638] 2 bi-wire speaker terminals with two independent inputs [Altronics P2019] 1 2400 × 1200 × 16mm sheet of MDF or similar, cut as per Fig.10 30 16mm-long 8G wood screws 1 2m length of 1mm2 (17AWG) figure-8 speaker [Altronics W1936] 2 5mm-thick sheets of dark felt, 300 × 200mm 1 5m length of 5-10mm wide soft foam sealing tape (for sealing driver and terminal holes) 1 2m × 1m acoustic wadding blanket [Lincraft “king size thick wadding”] 1 300mL tube of PVA glue 1 500mL tin of acrylic primer paint 2 350g cans of spray primer paint 2 350g cans of spray paint (for two or more top coats) ➊ If you’re ordering the drivers from Wagner and want to build the subwoofer, you can get the SB34SWNRX-S75-6 subwoofer driver at the same time (siliconchip.au/link/abfk). ➋ Increasing the value up to 220μF is beneficial but not required. Make sure the capacitors can handle the currents involved. Parts for optional stands (per pair, 800mm tall) 2 2m lengths of 120 × 19mm DAR pine 2 300 × 300 × 16mm sheets of MDF or similar 2 200 × 140 × 16mm sheets of MDF or similar 8 75mm-long 10G wood screws 8 50mm-long 10G wood screws 1 250mL tin of acrylic primer paint 1 350g can of spray primer paint 1 350g can of spray paint (for two or more top coats) Fig.14: cut the felt to this shape and glue it to the front of the speaker. It serves two purposes: to prevent sound from refracting from the edges of the drivers and to hide the difference in the mounting styles of the two drivers. Photo 7: the rear of the speaker, showing the texture you get if you don’t sand the undercoat. This also gives you a good view of the vent and the terminals. siliconchip.com.au Australia's electronics magazine November 2022  73 Speaker stands Where and how you use your Active Monitor speakers is a personal choice. That said, positioning is important; having them at ear level is a good idea. My speakers are in a listening room and I wanted some stands to set them at the right height. I made suitable speaker stands from off-cuts of MDF and 120 × 19mm DAR pine timber. I used angled braces to make them both stronger and more visually interesting. Fig.15 shows how you can make some of these simply and cheaply. I cut the timber as shown, primed, sanded and applied a top coat in a similar manner to the Active Monitor speakers. The overall height of the stands as specified is 800mm; you can tweak the height to suit your needs. Testing We’re getting a bit ahead of ourselves Fig.15: the details of the low-cost but sturdy and attractive stands I designed for the Active Monitor speakers. You can probably cut the base and top plates from off-cuts of the material used to make the speakers. 74 Silicon Chip Australia's electronics magazine siliconchip.com.au -27 150 -30 100 -33 50 -36 0 -39 -50 -42 -100 -45 1.0K 1.5k 2.0k 2.5k 3.0k 3.5k Frequency (Hz) 4.0k 4.5k 5.0k 5.5k 6.0k 6.5k 7.0k 7.5k 8.0k 8.5k 9.0k 9.5k Phase (deg) Amplitude (dB) here because you’ll need to build the amplifier/crossover system described in the article next month to test your new speakers properly. Still, this is an appropriate place to discuss how to check that everything has gone together properly, so let’s proceed on the assumption that you have already built the electronics. A good test for a crossover and speaker alignment is to invert the tweeter phase and see if there is a dip at the crossover frequency. Fig.16 shows a 10dB dip in the response at the crossover frequency when I invert the phase of the tweeter. This indicates that the time alignment is correct and that everything in the system is working as planned. -150 Fig.16: a major dip is seen in the frequency response when the tweeter phase is inverted. This sound cancellation shows that everything is well aligned and working as expected. Calibration and use Assuming you are setting the output level controls on the 3-Way Active Crossover, I recommend you use an oscillator and AC voltmeter. The oscillator could be your PC audio output. Be a little cautious using DVMs as an AC voltmeter as some do not respond to signals above 400Hz, so check you get sensible readings. The steps are: 1 - Unplug your Active Monitor speakers from the Active Crossover Amplifier. 2 - Set the woofer level to maximum. 3 - Set your oscillator to generate 400Hz at 1V RMS. 4 - Measure the woofer output of the active crossover or amplifier. These should be 0.65V/12.6V RMS respectively; ±1dB precision on these is 0.580.73V & 11.2-14.1V. 5 - Now set your oscillator to 5kHz. Check that your meter still reads 1V RMS at the input to the Active Crossover Amplifier. 6 - Adjust the tweeter volume control to get 0.24V/4.7V RMS on the active crossover or amplifier’s tweeter output; ±1dB precision on these is 0.21-0.27V/4.17-5.25V. 7 - Set your oscillator to 40Hz and check that your meter still reads 1V RMS at the input to the Active Crossover Amplifier. 8 - Adjust the subwoofer volume control to get 0.59V RMS on the subwoofer output; ±1dB precision on this is 0.48-0.61V. It is probably best to set the subwoofer output by ear as there can be huge differences between listening rooms. Adjust the level up until it siliconchip.com.au Photo 8: I used hot melt glue to attach the DC block capacitor for the tweeter to the back of the speaker terminal. I then soldered 600mm of heavy-duty speaker wire to the terminals, ready for attachment to the drivers. sounds ‘bassy’, then back it off until the sound is dry. The right level is in between those settings. If you have an SPL meter, use it, just be aware that your room will create all sorts of interesting peaks and dips. Some say that two subs can help fill these, but it is an expensive proposition. Still, there’s absolutely nothing stopping you using both subwoofer outputs from the Active Crossover Amplifier to drive one sub each. In that case, you’ll initially want to set the subwoofer output closer to 0.4V RMS. I adjusted the baffle step correction to achieve optimal subjective sound quality in my listening room. You might wish to tweak this to suit your room. This is because the baffle step corrects how much sound is heard at the listening spot – but remember that diffraction merely redirects the sound off Australia's electronics magazine to the side, and the sound is still in the listening room. So each room may demand a different correction. Increasing the 2.2kW resistor will reduce the amount of baffle step correction (and reduce the frequency at which the correction kicks in). The recommended value should be correct in many situations, but you may like to experiment with it. I trust that you will enjoy building and tweaking, then listening to these very high-quality speakers and possibly making your own version inspired by some of these ideas. Next month The second article next month will describe the Active Crossover Amplifier system for driving the Active Monitor speakers. After that, we’ll have an article on building the matching High Performance Subwoofer. SC November 2022  75