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High-Performance Part 2: By Phil Prosser Active Subwoofer For Hi-Fi at Home Last month, we showed the performance of the new ultra-high-fidelity Subwoofer design and provided all the cabinet construction details. In this second and final article in the series, we’ll finish off the Active Subwoofer by building and installing its internal 180W amplifier, finishing the wiring, installing the driver and adding some feet. A fter building the SC200, Ultra-LD Mk.3 or Mk.4 amplifier, most of the remaining work is in making the custom metal bracket, drilling the heatsink and combining the bracket, heatsink, amplifier and power supply into a compact amplification module. It then slots neatly into the 220 × 170mm rectangular cut-out that you would have already made in the rear of the Subwoofer. If you haven’t already built the amplifier module, it’s best to refer to the ‘IMPORTANT!’ box opposite (repeated from last month). You will also need to build the Multi-Channel Speaker Protector but with only one relay. You can also leave off the components surrounding the missing relay. For example, you could install RLY2 and leave off everything to the left of diode D2 and the 100kW resistor above it. With those two modules assembled, and the rest of the components gathered, you are ready to start putting it all together. I used nutserts to hold those pieces together as they make for an elegant result (they’re basically threaded rivets). However, you can use machine screws and nuts instead. The L-bracket for the Speaker Protector can be made by bending an aluminium sheet by hand in a vise. The larger bracket for the power supply is trickier; if you do not have access to metal folding equipment, I saw some brackets at our local hardware store that would work. Just remember that the power supply transformer is a heavy component and the mounting needs to consider shock loads such as being dropped. The power supply is straightforward; its circuit diagram is shown in Fig.14. Mains power comes in via CON1 and passes through fuse F1 and power switch S1 to transformer T1 (which may have a single 230V or dual 115V primaries, depending on which transformer you purchase). Its two 40V AC secondaries connect to bridge rectifier BR1 and a capacitor bank, producing ±57V DC rails. Since a subwoofer must deliver large amounts of power for extended periods, Fabricating the bracket I used a 3mm-thick panel of aluminium as the main plate for the chassis. To that, I mounted a folded bracket made from 1.5mm-thick aluminium for the transformer and an L-shaped panel for the speaker protector. You can see these panels assembled in Photo 11 (note some differences in the cut-out from the final version). All of the plate amplifier parts mount to those panels, mainly the central bracket. 34 Practical Electronics | February | 2024 IMPORTANT! What you need to build the Active Subwoofer. Plate amplifier construction I mounted the (Ultra-LD) amplifier to the main panel and heatsink combined. In other words, the 3mm base plate is between the output devices and the heatsink. You can see the arrangement in Photo 12. Provided your main panel is free of dents and scratches and the heatsink is mounted to this with a good layer First and foremost, you need the ‘active’ element – an amplifier. At the time of publication this was more complicated than expected. We intended to use the upgrade to the Ultra LD Mk.2 200W Power Amplifier published back in August 2010 with the Mk.3 (or its surface-mount follow-up, the Mk.4). However, unfortunately, the ‘pandemic silicon shortage’ is still affecting a few critical devices for those designs, so they will be published at a later date. Instead, we suggest you use the SC200 200W Amplifier Module we published back in 2018 (January to March). That circuit incorporates most of the features of modern amplifier modules, but uses easy-to-solder through-hole components. There are no tiny surface-mount components. Photo 11: The majority of the plate amplifier parts mount on this bracket. Do note that just like the Ultra-LD Mk.3 and Mk.4 there are component issues for the SC200. Fortunately, we have found good alternatives – see the box last month. You will also need the MultiChannel Speaker Protector (4-CH) from PE, January 2023, timber for the cabinet and acoustic wadding. we have 16mF of energy storage per rail. This reflects the ‘no compromise’ approach to the design. However, if you only install two 8000μF capacitors, it will still work reasonably well. The 270W 10W resistor is to drop the voltage to a level suitable for powering the Speaker Protector and also to reduce the dissipation in its regulator. Photo 12: Here, the Ultra-LD Mk.4 amplifier attached to the bracket, ready to be wired up – you can also use the Mk.3 or the easier SC200. L Subwoofer Amplifier Power Supply Fig.14: the subwoofer power supply is about as basic as it gets. I used a 300VA transformer, but it is no longer available, and 250VA is adequate. Practical Electronics | February | 2024 35 Fig.15 (left): the heatsink drilling details. The heatsink used is the same as in the original Ultra-LD Mk.3/4 articles, but the way the heatsink is mounted is different. These hole positions are suitable for any of the amplifier modules mentioned, including the SC200. Fig.16 (below): the rear plate for the amplifier is made from 3mm-thick aluminium cut and drilled, as shown here. It’s a good idea to paint it black when finished. Ensure the rectangular hole for the rocker switch is only as large as it needs to be for the switch to snap in. Again, this is for the Ultra-LD amplifier, but it will work fo the SC200. this, you can pull everything apart, knowing it will fit perfectly later on. of thermal paste, this will make fabrication easier and contribute to the overall heatsinking capacity. To ensure perfect alignment of the baseplate and the heatsink mounting holes to the transistors, I drilled and assembled the heatsink and main panel before building the amplifier and then mounted the transistors to that before soldering them to the PCB. This ensured that the transistors were perfectly aligned to the mounting holes and PCB. Do not use insulators at this point; we will add them later. Once you have soldered the transistors in like 36 Heatsink drilling Fig.15 shows where to drill the holes in the heatsink for the (Ultra-LD) amplifier. My approach was to mark all holes on the main panel first (see Fig.16), then drill and tap the four corner mounting holes into the heatsink and attach it to the main panel with M3 screws. I then drilled 2.5mm holes through the main panel and heatsink. This guarantees that the transistor mounting holes are perfectly aligned between both panels. I then took the heatsink off, tapped and deburred the holes in the heatsink, then drilled and deburred the main panel holes to 3.5mm. Details of the main bracket that attaches to the rear panel and holds the amplifier module and power supply are shown in Fig.17. The speaker protector bracket attaches via two of the heatsink mounting screws. This is fabricated from 1.5mm aluminium sheet folded at 90°; see Fig.18 for the details. I included a small clamp to hold the 270W 10W wirewound resistor to drop the 57V rails by about 15V. It is wired in series with the positive supply to the Speaker Protector module. Once the metalwork is ready, dryfit everything first and get your assembly plan in mind. Use Figs.19-21 and Photos 12-14 to see how everything fits. At this point, temporarily fit the amplifier board, screw the output devices to their mounting positions Practical Electronics | February | 2024 and diode bridge. Use a small amount of thermal paste under the diode bridge. Install 15mm standoffs for the amplifier module (only in the two corners furthest from the heatsink), 62 F OL D U P 10 10 20 20 57.4 F OL D U P 86.5 10 10 20 20 86.5 18 25 90 13 FOLD UP 42.5 43 146.5 43 40 40 * * 6 .5 60 ** 125 54.8 ** 28.2 F OL D U P 40 A * * 50 43 6 .5 43 110 10 20 62 28.4 10 20 25 F OL D U P making sure you countersink the hole for the screw that goes under the transformer and use a countersunk head screw. Cut a piece of Presspahn or similar and place it under the terminal 22.6 10 without insulators and solder the output devices to the PCB. This gets all the holes lined up. Start final assembly with the terminal block, the transformer, earth screw 6.8 50 32 HOLE A IS 6mm DIAMETER; ALL OTHER HOLES 3mm DIAMETER (* COUNTERSUNK, FOR ULTRA-LD MK.3/4 ** COUNTERSUNK, FOR SC200) 213 377 (SHOWN HERE AT 40% OF FULL SIZE) 32 50 ALL DIMENSIONS IN MILLIMETRES Fig.17 (above): and cut fold this support bracket from 1.5mm aluminium and paint it black. It attaches perpendicular to the rear plate. You only need to make two of the six countersunk holes, marked with a double asterisk for the SC200. Fig.18 (below): the larger bracket allows the Speaker Protector to be mounted in the space next to the amplifier. The smaller bracket clamps down the 10W resistor needed to drop the supply voltage to the Speaker Protector. Photo 13: the underside of the plate amplifier with everything in place but not wired up yet. Practical Electronics | February | 2024 37 strip to ensure that if anything shakes loose from the terminal strip, there is insulation surrounding it. Make sure it is mounted far enough away from the rear panel that it won’t interfere with the wiring to the IEC socket. It only needs to be a three-way terminal to connect the transformer primaries, including joining them in series. Now mount the capacitors. Keep all the negative terminals facing the same way to ensure a tidy build. Then you can finally mount the amplifier module. Flip the module and fit the amplifier using insulating bushes and washers. Screw this down to the 15mm standoffs you installed earlier, using shakeproof washers under the M3 screws. Next, install the Speaker Protector on its standoffs. Make sure you have connected a 200mm length of lightduty wire to the power input of the Speaker Protector, as this connector will be hard to get to later on. Remember to wire the 270W 10W resistor in series with the power input for the Speaker Protector. This reduces power dissipation in the regulator heatsink on the Protector. This is not strictly necessary if you have a single relay installed, provided you use an th specified (Altronics H0655) heatsink Photo 14: A view of the underside of the completed plate amplifier, showing all the wiring. Note though that this version uses a separate fuse holder and a toggle switch; build yours based on the revised design with the fuse holder in the IEC socket. on the Speaker Protector, but it doesn’t hurt either. With everything mounted, most of the remaining work is wiring it up, as shown in Fig.19 (mains wiring), Fig.20 (low-voltage supply wiring) and Fig.21 (amplifier module wiring). Use 7.5A mains-rated cable for all power wiring and insulate all mains connections to prevent accidental contact with high voltages. Do note that the final design is slightly different than what’s shown in the photos; instead of using a separate fuse-holder, we’re using an IEC input socket with an integral fuseholder and the toggle power switch is replaced by a rocker switch. That simplifies the wiring and also keeps all the mains parts away from the low-voltage side. So follow the diagrams in that respect, not the photos. You can use the following steps to guide you through the wiring. Fig.19: a view of the underside of the plate amplifier showing the mains wiring. Be sure to keep these wires short, tie them up and insulate all exposed mains junctions. When mounting the transformer, make sure it isn’t too close to the corner or it could interfere with the IEC mains wiring; this configuration should be used rather than what is shown in the photos on the prototype as it keeps all the mains connections away from the lower-voltage side. 38 Practical Electronics | February | 2024 Photo 15: this is how the amplifier side of the module looks after construction and wiring is complete. Again, this shows the Ultra-LD Mk.4 amplifier – you can also use the Mk.3 or the easier SC200. 1. Install the earth lug and connect the green/yellow striped earth wire from a solder lug to the IEC plug mains earth [we prefer using crimp eye terminals as, if crimped properly, they are more robust than solder joints – Editor]. You can and should locate the earth lug right near the IEC socket; we’ve only shown it further away to avoid cluttering up the diagram. The earth screw must connect the earth lug to the chassis and nothing else. Ensure there is no paint or other layer stopping the earth lug from making good contact with the chassis; if there is, scrape it away. Connect the second solder lug to a 10nF capacitor and a short green wire from the capacitor to 0V on the capacitor bank. 2.  Cut the transformer secondary wires to appropriate lengths to reach the bridge rectifier AC inputs. Crimp and plug or solder these to the bridge rectifier. 3. Using heavy-duty red and white wire, connect the bridge’s positive and negative outputs to the capacitor bank. Optionally, use crimp connectors for the bridge. 4. Cover the exposed metal strip on the IEC socket with neutral-cure silicone sealant. 5.  Using brown mains-rated wire, solder the live wire to the mains socket, and from there to one pole of the switch, then back to the terminal block. Make similar connections for neutral using blue mainsrated wire. Use heatshrink tubing to cover all exposed junctions. Twist these wires together and use cable ties to secure them, so that nothing can get loose should a connection fail. We do not suggest using spade lugs to connect to the mains socket (except possibly for the earth) because space is relatively tight due to the proximity of the transformer. Ideally, the wires should be soldered so they extend upwards and over the transformer body to go to the switch. You shouldn’t need to bend the IEC Fig.20: while similar to Fig.19, this diagram only shows the lower-voltage (~114V DC, so not that low) wiring for the power supply. It’s best to follow this diagram exactly to avoid the possibility of ripple injection in the DC supply to the amplifier module. Practical Electronics | February | 2024 39 Fig.21: the wiring to the amplifier module, mounted on the opposite side of the bracket to the power supply. socket lugs to get extra clearance but it could be done if necessary. You could use crimp spade lugs to connect to the switch since it sits just above the transformer. 6. Connect the transformer primary winding to the switched mains on the terminal strip. Again, tie wrap these securely. If the transformer has two primaries, join the two windings in series via another terminal on the terminal strip (ideally, Photo 16: I used a staple gun to attach a double layer of poly wadding. This is required to dampen rear emissions from the driver and reduce resonances. 40 between the terminals used for the other primary connections). 7. Now wire up the capacitors using heavy-duty red and black wire. Join all the capacitor grounds together using heavy-duty green wire, and connect them to the transformer centre tap wires. 8. Next, take 400mm lengths of red, black and green heavy-duty wire and twist them together gently. Connect this to the +57V, −57V and ground terminals of the capacitor bank, respectively. Route this to the power amplifier power input and trim to length. 9. Use neutral-cure silicone sealant to stick pieces of plastic sleeving over the exposed ±57V connections on the capacitors at this point. This will save you from a potential (no pun intended) 114V DC shock if you slip and come across them. 10.  Connect the +57V rail from the amplifier to the 270W resistor if you need this, and from the other end of the resistor to the positive input of the Speaker Protector. This can be done using light-duty wire. 11. Connect the amplifier ground to the GND input of the speaker protector. 12. Connect the amplifier output to the ‘AMP’ input on the Speaker Protector. The SPKR terminal goes to the positive side of the driver. 13. The amplifier ground output goes to the negative on the driver. Final assembly Assembly of the Active Subwoofer is very simple as all the work is in the enclosure and amplifier module. Install thick ply wadding on the sides, top and bottom of the enclosure as shown in Photo 16. Do not block the port as, when working hard, a lot of air is moving through it. Connect the amplifier’s output to the driver using heavy-duty speaker wire, being careful to connect the ‘+’ output of the amplifier to the red terminal of the driver. Then install the amplifier module after sticking foam sealing tape around the edge of the hole in the cabinet. Attach the module with eight 16mm screws. Fig.22 and Photo 17 show how it should look when installed in the cabinet. Last, fit the driver with foam tape around the hole with eight 16mm screws. I stuck large felt feet on our Active Subwoofer to protect our floor. This thing is not a lightweight piece of kit! Give your new Subwoofer a light workout to verify that everything is working as expected before you move onto earth-shaking bass! Practical Electronics | February | 2024 www.poscope.com/epe Fig.22: this is how the rear of the plate amplifier will look when you’ve finished. - USB - Ethernet - Web server - Modbus - CNC (Mach3/4) - IO - PWM - Encoders - LCD - Analog inputs - Compact PLC Reproduced by arrangement with SILICON CHIP magazine 2024. www.siliconchip.com.au - up to 256 - up to 32 microsteps microsteps - 50 V / 6 A - 30 V / 2.5 A - USB configuration - Isolated PoScope Mega1+ PoScope Mega50 Photo 17: A rear view of the finished Sub, slightly different from the final version. Practical Electronics | February | 2024 - up to 50MS/s - resolution up to 12bit - Lowest power consumption - Smallest and lightest - 7 in 1: Oscilloscope, FFT, X/Y, Recorder, Logic Analyzer, Protocol decoder, Signal generator 41