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1kW+ Class-D Part 2 by Allan Linton-Smith Image source: https://unsplash.com/photos/SP9HcRASMPE Mono Amplifier This seriously powerful mono amplifier module uses the International Rectifier IRS2092S Class-D controller and four IRFB4227 Mosfets. This amplifier module is available from DigiKey ready-built and can deliver over 1000W RMS! Having described how it works and is configured last month, we will now cover assembly and testing. W hen building such a powerful amplifier, the power supply is the biggest stumbling block. The only practical way to power this amplifier at a reasonable cost is by using multiple off-the-shelf switchmode power supplies. In this case, six 25V 20A DC supplies are wired in series, giving a total of 150V or ±75V when using a centre tap. Consider that you would need at least six 500VA transformers to provide the ±75V DC at 18A. Not only would that be extremely costly and heavy (over $1000 and 25kg), but the initial surge from switching it on would blow a fuse or circuit breaker unless some sort of soft starting was implemented. For comparison, the six switchmode supplies cost us $347, including delivery, and they weigh around 4kg in total. Our setup provides ±72V at a maximum of 20A DC. Each switchmode unit is an independent supply and is adjustable to 25V, which would give the recommended ±75V, although we didn’t find that necessary; we got plenty of power with the ±72V supply. The only supply adjustment we made was to match the positive and negative supplies within 0.1V to maximise the PSRR (power supply rejection ratio). Our performance tests were not conducted with any additional capacitance, although we will describe how you can add some if you want to. According to IR, it will lower the noise level, but the large capacitors required are a bit expensive. Photo 1 shows the all-important Class-D amplifier module with the basic connections made. Audio comes in via a double-sided RCA socket and is fed to a front panel volume control (a basic logarithmic response potentiometer) using an RCA plug lead. It then goes to the amplifier module via another similar lead. An internal volume control on the amplifier module PCB can set a maximum level, to frustrate ambitious volume twiddlers. Mains Wiring Caution This device uses connections to 230V mains power, so attention must be given to insulation and earthing. Only those who are experienced with mains-powered devices should attempt this project. Ensure you follow all our instructions regarding the mains wiring. High DC voltages (150V) will also be present during and after operation, and high voltages of up to 60V AC can be present at the speaker outputs. Avoid physical contact with exposed metal surfaces when operating the device and immediately afterwards. Switch off power and allow the supply rails to discharge before placing or removing measurement probes. 74 Silicon Chip Australia's electronics magazine The speaker output from the amp module connects to chassis-mounted binding posts via short lengths of heavy wire. You could customise it with a completely enclosed (Speakon type) speaker output socket. The latter would be a good idea since, at full power, the output can exceed 58V RMS, which is a shock hazard. The optional VU Meter mounts above the volume control; the needle enters the red zone when the output is over 1000W into 2W, 500W into 4W or 250W into 8W (see Photo 2). Housing it These parts are all housed in a metal case. We decided to use an aluminium toolbox as it was large enough, sturdy, not too expensive and convenient to carry around. We purchased ours from eBay (192790170418). The metal toolbox we used was made by “Sunrise”. It is 575 × 245 × 220mm, big enough for everything to fit snugly. It is sturdy, portable and has a latched lid for easy access, although it can be padlocked for safety. It is relatively easy to cut and drill. A handy plastic tool tray comes with the toolbox, although it is not used for this project. You might come up with a different idea; as long as it’s made of metal and large enough, it should do the job. Once you’ve obtained the case, power supplies, amplifier module and other bits and pieces, it’s time to siliconchip.com.au Photo 1: the IRAUDAMP9 ‘evaluation board’ with the Class-D amplifier IC (under the large heatsink) and support circuitry. The only required connections are the signal input at upper left, the speaker output at upper right and the ±72-75V DC supply rail inputs below that. If using the VU Meter, you’ll likely also terminate its signal wires to the two-way speaker terminal. Photo 2: while the VU Meter is a handy way to see how much of the amplifier’s power is being used, it needs to be calibrated for the particular load impedance to be accurate. start assembling it. Roughly, the steps will be: 1. Join the six switchmode supplies together into two sets of three and wire them together (see Photo 3). 2. Prepare the case by making the required holes and installing the chassis-mounting components. 3. Add the chassis wiring. 4. Mount the switchmode supplies in the case & wire them to the chassis. 5. Mount the amplifier module on top of the switchmode supplies and connect it up. into the threaded holes of each switchmode unit with M4 machine screws and flat washers. It is critical that the twelve M4 screws that fix the top straps to the switchmode power supplies are no longer than 10mm; otherwise, they will touch the internals and may damage the supplies. Different supplies can vary (even if they look similar), so we recommend checking the “free-depth” in the data sheet for the supplies you purchased to verify that the 10mm screws are short enough to avoid damage. Leave at least 13mm between the switchmode units so that you can insert M10 bolts to attach them to the floor of the case, as shown in Figs.12 & 13 and our photos (including Photo Initial assembly You can see the final result we are aiming for at the end of the article in Photo 9, which shows everything mounted inside the case and wired up. For mounting the six switchmode power supplies, cut four top straps from 25 × 3mm thick aluminium flat bar and four bottom straps from 20 × 10 × 2mm aluminium rectangular bar and drill 4mm holes, as shown in Fig.11. Note that the negative (left) supply bank mains selectors are up, whereas the LEDs face up for the positive supply bank. The reason for this is to give better cooling and airflow. Flat straps can be used at the top, but rectangular tubes should be used on the bottom to keep the banks 10mm above the floor. That improves the airflow too. The top and bottom straps screw Fig.11: the four straps holding the power supply banks together are made from 170mm lengths of aluminium bar and rectangular tube. The bottom straps are thicker to allow enough air to circulate under the supplies. Holes for mounting the amplifier module to the top straps are not shown, as they are marked once the supplies are in the case. siliconchip.com.au Australia's electronics magazine November 2023 75 Photo 3: here’s how to wire up the six supplies in series and make the mains wiring. Note that we have not used crimp connectors at this early stage. They are not strictly necessary but, if crimped correctly, they give more secure anchoring with less chance of accidental shorts. Don’t skimp on the cable ties once the wire is finished, especially on the bundles of mains wires. 9). We used two bolts, but four would be better! Case preparation Remove the plastic tool tray from the Sunrise toolbox and remove the front decal (attached with some sort of sticky, rubber-like adhesive) to make room for the VU Meter and the front volume control. Next, make the holes for the M10 retaining bolts in the bottom of the box. Mark their locations after you have inserted the power banks because there is quite a bit of fiddling required so that the amplifier module will fit neatly on top of the straps, with its mounting holes located over the straps. The easiest way to check that is to attach four tapped spacers to the amplifier module mounting holes using short M3 machine screws and place it on top of the straps. Verify that the module is not wobbly and that the spacers are centrally located on the straps. Once you are happy with the setup, mark the bottom of the box with a bit of paint on the bolts. Drill 10mm holes and check out how the retaining bolts will work. We used a nibbling device to sink the bolt heads neatly into a small square at the bottom so they wouldn’t turn during tightening. With the holes made, remove the power supply banks and start marking out the other holes and cutouts in the case, shown in Fig.14. Some of these holes are optional; for example, you don’t need to make the VU Meter cutout at the front unless you’re going to install the VU Meter. You could also omit the volume control if you will have an external control (although we recommend you fit it anyway as it will probably come in handy at some point). You could also only drill one pair of holes for binding posts if your load impedance will always be below 8W. For the vent, ensure the rectangular cutout isn’t too large and leave the four corner mounting holes until last. You can mark the positions using the actual vent as a template to ensure they’re accurately placed. When mounting the vent, use M4 machine screws and nuts with washers under each nut. Be accurate when making the hole for the switched, fused IEC socket because it has to snap into place – see Photo 4. To do this, scribe the hole and use a small drill to make a hole in each Fig.12: a side view showing how the power supplies, amplifier module and optional capacitor bank are installed in the case. Both Figs.12 & 13 are shown at 25% of actual size. Fig.13: an overhead view showing how the power supply components are arranged in our toolbox case. While we used one M10 bolt to hold each supply bank in place, we recommend you use two for each. 76 Silicon Chip Australia's electronics magazine siliconchip.com.au Photo 4: the amplifier’s rear panel, with the input connector at the top, the 2-4W output terminals below that and the 8W output terminals just above the switched and fused mains input socket. The vent we used was discontinued, so we’ve specified a slightly smaller one. Also note that the RCA input socket has been moved down as we had trouble with the lid hitting it on the prototype. corner (3mm is good), then drill a larger hole to allow a hacksaw blade in your jigsaw to pass through and carefully run the blade up to each corner. File it until the IEC socket snaps into place. The hole for the VU Meter is 85 × 44mm. You can also cut it with a jigsaw; drill a hole in the centre slightly bigger than the blade, then cut up to each corner. You can also use a nibbler tool (Jaycar TH1768). The round hole for the front volume control will be either 7mm or 8mm in diameter, depending on the potentiometer you are using. Make a hole for the fan and fan guard on the right-hand side of the box. You could use a different sized fan to ours (eg, you could go for 80mm or 150mm) but 120mm fans are widely available and often very quiet for the amount of air they move. Cut the required hole for your fan by drilling a small hole, then drill a larger hole to enable you to use a jigsaw fitted with a hacksaw. A nibbler tool can also be used. Make sure everything fits and deburr all the holes; you can use a large drill bit to deburr the round holes and a file, sandpaper or emery paper to smooth the others. Clean out the box carefully after doing that by vacuuming and then wiping it down with a damp cloth, as you don’t want any metal filings floating about inside the amp. Mount the RCA socket, binding posts, IEC mains socket, vent, fan and fan grille and ensure they are all secure before proceeding. Verify that the binding posts and RCA socket are insulated from the toolbox chassis. If you find that the lid hits the RCA socket when opened, attach a rubber foot above the RCA socket to limit how far the lid can open. Photo 5: the inside of the case rear. The audio input is a double chassis-mount RCA socket, while output connectors are binding posts. The lower output for 8W loads has an extra 75µH choke to prevent a spike in the upper end of the frequency response that could damage tweeters. Wiring Cut the RCA-RCA cable such that you have a sufficient length to go from the input socket at the back to the potentiometer at the front, then solder its outer braid to the potentiometer’s anti-clockwise lug and the inner conductor to the clockwise lug. The remaining cable section will go from the pot to the amplifier module, with its braid also soldered to the anti-clockwise lug and the inner conductor to the wiper. While you still have good access, assuming you’re fitting two sets of binding posts (as we did), partially unwind the 100µH inductor until it has 25 turns left to make it siliconchip.com.au Fig.14: the positions for the required and optional holes in the specified case. The cutout for the VU Meter and the second set of binding posts are two that you could omit. If your case is different, you could use a similar arrangement. Regardless, it’s best to check that everything will fit after marking the hole positions before cutting and drilling. We have shown the fan cutout as 120mm, but you might need a smaller or larger cutout depending on your fan. Australia's electronics magazine November 2023 77 approximately 75µH, then crimp eyelet lugs to its leads and connect it between the two red binding posts. Secure the inductor to the side of the case using some neutral-cure silicone sealant, as shown in Photo 5. For all the crimping in this project, use a good-quality crimping set and mains-rated wire for the mains connections. Leave wires long enough to allow you to make connections before mounting everything in the case. Cut a length of heavy-duty figure-8 speaker wire or two similar heavyduty wires to go from the binding posts to where the output connector will be located on the amplifier module. Crimp eyelets onto the ends and connect them to the 2-4W binding posts, as shown on the wiring diagram. Make up a second short length of heavy-duty wire with eyelets on each end and connect it between the two black binding posts. Now is also a good time to crimp a spade lug to one end of a length of 10A mains-rated green/yellow striped wire and an eyelet to the other. Push the spade lug onto the IEC connector Earth terminal and ensure it is secure. Drill a nearby hole in the base of the case and use an M4 machine screw and two nuts to connect the eyelet to the exposed metal of the case. The Earth screw must not be used to attach anything else to the case, although it’s OK to connect other Earths (such as for the switchmode supplies) to the same screw. Use a shakeproof washer between the case and eyelet to ensure a good electrical and mechanical connection. Tighten the top nut onto the other to make it a lock nut. Power supply wiring The power supply wiring is shown in Fig.15. Start by wiring up the switchmode power units in series. While you can screw bare wire into the screw terminals, it’s far better to crimp a fork lug onto the ends of the wires. For example, that prevents any stray wires from causing short circuits. Use a proper crimping tool so they are secure. Note that each unit has three terminals for each of the positive and negative outputs, which are common. So you can use any +24V positive or any 0V negative connector when wiring it up. The translucent window clips into place for protection when you’ve finished. Ensure the mains wires are long enough to reach the IEC input socket, while the DC wires will need to extend to the screw terminal on the amplifier module. Use cable ties to tie the mains wires together and insulate them as shown. The Earth wire will go to the chassis Earth lug (place its eyelet lug on top of the other), while the Active and Fig.15: note that the specified switchmode supplies have three pairs of DC output terminals. If using a 24V fan, connect it to the outputs of one of the switchmode supplies rather than the buck converter. You can omit the buck converter entirely if using a 24V fan and no VU Meter. A LED and 39kW 1W resistor can be connected across the ±75V supply to indicate voltage. All mains wire is rated at 10A; power supply & speaker wire must be minimum 15A rated. 78 Silicon Chip Australia's electronics magazine siliconchip.com.au Photo 6: make sure your Jiffy box covers the IEC socket and wiring like ours. The Jiffy box can be secured by using small right-angle brackets, screws and nuts. One of the optional 10,000µF capacitors is visible in this photo; it was added after all the performance measurements shown in the first article. Neutral wires will go to the switched IEC socket. After checking that they will be long enough to reach, you can crimp insulated spade connectors onto the Active and Neutral wires (see Photo 8) and an eyelet lug onto the Earth wire. You will also need two short Active and Neutral wires with insulated spade connectors at each end to connect the IEC connector’s switch and mains input terminals, as shown in the wiring diagram. When finished, plug them in and check they are secure. The buck converter Note that this converter is not required if you use a 24V fan and no VU Meter. In that case, the fan connects directly to one of the spare sets of DC output terminals of the nearest switchmode power supply. If using it, connect the input wires of the 24V to 12V buck converter to the outputs of one of the switchmode power supplies (the one closest to the fan is probably the best) using crimped fork connectors. Solder the 12V DC output wires to the fan wires (assuming you’re using a 12V fan) and insulate the joints with heatshrink tubing. At the same time, if you are using the VU Meter, solder its two outer supply terminals to the same 12V DC output wires (the polarity doesn’t matter as the backlight is a tungsten lamp). Final assembly The wiring in the case should now be sufficiently complete that you are ready to drop the switchmode supply assemblies into the case and fix them in place using the M10 bolts. While doing that, connect the mains wires for the switchmode supplies to the IEC mains socket terminals, as shown on the wiring diagram. Place the left (negative) bank of three supplies into the toolbox. It will be a tight squeeze, but it should fit if you angle the bank with the wired side slanting into the front and then push the back down until it sits on the bottom. Mount the buck converter on top of the right bank using foam-cored double-sided tape. Now cut 60mm off the end of a UB5 Jiffy box so you can place it over the mains connections, like in Photo 6. Later, once you’ve tested the amplifier and found it to be working, you will need to secure it in place using right-angle brackets, screws and nuts connected to the base. This is important; not only are there exposed mains conductors on the back of the IEC socket, but it’s also quite close to the input & output terminals. If one of those wires came loose and touched the IEC socket, it would be a severe hazard, so don’t skip this step. If using the optional chassis- mounting capacitors, you can install them now, in the middle of the case between the switchmode supplies. Wire them up to the DC bus being very careful to get the polarity correct. The positive terminal of one goes to the +75V rail, the negative of the other to the -75V rail and the two remaining terminals to the 0V rail. Getting this wiring wrong would be a disaster! It’s a good idea to test the power supply before installing the amplifier module. Double-check everything to ensure there are no errors and that none of the unterminated wires are in a position to short against anything (or each other). Also, if the switchmode supplies have a mains voltage range selector switch, ensure they are all set to the correct setting (220-240V for Australia & New Zealand). With the Jiffy box covering the mains terminals and the power supply Photo 7: the lefthand bank of (negative) supplies; you can see the orange trimmers that adjust the supply output voltage. The thick rail at the top goes into the bottom of the case (the supplies are flipped when installed) to allow cooling air to circulate under the supplies. This photo was taken before all the cable ties were added. Each cable should be tied in place and the mains should be separate from the other wiring siliconchip.com.au Australia's electronics magazine Photo 8: a close-up of the mains connections to the IEC input socket before the protective Jiffy box has been placed over them. November 2023 79 plastic shields clipped in place, connect mains power and turn it on. Use a DMM to check for 72-75V between the 0V wire and the other two DC supply wires. Verify that the polarity is correct for each too. You can now adjust the rails to within 0.1V using the adjusters on one or two switchmode units to ensure the lowest possible noise and distortion, but it is not critical. When finished, switch it off and let the capacitors discharge (connect wirewound resistors across the supply rails if necessary) until the outputs are below a couple of volts. Do not proceed to work on it until they are fully discharged. Amplifier module mounting It is time to mount the amplifier module on the upper supply rails. There are two basic approaches to mounting the module. The easiest is to attach the tapped spacers to the amplifier module, place them on top of the rails, and glue them to the rails using a generous amount of neutral-cure silicone sealant on each. That should give a secure anchoring (the module isn’t super heavy). The superior approach, which takes a bit more work, is to place the module on the rails and mark the four positions where the screw holes are located. Then you remove the rails from the switchmode supplies one at a time, drill 3mm holes and countersink them on the underside. Use short countersunk head M3 machine screws to attach the spacers to the rails, then reattach them to the supplies. You can then screw the spacers on top and use short panhead machine screws to attach the module once all four spacers are in place. With the amp module secured, you can complete the wiring. Plug the RCA input socket into the socket on the board and connect the +75V, 0V and -75V supply rail wires to its DC supply inputs, being very careful to connect them to the correct terminals. Connect the output wires to the binding posts you prepared earlier, as shown in Fig.15. That just leaves the VU Meter signal wiring, if you are using it. If so, connect its two inner terminals to the amplifier module’s output terminals as shown in the wiring diagram, with the required series resistor and diode connected inline with those wires, covered with heatshrink tubing (including the solder joints). The diode anode goes to the terminal on the meter labelled −. The 120kW resistor sets the VU redline at 1700W into 2W but you could use a lower-value resistor if your target output power is less, such as 33kW or 47kW. Heatsinking Given the forced airflow we’re providing with the fan, the heatsink on the amplifier module should be adequate. However, if you’re going to drive it flat out all the time, you might want to add more metal and area to the heatsink. The amplifier will cut out if the heatsink reaches 100°C. If doing this, make sure the heatsink you choose to add on will fit in the box with the lid closed. In this case, we recommend that you bolt it to the existing heatsink using a bracket, as shown in Fig.16, and use thermal compound between each heatsink and the bracket. Photo 9: the switchmode banks fit nicely into the aluminium toolbox and the kilowatt amplifier occupies a small area on top mounted on plastic insulators. The small module on the right provides 12V from the 24-25V output of any of the switchmode supplies, to power a 12V DC fan and the VU Meter backlight. 80 Silicon Chip Australia's electronics magazine Testing Now double-check all the wiring, especially the power connections to the amplifier. Once you’ve verified that everything is connected correctly, set the S1 & S2 switches on the PCB to their central positions (“on” and “self”) and also set the PCB-mounted volume control to the halfway position. Set the external volume control to the lowest position. Make sure the RCA cable is connected to the “CH1” RCA socket on the PCB. Begin the startup procedure: 1. Check that you have a 10A 250V rated fast-blow fuse in the IEC mains input socket fuse holder. If not, fit one now. 2. Connect a speaker to the output terminals. If you have two sets, make sure you use the right pair. 3. Connect a line-level signal source to the RCA input. Re-check that the volume control is at minimum. 4. After verifying that you are nowhere near any mains conductors, apply power. 5. The red LED (Protection) should turn on almost immediately and turn off after about three seconds. 6. The green LED (Normal) should then light up and stay on. 7. Slowly wind the volume control up and check that you get undistorted audio from the speaker. 8. If there is a problem, switch the amplifier off immediately, remove the plug from the mains and allow 15 minutes for the capacitors to discharge before investigating. If all is well, secure the Jiffy box with M3 screws and nuts. You’re ready to SC bring the house down! Fig.16: most users will find the heatsink supplied with the module adequate, but if you will be pushing the amplifier very hard, consider attaching a larger heatsink (or even just a strip of aluminium) to it. siliconchip.com.au