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500 POWER WATTS AMPLIFIER PART 3 BY JOHN CLARKE To finish our new 500W Amplifier, we shall now describe the power supply configuration and the complete assembly details. That includes mounting all the modules and parts in the enclosure, wiring it up, checking that it works and making the calibration adjustments. T he 500W Amplifier module we’ve described over the last two issues cannot operate alone. It needs a power supply and added circuitry to protect the loudspeaker and keep it cool. We are using two additional projects for these tasks: the Amplifier Clipping Indicator (April 2023) and the Fan Controller & Loudspeaker Protector (February 2023). The final circuitry to be described is the Amplifier’s power supply, and its circuit is shown in Fig.9. As you would expect for this Amplifier, the power supply uses a large transformer, rated at 800VA. The transformer has two independent 115V primary windings and two independent 55V secondary windings. The two 115V windings are connected in series so it can be powered from the (nominally) 230V AC mains. Similarly, the two 55V windings are connected in series with a centre tap so Practical Electronics | June | 2023 that after rectification and filtering, we get approximately ±80V DC. Considerable capacitance is used to filter the DC supply, with four 10,000uF 100V capacitors filtering the positive supply and another four for the negative supply. This is to remove much of the ripple from the DC supply rails, especially when under load, as the Amplifier can draw many amps when delivering the peak power it is capable of achieving. Three 15kW 1W resistors are connected in parallel from both supply rails to ground to discharge the capacitors when the amp is switched off. LEDs are included in series with one resistor on each side of the supply, as voltage presence indicators. They ensure that the capacitors do not remain charged to high voltages for too long after the power is switched off. This is for safety reasons since the total of around 160V DC is an electrocution risk. Additionally, a plastic cover over the capacitors (removed in some photos for clarity) prevents accidental contact with the high-voltage wiring and capacitor terminals. The bridge rectifier is rated at 35A 400V. This rating is sufficient to handle the initial surge current that charges Danger: High Voltage The 160V DC supply across the filter capacitor bank and the amplifier supply rails is potentially lethal! After the power supply wiring is complete and before you apply power, mount a clear pers­pex sheet over the cap­acitor bank to protect against inadvertent contact – now or in the future! Note and remember that the capacitors take some time to discharge after the power is switched off. 21 500W Power Amplifier Supply Fig.9: the only remarkable aspect of the power supply circuit is the large 800VA transformer and relatively high ±80V supply rails. Several 15kW discharge resistors are needed due to the high total capacitance. the capacitors at power-up, and the repetitive capacitor charging current peaks that occur near the peak of the rectified waveform for each mains half-cycle. The transformer is a toroidal type, and a slow-blow fuse is required to prevent it from blowing when power is initially applied, as the inrush current can be very high. For this transformer, a 3.15A M205 slow-blow fuse is specified. It is installed within the IEC power connector housing. This has a safety fuse enclosure, where the fuse cannot be accessed until the IEC power lead is unplugged. The power supply is installed and wired up within a 3U rack case that houses the Amplifier Module, heatsink fans, the Amplifier Clipping Detector, Loudspeaker Protector & Fan Speed Controller and other necessary components. Enclosure layout The internal layout for the Amplifier and associated parts is shown in Fig.10. The Amplifier is built into a 3U rack case with a solid baseplate and vented top lid. This layout allows the amplifier heatsink to be mounted inside the enclosure with three cooling fans that draw air in from one side of the lid and pass this air across the heatsink fins. That forces airflow upwards, to remove heat from the heatsink. The fans are taller than the heatsink, so any air coming up past the fins is blown sideways and then out through the vented lid on the other side. There are quite a few holes that need to be drilled for all the mounting hardware, various cutouts made for the power switch, XLR and IEC sockets, the loudspeaker terminals and clipping indicator LED. The locations for these are shown in Fig.10, and the close-up detail drawings in Fig.11-Fig.13. Begin with the front and rear panels. Some of the required cutouts are not circular; you can cut these by drilling a series of small holes around the inside of the required perimeter, knocking out the piece of metal and filing to shape. Note that you could dispense with the XLR input socket and just use an insulated panel-mount single RCA socket. This depends on your intended application; XLR would be better suited for PA use, while RCA might be fine for home use. If using an RCA socket, a single-core shielded cable is all that’s needed to connect internally to the Amplifier Module input. We specify an insulated RCA socket because the connections need to be isolated from the chassis. Otherwise, a hum loop will be caused by earthing the signal ground to the chassis in two places, since it is already earthed by the Amplifier Module. If using the XLR socket, the main XLR cutout can be made using a 22mm Speedbor drill and then filing the hole shape. Now make holes in the front panel for the power switch and clipping indicator LED bezel similarly. You can make a copy of the front panel label (Fig.14) and use that as a template for positioning those two holes. Next, prepare the insulating material sections to go under the transformer, the 3-way mains terminal strip and the 12V switchmode supply. The insulation for the transformer prevents voltage flash-over to the earthed chassis should there be an insulation breakdown. The other insulators prevent a live wire from contacting the chassis if it disconnects from its terminal. Cut the required insulation pieces from the 208 x 225mm sheet with scissors or a sharp knife and ruler. The sizes At left is a close-up of the power supply section of the Amplifier, with the rest of it, transformer and all, shown adjacent. 22 Practical Electronics | June | 2023 REAR PANEL (inside view) IEC CONNECTOR WITH FUSE E N A LOUDSPEAKER TERMINALS Note that inductor L1 is wound using 13.5 turns of 1.25mm diameter wire, not 30.5 turns or 1mm diameter as explained in last month’s PE. XLR INPUT SOCKET + INSULATION BOOT Piezo Transducer Cooling Fan and Loudspeaker Protector Controller Clipping Indicator SILICON CHIP Cooling Fan and Loudspeaker Protector Controller 01111211 C 2021 2 NC 1 4 .7 V COM 3 4148 2 15V 1 coil 5819 15V 4 To FAN1 * 3.9V To FAN2 NO 4004 5819 4 4.7V To FAN3 4148 * A 4004 3 4148 REV.C 4 REV.B C 2021 01112211 Earth 4.7V Clip SILICON CHIP Indicator 3 + 4004 2 + 1 5819 RLY1 + TP1 TP3 THIS SECTION SHOWN ENLARGED IN FIG.11 A To TH2 + To TH1 TP2 0.47W 5W CON2 0.47W 5W FAN 1 THIS SECTION SHOWN ENLARGED IN FIG.13 Earth 0.47W 5W 12V SUPPLY 0.47W 5W 0.47W 5W * 3.3kW 0.47W 5W A N E 0V12V ~ + 4148 + ALUMINIUM ANGLE 4148 WARNING! HIGH VOLTAGES PRESENT FAN 2 Earth ~ + + Insulation Board 0.47W 5W + CON1 0.47W 5W Transformer T1 0.47W 5W 0.47W 5W CLIPPING LED Practical Electronics | June | 2023 An 8mm hole is needed in the centre of the transformer insulator. That can be made using a wad punch (giving a 01107021 REV.B 0.47W 5W C 2021 0.47W 5W FRONT PANEL (inside view) required are 63 x 97mm for the 12V supply, 57 x 45mm for the 3-way terminals and 162 x 162mm for the transformer. RCA PLUG: RED TO CENTRE, BLUE TO BODY. 500W AMPLIFIER Mounting Plate FAN 3 M8 nut BASE PLATE THIS SECTION SHOWN ENLARGED IN FIG.12 A + + * Fig.10: here’s an overview of the chassis layout and wiring; more details are shown in the close-up drawings of Figs.11-Fig.13. Use this diagram to arrange the components in the chassis and to get an idea of where the wires and cables run, then use the following figures to determine where exactly each wire connects. Note: the wiring between the fans and thermistors TH1 and TH2 (mounted to the heatsink), the Cooling Fan Controller module and fan wiring has been omitted for clarity. The corner instrument feet mounting holes are also not shown. cleanly cut hole) or an 8mm drill, after which you can clean up the resulting furry edges with a hobby knife. 23 All of the various modules are attached to the case by mounting screws. The wiring between these modules is also cable tied to the case. It’s a good idea to be generous with cable ties as it keeps everything secure and neat. Note that the XLR input socket has a 560nF capacitor soldered to it, as shown above. 3mm holes are also needed in the other insulation pieces for the mounting holes of the 3-way terminals and those on the underside of the 12V supply. Again, a small wad punch is ideal for making these holes. A 3mm drill can be used instead, although the resulting holes will not be clean. Table 1: Screw and nut usage Arranging the parts At this point, it’s a good idea to place all the major components in the chassis and make sure you’re comfortable with all their positions. Mark them out with a fine felt-tip pen. That way, you can be sure everything is positioned correctly before you start making holes. When initially laying out the parts in the baseplate, be sure to allow room for the equipment feet to be secured in the corners with M3 screws and nuts. In particular, check that the transformer can be positioned without the screw and nut for the equipment foot in that corner interfering. 24 The holes required in the aluminium baseplate include the mounts for the four corner equipment feet (3mm), the three PCBs (3mm), the heatsink (3mm), the relay (3mm) and piezo transducer (2.5mm), the earth lug holes (4mm), capacitor mounting holes (4mm), the 12V supply (3mm), the three-way mains terminals (3mm), the bridge rectifier (4mm) and transformer (8mm). Refer to Fig.10-Fig.13 to see the locations. You’ll also need to make holes to hold the aluminium right-angle bracket for mounting the fans near the heatsink (4mm). It’s best to locate it after the heatsink has been mounted. The aluminium bracket itself will also need holes to attach the fans that are spaced evenly along the 400mm length, with one fan in the centre and the others at each end. The angle piece is secured to the base by two 4mm screws in the gaps between the fans. We made a small semicircular cutout for each fan to Equipment feet four M3 x 10mm machine screws, four hex nuts Amplifier PCB mounting six M3 x 5mm machine screws, three 9mm M3-tapped nylon standoffs Heatsink mounting four M3 x 10mm machine screws Speaker Protector PCB eight M3 x 5mm machine screws, four 9mm M3-tapped standoffs Clipping Indicator PCB eight M3 x 5mm machine screws, four 9mm M3-tapped standoffs 12V switch-mode two M3 x 6mm machine screws supply Capacitor mounting 20 M4 x 10mm machine screws, 32 M4 hex nuts, eight M4 x 50mm machine screws, four M4-tapped joiners (for mounting protective cover) 3-way mains terminal block two M3 x 15mm machine screws, two M3 hex nuts prevent the lower portion of the bracket from covering the fan blade area. But that is not strictly necessary; it’s just nice to have. While there needn’t be any particular order to install the parts within the case, it is easier to mount the lighter ones first. The transformer is the heaviest part, so attach it last. There is a list of the screws and nuts in Table 1 to help you select the correct hardware for each job. Install the IEC socket, the speaker terminals and the XLR socket on the rear panel. Then mount the switch and clipping indicator LED bezel on the front panel. However, leave these panels detached from the enclosure until the rest of the wiring is complete. Now is a good time to mount the thermistors for the Cooling Fan Controller. These are mounted against the amplifier heatsinks behind the Q25 and Q26 transistor clamp screws. The wires from the thermistors will need extending with an approximately Bridge rectifier one M4 x 20mm machine screw, one M4 hex nut Earth connections three M4 x 15mm machine screws, three 4mm star washers, four M4 hex nuts, 3 5.3mm diameter crimp eyelets Transformer mounting one M8 x 75mm bolt, M8 washer, M8 hex nut Aluminium angle two M4 x 10mm machine screws, two mounting M4 hex nuts Relay two M3 x 10mm machine screws, two M3 hex nuts IEC connector two M3 x 12mm countersunk head machine screws, two M3 hex nuts XLR connector two M3 x 12mm countersunk head machine screws, two M3 hex nuts two No.4 x 6mm self-tapping screws Piezo transducer or two M2 x 6mm machine screws and two M2 hex nuts Practical Electronics | June | 2023 Fig.11: a close-up of the chassis’ right rear corner showing the wiring between the three main PCB modules, the speaker protection relay, the warning piezo, the loudspeaker terminals and the XLR input socket. 350mm length of light-gauge figure-eight wire; insulate the joints with heatshrink tubing. Next, mount the three PCBs in the chassis on 9mm nylon standoffs using M3 x 5mm screws. The Amplifier Module’s primary mounting is via the screws into the heatsink. The Amplifier Module has three PCB mounting locations at the edge away from the heatsink that attach using spacers and short machine screws, but these should be installed last to avoid stressing the PCBs. Before mounting the capacitors, cut out the capacitor plastic covering piece measuring 295 x 125mm, place this on the base plate and mark out the four 4mm mounting holes. These coincide with the capacitor mounting Practical Electronics | June | 2023 clamp screws marked with asterisks in Fig.10 and Fig.13. Now mount the capacitors. These must be oriented with the correct polarity. The negative side is marked with a minus symbol down one side of the capacitor body. When oriented correctly, tighten down the clamps to prevent them from rotating. Note that the four capacitor bracket mounting locations marked with asterisks are secured using 50mm-long screws and nuts. Once all the capacitors are mounted, place M4 joiners on the end of these four 50mm screws, ready to attach the capacitor covering piece using four more M4 x 50mm screws. Now mount the 12V switchmode supply with the insulation board beneath it, and the three-way mains terminal block, also with the insulation underneath. Next, attach the fans to the aluminium bracket using the supplied securing screws, then mount the bracket and fan assembly to the baseplate. To improve heat transfer, when attaching the bridge rectifier to the base, smear a little heatsink compound on the mating surface and the chassis. Transformer mounting Place a washer onto the M8 bolt for the transformer and insert it from the underside of the baseplate. Place the insulation square onto the baseplate over the screw, then add a neoprene washer on top of this, followed by the transformer, the second neoprene washer, the mounting disc and then the M8 nut. Orient 25 terminals, as shown in the photos. Solder the wires securely to the terminals. Both sides of the filter capacitor bank have two 15kW 1W bleed resistors connected across them. Also, a red LED is connected across each side of the capacitor bank in series with another two 15kW resistors. The LEDs are positioned to protrude through 5mm holes in the capacitor cover plate. If your cover plate is made from clear or translucent plastic, you could skip making those holes. These LEDs indicate when voltage is present across the capacitors. As you will find, even with these bleed resistors, it takes quite a while for the capacitors to discharge after the Amplifier is switched off. The whole Amplifier uses single-­ point earthing, so it is important to follow the wiring details in Fig.10Fig.13 closely. Fig.12: a close-up of the chassis’ left front corner showing the wiring of the toroidal transformer, bridge rectifier, mains terminal block and front panel. the transformer as shown in Fig.10 and Fig.12 and tighten the nut. Wiring it up Most of the remaining work involves the heavy-duty power supply wiring. 26 Wire the two banks of four capacitors in parallel using strands of 0.5mm-diameter copper wire. We twisted two strands together using a drill and then bent this in half, interweaving the wire around the capacitor Mains wiring The mains supply is via the IEC power socket, then a length of twin-core mains flex rated at 7.5A or more. This wire needs to pass through the IEC insulation boot before being terminated (soldered) to the IEC socket terminals. The earth wire also passes through the insulation boot and is secured to the earth terminal on the IEC socket, and to the chassis using a crimp eyelet secured with an M4 screw, star washer and nut. Note that this earth continues to also connect to the baseplate via another eyelet. Tie the mains wires with a cable tie before placing the insulating boot cover over the rear of the IEC socket. A third eyelet and earth wire connects from the baseplate earth point to the star earth between the capacitor banks. The mains wires from the IEC socket connect to the power switch using crimp spade connectors on the top two terminals. It is important to wire this switch the right way around; otherwise, the neon LED will be lit, regardless of whether the Amplifier is on or off. We placed unused insulated crimp spade connectors on the lower two switch terminals just for safety. The mains wires from the power switch at the centre terminals run to an insulated three-way terminal block. Further mains wiring connects to the 12V switchmode supply. The mains wires must all be cable tied together so that if one comes loose, it will not cause a safety issue by shorting to chassis. Note that the 12V supply also has an Earth connection to the chassis using an eyelet secured to the baseplate with an M4 screw, star washer and nut. Practical Electronics | June | 2023 Transformer wiring We’ve shown the transformer wiring using the colour coding of the recommended transformer. But check on the transformer label that your winding colours are the same as we used; if not, wire it up according to the colours for your transformer. Connect the two 115V primary windings in series by joining the purple and grey wires using the centre terminal of the 3-way terminal block. Run the wiring to the filter capacitors from the bridge rectifier using the 2.9mm2 (cross-sectional area) wire with red for positive and black for negative. You will find that the yellow and black transformer wires are not long enough to reach the star earth point, so extend them using one of the 2.93mm2 figure-8 cables. The power supply wiring is basically complete at this stage, but it is not connected to the Amplifier Module. Check for continuity between the chassis and the earth connection on the IEC connector. You should get a reading very close to 0W. Next, install the 3.15A slow-blow fuse into the IEC socket. Check your work to ensure everything is connected correctly. Be sure that the capacitors are all oriented correctly. Check that the positive terminal on the bridge rectifier connects to the positive side of the capacitor bank, and that the negative terminal of the bridge rectifier connects to the negative side of the capacitor bank. Safety precautions After the power supply wiring is complete and before you apply power, we suggest that you mount the cover sheet over the bank of capacitors. This will prevent accidental contact with the 160V DC supply. The total DC supply is potentially lethal. The cover will also provide a degree of safety if one of the capacitors overheats and vents to the atmosphere. Before powering it up for the first time, wear safety glasses or a face shield. Briefly apply power and check that both LEDs light. Then switch off the power and allow the capacitors to discharge completely. It should take a while for the LEDs to stop glowing, and they should go out at around the same time. If all is OK, remove the capacitor safety shield and, taking great care (as the capacitor voltages are dangerous), switch on power again and measure the capacitor voltages. The readings you get should be close to ±80V DC. Check also that the 12V supply provides 12V DC at its output terminals. Switch off the power, and again, wait for the voltage to drop to near zero. Practical Electronics | June | 2023 Fig.13: a close-up of the chassis’ left rear corner showing the wiring of the capacitor bank, 12V switch-mode supply, mains IEC input socket and the earthing. Now you can complete the remaining wiring. Run the wiring from the filter capacitors to the Amplifier using the 2.9mm2 wire, with red for positive, black for negative and one of the 2.93mm2 figure-8 wires for the 0V connection. Similarly, use 2.9mm2 or 2.5mm2 wire for the loudspeaker output wiring to the speaker terminals via the relay. 27 For safety, the capacitor bank needs a sheet of perspex mounted on top of it to prevent accidental contact. This photo shows the capacitors without the cover, to clearly present how they are arranged. The rest of the wiring can use lighter-duty wire. Follow the wiring diagram carefully to complete it. Use cable ties and chassis mount ties to bundle wires together where needed. We don’t show all the cable ties on the diagram; be generous and use them wherever required. Connect the XLR input socket to the amplifier module via dual-core microphone shielded cable, as per Fig.15. If using an RCA input socket instead, use single-core shielded cable. Now attach the side, rear and front panels to the baseplate. Final checks and adjustments You are now ready to power up the Amplifier Module and make voltage checks. First, double-check all your wiring against the circuits and diagrams in this series of articles. Then reattach the capacitor safety shield. Remove fuses F1 and F2 on the Amplifier Module and replace these with blown fuses with 390W 5W resistors soldered across the fuse ends. Ensure that trimpot VR2 is rotated fully anti-clockwise. Apply power and measure the voltage on the Amplifier speaker output, at one of the 56W 1W resistor ends closest to the edge of the Amplifier PCB. There should be less than ±20mV DC at the output. You can adjust this using VR1, to get a reading close to 0V. Now connect your multimeter across the 390W 5W resistor across fuse F1, and adjust trimpot VR2 clockwise to obtain 30V. This provides a total quiescent current of 77mA or about 13mA per output transistor. Now measure the voltage across the other 390W 5W resistor in place of fuse F2. It should be within 10% of the reading across F1. Fig.14: the front panel label (shown at 85% actual size) can be used as a template to drill the holes for the power switch and the clipping indicator LED. Or, download a copy from the June 2023 page of the PE website at: https://bit.ly/pe-downloads and print it on overhead transparency film or photo paper (laminated after printing) and affix it to the front of the Amplifier. This label only covers the left-hand half of the panel, as it would be too wide to easily print otherwise. The 120mm PWM fans for the 500W Amplifier are attached via a metal bracket on the base of the case. These types of fans are quite common in computers, and be purchased at low cost. Smaller fans (eg, 80mm) could be used, but they will probably be louder and, due to how common 120mm fans are, likely more expensive too. 28 Practical Electronics | June | 2023 It’s important to check the winding colours for the transformer, as your transformer might not match the colours we’ve used here. You now need to leave the Amplifier running for at least an hour. This will allow it to warm up gradually. Measure the voltage across the 390W resistors again and readjust VR2 to give 30V. Troubleshooting If these voltages cannot be realised, switch off the power and recheck your construction and wiring. You will need to measure voltages around the Amplifier Module to see where there could be a problem. To do this, first reduce the quiescent current by turning VR2 anti-clockwise so that there is minimal voltage across the Reproduced by arrangement with SILICON CHIP magazine 2023. www.siliconchip.com.au 390W resistors that are across each fuse holder. First, check for +80V, 0V and −80V at the Amplifier Module supply terminals. Check the voltages across the 470W resistor at Q5’s emitter and the 39W resistor at Q7’s emitter. If these aren’t between 0.6 and 0.8V, check transistors Q5 and Q6 for about 0.60.7V between the base and emitter of each. If not correct, verify that they are the right transistor types. Additionally, the voltages across the 68W emitter resistors for Q3 and Q4 should be about 50-60mV each, and both voltages should be the same provided VR1 is adjusted for minimum output offset. If these are not correct, check the transistors for the correct type. If the correct transistors are in place, but the voltages are incorrect, consider replacing these transistors with reputable brand-name devices. Once the problems are found/fixed, you can adjust the quiescent current again for 30V across the 390W resistors. When it all checks out, power it down, wait for the capacitor banks to fully discharge, then remove the fuses with the 390W resistors attached and install the correct ceramic fuses; 5A for use with 8W speakers or 10A for 6W or 4W speakers. Finally, follow the instructions for setting up the Fan Controller & Loudspeaker Protector in the February 2022 issue. Fig.15: the wiring details for the XLR socket. For home use, an RCA socket could be fitted instead, in which case you could even use a panelmount female-female RCA socket. A standard RCA cable can then connect from the inside of this socket to the Amplifier Module input, thereby avoiding soldering. Practical Electronics | June | 2023 29 The completed 500W Amplifier with its vented lid attached. The functions of the three connections on the rear of the case can be made more obvious by printing out small labels. The 500W Amplifier chassis, as presented, is designed to operate with a reasonable amount of free air above the case, as the fans draw in cool air and exhaust hot air through the substantial vent area in the lid. If it is installed in a constricted space, such as an equipment rack or cabinet without much space above the lid, modifications need to be made, especially if it’s run flat-out. Airflow in a rack can be increased by expanding the small holes on either side of the case in front of and behind the fans. Internal ducting may also be required to prevent hot air recirculation. 30 Practical Electronics | June | 2023