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Building the 500W Audio Power Amplifier Last month, we presented the circuit details of this new 500W amplifier and in this issue we present it in its final form, in a large chassis with its brute force power supply, loudspeaker protector module and fan-forced cooling. Pt.2: By LEO SIMPSON & BOB FLYNN I T IS ONE THING to see a large PC module with 14 power transistors on the heatsink and quite another to see that module assembled into a chassis with the necessary power supply, cooling fan and all the other hardware. Whichever way you look at it, this amplifier’s a brute. It is brutishly heavy, it has a brute force power supply and pity help the loudspeakers that can’t handle its brute output. The amplifier chassis is a 3-unit high rack-mounting case which is 406mm deep, not including the handles and the loudspeak­er terminals. The front panel is bare except for the handles and the large illuminated power switch. On the rear panel is the 120mm cooling fan, XLR input socket, loudspeaker terminals, mains fuse and 3-pin IEC power socket. Inside the chassis, the large toroidal 54  Silicon Chip transformer is mounted at the front with the bank of filter capacitors running down the lefthand side. The amplifier module is mounted on the righthand side but the single side finned heatsink is enclosed totally within the chassis. Fan cooling To ensure effective fan cooling, the heatsink is raised off the floor of the chassis by 5mm. The diagram of Fig.1 shows how this is done – the heatsink is shown in end elevation. This allows the fan to force air under the amplifier module and heat­sink base and then up through the fins to the ventilation slots in the lid of the case. The result is an effective cooling system with no ugly fins outside the case. This is big advantage when the amplifier has to be moved frequently, as it will be if September 1997  55 it is used for band and disco work. Now let’s move on to the assembly details for the amplifier module. Fan-Cooling Airflow Fig.1: to ensure effective fan cooling, the heatsink is raised off the floor of the chassis by 5mm. This allows the fan to force air under the amplifier module and heatsink base and then up through the fins to the ventilation slots in the lid of the case. Fig.2 (below): the component overlay for the PC board. Note that the U-shaped heatsinks for Q6 & Q8 should be left off until the output transistors are mounted on the heatsink. 56  Silicon Chip PC board assembly Because there are so many power transistors and because they need a practical spacing between each one, the PC board is quite long at 362mm and is 99mm wide. It is coded 01208971. All the power transistors mount along one edge and are fixed to a large single-sided heatsink which is made in two parts, each 200mm long and 118mm high. These are tied together with a small fishplate at the top to make one long heatsink assembly. Fig.2 shows the component overlay for the PC board. The suggested procedure for assembling the PC board involves mounting all the small components first and then the power transistors, although there are a few wrinkles along the way. The first step is to check the PC pattern for any defects such as broken tracks or undrilled holes. Fix any defects before proceeding and then install all the wire links. Then install the diodes and zener, making sure that they are installed with cor­rect polarity and don’t confuse the 1N914s with 1N4936s or the zener diode. Next, insert the small resistors (not the wirewounds) and the capacitors. Make sure that the electrolytics are all installed the right way around on the PC board. Note that the 100pF ceramic capac- itor between the collector of Q8 and the base of Q9 has a rating of 500V. In practice, it does not have to be that high but it does need to be more than 200VW. The rating of 275VAC for the 0.15µF capacitor in the output filter might seem excessive but anything less than 250VAC would be very risky. A lower rated capacitor could be blown when the amplifier is delivering high power at high frequencies. The 6Ω 3W resistor in the output filter is made up of three 18Ω 1W resistors in parallel. Choke L1 is wound with 21.5 turns of 1mm enamelled copper wire on a 13.7mm plastic former (see parts list). When installing the fuse clips, note that they each have a little lug at one end which stops the fuse from moving. If you insert the fuse clips the wrong way around, you will not be able to fit the fuses. A 390Ω 5W resistor is installed in parallel with each of the power supply fuses. These resistors serve no purpose when the amplifier is working normally but they are used when the quies­cent current is initially adjusted (without the fuses fitted). Next, mount all the small transistors; ie 2N5401s, BC5XX series and MJE340/350s. Don’t install Q9 at this stage as it will be mounted on the heatsink. Both Q6 and Q8 will be fitted with U-shaped heatsinks but these should not be fitted until all the output transistors are mounted on the main heatsink; otherwise the U-shaped heatsinks will just get in the way. Note that the transistor pairs Q1/ Q2 and Q4/Q5 are thermal­ly bonded; the pairs are mounted on the board so that their flat surfaces are touching. The thermal bonding is assisted with a smear of heatsink compound. Solder in one of the pair so that it is angled very slight­ly towards where its mate will be and then smear a little heat­sink compound over its flat surface. Then solder in the collector and emitter of its mate and push the two together before solder­ ing the base lead, to lock the two transistors together. Repeat this process for the other pair of transistors. Incidentally, note that the pinouts of the 2N5401s are reversed from those of the BC546s; don’t insert the wrong tran­ sistors in the wrong positions, otherwise they’ll blow as soon as the amplifier is turned on. Mounting the power transistors Before the power transistors can be mounted on the main heatsink, it must be fully drilled and tapped where necessary. Each transistor is secured to the heatsink with an M3 machine screw. Fig.3 gives the full details of the heatsink dimensions, the hole sizes and so on. As previously stated, this heatsink comprises two parts measuring 200mm long by 118mm high. The two parts are attached at the top by a small aluminium fishplate. Fig.3 also shows the drilling detail for the righthand end of the heatsink assembly. Two holes (B on diagram) are drilled to clear the heads of the handle on the front panel. The way to mount the 14 power transistors is as follows. First, attach each power transistor to the heatsink using an M3 (3mm) screw, washer and silicone heatsink pad. The details are shown in Fig.4. Make sure that each transistor is straight and parallel to its neighbour. All the transistor leads should be straight and parallel as well. This done, sit the heatsink on a flat surface and “introduce” the PC board to the power transistor leads. The PC board should be at rightangles to the heatsink. Make sure that each trio of transistor leads pass through their respective holes in the PC board. Push the board onto the leads so that the bottom surface of the board is 10mm from the lower edge of the heatsink. Make sure that the PC board is paral­lel to the heatsink and then solder one lead of one transistor at each end of the board. This done, recheck that the board is oriented exactly as you want it and then tighten all the transistor mount­ing screws. Finally, solder all the remaining transistor leads and cut the excess pigtails off. Q9, the MJE340 Vbe multiplier transistor, is mounted on the heatsink instead of on the PC board as pictured in last month’s article. The reason we have mounted it on the heatsink is that it gives slightly better thermal compensation of the quies­ cent current. Accordingly, Q9 is mount­ed on the heatsink with the same details as in Fig.4 and connected to the PC board with three flying leads. Note that each lead should be sleeved to prevent any possibility of shorts. The other visible difference between the amplifier module pictured last month and the way it is finally shown in the chas­ sis involves the wiring of the temperature sensor on the heat­sink. This is not wired to the amplifier PC module but connects to the loudspeaker protector board. We’ll talk about this later. Next, fit the U-shaped heatsinks to transistors Q6 & Q8 and the amplifier module is essentially finished apart from mounting it in the chassis. Chassis assembly Quite a lot of work has to be done to the chassis before any componentry September 1997  57 possibility of shorts and to make connections easy; it can be quite awkward trying to make speaker connections with heavy wires when the terminals are close together. The general layout of the components in the chassis is shown in Fig.5 and this shows all the wiring as well. Most of the work in the chassis involves the power supply and its heavy duty wiring. The circuit of the power supply is shown in the diagram of Fig.6. The chassis mount mains fuse is a 5-amp slow blow type. This is most important because a standard quick blow fuse will fail at the first switchon because of the high inrush currents into the 800VA toroidal power transformer. This is compounded by the fact that the 80,000µF filter capacitor bank will also have a very high inrush current at switch-on. In fact, it is normal to see the 240VAC lights on the same circuit momentarily dim when the power supply is switched on. Instead of using a 3-core power flex anchored with a cordgrip grommet and so on, we have used an IEC power socket. The Active wire from the power socket goes to the fuse and then the Active and Neutral are twisted together and run to the DPDT rocker switch on the front panel. It is important to wire this switch the right way around otherwise the neon illumination will be on, regardless of whether the amplifier is on or off. The mains wires from the power switch run to an insulated 3-way terminal block which also accommodates the .01µF 275VAC suppression capacitor which is wired directly across the trans­former primary. The 240VAC wires to the fan also connect to this terminal block but should not be connected at this stage. Before you mount the terminal block, place a piece of Presspahn (or Elephantide) measuring about 30 x 40mm between it and the chassis. This will prevent any possibility of flash­overs to chassis. Fig.3: these are the drilling details for the heatsink assembly. It two parts measuring 200mm long by 118mm high and these are attached at the top by a small aluminium fishplate. can be installed. All the holes need to be drilled for all the hardware, the circular cutout made for the fan and various cutouts made for the power 58  Silicon Chip switch, the XLR and IEC sockets and the mains fuse. Note that the heavy duty speaker terminals are mounted 40mm apart. We did this to reduce the Transformer mounting Mounting the toroidal transformer in the chassis does pres­ent a problem because of the large securing bolt. Because we have used a rack-mounting chassis and because it must be assumed that at least some users will want to mount the amplifier in a rack, the bolt head cannot protrude from the This chassis view clearly shows the revised mounting details for Q9, the MJE340 Vbe multiplier transistor. The transistor must be mounted as shown in Fig.4 and is connected to the PC board via three flying leads. base panel of the chassis. To solve this problem, we mounted the transformer on a separate panel within the chassis but raised on suitable pillars to clear the bolt. By the way, because of the weight of the transformer, the base panel should be made of steel at least 1.6mm thick. When mounting the bridge rectifier, smear a little heatsink compound on the mating surface and the chassis, to improve heat transfer. All the wiring to and from the filter capacitors should be run in heavy duty hookup wire while the busbars connecting all the filter capacitors together should be made with two strands of 16 gauge tinned copper wire. Note that the whole amplifier has single-point earthing so it is important to follow the wiring details of Fig.5 quite closely. Note also that the transformer wiring runs down the side of the transformer, behind the power switch. This is important because these heavy wires can otherwise radiate rectifier buzz into the amplifier module. Incidentally, toroidal transformers do have a significant hum leakage field and it is important to orient them for minimum hum pickup by the circuit. The orientation shown in the photos is close to optimum for this circuit. Both sides of the filter capacitor bank have two 15kΩ 1W bleed resistors connected across them. As well, a red LED is connected across each side of the capacitor bank in series with a 15kΩ resistor. The LEDs indicate when voltage is present across the capacitors, and as you will find, even with the bleed resis­tors connected, it takes quite a while for the capacitors to discharge after the amplifier is turned off. Safety precaution After the power supply wiring is complete and before you apply power, we suggest that you mount a clear Perspex sheet over the bank of capacitors. Fig.4: attach each power transistor to the heatsink using an M3 (3mm) screw, washer and silicone heatsink pad. September 1997  59 Fig.5: this diagram shows the general layout of the components in the chassis and all the interconnecting wiring. Take care with the mains wiring and note that the 160V DC developed across the filter capacitor bank and the amplifier supply rails is potentially lethal – see text and warning panel. 60  Silicon Chip Fig.6: this is the circuit of the power supply. The 5-amp fuse must be a slow blow type to cope with the switch-on surge currents. This will prevent accidental contact with the 160V DC supply. Note that the full DC supply is potentially lethal! The Perspex shield will also provide a degree of safety if one of the capacitors suddenly overheats and vents to the atmosphere! At this stage, the power supply wiring is complete but the amplifier module and the loudspeaker protection module is not installed. Now apply power and check that the supply voltage is close to ±80V. Both LEDs should light. Then switch off and allow the capacitors to completely discharge. This will take about 10 minutes. adjust the locknuts so that the top of the heatsink is level with the top of the case sides. When the lid is finally installed on the case, the heatsink is prevented from moving by the countersunk screws which secure it to the lid. Connect the XLR input socket to the amplifier module via shielded cable Mounting the amplifier module 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 that the capacitors take some time to discharge after the power is switched off. As noted above, Fig.2 shows the scheme for mounting the amplifier module. You will need the four M3 countersunk heatsink support screws in place and the two pillars which support the front of the PC board. The heatsink should have four clearance holes drilled in the lower edge (see Fig.3) to mate with the support screws. What happens is that locknuts are fitted to each of the four support screws and then the heatsink merely sits on top of the nuts. You need to Fig.7: follow this diagram when wiring the XLR input socket. Note that shielded microphone cable is used to make the connections to the amplifier module. as shown in Fig.7. Do not make any connections to the amplifier output at this stage and do not install the loud­speaker protection module. Make the positive and negative 80V supply connections to the amplifier module. Switching on 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 article. This done, remove fuses F1 and F2 on the amplifier module. The 390Ω 5W resistors across the fuses should be in place and trimpot VR2 should be rotated fully anticlockwise. Apply power and measure the voltages shown on the circuit featured last month (Fig.1, page 26). There should be less than ±20mV DC at the output. Now connect your multi­meter across the 390Ω 5W resistor (across fuse F1) and adjust trimpot VR2 to obtain 30 volts. This provides a total quiescent current of 77mA or about 13mA per output transistor. Now measure the voltage across the other 390Ω 5W resistor (across fuse F2). It should be within 10% of the value across F1. You now need to leave the amplifier running for at least an hour. This will allow it to gradually warm up. Measure the voltage across the 390Ω resistors again and adjust VR2 to again give 30V. Next month, we will provide details of the loudspeaker protection module (based on the article in the April 1997 issue of SILICON CHIP) and will include the artwork for the main PC board. We also intend to describe a temperature operated switch for the fan so that it will only operate when needed. SC September 1997  61