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Pt.2: By LEO SIMPSON & PETER SMITH Building the: Studio 350 Power Amplifier Module Last month, we introduced our rugged new 350W power amplifier module and gave the circuit details. This month, we show you how to build it and describe a matching power supply. T O HELP ENSURE that everything goes together without a hitch, it’s a good idea to read the following information in its entirety before reaching for your soldering iron! 22  Silicon Chip Referring to the overlay diagram in Fig.1, begin by installing all the wire links. There are 15 links in total, 11 of which must be formed from 1mm tinned copper wire. Use 0.7mm wire for the remaining four links. The over- lay diagram shows the larger (1mm) links in red. Set aside all of the heatsink-mounted transistors (Q4-Q17), the two 470µF electrolytic capacitors, choke (L1) and 6.3mm spade lugs for the moment. We’ll deal with these in more detail shortly. All other components can now be installed, progressing from smallest to largest. The 1W and 5W resistors should be mounted about 1mm proud of the PC board to aid heat dissipation. Also, be sure to orient the cathode (banded) ends of diodes D1-D5 as shown. www.siliconchip.com.au When installing the fuse clips, note that the small retaining lug on each clip must be positioned to the outer (fuse end) side, otherwise fuse installation will be impossible. If you intend mounting the output transistors horizontally, then it’s also necessary to install 3-pin header strips in the mounting positions for Q8 & Q9. As we’ll see shortly, these are required because the transistor leads are too short to extend all the way through the PC board holes. TO-220 heatsinks Transistors Q4, Q5 & Q6 must be attached to TO-220 heatsinks before fitting them to the PC board. First, smear a thin film of heatsink compound to both the rear (metal) area of each transistor as well as the mating areas of the heatsinks. That done, fasten them to the heatsinks using M3 screws, nuts and washers (see Fig.2) but don’t fully tighten the screws just yet. Note that insulating pads are not required here. Now slip each assembly into place in its PC board holes, taking care not to mix up the BF469 and BF470 types. The tabs of the heatsinks should fully engage the holes in the PC board, such that all of the heatsink edge contacts the PC board surface. Finally, push the transistors all the way down the slots in the heatsinks and then tighten up the screws. The transistor leads can now be soldered, taking care that the assemblies remain in place when the board is turned over. Winding the choke If you’ve building your amplifier module from a kit, the 6.8µH choke may have been supplied pre-wound. If so, all you’ll need to do is scrape the enamel insulation off the wire ends, tin them and solder the part in place. Alternatively, it’s a relatively simple matter to wind the choke yourself. You’ll need a 13mm I.D. plastic former (bobbin) and about three metres of 1mm enamelled copper wire. Begin by bending the wire at right angles, about 10mm from one end. This will be the starting end. Slip it into the bobbin and position the end in one of the slots. Now wind on 23.5 turns as evenly and tightly as possible, then pass the remaining wire length out through the opposite slot and cut off any excess, leaving about 10mm protruding. Finally, wind on a couple of turns of www.siliconchip.com.au Parts List 1 PC board coded 01102041, 136mm x 241mm 1 6.8µH air-wound choke (L1) (see text) 1 2-way 2.54mm terminal block (CON1) 2 3-way 2.54mm pitch SIL headers (for Q8 & Q9) 3 TO-220 heatsinks, 25mm x 12.5mm with PC board tabs 1 diecast heatsink, 300 x 75mm, 35mm shelf (0.4°C/W or better) 8 TO-3P or TO-264 siliconebased insulating pads 2 TO-220 silicone-based insulating pads & washers 1 TO-126 silicone-based insulating pad 350mm (approx.) 1.0mm tinned copper wire for links 70mm (approx.) 0.7mm tinned copper wire for links 4 M205 PC-mount fuse clips (F1, F2) 2 M205 5A slow-blow fuses 5 6.3mm chassis-mount spade lugs Semiconductors 1 BC556 PNP transistor (Q1) 2 2SA1084 PNP low-noise transistors (Q2,Q3) 2 BF469 NPN transistors (Q4, Q5) 1 BF470 PNP transistor (Q6) 1 MJE340 NPN transistor (Q7) 1 MJE15030 NPN transistor (Q8) 1 MJE15031 PNP transistor (Q9) 4 MJL21194 NPN transistors (Q10, Q12, Q14, Q16) 4 MJL21193 PNP transistors (Q11, Q13, Q15, Q17) 3 1N4148 small-signal diodes (D1-D3) 2 1N4936 fast-recovery diodes (D4, D5) Capacitors 2 470µF 100V PC electrolytic (Farnell 319-9149) 1 47µF 16V non-polarised PC electrolytic 1 1µF 16V non-polarised PC electrolytic insulation tape to hold everything in place. You can now test-fit the assembly in position, bending the leads as nec- 10 220nF 100V MKT polyester 1 150nF 250V MKT polyester 1 100nF 63V MKT polyester 1 12nF 100V MKT polyester 1 330pF ceramic disc 1 68pF 250V ceramic disc (or mica) (Farnell 867-871) 1 10pF ceramic disc Resistors (0.25W 1%) 2 22kΩ 1 1kΩ 1 18kΩ 1 680Ω 1 15kΩ 1W 1 470Ω 1 6.8kΩ 1W 10 100Ω 2 4.7kΩ 1 10Ω 1 2.2kΩ Wirewound resistors 2 470Ω 10W wirewound (for setup) 1 6.8Ω 5W wirewound 8 0.47Ω 5W wirewound Trimpots 1 200Ω 25-turn miniature horizontal trimpot (VR2) 1 100Ω 25-turn miniature horizontal trimpot (VR1) Screws & nuts 8 M3 x 20mm pan-head screws 3 M3 x 15mm pan-head screws 3 M3 x 10mm pan-head screws 14 M3 nuts 28 M3 flat washers 5 M4 or 3BA x 10mm pan-head brass screws 5 M4 or 3BA brass nuts 10 M4 or 3BA internal star washers (brass or stainless steel) Power supply 1 50V+50V 500VA toroidal mains transformer (Altronics Cat. M-5750) 1 35A 400V chassis-mount bridge rectifier 6 8000µF 75V chassis-mount electrolytic capacitors (Altronics Cat. R-6722) 2 470nF 100V MKT polyester capacitors 4 15kΩ 1W resistors essary to get the bobbin to sit down on the PC board surface. That done, scrape the enamel insulation off the wire ends with a scalpel blade or February 2004  23 24  Silicon Chip www.siliconchip.com.au Fig.1: use this diagram when assembling and wiring the amplifier module. The ±70V wiring is routed underneath the board and attached to it with cable ties once testing is complete. yet to be installed are the two 470µF electrolytic capacitors. These can go in now, with an eye to correct orientation. Take particular care here, as they’re oriented differently to one another. If you get one the wrong way around, it will be damaged at power up and may even explode! Horizontal heatsink mounting The amplifier module was designed for mounting to the horizontal shelf of a diecast heatsink. However, a verticalmounting configuration is also possible – see the panel entitled “Using Different Heatsinks” for a discussion of this alternative method. We recommend an Altronics 300mm diecast heatsink with 35mm shelf (Cat. H-0452), as used on the prototype. So let’s look at how the PC board and transistors are attached to this heatsink. The only guaranteed way of getting all the heatsink holes in the right places is to use the PC board as a drilling template. First, find the smoothest side of the heatsink shelf and place it upwards. That done, position the PC board on the top of the shelf and butt it right up against the main body of the heatsink, centred left to right within the available space. Next, making sure that nothing moves (clamp the board to the shelf if necessary), use a sharp pencil to mark through all 11 transistor mounting holes. Be sure to mark a clean circle around the circumference of each hole, so that you’ll easily be able to find the centre. Remove the PC board and gently centre-punch your marks before drilling. A strip of cardboard cut to the correct width (7mm) makes a handy bending guide for the leads of the heatsinkmounted transistors. similar and tin them before soldering the choke permanently in position. Lug terminations Except for the audio line input, all connections to the PC board are made via 6.3mm spade lugs. If the lugs are double-ended, then cut off one end using electrician’s sidecutters. Position each lug as shown on the overlay diagram and fasten it securely to the PC board using the method depicted in Fig.3. We recommended raw brass (rather than nickel-plated) screws and nuts for securing the lugs. As noted in several of our recent high-power amplifier designs, these return a slightly lower distortion figure at the high-power end of the spectrum. Apart from the main heatsinkmounted transistors, the only parts Fig.2: transistors Q4-Q6 must be attached to TO-220 heatsinks as shown here. Insulating pads are not necessary, but you should apply heatsink compound to the mating surfaces. Fig.3: here’s how to bolt up the spade lugs. If you have doublesided lugs, cut off one side with heavy-duty sidecutters first. Tighten them up enough so that they don’t move around when the receptacles are pushed on. Initially, drill a pilot hole at each mark, using a 1mm bit. Finish with a 3.3mm bit, then deburr the holes by hand using a much larger drill size. Both sides of the shelf must be completely free of swarf and sharp edges. Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o No. 2 1 1 1 2 1 1 1 1 10 1 2 1 8 www.siliconchip.com.au Value 22kΩ 18kΩ 15kΩ 6.8kΩ 4.7kΩ 2.2kΩ 1kΩ 680Ω 470Ω 100Ω 10Ω 470Ω 10W 6.8Ω 5W 0.47Ω 5W 4-Band Code (1%) red red orange brown brown grey orange brown brown green orange brown blue grey red brown yellow violet red brown red red red brown brown black red brown blue grey brown brown yellow violet brown brown brown black brown brown brown black black brown not applicable not applicable not applicable 5-Band Code (1%) red red black red brown brown grey black red brown brown green black red brown blue grey black brown brown yellow violet black brown brown red red black brown brown brown black black brown brown blue grey black black brown yellow violet black black brown brown black black black brown brown black black gold brown not applicable not applicable not applicable February 2004  25 Fig.4: the mounting details for the TO-126 (Q7) and TO-264 (Q10-Q17) transistors. Don’t solder the leads until the screws have been tightened to their final torque. Fig.5: the leads of the TO-220 (Q8 & Q9) packages are too short to reach all the way through the PC board. Simply bend the leads so that they touch the header pins instead. Again, don’t solder the leads until the mounting screws have been tightened. Insulated TO126 Packages Transistor Q7 (an MJE340) is supplied in a “plastic” TO-126 pack­age. These packages usually include a small rectangular metal area on the rear. This area is electrically connected to the collector and therefore must be isolated from the heatsink with an insulating washer (see Fig.4). However, some TO-126 packages do not have this metal area – they’re “plastic” on both sides. This isolated type package should be mounted without an insulating washer. Simply smear its mating surface with a small amount of heatsink compound and bolt it directly to the heatsink. By the way, a drill press is mandatory for this job, as drilling accurate holes in thick aluminium with a hand drill is extremely difficult. Attaching the transistors Now position the PC board beneath the heatsink shelf and insert two M3 x 20mm screws in the extreme left and righthand holes. Fit M3 washers and nuts (on the PC board side) and wind them up barely finger tight. The idea here is not to clamp the board against the heatsink shelf too tightly; it must be allowed to move at this stage. These screws are temporary placeholders 26  Silicon Chip and can be removed when necessary. All transistors must be insulated from the heatsink with silicone-based pads. The TO-220 devices (Q8 & Q9) also require insulating bushes for the screws. Figs.4 & 5 shows how to mount each transistor type. As you can see, the leads of each transistor must be bent at right angles before installation. The position of the bend should be placed so that the leads slip easily into the PC board holes while the mounting holes line up with the holes in the heatsink and the PC board underneath. A strip of cardboard cut to the appropriate width makes a handy bending guide (see photo). Mount the TO-126 package (Q7) first, then progress outwards in left and right pairs (Q8 & Q9, Q10 & Q11, etc). The two TO-220 transistors (Q8 & Q9) present a special case. Their leads are not long enough to reach all the way through the PC board holes, so instead must be soldered to the 3-pin headers installed earlier. However, do not solder to the header pins just yet. Simply bend the device leads so that they just make contact with the rear of the header pins. You’ll probably find that you need to trim a little off the leads so that they don’t interfere with the plastic base of the header strips. Wind up the nuts only finger tight during installation. Once they’re all in place, go back and tighten each one to the final torque, starting in the middle and working towards the sides. Don’t overtighten – about one click of the elbow is more than enough! That done, set your meter to read Ohms and measure between the heatsink and the centre lead (collector) of Although featuring a different amplifier module, this picture shows the vertical mounting method for the output and driver transistors. www.siliconchip.com.au Fig.6: the power supply wiring is quite straightforward. Take particular care that you have the positive (+) and negative (-) terminals of the capacitors connected as shown. The same goes for the bridge rectifier, also noting that it must be bolted firmly to a metal surface for heatsinking. Note the safety warning. each device. You should get an open circuit reading in all cases. If everything checks out, then solder all transistor leads to complete the assembly. Note that the mounting screws must be tightened up before soldering the leads. If this is done in reverse order, then stress will eventually crack the solder joints and perhaps even delaminate the PC board copper. DANGER: HIGH VOLTAGE! The 100VAC from the transformer secondaries (2 x 50VAC) and the 140V 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 also that the capacitors take some time to discharge after the power is switched off (check the voltage with a multimeter). Vertical heatsink mounting Details for vertical mounting will vary according to the style of heatsink. However, we’ve included a rough guide to get you started. Of course, you must have already modified the PC board as described in the “Using Different Heatsinks” panel! To begin, use what ever you have on hand to raise the PC board to the required mounting height. A pair of 3mm holes is provided at the rear of the board for tapped spacers but you’ll also need to place something under the front of the board to bring it back to the horizontal position. Next, fit the 11 transistors (Q7Q17) into their respective mounting holes but don’t solder or cut any of their leads just yet! That done, butt the assembly up against your chosen heatsink and centre it roughly within the available space. Note that the transistors should be mounted as close to the centre of the heatsink as practical although this will be affected by the www.siliconchip.com.au available transistor lead length. If possible, line up the transistors so that the mounting holes will fall between the heatsink’s cooling fins. This way, you can avoid the additional task of thread tapping. Once you’re happy with the positioning, mark through each transistor mounting hole with a sharp pencil. Now centre-punch each mark and drill 1mm pilot holes. Redrill to 3.3mm if you’ll be using screws with nuts, or use a smaller, 2.5mm bit size in preparation for M3 thread tapping. After drilling, deburr the holes by hand using a much larger drill size so that the mating surface is entirely smooth. Attaching the transistors Loosely attach the transistors to the heatsink using insulating pads and bushes where necessary. The requirements here are similar to those shown for horizontal mounting as shown in Figs.4 & 5. Be sure to check that the PC board is sitting horizontal and at right angles to the heatsink before tightening up the screws. It’s then just a matter of turning the assembly over and soldering all transistors in place. Finally, it’s a good idea to make sure that all transistor collectors are indeed isolated from the heatsink. To do this, set your meter to read Ohms and measure between the heatsink and the centre lead (collector) of each Table 2: Capacitor Codes Value 220nF 150nF 100nF 12nF 330pF 68pF 10pF μF Code 0.22µF 0.15µF 0.1µF 0.012µF    –    –    – EIA Code   224   154   104   123   331    68    10 IEC Code 220n 150n 100n   12n 330p   68p   10p February 2004  27 Using Different Heatsinks As shown in the various photos, the transistors on our prototype are mounted horizontally, on the shelf of a large diecast heatsink. This method of mounting is mechanically robust and relatively easy to assemble but obviously unsuitable for heatsinks without a shelf. Suppose, for example, that you’ve decided to build a stereo unit, utilising a pair of Jaycar’s fan-cooled tunnel heatsinks (Cat HH-8532). In this case, the transistors must be mounted vertically along the edge of the PC board, allowing them to be bolted directly to the heatsink faces. With just one modification, the PC board can accommodate this alternative, vertical mounting style. This modification involves cutting off a portion of the PC board so that the transistors are just a few millimetres from the PC board edge. This must be done before any components are mounted on the PC board! A thin broken track has been included on the PC board as a cutting guide. Note that there should be about 0.5mm of space between the pads/tracks and the board edge. This ensures that once the unit is assembled, the bare copper tracks can not short out on the face of the heatsink. For this reason, we suggest cutting along the device. You should get an open circuit reading in all cases. Power supply assembly Due to the weight of the mains transformer, the power supply components must be mounted on a substantial metal baseplate. Typically, this will be the base of a rack-mount case or similar. If deemed necessary, the base can be strengthened with an additional plate to achieve sufficient rigidity. The suggested wiring for the bridge rectifier (BR1) and capacitor bank is shown in Fig.6. The bridge rectifier Fig.7: the mains earth should be securely attached to the base of the metal chassis as shown here. Tighten the first nut very firmly before winding on the second “locknut”. The earth wire from the capacitor bank also connects to this point. 28  Silicon Chip Fig.8: to enable vertical transistor mounting, cut off the entire front section of the PC board as shown here. You do not need to do this for the horizontal mounting style shown in the various photographs! outside of the line, to allow for the width of the cut and any subsequent filing (see Fig.8). must be attached directly to a flat area of the metal chassis for heatsinking. Smear the face of the rectifier and the contact area with a thin film of heatsink compound before assembly. The 8000µF capacitors are attached to the baseplate using circular clamps. They should be positioned as close together as practical, with their terminals in line to allow hookup with lengths of solid-core wire. Use two strands of 0.7mm tinned copper wire or similar for a total wire diameter of at least 1.4mm for each connection. If you only ever intend driving 8Ω speakers, the filter capacitor count can be reduced by two for a worthwhile saving. For 4Ω speakers, the full complement of six capacitors is required to achieve the listed power and distortion figures. Connections to and from the capacitor bank should be made with extraheavy duty (10A) multi-strand cable. The +70V, -70V and 0V wires leading away from the bank should be twisted tightly together to minimise radiated noise and improve appearance. Safety precautions Before applying mains power, the capacitor bank must be covered with a rigid, non-conductive shield. A section of clear perspex is ideal for the Where To Get The Parts Kits for this amplifier project will be available from Altronics and from Jaycar Electronics. Check out their websites at www.altronics.com. au and www.jaycar.com.au for further details. Individual items can be obtained from the usual kit suppliers, including DSE, Altronics and Jaycar. The 2SA1084 low-noise transistors are available from WES Components, on the Internet at www. wescomponents.com or phone (02) 9797 9866. Parts shown with a Farnell catalog number can be ordered on-line at www.farnellinone.com.au or phone 1300 361 005. job. This step is very important, as simultaneous contact with the +70V & -70V rails could easily kill you (or someone else)! Note also that the 100VAC produced by the transformer secondaries (2 x 50VAC) is also potentially lethal, so don’t get across these windings. As shown on the wiring diagram, four 15kΩ 1W resistors must be inwww.siliconchip.com.au stalled across the ±70V rails. These will gradually discharge the capacitors after power is switched off. However, before working on any part of the circuit, always measure the supply rails with a multimeter first to make sure that it is safe to do so. Wiring Housing and wiring of the amplifier modules is totally up to you. However, we’ve outlined a few points below that will help you to get the most from your amplifier. First, never take shortcuts with mains wiring. Always use mains-rated cable and be sure to insulate all exposed connections. This includes the use of rubber boots (or equivalent) on the rear of IEC sockets, switches and fuseholders. The mains earth must be connected to the metal chassis using the arrangement shown in Fig.7. Return all earth wires to this point to eliminate potential earth loops. Use extra heavy-duty (10A) multi-strand cable (or larger) for all power and speaker connections. The wire ends need to be terminated with 6.3mm push-on receptacles to suit the board-mounted lugs. These are available in insulated and noninsulated varieties. For the insulated type receptacles, you’ll need a ratchet-driven crimping tool, such as the Altronics T-1552, Jaycar TH-1829 or DSE T-3535. Don’t be tempted to use a cheaper (non-ratchet style) crimper, as they’re just not up to the job. If you don’t want to cop the expense of a new crimper, then you can use the non-insulated style receptacles and solder them on instead. These are available from DSE (Cat. H-5012) and most electrical wholesalers. While you’re at it, get some terminal covers to suit (Cat. H-5022). Supply wiring The +70V, -70V and 0V connections to the amplifier module should be twisted tightly together and positioned as shown on the overlay diagram. Note how the www.siliconchip.com.au February 2004  29 This is what the completed amplifier module looks like. Be sure to mount the 5W wirewound resistors about 1mm proud of the PC board, to allow the air to circulate beneath them for cooling. The spare holes in the PC board allow the supply power wiring to be secured in position using cable ties. 0V wire connects to the centre lug, whereas the ±70V wires continue beneath the PC board. Small cable ties are then used to secure the wires in place underneath the PC board. Positioning the wires as shown helps to cancel the fields resulting from currents flowing in the PC board tracks. This produces the lowest possible signal distortion. Setup & testing With nothing connected to the power supply output, apply mains power and measure the positive and negative rails. Both readings should be close to the 70V mark, depending on mains fluctuations. The next task is to zero the amplifier’s input offset voltage and set the quiescent current in the output transistors. To protect the amplifier in case of faults and to simplify adjustment, remove both fuses from the board and solder a 470Ω 10W resistor across each 30  Silicon Chip fuseclip pair. Alternatively, you may find it easier to tack solder the resistors on the rear (copper) side of the PC board. Note that nothing should be connected to the input or output terminals until these checks are complete. Set VR2 fully anticlockwise and then apply power. With your multimeter set to read millivolts, measure the voltage across the output (speaker) terminals. Adjust VR1 for a reading of 0V ±2mV. That done, set your meter to read 70V or more and measure the voltage across one of the 470Ω 10W resistors. It’s not important which one you choose. Rotate VR2 clockwise until you get a reading of 47V. This gives a total quiescent current of 100mA. Now give the amplifier about 10 minutes to warm up, then readjust VR2 if necessary. It’s normal for this reading to vary by a few volts as circuit temperature varies. To check that each output transistor is doing its job, you can measure the voltages across the 0.47Ω emitter resistors. With about 25mA flowing in the emitter legs, you should get a reading near 11mV across each of these resistors. Note that the innermost pair of resistors also carry the driver transistor (Q8 & Q9) emitter current, so these two will read a few millivolts higher. Problems? If you’re unable to adjust VR1 or VR2 for the specified readings, then there is a fault somewhere on the board. We’ve provided voltage readings for various points on the circuit that may help you to track down the problem (see Fig.7, Pt.1). Your readings should fall within ±10% of our listed values. If everything checks out OK, switch off the power, remove the 470Ω resistors and install the fuses. That’s it – your Studio 350 Amplifier is now SC ready for use! www.siliconchip.com.au