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Stereo Class-A Amplifier; Pt.5 By GREG SWAIN Ch as s is as s em b l y, w ir in g & a d j us t m en t de t a ils 66  Silicon Chip siliconchip.com.au In this article, we show you how to build a high-performance 20W Class-A Stereo Amplifier using the modules described over the last few months. The unit is available as a complete kit from Altronics and the assembly is straightforward. I N THE MAY & JUNE 2007 issues, we published the circuit and assembly details for our new high-performance 20W Class-A Stereo Amplifier modules, along with a suitable Power Supply module. Then in the July issue, we described a Speaker Protection & Muting module and followed that up in August 2007 with a LowNoise Preamplifier & Remote Volume Control. This month, we show you how to assemble everything into a custommade steel chassis that’s been designed by Altronics. This precision laser-cut chassis is supplied with all the holes drilled and with pre-punched front and rear panels with screened lettering. This case is similar to their “2U” deluxe rack cases (but is much deeper) and features a bevelled front panel. The completed amplifier looks very professional, although at 420 x 425 x 88mm (W X D x H) it’s really quite a siliconchip.com.au large unit. This size is necessary to accommodate the large finned heatsinks used for the power amplifiers and to allow the various modules to be logically placed (and separated) inside the chassis. The large chassis size is also important to aid ventilation, as the main heatsinks run quite hot in operation (about 30° above ambient). In addition, the bottom of the chassis and the lid have large ventilation slots which line up with the heatsink fins, to allow the air to circulate through them. That’s one of the drawbacks of a class-A amplifier – they generate lots of heat that has to be dissipated. OK, let’s assume that you’ve completed all the modules and that you’re ready to mount them in the chassis and install the wiring. Here’s how to go about it. Preparing the case As supplied, the case is finished in a tough powder-coating that’s also a good insulator. However, you must ensure that all sections of the case, including the side panels and the front & rear panels, are correctly earthed and that means ensuring they make good electrical contact with each other. There are two reasons for this: (1) all sections of the case must be connected to the mains earth to ensure safety; and (2) correct earthing is necessary to keep RF interference out of the audio circuitry. The first job is to ensure that the two side panels, the front & rear panels and the lid are all earthed to the bottom section of the chassis. This is done by using an oversize drill to remove the powder coating from the countersunk screw holes. Use a drill that’s slightly smaller in diameter than the screw heads and be sure to remove the powder coating right back to the bare metal. Don’t just do this for one or two holes – do it for all the holes in each panel. Provided you use a drill that’s not too big, the bare metal will later be covered by the screw heads. Next, scrape away the powder coating around the screw holes inside the panels, the underside of the lid and from the matching contact areas around the screw holes in the chassis. This includes the contact areas around the screw holes on the inside folded September 2007  67 OUTPUT + + – – E B C E B B C 17070210 E -NIKPSR C E B -NIKPSL +NIKPSL B B SPEAKER PROTECTOR BOARD + B C E CABLE TIE C E C +NIKPSR +TUOKPSL E C B E C E C C E B CON3 CON2 CON1 ~ RECTIFIER BRIDGE + SPKR– LEFT CHANNEL POWER AMPLIFIER E E B SPKR+ B + +22V C E +22V ~ SPKR– SPKR+ – E B + C C B E + C B E + GN D GND 1N 4148 1N 4148 CON1 LEFT AMPLIFIER INPUT C E B B C C E E B + – 22V + * B C E C B E C B E E B C B E + 1k + 68  Silicon Chip B * 7002 C Installing the hardware You can now start installing the hardware in the case – see Fig.1. Begin by securing the IEC power socket to the rear panel using the two 6g x 12mm countersunk self-tappers supplied. That done, mount the two insulated RCA input sockets and the two loudspeaker terminal panels. Note that the white (left) colourcoded RCA socket goes to the top, while the red (right) socket goes to the bottom. The loudspeaker terminal pairs go in with their red (positive) terminals towards the top and are again secured using 6g x 12mm countersunk self-tappers. The 35A bridge rectifier can go in next. Because it uses the chassis for heatsinking, it’s important to ensure good metal-to-metal contact. It’s mounting area should be completely free of powder coating but if not, mark out the area and remove the powder coating using a small grinding tool. Now smear the underside of the 35A bridge rectifier (BR1) with heatsink compound and bolt it to the chassis using an M4 screw, star washer and RIGHT SPEAKER OUTPUT +TUOKPSR sections of the front panel. That way, when the case is assembled, earthing takes place via the screws themselves and also via direct metal-to-metal contact between the various sections. The transformer mounting bolt must also be earthed and this means that you have to remove some of the powder coating from around the mounting hole on the outside of the chassis (ie, from under the bolt head). The same goes for all other mounting screws that go through the bottom of the chassis. In particular, make sure that you clear away the powder coating from around the six heatsink mounting holes. Once you’ve done all this, remove the front panel, wrap it up and put it to one side, so that it doesn’t get scratched or damaged. It doesn’t take much of an accident to spoil the panel’s appearance while you are installing the parts in the chassis and completing the wiring. LEFT SPEAKER 1N 4148 Fig.1: follow this diagram and the photos to install the parts in the chassis and complete the wiring. Note that the supply leads to the modules, transformer and mains switch are twisted together – see photos & text. –22V EARTHING LUG NOTE: ALL FEMALE SPADE QUICK CONNECTS MUST BE FULLY INSULATED (NOT SHOWN FOR CLARITY) LEFT CHANNEL HEATSINK * ADHESIVE CABLE TIE-DOWN POINT siliconchip.com.au IEC MAINS CONNECTOR (FUSED) * INSULATE FROM L EF T REAR PANEL GRN INPUT BLUE RIGHT INPUT B RN POWER TRANSFORMER G RN NYLON P-CLAMP EARTHING LUGS * GRN SLEEVE WITH HEATSHINK TUBING - SEE TEXT * NYLON P-CLAMP * +22V SPKR– +22V G ND – 22V SPKR+ NYLON P-CLAMP E +2 2V SPKR– B + E E + + B B C C + B E POWER SUPPLY BOARD E RIGHT CHANNEL POWER AMPLIFIER C SPKR+ B + + + SLEEVE SWITCH LEADS WITH HEATSHINK TUBING - SEE TEXT E + B C GN D GND + + R1 1N 4148 B B C C E B E E +22V E B C C E B GN D + C O N1 NYLON P-CLAMP LED1 LE D 2 K – 22V + RIGHT AMPLIFIER INPUT A K A TO AMPLIFIER(S) 1N 4148 B E C + + C –22V B B E +22V C –22V E 100k BEAD 22F NP 1F NP 100nF 22F NP 1F NP X1 REG1 22pF 1k 100F 16V 100F 16V REG3 100F 25V 10F 16V 1k 10 1k D1 RE G 2 LK2 LK 1 22pF 100 PREAMP & VOLUME CONTROL BOARD siliconchip.com.au 22 D2 100 B E AD 22k 560pF 100 25V 220F 1k 1k 1k 10k 1k 100nF 25 V 220F * VR 2 1R 0O 0nTFOM Q2 1k 100nF BEAD 4.7k 4.7k 22F NP Q4 Q1 IC3 PIC16F88-I/P 100k 270pF 100 Q3 CON6 CON5 100nF 560pF 22F NP CON4 IC2 NE5532 4.7k 4.7k 4.7F NP IC1 N E 5532 100k 270pF 100 CON3 100k CON2 100 4.7F NP B E AD CON1 10nF 22k OERETS ESION WOL REIFILPMAERP 17080210 –22V LK4 LK3 18k 100 100F 16V POWER SWITCH RIGHT CHANNEL HEATSINK September 2007  69 Fig.2: the Altronics case will come with the main earth lugs mounted in place. However, if you’re using a different enclosure, here’s how to install the chassis earth point. Two nuts are used to permanently lock the assembly in place. Make sure that it forms a sound electrical contact with the chassis. nut. Fig.1 and the photos show BR1’s mounting location and orientation. It’s mounted with its positive DC output at top right. Chassis earth lugs Fig.2 shows the mounting details for the main chassis earth lugs. This assembly consists of two double-ended quick-connect terminals which are bolted to the chassis using an M4 x 10mm machine screw, two star washers and two nuts. The second nut on top locks the first nut in place, so that there’s no possibility of the earth terminals coming loose. Altronics has indicated that the chassis supplied with the kit will come with the earth lugs mounted in position. However, if you are using a different case, then you will have to remove the powder coating yourself. To do this, temporarily bolt one of the double-ended quick connects to the chassis and use a pencil to outline the contact area. The quick connect can then be unbolted and the powder coating removed using a sharp implement or a small grinding tool. The two double-ended earth lugs can then be bolted in position. Be sure to do the nuts up nice and tight, to ensure a reliable earth. A second earth lug is mounted at the front of the chassis, to the left of the preamp board. This is a single-ended lug and is used to ensure a reliable earth connection for the body of the volume pot. Installing the modules The five PC-board modules can now 70  Silicon Chip be installed in the chassis. Note that these modules are all mounted on 10mm tapped stand-offs, except for the preamplifier module which mounts on three 25mm tapped stand-offs. Begin by mounting the left and right channel power amplifier modules. These should already be attached to the heatsinks and it’s just a matter of lining these up with their mounting holes in the chassis and bolting them into position using M4 x 10mm machine screws and star washers. The star washers go under the heads of the screws and bite into the chassis to ensure that the heatsinks are securely earthed. Note: do not over-tighten these screws. The heatsinks are made of aluminium and it’s all too easy to strip the threads if you are ham-fisted. Once the heatsink screws are in, the spacers fitted to the power amplifier boards can be secured to the chassis using M3 x 6mm screws and flat washers. Loosen off the heatsink screws under the chassis to get everything to line up if necessary, then do the screws up nice and tight. That done, the power supply board and the preamplifier module can be mounted. As previously mentioned, the preamplifier is secured using only three of its four mounting holes – the hole adjacent to the volume control pot is not used. This avoids placing strain on the pot’s soldered joints and in any case is unnecessary, since the pot’s ferrule is also secured to the front-sub-panel. Before mounting the preamp, fit a flat washer over the pot’s threaded ferrule. That done, fit the three 25mm tapped spacers, then slip the preamp board into position and secure it to the front sub-panel by fitting the nut and a shakeproof washer to the pot. Do this nut up firmly but don’t over-tighten it, to avoid stripping the thread. Finally, the three spacers can be secured to the chassis using M3 x 6mm machine screws and washers. The three indicator LEDs and the IR LED on the preamp board all go through a cutout in the front sub-panel. Provided you’ve installed them correctly, as shown in Pt.4 last month, they should all line up neatly with their respective holes when the front panel is later installed. Note that the photos show these parts going through separate holes in the front sub-panel. The chassis supplied for kits will feature a large cut-out in the sub-panel instead. Note also that if the infrared receiver module (IRD1) has a metal shield, then it must be insulated from the front panel (see p22, August 2007). The power supply board is next on the list but first you have to add some extra quick connectors. First, you need to install three extra single-ended connectors at the DC output end of the board, to go with the existing doubleended connectors. That done, install an extra double-ended connector at the GND terminal (to go with the existing single-ended connector) at the input end of the board. Make sure that the screws that hold these connectors in place are done up nice and tight. You will need a screwdriver to hold the head of each screw in place and a ratchet-driven socket to tighten up the nuts. Having added the extra connectors, the power-supply board can be mounted in position. Secure it using M3 x 10mm screws and flat washers. Leave the power transformer out for the time being – that step comes a little later, after you’ve installed the low-voltage DC wiring. Wiring up Fig.1 shows the wiring routes. It’s not nearly as intimidating as it appears at first sight as most of it simply consists of DC supply wiring to the various modules. In addition, there’s a small amount of audio signal cabling, plus the loudspeaker cabling and the mains wiring. As can be seen, most of the supply wiring is terminated using insulated female spade connectors. These simply plug into the quick connect spade terminals on the various modules. Screw terminal blocks handle most of the other terminations, the exceptions being the RCA input sockets on the two audio amplifier boards. By the way, a ratchet-driven crimping tool (see photo) is an absolute necessity when it comes to installing the crimp connectors. Low-cost automotive type crimpers are definitely not suitable here, as their use would result in unreliable and unsafe connections – particularly where the mains wiring is concerned. As shown in the photos, all the supply leads are tightly twisted together. This not only keeps the wiring neat but also minimises hum pick-up, siliconchip.com.au This chassis view clearly shows the routing of the loudspeaker cable from the right channel power amplifier. It runs along the bottom of the rear panel and is secured using adhesive cable tie mounts and cable ties. since the hum fields are effectively cancelled out. There’s an easy way to twist the leads together and that’s by using a hand drill. All you have to do is secure one end of the leads in a vice and the other end in the drill chuck. You then rotate the drill handle until you get a nice even lead twist along the full length of the cable. Make the twists reasonably tight but don’t overdo it – the wire will break through the insulation if you do. Once it’s done, trim the ends to remove any damaged insulation and fit spade connectors to the leads at one end of the cable only. The spade connectors are fitted as follows: (1) trim 6mm of insulation from the end of a lead and twist the wire strands together; (2) push the lead into the connector until the insulation siliconchip.com.au hits the internal collar; (3) crimp the connection using the crimping tool; and (4) check that the connection is secure and properly insulated, with no wire strands outside the connector (this is particularly important for the 240V AC wiring). The leads at the other end of each cable are also later fitted with spade connectors, after they have been run to their destinations and cut to the correct length. It’s best to install the low-voltage DC wiring first. This can go in as follows: (1) Install the supply wiring to the two power amplifiers. These cables should be run using extra heavy-duty red, green and black leads. Twist the leads together and initially fit spade connectors to the power supply ends only. That done, plug each cable into the power supply board and route it to its respective power amplifier board. When it reaches the amplifier board, cut the green lead to length, fit it with a spade connector and plug it in. The red and black leads then continue under the amplifier to the centre of the board. They then diverge at right angles and are routed to the +22V and -22V terminals. (2) Install the +22V, 0V & -22V supply wiring between the power supply board and the preamplifier. (3) Install the +22V and 0V wiring between the preamplifier and the loudspeaker protector module. Note that this wiring actually runs behind the bridge rectifier (Fig.1 shows it in front for clarity) and is tied down to one of the transformer ventilation slots. An adhesive cable tie mount at the front left corner of the power supply board September 2007  71 You Need A Ratchet Type Crimping Tool One essential item that’s required to build this amplifier is a ratchetdriven crimping tool, necessary for crimping the insulated quick-connect terminals to the leads. Suitable crimping tools include the Altronics Cat. T-1552, Dick Smith Electronics Cat T-3535 and the Jaycar TH-1829. These all feature doublejaws so that the bared wire end and the lead insu­lation are crimped in a single action. Don’t even think of using one of the cheap (non-ratchet) crimpers that are typically supplied in automotive crimp kits. They are not up to the job for a project like this, as the amount of pressure that’s applied to the crimp connectors will vary all over the place. This will result in unreliable and unsafe connections, especially at the mains switch and IEC socket terminals. By contrast, a ratchet-driven crimp­ i ng tool applies a preset amount of pressure to ensure consistent, reliable connections. provides a second anchorage point. (4) Install the ±22V wiring between the bridge rectifier (BR1) and the power supply board. (5) Install earth leads from the power supply board to the main chassis earth point and from the preamplifier board (near the volume pot) to its adjacent chassis earth. AC sense leads The two “AC-Sense” leads that run from the bridge rectifier to the loudspeaker protector are next on the list (these are the blue leads that run to BR1’s AC terminals in Fig.1). First, twist the two leads together and fit one end of each lead with a piggyback crimp connector (see photo). That done, plug these into the AC (~) terminals of the bridge rectifier, then route the leads to the loudspeaker protector and trim them to length. Finally, strip about 5mm of insulation from the ends of the leads and tin them before connecting them to the screw terminal block (CON2). If you route these leads as shown in the photos, they can be secured to the chassis using a cable tie that passes through one of the transformer ventilation slots. A second cable tie adjacent to the CON2 is also a good idea. Audio input wiring The audio input signal leads can now be run from the rear panel to the preamplifier. These leads should be run using figure-8 (stereo) audio cable (ie, with the inner conductor individually shielded). Route these leads exactly as shown and secure them using cable ties and adhesive cable tie mounts. The locations of the latter are indicated on Fig.1. Note that the shield leads are separately connected to their respective solder lugs on the insulated RCA input sockets. Do not connect these shield leads together or to chassis, otherwise you’ll get an earth loop. At the preamplifier end, trim each conductor to length, then strip about 14mm of the outer insulation away from each conductor in turn and care- Where To Buy Complete & Shortform Kits A kit of parts for the 20W Stereo Class-A Amplifier (Cat. K5125) is available from Altronics, 174 Roe St, Perth, WA 6000, Australia. The kit is complete and includes the five modules (unassembled) and a pre-punched steel chassis similar to that shown in the photographs. Alternatively, you can purchase individual kit modules (but not the chassis) separately. Check the Altronics website at www.altronics.com.au for further details. Note: the kit does not include an infrared remote control handpiece. This must be purchased separately. Almost any universal remote should be suitable; eg, Altronics Cat. A 1009. 72  Silicon Chip fully separate and twist the shield wire strands together. That done, strip about 10mm of insulation from each inner conductor, then double each bared end back on itself, twist it together and lightly tin with solder. The shield wires can also be “doubled up”, twisted and tinned. Now secure the audio input leads to the screw terminal blocks. Note that it’s important to do these screw connections up nice and tight, otherwise the signal-to-noise ratio will be compromised. Some of the left over figure-8 audio cable can now be used to make the two audio leads that run from the preamplifier to the power amplifiers. Separate the cable into two separate leads and fit an RCA plug to one end of each lead (red for the right channel, black for the left). Make sure that each shield wire connects to the “earthy” side of its RCA plug (ie, to the terminal that connects to the outer collar). The other ends of these cables can then be trimmed to length and connected to screw terminal blocks CON2 & CON4 on the preamplifier. Be sure to tin the leads as before and again make sure the connector screws are done up tightly. Loudspeaker cabling The loudspeaker leads, both to and from the loudspeaker protector, are run using heavy-duty 90/0.18 speaker cable. The cables are terminated at both ends using female spade connectors and must be routed exactly as shown in Fig.1 and the photos. In particular, note the path for the loudspeaker cable from the right channel power amplifier. This must be kept as far away as practical from the mains wiring between the IEC socket and the power transformer. As shown, it runs around the chassis earth terminal and then runs along the bottom section of the rear panel (behind the transformer) to the loudspeaker protector module. The cable is anchored in position using several adhesive cable tie mounts. Two of these are attached to the bottom of the rear panel, while the third sits in front of the chassis earth lugs. Mounting the transformer The toroidal mains transformer can now be bolted into position. This transformer is supplied with two siliconchip.com.au Parts List For Class-A Stereo Amplifier If your infrared receiver module has a metal shield like this one, then be sure to insulate it from the front panel as described last month. The “AC Sense” leads from the loud­ speaker protector are terminated in piggyback crimp connectors at the bridge rectifier end, as shown here. neoprene rubber washers – one sits under the transformer (ie, between the transformer and chassis), while the other sits on top. A metal cup washer is then placed over the top rubber washer and the whole assembly secured using a large bolt that passes up through the centre of the transformer. Before installing the mounting bolt, check that the powder coating has been cleared from around its hole at the bottom of the chassis (this is necessary to ensure the bolt is correctly earthed). That done, install the bolt and do the nut up finger tight, then rotate the transformer so that its yellow secondary lead is exactly in line with the GND (centre) connection on the adjacent power supply board. Finally, do the nut up firmly but don’t over tighten it, otherwise you’ll distort the metal chassis. (Note: Fig.1 shows both the transformer and the preamplifier module offset to the right, compared to their true locations in the chassis. This has been necessary to keep these parts clear of the magazine centre.) As previously mentioned, the trans­ former leads are all the correct length to reach their destinations and are siliconchip.com.au 1 custom pre-punched steel case with screened front & rear panels 1 32mm black aluminium knob with grub screw (Altronics H 6236) 1 16V + 16V 160VA magnetically-shielded toroidal transformer (Altronics MA 5417) 1 SPST 10A 250VAC rocker switch (Altronics S 3224) 1 chassis-mount fused male IEC socket (Altronics P 8324) 1 M205 4A 250VAC slow-blow fuse 1 240VAC 3-pin IEC mains power lead 3 6.3mm double-ended chassis-mount spade lugs (Altronics H 2261) 5 6.3mm single-ended chassis-mount spade lugs 40 6.3mm female spade fully-insulated connectors (Altronics H 2006A) 2 piggyback crimp connectors (Altronics H 2016A) 1 red RCA plug (Altronics P 0200A) 1 black RCA plug (Altronics P 0201A) 2 chassis-mount insulated RCA sockets (red & black) (Altronics P 0218 & P 0220) 2 2-way loudspeaker terminal panels (Altronics P 2016) 1 pot nut and washer 5 P-clamps (Altronics H 4211) 20 M3 x 6mm screws 20 M3 shakeproof washers 20 M3 flat washers 13 M4 x 10mm screws 1 M4 x 16mm screw (to secure bridge rectifier BR1) 22 M4 flat washers 1 M4 shakeproof washer (for bridge rectifier BR1) Heatsink compound for BR1 Modules 1 right-channel class-A power amplifier module (Altronics K 5126) 1 left-channel class-A power amplifier module (Altronics K 5127) 1 power supply module (Altronics K 5128) 1 preamp and remote volume control module (Altronics K 5129) 1 loudspeaker protector module (Altronics K 5124) Wire & cable, etc 1m brown 32/0.20 extra heavy-duty hook-up wire (Altronics W 2280) (mains rated) 2m red 32/0.20 extra heavy-duty hook-up wire (Altronics W 2283) 1m black 32/0.20 extra heavy-duty hook-up wire (Altronics W 2284) 2m green 32/0.20 extra heavy-duty hook-up wire (Altronics W 2285) (mains rated) 250mm 24/0.20 heavy-duty blue hook-up wire (Altronics W 2275) 1m figure-8 shielded audio cable (Altronics W 3022) 1m heavy-duty 90/0.18 speaker cable (Altronics W 2130) 40 small nylon cable ties 7 adhesive cable tie mounts (Altronics H 4107) 500mm of 10mm-diamter heatshrink tubing pre-fitted with female spade quick connects. We’ll deal with the secondary wiring first – all you have to do is twist the various lead pairs together and plug them into the relevant quick connect terminals on bridge rectifier BR1 and the power supply module. First, twist the white and black leads together (to form the 0V centre-tap) and connect them to the adjacent GND point on the power supply module. That done, twist the red and yellow secondary leads together and plug them into the piggyback spade connectors on the AC terminals of BR1. There’s one important wrinkle you have to watch out for when plugging in the transformer secondary leads – it’s all to easy to push the male lug of each piggyback connector down September 2007  73 The RCA input sockets must be fully insulated from the chassis. The audio cable shield wires go to the individual solder lugs – do not join them or connect them to chassis at this point, as this would create an earth loop. The transformer’s red and yellow secondary leads plug into the piggyback connector at the bridge rectifier (BR1) as shown here. They should then be strapped using cable ties, so that the connectors can not short against BR1’s metal case. so that it shorts against the metal case of BR1. To avoid this, bend each male connector upright after plugging in the transformer lead and secure it in this position using a cable tie (see photo). It’s also a good idea to fit cable ties at both ends of the twisted pairs to keep the wiring tidy. Mains wiring It’s now time to connect the transformer primary leads (brown & blue) and install the rest of the mains wiring. Take particular care with the mains wiring – your safety depends on it. In particular, be sure to use fully insulated spade connectors for all connections to the IEC socket and mains switch. 74  Silicon Chip As shown in Fig.1, the transformer’s blue primary lead connects to the Neutral terminal on the IEC socket, while its brown primary lead runs directly to the top terminal of the mains switch. In addition, you need to run a heavy-duty (32/0.20) mains-rated cable (brown) between the bottom terminal of the mains switch and the Active terminal of the IEC socket. The best place to start this wiring is at the power transformer. Here’s the procedure, step-by-step: STEP 1: run the primary leads straight down the side of the transformer to the chassis and secure them together at top, bottom and centre using three cable ties. STEP 2: cut a 600mm length of brown 32/0.20 heavy duty cable (this will be used to connect the IEC socket Active terminal to the mains switch). STEP 3: twist this lead together with the brown primary lead. Start of the primary lead’s quick connector and twist the leads together all the way back to the base of the transformer. STEP 4: Slip a 320mm length of 10mmdiameter heatshrink tubing over this twisted pair and lightly shrink it into place using a hot-air gun. Be careful not to apply too much heat – you don’t want the cable insulation to melt (gently does it)! STEP 5: Secure this cable in position using the Nylon P-clamps as shown in Fig.1. Note the orientation of the Pclamps – the cable should run adjacent to the power supply board, so that it is well away from the righthand power amplifier. The switch end of the cable runs under the preamp and must be routed exactly as shown. STEP 6: Trim the switch end of the added brown lead to the same length as the brown primary lead and crimp on a fully-insulated spade connector. Make sure that all the wire strands go inside the connector – a stand outside the connector will be dangerous. STEP 7: Attach the front panel to the amplifier chassis and clip the mains switch into position. The two switch terminals go towards the top of the panel. STEP 8: Connect the two spade connectors to the switch terminals. The transformer’s primary lead goes to the top terminal. Use a cable tie to secure the leads at the switch terminal. STEP 9: Twist the added brown lead with the blue primary lead all the way to the latter’s spade connector. STEP 10: Slip a 120mm length of 10mmdiameter heatshrink tubing over this twisted pair and lightly shrink it into place using a hot-air gun (gently does it). STEP 11: Secure this section of the cable in position using another two Nylon P-clamps. As before, these should be orientated exactly as shown in Fig.1. STEP 12: Trim the brown cable to length and crimp on a female spade connector. STEP 13: Plug the connectors into the IEC socket. The blue lead goes to the Neutral terminal while the brown lead goes to Active. STEP 14: Further secure the leads using cable ties – two between the transformer and the first P-clamp and one right at the IEC socket. STEP 15: Prepare a 100mm-long earth lead with female spade connectors at either end and connect it between the earth terminal on the IEC socket and a spare chassis earth lug. STEP 16: Fit a 4A slow-blow M205 fuse to the IEC socket. STEP 17: Secure all the wiring in the amplifier by fitting cable ties as shown in Fig.1. This not only improves the appearance by keeping everything tidy but ensures reliability as well. In particular, make sure that the 240V wiring is properly secured by the P-clamps and by fitting cable ties immediately behind the spade connectors at the IEC socket and the mains switch. Initial checks That completes the wiring but there are a few things to check before plugging in a mains cord and switching on. Just follow this step-by-step checklist: (1) Check the 240V wiring to the IEC socket, mains transformer and mains switch to ensure all is correct. In particular, the female spade connectors should all be tightly crimped, the connectors must be fully insulated and there must be no wire strands outside these connectors. In addition, all spade connectors should be a tight fit onto their lugs, especially at the IEC socket, the mains switch and the bridge rectifier. Retension any connectors that slide on too easily. (2) Check that BR1’s positive and negative terminals connect to the correct terminals on the power supply board. siliconchip.com.au Ditching The Preamp & Using A Conventional Volume Pot Instead One of the options that you have in building this unit is to ditch the Preamplifier & Remote Volume Control module and use a dual 10kW log pot as the volume control instead. This option would typically be used if you want to use a CD player to drive the power amplifier modules – the signal output from a CD player is usually (but not always) sufficient to drive the amplifier modules to full power output. The advantage of this scheme is that you save money (ie, the cost of the preamplifier) and construction time. But there are a couple of disadvantages. First, as mentioned last month, using a simple volume control varies the input impedance to the power amplifiers, thereby slightly degrading the signal-to-noise ratio. And second, your CD player may not be able to drive the amplifiers to full output power on CDs that give below-average output signal levels. Another disadvantage is that you no longer have the convenience of remote volume control. If you do want to omit the preamplifier, Figs.3 & 4 show how it’s done. As shown, the incoming left and right channel signals are fed to the top of the pot and the attenuated signals on the pot wipers are then fed directly to the power amplifier inputs. Fig.3 shows the circuit, while Fig.4 shows the wiring diagram. Fig.3: this circuit shows how to use a dual 10kW W log pot as the volume control. Fig.4: follow this diagram to wire the volume pot in place. External preamp If you intend using an external preamplifier (eg, the SILICON CHIP Studio Series Stereo Preamp­lifier), you can omit the volume control altogether and simply run the audio input leads direct to the class-A power amplifiers. What about all those blank holes on the front panel? Easy – just mount the necessary parts to fill in the holes but don’t wire them up. The LEDs can be secured at the rear using epoxy resin. (3) Check that all the electrolytic capacitors on the power supply board are installed with the correct polarity. These things have a nasty habit of exploding if they’re in the wrong way around. The same goes for other electrolytics across the supply rails on the other modules. In fact, it’s not a bad idea to wear safety glasses when switching on for the first time, just in case you do have a capacitor in the wrong way around or you accidentally reverse the supply polarity. Exploding capacitors and eyeballs generally don’t mix too well! (4) Use a multimeter to confirm that all the chassis panels are correctly earthed. Do that by checking for continuity between the earth terminal of the IEC socket and each of the panels in turn (remove some of the powder coating from an inside surface of each panel to make these checks, if necessary). siliconchip.com.au Similarly, check that the heatsinks are earthed to the chassis and that all external screw heads are earthed. (5) Use a multimeter to confirm that the output transistors (Q12 & Q14) are correctly isolated from the heatsink of each power amplifier module. Test & adjustment There are three basic procedures to go through here. First, you have to check that the power supply module is delivering the correct voltages. You then apply power to each power amplifier module in turn and adjust its quiescent current. And finally, you power up the preamplifier and loudspeaker protector modules and check their operation. Here’s the procedure: STEP 1: disconnect all nine spade connectors from the +22V, 0V, -22V terminals at the output end of the power supply module. STEP 2: disconnect the loudspeaker leads from the power amplifier modules. STEP 3: connect an IEC power cord to the amplifier and use a multimeter to confirm continuity between the earth pin of the plug and the chassis earth. That done, plug the cord into a mains socket and switch on. Warning: don’t go poking around the rear of the IEC socket with power applied. The metal strap that runs from the Active terminal to one end of the fuse has 240V AC on it. STEP 4: check the unregulated ±22V rails at the output of the power supply module. These rails should both be measured with respect to the 0V terminal and should be a little high at around ±24V (since they are unloaded). If the meter reads 0V, switch off immediately and re-check the connections to BR1. STEP 5: switch off and connect the September 2007  75 Adjusting The Quiescent Current Through The Power Amplifiers The quiescent current flowing in the output stage of each power amplifier is initially adjusted by installing 1.5W 5W resistors in place of the fuses. The voltage across one resistor is then monitored and trimpot VR1 adjusted for a reading of 1.68V – equivalent to a quiescent current of 1.12A. The easiest way to connect the resistors is to “blow” the supply leads (+22V, 0V, -22V) for the righthand power amplifier to the power supply module. STEP 6: remove the two fuses from the righthand power amplifier and install 1.5W 5W resistors in their place – ie, one in series with the +22V rail and one in series with the -22V rail. The best way to do this is to solder these resistors across a couple a spare M205 fuses, after first destroying the internal fuse wires. First, drill a hole in each end cap, breaking the fuse wire in the process. The resistor leads can then be bent to shape, fed through the end caps and soldered (see photos). The modified fuses with their resistors are now plugged into the fuseholders. These 1.5W resistors protect the output transistors by limiting the current through them if there is a fault, eg, if the VBE multiplier circuitry (Q10) is not functioning correctly. STEP 7: wind trimpot VR1 on the right channel power amplifier fully anticlockwise, switch on and check that the amplifier’s output voltage (ie, between the loudspeaker terminals) is less than ±50mV. If not, check the baseemitter voltage of each transistor in the amplifier; they should all be 0.6-0.7V. Check also that the correct transistor is installed at each location and that they are all the right way around. STEP 8: assuming the output voltage is OK, monitor the voltage across one of the 1.5W 5W resistors and wind VR1 slowly clockwise until the meter reads 1.68V. This is equivalent to a quiescent current of 1.12A. 76  Silicon Chip fuse wires in a couple of spare M205 fuses, then drill holes in the end caps and solder the resistors in place as shown. The original fuses can then be removed and the “modified” fuses clipped into place – see photos. That done, let the amplifier run for about five minutes or so and then check the voltage again. During this time the amplifier heatsink will become quite warm and the quiescent current will drift slightly. Readjust VR1 to obtain 1.68V again. STEP 9: switch off, remove the 1.5W 5W resistors and install the 3A fuses. STEP 10: repeat steps 5-8 for the left channel power amplifier. STEP 11: let the amplifiers run for about 30 minutes (so that the heatsinks get nice and hot), then check the voltage across one of the 0.1W 5W resistors in the right channel power amplifier. Adjust VR1 for a reading of 112mV. Now check the voltage across the other 0.1W resistor – these resistors have a tolerance of about 10%, so set VR1 so that the average voltage across them is 112mV. STEP 12: repeat step 11 for the left channel power amplifier. STEP 13: switch off and reconnect the preamplifier’s +22V, 0V, -22V leads to the power supply module. STEP 14: check the preamplifier and remote volume control for correct operation, as described in the August issue (skip this step if you’ve already done this). STEP 14: check the operation of the loudspeaker protector module if this hasn’t already been done. To do this, apply power and check that the relay turns on after about 5-7 seconds. If it does, temporarily short the temperature switch input – the relay should immediately switch off. Similarly, the relay should immediately switch off if you disconnect one of the leads to the “AC Sense” input. Now check that the relay switches off if a DC voltage is applied to the loudspeaker terminals (this simulates an amplifier fault condition). This is done by connecting either a 3V, 6V or 9V battery (either way around) between the LSPKIN+ terminal and the ground terminal of CON1. The relay should immediately switch off. Repeat this test for the RSPKIN+ terminal, then reverse the battery polarity and do these two test again. The relay should switch off each time the battery is connected (see also pages 74-75, July 1007). Note: the reference to testing the loudspeaker protector using a 1.5V battery in the July issue is incorrect. You need at least a 3V test battery to bias on the transistors in the DC detection circuit. STEP 15: switch off and reconnect the loudspeaker leads to the power amplifier modules. Watch the ventilation That’s it, your new 20W Class-A Stereo Amplifier is now ready for action. Just one final thing – as previously mentioned, the heatsinks get quite hot and the air must be allowed to flow freely through the bottom and top ventilation slots. This means the amplifier must sit out in the open on a hard, level surface. Do not enclose it in a cabinet and do not stack anything on top of it. SC siliconchip.com.au