Fullscreen HTML5Med Res (??MB)HTML4

Ultra-LD Mk.3 135 stereo amplifier Ch as s is as s em b l y a n d w ir in g de t a ils A T THE HEART of this new stereo amplifier are two Ultra-LD Mk.3 power amplifier modules, as described in the July and August 2011 issues of SILICON CHIP. The Mk.3 is by far the best class-AB amplifier module ever published and features astonishingly low total harmonic distortion and noise (THD + N) figures and a power output of 135W RMS into 8Ω. Alternatively, it can deliver around 200W RMS into 4Ω. We followed that with a suitable power supply module in September 2011, then described a loudspeaker protector and muting module in October 2011. The latter is basically a slightly modified version of the very successful design used in our Class-A 32  Silicon Chip Stereo Amplifier described from June 2007 to September 2007. The preamplifier and input selector modules followed in November and December 2011. As with the loudspeaker protector, the preamplifier is a tweaked and enhanced version of the high-performance unit originally designed for the Class-A Stereo Amplifier. This month and next month we will show you how to assemble every­thing into a custom steel case that’s been designed in conjunction with Altronics. This precision laser-cut chassis is supplied with all holes drilled and with pre-punched front and rear panels with screened lettering. Also next month we will present the perfor- mance measurements of the complete amplifier. Optimised layout The final layout of the modules inside the case and the wiring between them has been carefully optimised to give the best overall performance from the individual modules. It is not simply a matter of plonking the modules inside the case and wiring them in a haphazard way. The details of the wiring must be closely followed. The result is a powerhouse stereo amplifier with state-of-the-art performance. In fact, we feel very confident in stating that there are very few commercially available amplifiers that would go close to it in performance, siliconchip.com.au 5W/ch Where To Buy Kits A kit of parts for the Ultra-LD Mk.3 Stereo Amplifier is available from Altronics, 174 Roe St, Perth, WA 6000, Australia. The kit is complete and includes the seven modules (unassembled) and a pre-punched steel chassis similar to that shown in the photographs. Alternatively, you can purchase the individual kit modules and the chassis separately – see the Altronics website at www.altronics.com.au for further details. Note that the kit does not include an infrared remote control handpiece. The unit will work with most universal remotes including the Altronics A1012. Pt.1: By GREG SWAIN Based on the series of high-performance modules described last year, this superb stereo amplifier can deliver 135W RMS per channel into 8Ω and boasts extremely low noise and distortion figures. It includes infrared remote control of volume and input selection and is straightforward to assemble. regardless of price. In fact, money cannot buy the fully assembled version of this Ultra-LD Mk.3 Stereo Amplifier – you have buy the kit or all the modules and assemble it yourself. At the end of that process, you will have a stereo amplifier with unmatched performance. Having said that, note that it will take a lot of time to fully assemble and test each module and then put it all into the case. We estimate that it might take up to 40-50 hours to do the whole job. It’s all pretty straightforward to do. Don’t rush the job and you will end up with a professional result. As shown in the photos, the case is finished in a tough black powder coating and the front panel features siliconchip.com.au bevelled top and bottom edges to give it a professional appearance. At 420 x 425 x 88mm (W x D x H), it’s really quite a large unit but that’s necessary to accommodate the large finned heatsinks used for the Ultra-LD Mk.3 amplifier modules and to allow the various modules and the large toroidal mains transformer to be logically placed inside the chassis. The large chassis also aids ventilation and cooling. The bottom of the chassis and the lid also have large ventilation slots which line up with the gaps between the heatsink fins, to allow the air to circulate between them. The modules To summarise, these are the modules that you need to build this amplifier: • 2 x Ultra-LD Mk.3 power amplifier modules (July-August 2011); • 1 x power supply module (September 2011); • 1 x loudspeaker protector module (October 2011); • 1 x stereo preamplifier module (November-December 2011); and • 1 x 3-way stereo input selector module plus its companion switch board (November-December 2011). Each of these modules will be available separately in kit form from Altronics and the chassis will also be available separately. In addition, Altronics will be selling a complete kit of parts for the new amplifier. This will include all the above kit modules, March 2012  33 10-WAY RIBBON CABLE UNDER 14-WAY CABLE 9 10 13 14 4004 SELF-ADHESIVE FLAT CABLE CLAMP + TUP NI REIFILP MAERP INPUT 2 4004 INPUT 3 LEFT OUTPUT + RIGHT OUTPUT 21111110 4004 1 INPUT 1 FIT ADDITIONAL SELF-ADHESIVE FLAT CABLE CLAMPS TO SPEAKER CABLES – SEE PHOTO 1 2 1 2 INPUT SELECTOR BOARD + 4148 CON2 4148 0V –57V CON1 B * B 17060210 E C + C B B E C E C C E CT CT –IN -IN B 4004 E B E C +IN 11190110 LEFT SPEAKER TERMINALS 1N 4148 B 30VAC LOUDSPEAKER PROTECTOR BOARD –57V 0V +57V 15V CT 15V Output 1 30VAC 15 ~ CT 15VAC – CT + - 0 + MK3 POWER SUPPLY BOARD GND SECONDARY EARTH POINT 1 –57V ~ GREEN/YELLOW (EARTH) CON2 + BR1 ~ – S MAIN CHASSIS EARTH 40 V +57V 4148 240V PRIMARY LEADS Output 2 +57V 0V –57V +IN uS r e woP reifilpmA 2.k M DL-artlU 01109111 E B +15V 0 –15V 4148 C +20V 0 4004 E 4004 C 4004 C - + +20V 0 +15V 0 -15V C 0 B B E Ultra-LD Mk.3 Power Supply E G 0V © 2011 0V F 40 V 15 V 0V 0V 1 + 5V + 11170110 MALE IEC CONNECTOR WITH INTEGRAL FUSE 3.K M REIFILP MA DL-ARTLU T1 A E N 34  Silicon Chip siliconchip.com.au + RIGHT SPEAKER TERMINALS +57V Fig.1: follow this wiring diagram to assemble the amplifier and complete wiring. Route the various leads as shown to ensure optimum performance and take particular care with the mains wiring. HEATSINK 11170110 3.K M REIFILP MA DL-ARTLU SECURE HEATSINKS TO BOTTOM OF CHASSIS USING M3 x 10MM PAN-HEAD MACHINE SCREWS & WASHERS the transformer and the chassis, plus other sundry parts OK, let’s assume that you’ve completed all the modules as previously described and that you’re ready to mount them in the chassis and install the wiring. We’ll get to that shortly but first, you have to do some work on the case itself. RIGHT CHANNEL POWER AMPLIFIER BOARD INPUT PREAMP AND CONTROL BOARD 02 © 9 10 1 2 _ + Preparing the case GND +15V 0V 14 13 RIGHT OUTPUT 2 1 9.5mm P-CLAMP – SEE TEXT RIGHT INPUT SWITCH BOARD LEFT CHANNEL OUTPUT LEADS LEFT OUTPUT LEFT INPUT 01111111 PREAMPLIFIER LOW NOISE STEREO CON1 LEFT CHANNEL POWER AMPLIFIER BOARD SECONDARY EARTH POINT 2 COVER REAR OF SWITCH S1 WITH INSULATION BOOT SPADE CONNECTOR ON FRONT PANEL S1 (REAR) HEATSINK siliconchip.com.au As mentioned, the case is finished in a tough powder-coating. That also acts as a good insulator so you must take steps to ensure that all sections of the case, including the side panels, the front and rear panels and the lid, are correctly earthed. 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 step is to ensure that the two side panels and the lid are earthed to the bottom section of the chassis. That’s done by using an oversize drill to remove the powder coating from the countersunk screw holes. Use drills that are 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 side panels and from around the three holes on the underside of the lid at the rear. That done, scrape away the powder coating from the matching contact areas around the screw holes in the chassis and at the top of the rear panel. When the case is subsequently re-assembled, earthing takes place via the screws themselves and also via direct metal-to-metal contact between the various sections. The front and rear panels are earthed by running leads from quick connect March 2012  35 You Must Use A Ratchet-Driven Crimping Tool One essential item that’s required to build this amplifier is a ratchet-driven crimping tool, necessary for crimping the fullyinsulated quick-connect terminals to the leads. Suitable crimping tools include the Altronics Cat. T-1552, and the Jaycar TH-1829. These all feature double-jaws 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 crimp­ing tool applies a preset amount of pressure to ensure consistent, reliable connections. An oversize drill can be used to remove the powder coating from around the screw holes – see text. terminals back to separate earth terminals bolted to the bottom of the chassis (see wiring diagram). In the case of the rear panel, the quick connect terminal is bolted into position using an M4 x 10mm screw, two starwashers and two nuts. Be sure to scrape away the powder coating around the mounting hole on the inside of the panel before bolting the quick connector into position as shown in Fig.3 – ie, the quick connector must contact bare metal. By contrast, the front panel quick connect lug is welded into position, not far from the power switch position. It’s accessed through a cut-out in the front sub-panel (not shown on the prototype). It’s also a good idea to remove the powder coating under the screws that are used to secure the front and rear panels in place. That way, these panels will be earthed to the chassis via both the mounting screws and the quick connector leads to the earth points when they are later installed. Transformer mounting bolt The transformer mounting bolt must 36  Silicon Chip also be earthed and this means that you have to remove the powder coating from around the mounting hole on the bottom 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 and from around any screw holes adjacent to the IEC socket and the mains switch. 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 while you are installing the parts in the chassis and completing the wiring. Installing hardware You can now install some of the hardware. Fig.1 shows the general layout inside the case. Begin by securing the IEC power socket to the rear panel using the two 6g x 12mm countersink self-tappers supplied. Once it’s in place, smear some neutral-cure silicone sealant over the metal strip that runs down one side of the IEC socket between the Active input and one end of the fuse (see photo). This is a worthwhile precaution since that strip has 230V AC on it when power is applied. That done, secure the loudspeaker terminals (with their leads attached) in position, again using 6g x 12mm countersunk self-tappers. The 35A bridge rectifier (BR1) can now go in. It uses the chassis for heatsinking, so it’s important to ensure good metal-to-metal contact. This metal strip on the IEC socket operates at 230VAC and should be insulated using silicone sealant. Its chassis-mounting area must be completely free of powder coating; if not, mark out the area and remove the powder coating using a small grinding tool and a sharp scalpel. The powder coating should also be removed from around the mounting hole on the bottom of the chassis, so that the head of the mounting screw will also make good contact. Once that’s done, smear the underside of the bridge rectifier with heatsink compound and bolt it to the chassis using an M4 x 20mm machine screw, star washer and nut. Fig.1 shows BR1’s mounting location and orientation. Chassis earth connections As shown in Fig.1, there are three earth points on the bottom of the chassis – two at the rear (to the right of the power transformer) and one at the front, near the mains switch. The rearmost earth point is used exclusively for the mains earth (more on siliconchip.com.au LOCATING SPIGOT UNDER 10-WAY IDC SOCKET 10-WAY IDC SOCKET 510mm x 10-WAY IDC RIBBON CABLE CABLE EDGE STRIPE LOCATING SPIGOT UNDER 14-WAY IDC SOCKET 14-WAY IDC SOCKET 630mm x 14-WAY IDC RIBBON CABLE CABLE EDGE STRIPE Fig.2: attach the header sockets to the IDC test cables exactly as shown in this diagram. The sockets are clamped using an IDC crimping tool or you can use a vice. Don’t forget to fit the locking bar after each header is clamped. this next month). The adjacent chassis point (to the right) is used to earth the power supply module and the rear panel. This latter point is fitted with a double-ended quick connect terminal as shown in Fig.3. Secure it using an M4 x 10mm machine screw, two star washers and two nuts. The top nut locks the bottom one in position so that there’s no possibility of the assembly coming loose. The front chassis earth point is also fitted with a double-ended quick connector and is secured in exactly the same manner. This point is used to earth the front panel and the metalwork of the pot. As supplied, the chassis should be clear of powder coating around the earth mounting holes, to ensure good metal-to-metal contact. If not, be sure to scrape away the powder coating as necessary. Making the IDC cables Before mounting the various modules in the chassis, you need to make up a number of simple cable assemblies. So that’s what we’ll do now. The first step is to fit the headers to the two IDC (flat ribbon) cables. Fig.2 shows how these are made. Note that pin 1 on the header sockets is indicated by a small triangle in the plastic moulding and the red stripe on each cable must always go to this pin. Start with the 10-way IDC cable. It should be cut to a length of 510mm and then attached to the IDC headers using either a vice or an IDC crimping siliconchip.com.au tool (eg, Altronics T1540 or Jaycar TH1941). The cable is then folded back across the headers at each end and the locking bars installed. Take care when fitting the cable to the headers. In particular, note that after each crimping operation the cable is folded back through 180° before the locking bar is fitted. Study Fig.2 carefully; it’s all too easy to feed the cable to the header from the wrong direction. The 14-way (630mm-long) IDC cable is made up in similar fashion. Shielded audio cables The twin audio cable that runs from the input selector board to the preamp can now be made. As shown in Fig.1, this is fitted with right-angle RCA plugs at either end – red for the right channel and black for the left. To fit these plugs, first pull off the plastic cable entry collars, then split the casings to gain access to the solder terminals. That done, cut the twin shielded audio cable to 600mm and separate the individual leads at either end over a distance of about 40mm. The cable ends can then be stripped and terminated to the plugs – red wire to the red plugs, white wires to the black plugs. Make sure that each shield wire goes to the “earthy” side of its RCA plug (ie, to the terminal that’s connected to the outer collar). Next, cut four 10mm lengths of 20mm-diameter heatshrink tubing and slip one over each plug and down the leads. The plastic casings can then be QUICK CONNECT LOCKING NUT STAR LOCKWASHERS M4 x 10mm SCREW& NUT CASE NB: CLEAN PAINT AWAY FROM MOUNTING HOLE Fig.3: the two double-ended quick connect terminals are each bolted to the chassis using an M4 x 10mm screw, star lockwasher and two M4 nuts. The same arrangement is used to secure the single-ended quick connect terminal to the rear panel. refitted to the plugs, after which the heatshrink pieces are slid over these casings and shrunk down to secure them in place. The original cable entry collars are discarded. By the way, be sure to use figure-8 (stereo) audio cable for this lead – ie, with the inner conductors individually shielded. Don’t use 2-core cable with a single shield. And if you’re wondering how to strip the very fine inner conductor, the answer is to use an adjustable wire stripper such as the Altronics T1510. Once the RCA plugs have been attached, fit cable ties to the figure-8 cable at both ends, to prevent the leads from separating any further. 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 amplifier. Separate the March 2012  37 This photo shows how the input audio cable is secured to the floor of the chassis using 5mm-diameter P-clips, so that it runs under the rear edge of the preamplifier module. A 9.5mm-diameter P-clip also secures the speaker cable from the left-channel power amplifier and this is fastened using a shared mounting screw with one of the 5mm-diameter P-clips. cable and cut off 85mm and 185mm lengths for the left and right channels respectively. Fit black right-angle RCA plugs to the 85mm cable and red RCA plugs to the 185mm cable and secure the plastic casings with heatshrink as before. The two speaker cables are made by cutting 490mm and 300mm lengths of heavy-duty (either 41/0.20mm or 102/0.12mm) figure-8 cable. That done, remove about 4mm of insulation from the wires at each end and pre-tin them so that they can later be soldered to the amplifier and loudspeaker protector modules. Once these cables have been prepared, cut two 80mm lengths from the leftover cable and solder them to the loudspeaker terminal panels. the screw terminal blocks on the power supply module. Be careful not to get these connections mixed up – you could fry the transistors on the power amplifiers if you do. In addition, you have to solder a 110mm green earth lead to the front of the power supply module (this lead later connects to the 0V rail on the preamp PCB). Note that this 0V (earth) connection point was absent from early versions of the power supply PCB but should be included on PCBs sold with the kit. If you do have an early version of the power supply PCB, simply drill a 1.5mm hole in the location shown but be careful not to damage the adjacent electrolytic capacitors. It’s then just a matter of scraping away the masking from around this hole to expose the bare copper before soldering the lead. Power amplifier supply cables Earth leads You can now make the two short power supply cables for the Ultra-LD Mk.3 amplifier modules. These each consist of three 80mm heavy-duty (32/0.20mm) leads (red, black and blue) which are connected to Molex Mini-fit Jr 3-pin female line plugs. Crimp and lightly solder the leads to the pins before inserting them into their matching locations in the plastic plug shells (see Fig.1). Be sure to use a red lead for the +57V connection, black for 0V and blue for -57V. Once the Molex plugs have been fitted, strip about 5mm from the free ends of the leads and secure them to The next step is to make up the front and rear panel earth leads. These are made using 75mm and 70mm lengths respectively of green 32/0.20mm wire and are fitted with fully insulated spade connectors at each end (strip about 5mm of insulation from the wire ends before fitting the connectors). A 75mm-long lead is also used to earth the volume control metalwork and this should also now be made. You can also make the earth lead that connects the TERM2 (or CT) terminal on the power supply module to chassis. This earth lead is 160mm long and is again fitted with fully in- Speaker cables 38  Silicon Chip sulated spade connectors at each end. Finally, you can make the mains earth lead (ie, the one from the IEC connector to chassis). This lead is 175mm long and is terminated in a crimped 4mm eyelet connector at one end and a fully-insulated spade connector at the other. Installing the modules Now comes the fun part – installing the modules, hardware and cables into the chassis. Note that all modules except for the preamplifier are mounted on 10mm tapped spacers and generally secured using M3 x 6mm machine screws and flat washers. The preamplifier is mounted on 25mm tapped spacers. Start with the input selector module. This is fitted by first securing its stereo RCA sockets to the rear panel using three M2 x 12mm machine screws and nuts, or you can use No.3 x 10mm selftappers if supplied. It’s then attached at the front on two 10mm spacers (the other two mounting holes are not used). Once this module is in position, plug the stereo audio cable into the selector module’s output sockets, then secure the cable to the floor of the chassis using five 5mm-diameter Nylon P-clamps (see Fig.1). These clamps are secured using M4 x 10mm machine screws, flat washers and nuts. This next bit is important – you must fit two M4 washers under the head of the screw that secures the P-clamp under the amplifier module siliconchip.com.au Fig.5: this side elevation shows how the switch board and the preamplifier module are mounted. In each case, M3 flat washers are used as spacers. RIGHT CHANNEL OR LEFT CHANNEL POWER AMP BOARD NYLON P-CLAMP MAINS WIRES OR SIGNAL CABLES FRONT SUB-PANEL 3 x M3 FLAT WASHERS AS SPACERS M4 NUT & WASHER M3 x 10mm SCREWS AND NUTS BOTTOM OF CASE 2 x M4 FLAT WASHERS M4 x 10mm SCREW Fig.4: the P-clamps that go under the power amplifier modules are mounted as shown here. Two M4 washers must be fitted under each screw head, so that the screw cannot short against the PCB. – see Fig.4. This is necessary to provide sufficient clearance between the end of the screw and the underside of the amplifier module when it is later installed. Note that one of the P-clips sits under the preamplifier and is not shown in Fig.1. However, its location is clearly shown in one of the accompanying photos. Note that the screw that secures this 5mm P-clip is also used to secure a 9.5mm-diameter P-clip for the left channel loudspeaker cable when that’s later installed. Having secured the audio cable, install the switch board in the chassis. Fig.5 shows the mounting details – it’s secured to the front sub-panel using four M3 x 10mm machine screws, nuts and washers, with three additional washers used as spacers at each corner. Centre the board vertically in its slotted mounting holes before tightening the screws. The preamplifier can now be installed but first you have to fit a 2-way screw terminal block to its CON5 position (ie, adjacent to the 3-way screw terminal block). This terminal was originally left off the board but is now necessary so that the preamp supply earth can be connected to the power supply earth. As previously stated, the preamp is mounted on M3 x 25mm spacers. In addition, three M3 flat washers must be fitted between each spacer and the chassis – see Fig.5. This raises the preamp slightly, so that the centre of the pot shaft is centred in its hole when the front panel is later fitted. In practice, it’s easier to loosely fit the spacers and washers to the chassis first, then drop the preamp module into place and install its mounting screws. You then push the assembly back in its slotted chassis holes as far siliconchip.com.au M3 x 6mm SCREWS INPUT PREAMP & CONTROL BOARD M3 x 25mm TAPPED SPACERS BOTTOM OF CASE M3 x 6mm SCREW INPUT SELECTOR SWITCH BOARD 3 x M3 FLAT WASHERS M3 FLAT WASHERS as it will go before tightening all the screws. Don’t fit the washers directly under the PCB, as they could cause short circuits. It’s now a good idea to test fit the front panel, to make sure the pot shaft and the switch buttons are correctly centred in their holes and that the three LEDs protrude the correct distance through the panel. The switch buttons should either sit flush with the panel or just protrude slightly. Note that the top of the front panel is pulled back slightly when the lid is fitted, so be sure to make allowance for this. If the switch buttons are off-centre, it’s simply a matter of raising or lowering the switch board until they are correct. You can also add or remove spacer washers if necessary so that the switches sit flush with the front panel. The preamp height can be similarly adjusted, if the pot shaft isn’t centred vertically in its front-panel hole. Once everything is correct, remove the front panel again and put it away where it won’t get damaged. Flat ribbon cables The next step is to fit the 10-way and 14-way flat ribbon (IDC) cables. The 10-way cable goes in first. Plug it into the IDC header on the preamplifier, then drop it down and run it straight back along the floor of the chassis as shown in Fig.1. The cable is then folded at right angles and routed towards the side panel, then folded at M3 x 6mm SCREW right angles again and plugged into the matching 10-way IDC header on the input selector board. Once it’s in place, plug the 14-way cable into the switch board, fold it at right angles as shown in Fig.1 and run it towards the rear of the chassis on top of the 10-way cable. The cable is then folded at right angles twice more and plugged into the 14-way IDC on the selector board, adjacent to the rear panel. Finally, push the IDC cables into two self-adhesive flat cable clamps and anchor them to the floor of the chassis in the positions shown. Adding more modules The power supply is the next module to install. That done, you can connect the earth lead from the power supply module to the negative (-) terminal of CON5 on the preamplifier module and install the +15V, 0V, -15V wiring. This consists of three heavyduty (32/0.20mm) leads (red, green and black) which are twisted together, cut to length and terminated at either end in screw terminal blocks. Twisting these leads together keeps them nice and neat. The best way to do this is to first clamp them at one end in a vice. The free ends are then secured in a hand-drill chuck and the handle rotated until you get a nice even lead twist along the full length of the cable. That done, the wires can be trimmed, stripped and secured at each end using heatshrink or cable ties. They are then March 2012  39 This is the view inside the completed Ultra-LD Mk.3 Stereo Amplifier. Note that the mains transformer used here was fitted with an external copper strap held in place by a ring clamp. By contrast, the transformer supplied with kits will come with an integral flux band, so constructors will not have to add this (see panel next month). 40  Silicon Chip siliconchip.com.au WARNING! HIGH VOLTAGE High AC and DC voltages are present in this circuit. In particular, mains voltages (230V AC) are present on the IEC socket and the primary side of the transformer (including the wiring to the power switch). In addition, the transformer secondary provides an 80V AC output (2 x 40V AC centre-tapped) and the amplifier power supply rails total 114V DC. Do not touch any part of the amplifier circuitry when power is applied otherwise you could get a severe electric shock. The two LEDs on the power supply board indicate when power is present. If they are alight, the power supply and amplifier boards are potentially dangerous. secured to the floor of the chassis at the preamp end using a 5mmdiameter P-clamp. That done, the loudspeaker protection module can be mounted in position and the 20V DC supply wiring and 30V “AC sense” wiring run from the power supply board. Use two self-adhesive cable-tie mounts and cable ties to secure the DC supply wiring as shown. The twisted “AC sense” can be cable-tied to the DC supply wiring, close to the loudspeaker protector module. Note that you will have to temporarily disconnect this wiring from the loudspeaker module later on, so that it can be lifted up to make the solder connections for the loudpeaker cable wiring. As explained next month, these connections must all be soldered, as quick connectors can cause distortion. At this stage, you can also install the wiring between the power supply board and the DC terminals on the bridge rectifier. This wiring consists of red and black heavy-duty leads which are twisted together and terminated at both ends in spade connectors. Connect these leads as shown in Fig.1, then secure both ends with cable ties. Transformer mounting The next step is to install the large toroidal transformer. As supplied, the toroidal transformer has a 620mm brown primary lead that’s long enough to reach the mains switch. The other primary lead (blue) is about 240mm long and this goes to the Neutral terminal of the IEC socket (more on this next month). Before fitting the transformer, check that the powder coating has been scraped away around its mounting hole underneath the chassis. As stated earlier, this is necessary to ensure that the head of the bolt contacts bare metal, so that the bolt is correctly earthed. Having done that, bolt the transformer in place. It’s supplied with two neoprene rubber washers and these must go between the toroid itself and the two dished metal cup plates. In other words, one metal plate sits against the chassis. A rubber washer then sits on top of this, then the toroid is placed in position. The second rubber washer is then fitted and finally the second metal plate sits on top of this and the whole assembly secured using the large mounting bolt. Do the nut up finger-tight to begin with, then rotate the transformer so that the blue and brown primary leads point directly towards the left-channel speaker terminals. That done, do the nut up firmly but don’t over-tighten it otherwise you’ll distort the metal chassis (and possibly even damage the transformer). Next month That’s all we have space for this month. Next month, we’ll complete the construction of the new Ultra-LD Mk.3 Stereo Amplifier and describe the test and set-up procedure. We’ll also publish the parts list SC and give the specifications. siliconchip.com.au March 2012  41