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Last month, we introduced our new high-performance stereo amplifier and described the circuit details. This month, we begin the construction by showing you how to build the preamp, RCA input & power amplifier modules. The full performance details are also provided. Building the Ultra-LD 2 x 100W Stereo Amplifier A LTHOUGH THERE’S A LOT of work in building this new ampli­fier, it’s really all quite straightforward. Most of the work involves assembling the six PC board modules but there’s also a fair bit of wiring do be done. Make no mistake – this project will gobble up quite a few man-hours and is no job for the fainthearted. To make the job as easy as possible, Altronics in Perth will be offering complete kits for this new high-performance amplifier and these are expected to be available by the end of December. The Altronics kit will come with a professional rack-mount­ing case that’s been custom-made for the job. The case is powder-coated, all the holes are pre-drilled and the front and rear panels are supplied with screened lettering. In short, there’s absolutely no metalworking to do. Note that although our prototype shows rack-mounting flang­es and handles, the Altronics metalwork will give you the option of leaving these items off, if that is what you prefer. These parts will still be supplied with the metalwork – it’s up to you to decide whether or not to use them. The detachable front panel really looks the part. The top and bottom edges are curved to improve the appearance and the design is “relieved” by the vertical slots at either end and the slots running horizontally along the bottom. And in case you’re wonder­ing how the front panel is secured on the prototype, it’s mounted on spacers on the front of the chassis and is fastened at either end by screws that pass through the front of the chas- sis and into the handles at either end. This means that no screw heads are visible on the front panel. Alternatively, if you choose not to use the handles, the front panel can be secured using the attractive Allen Key screws supplied with the kit. Inside, the large fan-cooled heatsink is supplied drilled and tapped, and has the necessary channels milled into its base to allow the wiring to run between the front and rear sections of the case. The power supply heatsink is also supplied cut to size and predrilled. Other highlights of the Altronics kit include fibreglass PC boards with solder-masked tracks, gold-plated heavy-duty loud­ speaker terminals, gold-plated RCA input connectors and a custom-made power-transformer with all leads made the correct length Part 2: By GREG SWAIN & LEO SIMPSON 54  Silicon Chip www.siliconchip.com.au The completed power amplifier modules are bolted to the large fan-cooled heatsink, with the thermal switch between them. The fan only cuts in if the heatsink temperature reaches 60°C – hardly ever (if at all) for normal domestic use. to reach their destinations and fitted with quick connects. Preparing the quick connects All the male quick connects supplied in the kit are double ended but most locations require a single-ended connector. There­fore, the first job is to convert 48 of the male quick connects to single-ended connectors by cutting off one of the lugs. This can be done using a sharp pair of tinsnips – the quick connects are made of brass and are quite easy to cut. Clean the sharp edges up using a light file after cutting off each lug. Semiconductor numbers Before starting the board assemblies, it’s important to note that some of the transistor, LED, diode and regulator numbers shown on the circuits (and overlays) are duplicated across the various modules. That’s because most of the circuitry is based on previously www.siliconchip.com.au published designs and we’ve retained the original numbering schemes. For this reason, always be sure to check the relevant circuit or section of the parts list for that module when looking up the type numbers for the semiconductors. Building The Preamplifier & LED Display Module Fig.6 shows how to build the Preamplifier & LED Display Module. You can start the assembly by installing the wire links, resistors and diodes, taking care to ensure that the latter are correctly oriented. Table 1 shows the resistor colour codes but it’s always a good idea to check each value using a digital multimeter, as some of the colours can be difficult to decipher. Note that the two 150Ω resistors near rotary switch S1 have RF suppression beads slipped over one of their leads before they are installed on the board. Once these parts are in, you can fit the five ICs and the two 3-terminal regulators (REG1 & REG2). ICs can be static sensi­tive, so always touch a grounded object before handling them and try not to touch any of the pins. This particularly applies to the NE­5534A devices (IC1 & IC3). Note that IC1 & IC3 can be labelled either NE5534AN or NE5534AP. The “A” in the suffix designates a very low noise device. Don’t use devices without an “A” in the type number (eg, NE5534N), as these have inferior noise performance. Regulators REG1 & REG2 are mount­ ed flat against the PC board and are secured using 10mm x M3 screws, nuts and star washers. This means December 2001  55 Fig.6: install the parts on the Preamplifier & LED Display board as shown here. Note that potentiometer VR1 and the LEDs must be stood off the board – see text and photos. 56  Silicon Chip Fig.7: this diagram shows how the male quick connects are installed on the back of the preamplifier PC board. Left: bend the leads of the 10µF electrolytic capacitors using needle-nose pliers, before installing them on the PC board (Note: some brands are supplied with the leads preformed). that you have to bend their leads down by 90° before installing them on the board. This is best done by slipping an M3 screw through the device tab, positioning it on the board and then gripping one of the leads with a pair of needle-nose pliers just before it reach­ es its mounting hole. The device is then lifted clear of the board and the lead bent at right angles, after which the proce­dure is repeated for the next lead. The four quick connect terminals can be installed next. Unlike the quick connects on other boards, these are installed on the copper side of the Preamplifier PC board and are each secured using an M4 x 10mm machine screw, a star washer and a nut. Fig.7 shows the details. Do up the nuts tightly so that the star washers bite into the quick connects. This should be done using a Phillips-head screwdriver on one side and a 4mm socket or nutdriver on the other. It’s also necessary to get things nice and tight so that the quick connects cannot twist on the board. The capacitors can be now be installed. You can install the four 10µF non-polarised (NP or BP) capacitors either way around but make sure that the polarised electrolytics are correctly oriented. The two 100µF capacitors near REG1 & REG2 should be mounted about 5mm proud of the PC board, so that their bodies clear the regulator leads. Don’t push the 10µF electrolytic capacitors too far down onto the board – applying undue stress to the capacitor leads can damage the internal connections. Instead, the bodies of these www.siliconchip.com.au Fig.8: the mounting details for the Preamplifier & LED Display module. This module must be installed so that the LEDs and volume pot (VR1) can be pushed into position before soldering their leads – see text. Above: take care to ensure that switch S1 is correctly oriented when installing it on the Preamplifier board. Pins 2 & 3 must be towards the bottom – see Fig.6. capacitors should sit 2-3mm above the board. A better scheme is to preform the capacitor leads before fitting them, so that they drop straight in the holes (note: some capacitor brands are supplied like this). You can do this using a pair of needle-nose pliers – see photo. The next step is to fit the two terminal blocks, followed by the 26-way pin header (ie, 2 x 13 pins). This item will prob­ably be supplied as a 2 x 40pin header but it’s easily cut to size using a sharp hobby knife. You will need to cut two sections – one for the preamplifier board and one for the RCA input board. A fine-tipped soldering iron and a good light are necessary when installing the pin headers, as the pins are quite close together and some have tracks running between them. If you have a magnifying lamp, then so much the better. Take care to ensure that there are no shorts between the This photo shows the male quick connects mounted on the prototype Preamplifier & LED Display board pictured last month. We fed the mounting screws through from the other side in the final version. www.siliconchip.com.au pins and any adjacent tracks. By the way, you will notice that many of the tracks around the pin header and rotary switch S1 are not connected at one end. These are earth “guard” tracks and are included to improve the channel separation and to reduce interference from unused signal sources. Don’t join any of these earth tracks together. You could get an earth loop if you do. Now for the rotary switch. Cut its shaft length to 23mm using a small hacksaw, then install it on the PC board with pin 1 positioned exactly as shown on Fig.6 (ie, pins 2 & 3 mount closest to the pin header). Make sure that the switch has been pushed all the way down and is correctly seated against the PC board before soldering any of its pins. It’s a good idea to solder two diametrically opposite pins first, then check that the switch is properly seated before soldering the remaining pins. Once the switch is in, rotate the shaft fully anti-clock­ wise, remove its locking nut and star washer, and move the indexing collar one position anti-clockwise (see photo). Finally, replace the star washer and mounting nut to lock the collar down. The switch should now operate over five positions (instead of six), with the flat section of the shaft facing down for position 3. Installing VR1 & the LEDs The LEDs must all be stood off the Fig.9: the mounting details for the volume control pot (VR1). Note particularly how the wire leads are attached to the pot terminals. PC board so that they later protrude through their matching holes in the front panel when the PC board is mounted in the chassis. This might seem awkward to arrange but it’s really a snack. All you have to do is insert the LEDs into the PC board, then mount the board as shown in Fig.8, push the LEDs through their front panel holes and solder one lead for each LED. After that, it’s just a matter of removing the board and soldering the re­maining leads. Make sure that the LEDs are all correctly oriented (the anode lead is the longer of the two) when installing them on the PC board. Remove the front panel if necessary (by removing the han­dles) so that you can attach the nuts and spacers to the chassis. These should all be tight before mounting the Preamplifier PC board and soldering the LED leads. Once the LEDs are in, you can install VR1 in similar fash­ion. This pot needs to have short lengths of tinned copper December 2001  57 Right: the source switch is changed from a 6-position type to a 5-position type by undoing the locking nut and moving its indexing collar. The volume pot (VR1) is mounted by first soldering tinned copper wire leads to its terminals. It is then loose-fitted to the PC board, the board mounted in the chassis and the pot fastened to the front of the chassis before soldering its leads. INDEXING COLLAR wire soldered to its terminals so that it can be stood off the PC board – see photo. The pot itself is secured by mounting it on the front panel (the tinned copper leads simply make the connec­tions back to the PC board). Begin by straightening a 300mm length of tinned copper wire. This is done by clamping one end of the wire in a vyce and then stretching it slightly by pulling on the other end with a pair of pliers. Cut this wire into six 40mm lengths and bend three at right angles 10mm from one end and the other three at right angles 3mm from one end. These six wires can now be soldered to the pot terminals as shown in Fig.9. Note that the wires with the 10mm sections solder to the top of their pot terminals, while those with the 3mm sec­tions solder to the bottom of their terminals. Also, in the latter case, the right angle bend sits closest to the pot body. You will need to devise some method of securing the pot while you attach the wires – eg, by lightly clamping it in a small vyce or by mounting it on some scrap aluminium. The pot can now be installed by inserting the leads into the PC board, then mounting the board in the chassis and pushing the pot through its mounting hole. Check that the pot’s anti-rotation spigot is correctly located in its hole before doing up the nut and soldering the leads at the back of the board. The Preamplifier board can now be removed from the chassis and placed to one side while you build the other modules. RCA Input Module Assembly Fig.10: the RCA Input Module carries the three RCA 2 x 2 stereo socket pairs and a 26-way pin header. Snip the plastic locking “tangs” off the socket pairs before installing them on the PC board. This one’s a snack, since the board carries just three RCA 2 x 2 stereo socket pairs and a 26-way pin header – see Fig.10. The first step is remove the plastic locking tangs on the RCA socket pairs using a pair of sidecutters. This done, fit the RCA sockets to the board and check that their plastic bodies are flush with the edge of the PC board. If the board protrudes slightly, remove the sockets and file the board down until it is “spot on”. The RCA sockets pairs can now be soldered in place, taking care to ensure that they are all seated correctly. Finally, complete this board assembly by soldering in the 26-way pin header. Power Amplifier Module Assembly You will need to fit a fine tip to your soldering iron before installing the 26way pin header, as the pins are quite close together. Check your work under a magnifying glass when finished, to make sure that are no solder bridges between adjacent pads or tracks. 58  Silicon Chip Now let’s build the Power Amplifier modules. Fig.11 shows the assembly details. Begin each board assembly by installing the wire links, resistors and capacitors but don’t install the two 1000µF elec­trolytic capacitors at this stage. You can then install the 3.3V zener diode (ZD1), the fuse clips and the four PC stakes which terminate the 220Ω 5W test resistor leads (note: www.siliconchip.com.au these resistors are only installed temporarily when adjusting the quiescent current). Make sure that the electrolytic capacitors and the 3.3V zener diode are installed with the correct polarity. Note that the 1W resistors should all be mounted about 1mm proud of the board, to allow the air to circulate beneath them for cooling. The easiest way to do this is to use a strip of cardboard (about 14mm wide and 1mm thick) as a spacer. You just push the resistor body all the way down onto the cardboard, then slide the cardboard back out after soldering the leads – see photo. The same technique can be used for mounting the 2.7kΩ 5W resistor, which should sit about 2mm above the board surface. If the amplifier is intended for continuous high-power delivery at frequencies above 10kHz, then the 6.8Ω resistor in the output filter should be a wirewound type with a rating of at least 5W, otherwise it may burn out. The 1W resistor specified will be perfectly adequate for normal domestic situations. When installing the fuse clips, note that they each have little lugs on one end which stop the fuse from moving. If you install the clips the wrong way, you will not be able to fit the fuses. Next, mount the small-signal transistors; ie, BC546, BC556, BF469 and BF470 (Q1-Q9). These should be push­ed down onto the PC board as far as they will comfortably go before soldering their leads. Transistor pairs Q1 & Q2 and Q5 & Q6 are mounted with their flat faces touching each other, to ensure thermal tracking. Install one transistor of each pair first, then smear their faces with thermal grease before installing the other two transistors. The mounting holes have been carefully positioned on the PC board so that the flat faces of the transistor pairs touch each other when they are installed normally. Transistors Q8 & Q9 must be fitted with U-shaped heatsinks before soldering them to the PC board – see Fig.11. The best way to go about this is to first fit each transistor in position and smear its mating surface with thermal grease, then loosely attach the heatsink using an M3 x 10mm screw, nut and star washer. This done, the assembly can then be pushed down www.siliconchip.com.au Fig.11: the parts layout for the Power Amplifier boards. Note that brass screws, nuts and star washers should be used to mount the quick connect terminals for the two loudspeaker outputs. until the heatsink contacts the PC board, the transistor leads soldered and the mounting screw tightened. Note that insulating washers aren’t required here, since the heatsink doesn’t touch any other components. Choke L1 is wound with 23.5 turns of 1mm enamelled copper wire on a 13mm plastic former. You can straight­ en the wire before winding on the turns by clamping one end in a vyce and then stretching it slightly by pulling on the other end with a pair of pliers. December 2001  59 Fig.12(a): mounting details for the MJL3281A & MJL1302A output transistors. Fig.12(b): mounting details for the MJE15030 & MJL15031 driver transistors. Fig.12(c): mounting details for the MJE340 Vbe multiplier transistor. Fig.13: this diagram shows how the male quick connects are installed on the Power Amplifier, Power Supply and Loudspeaker Protection modules. Be sure to use brass screws, nuts and star washers where indicated on the parts layout diagrams. Bend one end of the wire at right angles (about 10mm from the end) and feed it through one of the end slots in the plastic former, then wind the turns on as neatly and evenly as possible. Finally, bend the free end of the wire through 90°, feed it out through the opposite slot and finish off with a few layers of insulation tape to hold everything in place. The individual coil leads can then be cleaned (by scraping away the enamel) and tinned before soldering the component in place. You can now install trimpot VR1, the 2-way terminal block and the nine male quick connects. Fig.13 shows the mounting de­ tails for the quick connects – each is secured using an M4 x 10mm screw, two star washers and a nut. Note that brass screws, nuts and star washers are used to secure the quick connects for the loudspeaker output terminals. The reason for this is explained in the section where we detail the performance. Do up the nuts tightly so that the star washers bite into the quick connects and the copper pads – and so that the quick connects cannot twist on the board. Note that a double-ended quick connect is used in the 0V position. These three photos show how the leads off the output and driver transistors are bent using pliers and a cardboard spacer, prior to installing them on the PC board. Make sure that the cardboard strip is cut exactly to 4mm before bending the leads at right angles – see also Fig.12 above. 60  Silicon Chip www.siliconchip.com.au This photo shows one of the completed Power Amplifier modules, prior to attaching it to the heatsink. Make sure that the mounting screws for the quick connect terminals are tight (use a screwdriver to stop the terminals from twisting while you do up the nuts with a socket). The other eight quick connects are all single-ended. Heatsink transistors OK, now for transistors Q10-Q16. These transistors are at­ tached to a channel which runs across the centre of the heatsink. This makes it necessary to bend their leads as shown in Fig.12, before mounting them on the PC board. Start by bending the leads for transistors Q11-Q16. As shown, the leads of each device must be bent at right angles away from the metal face, some 4mm from the plastic body. This done, the leads are then bent downwards by 90° a further 4mm away. There’s any easy way to do this and that’s to use a strip of cardboard as a spacer. First, cut a strip of thin cardboard exactly 4mm wide, fold it in half and wrap it around the transis­tor leads so that it butts up against the plastic body. This done, you can then grip the leads right at the edge of the card­board using needle-nose pliers and bend the leads up at right angles. Finally, reposition the cardboard so that it butts against the bend before bending the leads down at right angles. The accompanying photographs show how it’s all done. Transistor Q10 (MJE340) has its www.siliconchip.com.au leads bent as shown in Fig.12(c). By the way, don’t bend any of the transistor leads right at the point where they narrow after emerging from the plastic body – it’s all too easy to break the lead if you do. Once all the leads have been bent, mount transistors Q13-Q16 in position by pushing them all the way down onto the PC board with their metal surfaces facing outwards. Solder one lead of each transistor initially, then test fit it on the heatsink before soldering the other leads. If you haven’t already done so, you will have to remove the heatsink to do this – it’s held in the case by two self-tapping screws at either end and by a self-tapper underneath. A strip of cardboard makes a handy spacer when installing the 1W resist­ ors on the PC boards. Just push the resistors down onto the spacer and withdraw it after soldering the leads. Once it’s out, position the output transistors on the heat­sink and install M3 x 10mm mounting screws to hold the board in position (don’t worry about the silicone washers at this stage). Now inspect the underside of the board – the soldered joints and tracks should all clear of the heatsink channel by several milli­metres. If the board fouls the heatsink, then the transistor leads have been bent incorrectly. Assuming that everything is OK, the remaining leads can all be soldered and trimmed. Transistors Q10, Q11 & Q12 can then be installed on the board in similar fashion. Once all the transistors have been installed, undo their mounting screws and fit each transistor with a thermal washer to isolate it from the heatsink. Transistors Q11 & Q12 also require insulating bushes to ensure isolation – see Fig.12(b). Be sure to use a flat washer under each screw head and don’t overtighten the mounting screws. Now switch your multimeter to a high ohms range and check that there are no shorts between the heatsink screws and any of the transistor leads or metal tabs. If you do find a short, undo each transistor mounting screw until the short disappears. It is then a matter of locating the cause of the short and remounting the offending transistor. Once everything is together, you can continued on page 64 December 2001  61 Ultra-LD 2 x 100W Stereo Amplifier – Performance AUDIO PRECISION AMPRESP 2-CHAN(dBr) & LEVEL(dBr) vs FREQ(Hz) 02 NOV 2001 03:13:43 5.0000 4.0000 3.0000 2.0000 1.0000 0.0 -1.000 -2.000 -3.000 -4.000 -5.000 20 100 1k 10k 100k Fig.14: frequency response in both channels at 1W into 8Ω. It is about 0.3dB down at 20Hz and about 0.5dB down at 20kHz. AUDIO PRECISION AMP-THD THD+N(%) & THD+N(%) vs FREQ(Hz) 5 02 NOV 2001 15:38:23 1 0.1 0.010 0.001 20 100 1k 10k 20k Fig.15: total harmonic distortion (THD) vs. frequency – both channels driven into 8Ω loads at 90W. T HE MAIN SPECIFICATIONS for the Ultra-LD 100W amplifier are shown in an accompanying panel but the graphs of Figs.14-17 give a more detailed picture. Fig.14 shows the frequency response 62  Silicon Chip in both channels at 1W into 8Ω. It is about 0.3dB down at 20Hz and about 0.5dB down at 20kHz. Note that there are slight differences between the two channels. As in the previous amplifier, we have deliberately cur­tailed the high frequency response beyond 20kHz. This is most important for low EMI (electromagnetic interference) susceptibil­ity which means that signals from computers and video monitors are less likely to be a problem. Nor will the amplifier respond to extraneous signals such as 38kHz multiplex from stereo FM tuners or over-sampling artefacts from CD players or other digi­tal program sources. By the way, all of these performance graphs are for signals passing through both the preamplifier and amplifier stages. Therefore readers who have access to the previous series of articles on the Ultra-LD 100W amplifier in the March, May and August 2000 issues will notice that there are some differences in performance. Specifically, because the audio signals now pass through more stages of amplification and because of the new amplifier’s much more compact wiring layout, the distortion performance is not quite as good as the earlier, simpler design. Nevertheless, the performance is still pretty good and far better than the vast majority of commercial hifi amplifiers, many of which cost much more. Fig.15 shows the total harmonic distortion in both channels at a power level of 90W into 8-ohm loads and with an input signal of 1.8V RMS for the frequency range from 20Hz to 20kHz. These curves are taken with a measurement bandwidth of 10Hz to 80kHz. Fig.16 shows the total harmonic distortion on both channels at 1kHz over a range of powers from 500mW to well beyond clip­ ping, with both channels driven simultaneously into 8-ohm loads. As can be seen, one channel clips (ie, overloads) at about 90W while the other clips at around 98W. Naturally, somewhat more power is delivered from both channels under normal program condi­tions. Measurement bandwidth is 10Hz to 22kHz. As can be seen, for most powers at 1kHz, the total harmonic distortion is between .002% and .003%. The rise in the curve at lower powers is due to residual noise and is not due to an actual increase in distortion. As mentioned in previous articles on this amplifier design, we do not recommend it for use with 4-ohm loudspeakers, as the distortion is quite a bit higher. Signal-to-noise ratio is -103dB www.siliconchip.com.au un-weighted (20Hz to 20kHz) or -109dB A-weighted. This is taken at maximum sensitivity (ie, volume control fully clockwise) and is even better at normal settings. It is really very quiet. Fig.17 shows the separation between channels and as you can see, it is just on -60dB over the whole frequency range from 20Hz to 20kHz, in both directions; ie, from left to right and from right to left and this applies for any source, at any normal program level or power output. Finally, the damping factor is just over 100 for all fre­quencies below 1kHz, dropping to a bit over 60 at 10kHz. Again, these are very good figures. AUDIO PRECISION SCTHD-W THD+N(%) vs measured LEVEL(W) 10 02 NOV 2001 17:19:43 1 0.1 0.010 Brass screws and nuts In measuring the performance of the prototype amplifier we found that the initial distortion measurements were somewhat disappointing. While they were respectable, they were not quite as good as we were hoping for. In order to track down the problem, we then decided to measure the harmonic distortion through the power amplifi­ers only. So we connected the Audio Precision input signals directly to the inputs of the amplifier modules and measured at the speaker output terminals at the rear of the case. Guess what? The distortion was still high; about twice what we have come to expect from these modules. Eventually, we decided to measure the distortion right at the output of the amplifier modules themselves. What a revelation! Here the distortion was of the same magnitude as published in the original design last year. So what was happening? To cut a long story short, the distortion arose from the various bolted “quick connect” spade lug connections from the amplifier and loudspeaker protection PC boards. We were using bright nickel-plated screws and bolts with plated brass “quick connect” spade lugs and each of these dissimilar metal joints were obviously non-linear; ie, the resistance across each joint varied with the current and this caused a small but significant increase in distortion. Replacing all screws and nuts for the quick connects in the output signal path with brass equivalents eliminated those increases in distortion, giving a very worthwhile improvement in performance. These brass screw connections are shown in different colours on both the Power Amplifier modules and the Loudspeaker Protection board. www.siliconchip.com.au 0.001 .0005 0.5 1 10 100 200 Fig.16: total harmonic distortion (THD) vs. power at 1kHz – both channels driven simultaneously into 8Ω loads. AUDIO PRECISION SCCRSTK XTALK(dBr) & XTALK(dBr) vs FREQ(Hz) 0.0 02 NOV 2001 17:28:53 -20.00 -40.00 -60.00 -80.00 -100.0 -120.0 20 100 1k 10k 20k Fig.17: channel separation across the frequency range from 20Hz to 20kHz. Specifications Of Prototype Output power.................. 100 watts per channel into 8-ohm loads Frequency response....... -0.3dB down at 20Hz; -0.5dB at 20kHz (see Fig.14) Input sensitivity............... 0.5V RMS (for full power into 8Ω) Harmonic distortion......... see Fig.15 & Fig.16 Signal-to-noise ratio........ -103dB unweighted (20Hz to 20kHz); -109dB A-weighted .........................................(with respect to 100W) Damping factor............... >100 at 100Hz and 1kHz; >60 at 10kHz Stability........................... unconditional December 2001  63 Table 1: Resistor Colour Codes Preamplifier & LED Display Module   No.    2    2    2    2    2    2    2    2    2    3    4    2    2    1 Value 680kΩ 330kΩ 220kΩ 150kΩ 100kΩ 6.8kΩ 4.7kΩ 2.2kΩ 1.8kΩ 1.2kΩ 150Ω 100Ω 33Ω 10Ω 4-Band Code (1%) blue grey yellow brown orange orange yellow brown red red yellow brown brown green yellow brown brown black yellow brown blue grey red brown yellow violet red brown red red red brown brown grey red brown brown red red brown brown green brown brown brown black brown brown orange orange black brown brown black black brown 5-Band Code (1%) blue grey black orange brown orange orange black orange brown red red black orange brown brown green black orange brown brown black black orange brown blue grey black brown brown yellow violet black brown brown red red black brown brown brown grey black brown brown brown red black brown brown brown green black black brown brown black black black brown orange orange black gold brown brown black black gold brown Power Amplifier Modules   No.    4    2    2    2    2    2    4    4    6    8    4    2    2  16 Value 18kΩ 12kΩ 3.3kΩ 1.2kΩ 1kΩ 390Ω 330Ω 150Ω 120Ω 100Ω 47Ω 10Ω 6.8Ω 1.5Ω install the two 1000µF 63VW electrolytic capacitors on the power amplifier module. There are two reasons why these aren’t installed earlier on: (1) they’re easily bumped and damaged; and (2) they make it awkward (but not impossible) to do up the mounting screws for Q11 & Q12. The other power amplifier module can now be built and mounted on the heatsink in exactly the same fashion. Finally, complete the power amplifier assembly by attaching a 25mm tapped brass spacer to each of the front mounting hole positions. Thermal cutout There’s just one more part to attach to the heatsink and that’s the 60°C thermal switch for the heatsink fan. This 64  Silicon Chip 4-Band Code (1%) brown grey orange brown brown red orange brown orange orange red brown brown red red brown brown black red brown orange white brown brown orange orange brown brown brown green brown brown brown red brown brown brown black brown brown yellow violet black brown brown black black brown blue grey gold brown brown green gold brown Table 2: Capacitor Codes Preamp & LED Display Module       Value IEC Code EIA Code 0.22µF   220n   224 .01µF   10n  103 390pF 390pF   390 33pF  33pF   33 10pF  10pF   10 Power Amplifier Modules      Value IEC Code EIA Code 0.15µF   150n   154 0.1µF   100n   104 .0012µF   12n  121 100pF   100pF   100 5-Band Code (1%) brown grey black red brown brown red black red brown orange orange black brown brown brown red black brown brown brown black black brown brown orange white black black brown orange orange black black brown brown green black black brown brown red black black brown brown black black black brown yellow violet black gold brown brown black black gold brown blue grey black silver brown brown green black silver brown is bolted to the centre of the heatsink, between the two power amplifier modules using two M3 x 10mm machine screws (see photo). As supplied, the quick connect terminals on the thermal switch run parallel to its metal mating surface. These terminals should be bent back about 70° (to make them easier to access later on) using needle-nose pliers before bolting it to the heatsink. Next month That’s all we have space for this month. Next month, we'll describe the assembly of the Power Supply and Loudspeaker Protector modules and give the wiring details. We’ll also publish a complete parts list for those determined to start from scratch. SC www.siliconchip.com.au