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CLASSiC DAC Pt.3 Third article has the full PCB layout & the assembly details Last month, we described all the features of the CLASSiC DAC and how it works in some detail. Now it’s time to put it together and get it going. There are quite a few parts to install, so let’s start building it. T HE CLASSiC DAC is built on two PCBs: a main PCB coded 01102131 (157 x 198mm) and a front-panel PCB coded 01102132 (219 x 35mm). A third PCB coded 01102133 (219 x 35mm) is used for the rear panel and the entire assembly is housed in a standard lowprofile instrument case measuring 225 x 165 x 40mm. Fig.11 shows the parts layout on the main PCB. This holds most of the parts, including all the SMDs which must be installed first. Alternatively, if a kit is made available, the PCB will probably be supplied with the SMDs pre-loaded. So if you are building one of these kits, you can skip the following section and move straight on to fitting the through-hole components. 36  Silicon Chip There are five fine-pitch SMDs to install but if you are careful and follow our instructions, you should be able to reliably hand-solder them. Start with ICs1-3 and IC7, all of which are in similar Shrink Small Outline Packages (SSOP), although IC7 has 20 pins while the rest have 28. First, remove one of the chips from its protective packaging and place it alongside the appropriate set of pads with the marked side up. Check the part code with a magnifying glass to ensure it’s going in the right place and find the small divot or dot in the corner which indicates pin 1. Once you have located it, gently rotate the part so that this dot lines up with the pin 1 indicator on the PCB layout. We’ve described SMD soldering techniques on a number of occasions in the past so we’ll just cover the basics here (for more information, refer to pages 80 & 81 in the June 2012 issue of SILICON CHIP). You will need a soldering iron with a reasonably fine tip (medium conical or chisel is fine), good quality solder of 0.7mm diameter or less, solder wick, flux paste, tweezers, a desk lamp or other strong source of light, a magnifier of some sort and ideally, a syringe of flux paste (“no-clean” type if possible). Start by carefully applying a tiny bit of solder to one of the IC pads. If you are right-handed it’s generally best to start with the upper-right pad or if you are left-handed, the upper-left. Then, siliconchip.com.au By NICHOLAS VINEN using tweezers, slide the chip into place while heating the solder on that pad. That done, remove the iron and check that the part is correctly orientated and that it is centred on its pads. All pins must be over their corresponding pads and not too close to an adjacent pad. In particular, check that the IC is not rotated too much, ie, the pins should all be parallel to their pads (you will require a magnifying glass to check this properly). If the position isn’t good enough (it rarely is on the first attempt), reheat the soldered pin and gently nudge the chip into place. Once it’s correctly aligned, solder the diagonally opposite pin. It’s a good idea to now recheck the orientation of the IC and, if necessary, reheat that second pin since it’s easy to rotate the chip slightly during the soldering process. You can then solder the rest of the pins, starting at one of the other corners. Don’t be concerned if you bridge some of the pins during siliconchip.com.au this process, as that can be easily fixed up later. Once all the pins have been soldered, refresh the solder on the first couple of pins you soldered. Now use solder wick to clean up any bridges. A dab of no-clean flux paste applied to the bridge beforehand makes this a lot faster and easier. In fact, we recommend applying flux paste along both edges of the IC and then cleaning the pins up with solder wick even if there are no apparent bridges as this reduces the chance of bad joints. When finished, carefully examine the chip under magnification to ensure that no bridges are left (also check the top of the pins, where they enter the IC package). It’s also a good idea to examine the area where the pins are in contact with the PCB pads to ensure that the solder has flowed onto the pads properly. Repeat this procedure for the other three SSOP ICs. Once they are all in place, the remaining flux paste can be removed from the PCB and the ICs using isopropyl alcohol and a lint-free cloth. However, this isn’t strictly necessary if you used no-clean flux paste. Remaining SMDs The only remaining fine-pitch chip is microcontroller IC5. This has a similar pin pitch to the aforementioned ICs (slightly smaller, in fact) and it also has pins on all four sides. The soldering technique described above works just as well for this device but you will need to be extra-careful to check the alignment of the pins on all four sides before you solder the second pin. Note that the pin 1 dot is in one corner and a corresponding dot is shown on the PCB, just outside the quad flat-pack outline. The next job is to solder the small outline integrated circuit (SOIC) parts. These include IC8 (14 pins), REG4 April 2013  37 The main PCB holds most of the parts, including all the ICs (some surface-mount), the connectors, the volume control, the headphone socket and the SD card socket. Resistor Colour Codes o o o o o o o o o o o o o o o o o o o o No.   4 18   1   6   8   7   3   6   1   6   2   7   2   1   7   3   6   2   1 Value 1MΩ 100kΩ 47kΩ 22kΩ 10kΩ 4.7kΩ 3kΩ 1.5kΩ 1.2kΩ 1kΩ 750Ω 470Ω 240Ω 180Ω 100Ω 75Ω 22Ω 10Ω 2.2Ω (5%) (8 pins) and the six dual Mosfets (8 pins). These are much easier to install than the previous ICs, as their pins are further apart. However, while their pins are spaced widely enough to be soldered individually, it’s easier and more reliable to use the same technique 38  Silicon Chip 4-Band Code (1%) brown black green brown brown black yellow brown yellow violet orange brown red red orange brown brown black orange brown yellow violet red brown orange black red brown brown green red brown brown red red brown brown black red brown violet green brown brown yellow violet brown brown red yellow brown brown brown grey brown brown brown black brown brown violet green black brown red red black brown brown black black brown red red gold gold described above; ie, apply a generous amount of solder to all the pins, then clean up the joints using solder wick and flux paste to remove any solder bridges. REG5 goes in next and it has an even wider pin spacing so you should be able to solder its pins individually 5-Band Code (1%) brown black black yellow brown brown black black orange brown yellow violet black red brown red red black red brown brown black black red brown yellow violet black brown brown orange black black brown brown brown green black brown brown brown red black brown brown brown black black brown brown violet green black black brown yellow violet black black brown red yellow black black brown brown grey black black brown brown black black black brown violet green black gold brown red red black gold brown brown black black gold brown not applicable without bridges. That done, install the five SMD ceramic capacitors but be careful to let the first joint solidify before you try to make the second, otherwise surface tension can pull the part out of place. The last SMD part to fit is the SD card socket which may or may not siliconchip.com.au 220 µF 35V 100nF Q16 Q10 BC337 1k 2.2Ω 10k D8 IRF7309 CLASSiC DAC D9 REG3 7805 REG1 7815 10k 1k REG2 7915 1M 192kHz 7-input Stereo DAC with SD Card Playback & Headphone Amplifier 10k CON13 1 A k Sw Sw To Power Switch LED8 LED7 LED6 22k 100k 470Ω 1.5k 22k 100k 100k 22k 100k 22k 100Ω LED5 ICSP 1 22k 100k LED4 100nF CON11 On Off 100nF 1 22k 100k LED3 10Ω dsPIC33FJ128GP306T 3.0k 10k LED2 100nF IC5 LED1 10 µF 100k 4004 VR1 10k LOGx2 Q15 VR2 2k BC337 1k 3.0k 10k CON10 SD Card Power On Auto-sel IR codes Stby. LED IC8 74LV74 Q13 1 µF 100k 4004 Headphones 100nF MKT 100nF S2 D2 10V Q11 LEDs (green) 220 µF 1 2 3 4 REG5 100 µF 100nF 25V 4004 D4 470 µF 16V 1M 100 µF 100 µF 100nF Q12 REG4 100k 5819 1k 22 µF D1 D3 47 µH 0.5A LK1 5V D7 D5 L5 D6 100nF + 4004 100nF Q14 BC559 Si4804 22 µF + 4004 AP5002 LK2 (3.3V) 100 µF ZD6 10 Ω/L9 * 100nF 100nF L6 470 µH 0.5A 4.7k 470 µF 220 µF 25V 4004 330pF 33pF 33pF 100k 100k 1.2k 100nF JP1 5V 3.3V MIC39100 –3.3 4004 470Ω 1 µF 100 µH 100 µH 1 µF x2 10 µF 10 µF 10 µF 4004 33pF X1 IC2 2 L3 L2 CON9 9VAC 33pF 10V 22Ω 22Ω 100nF X2 27MHz 100nF 12MHz PCM2902E L7 100 µH 3 2 1 L1 RX1 1 4 3 2 1 10 µF + RX2 1 µF 1 µF 100nF 3 2 1 22µH 100nF RX3 1.5k 22Ω 22Ω 2.2Ω 100nF 3 USB 1M 100k CON1 100nF 10nF GND 10 100nF 0nF MKT BC327 100k 100 µF 100nF 4.7k CON8 10 Ω/L8 * BC3 2 7 K1 K2 K3 K4 ZD8 Q7 Q9 VR3 2k 470Ω 180Ω 100 µF 100 µF CON5 4.7k 4.7k IC3 CS4398CZ 100k 47k 10V ZD5 BC337 Q8 CON12 A 1 ZD7 22Ω C 2013 01102131 100pF 100Ω 10k GAIN (R) 100 µH IC7 In4 75Ω 12dB 0dB L4 10V 22Ω Q6 100nF JP3 PLL1708 CON2 BC559 100pF 100 µF CS8416-CZ In5 75Ω IC4 LM833 GND 2x 100 µF 4.7k BC327 GAIN (L) 100k IC1 75Ω CON3 10nF 10nF 10nF 10nF 10nF 100nF 10nF 220nF 1k 10nF 10nF 10nF 100k 10k 100k 4.7nF 240Ω 470Ω 22nF 470Ω 100Ω In6 12dB 0dB Q3 Si4804 BC337 Q5 100nF JP2 100nF 10 µF Si4804 4.7k 4.7k 18V 100nF 1k 3.0k 10k 18V 2x10 µF 50V 100k 100nF 100 µF 100nF 220nF ZD2 ZD1 2x 100 µF 100nF 1.5k Left Out 6.8nF Q1 Si4804 Q4 BC559 4.7nF 100nF 47 µF 10 µF 10nF 50V 100Ω CON6 220 µF IC4 LM833 1.5nF 220 µF 100Ω 1.5k 750Ω CON7 47 µF 100Ω Q2 1.5k Si4804 Right Out 470Ω 240Ω 470Ω 22nF 18V 100Ω 1.5k 750Ω ZD3 6.8nF ZD4 18V K A K A K A K A K A K A K A K A 100nF IRD1 1M Fig.11: the parts layout for the main PCB of the CLASSiC DAC. Nearly all the parts are mounted on this PCB and all parts are installed on the top side of the board. The DAC IC circuitry is at upper-left, with the headphone amplifier at upperright. The digital audio receiver is below the DAC and then further down is the USB audio chip (IC2) and below that, in the bottom-left corner, the power supply. The control circuitry, including microcontroller IC5, is at mid to lower-right. Errata: if you find that one or more of the TOSLINK input LEDs light up when there is no signal present, a 30pF capacitor can be installed across the empty pair of pads near the TOSLINK receivers. come pre-soldered in a kit. This has two small plastic posts which fit into holes on the PCB. Remove the internal plastic frame which protects it during transport. You should find that it won’t siliconchip.com.au move much once the posts go in the holes and it’s then just a matter of carefully soldering the SMD pins one at a time, starting with the two large ones on either side near the edge of the PCB. It’s probably a good idea to check its orientation after soldering the first pin to make sure it’s properly aligned with the edge of the PCB. There are 11 pads to solder on the inside of the April 2013  39 CLASSiC DAC Par t s Lis t 1 main PCB, code 01102131, 157 x 198mm 1 front panel PCB, code 01102132, 219 x 35mm 1 rear panel PCB, code 01102133, 219 x 35mm 1 low-profile instrument case, 225 x 165 x 40mm (Altronics H0474, Jaycar HB5972) 4 100µH axial RF chokes (L1-L4) 1 47µH 500mA+ bobbin inductor (L5) 1 470µH 100mA+ bobbin inductor (L6) 1 22µH axial RF choke (L7) 1 10kΩ dual-gang 16mm log potentiometer with D-shaft (VR1) 1 knob to suit VR1 (eg, Altronics H6211) 2 2kΩ mini horizontal sealed trimpots (VR2,VR3) 1 12MHz HC-49 crystal (X1) 1 27MHz HC-49 crystal (X2) 1 4-way DIP switch (S2) 3 3-way pin headers with shorting blocks (JP1-JP3) 3 16Mbps TOSLINK receivers (RX1-RX3) 3 black switched PCB-mount rightangle RCA sockets (CON1CON3) 1 PCB-mount right-angle type B USB socket (CON5) 1 white switched PCB-mount rightangle RCA socket (CON6) 1 red switched PCB-mount rightangle RCA socket (CON7) socket plus the two aforementioned mounting pins on either side and two at the rear. Now make a final inspection of all the SMD joints since it’s going to be harder (although by no means impossible) to fix any solder bridges or bad joints later when the adjacent throughhole parts have been fitted. Through-hole parts Now fit all the 0.25W resistors. It’s a good idea to check their values with a DMM beforehand since it can often be hard to distinguish certain colours. There are about 90 resistors, so it will take some time to install them. The diodes are next on the list. Make sure that you don’t get the four different types mixed up and check that they 40  Silicon Chip 1 PCB-mount 6.35mm jack socket with long pins (CON8) 1 PCB-mount DC socket (CON9) 1 Oupiin PCB-mount SD card socket (CON10) [Altronics P5720] 1 5-way pin header, 2.54mm pitch (CON11) 1 5-way polarised pin header, 2.54mm pitch (CON12) 1 4-way polarised pin header, 2.54mm pitch (CON13) 1 5-way right-angle polarised pin header, 2.54mm pitch (CON14) 1 4-way right-angle polarised pin header, 2.54mm pitch (CON15) 2 5-way polarised plugs with crimp pins 2 4-way polarised plugs with crimp pins 1 200mm length 10-way ribbon/ rainbow cable 1 infrared receiver (IRD1) [Jaycar ZD1952, Altronics Z1611A] 1 panel-mount momentary pushbutton switch with integral LED (S1) (Altronics S0933) 2 8-pin DIL sockets 1 2m length 0.4mm diameter enamelled copper wire 1 40mm length 10mm diameter heatshrink tubing 1 60mm length 0.7mm diameter tinned copper wire 3 M3 x 6mm machine screws with nuts and shakeproof washers 6 No.4 x 6mm self-tapping screws or M3 x 6mm machine screws are orientated as shown on the layout diagram. Virtually all of the diodes go in with their cathode stripe towards the front of the PCB (ie, right side on the diagram) but two of the 10V zeners are installed the other way around, so be careful with those. Now fit the five axial inductors, noting that L7 has a different value from the other four. Follow with the two 8-pin DIL sockets for IC4 and IC6 (orientate the notches as shown), then solder crystals X1 (12MHz) and X2 (27MHz) in place (don’t get them mixed up). Follow with the three TO-220 reg­ ulators. In each case, bend the pins down through 90° 6mm from the body and then attach the tab to the PCB using an M3 x 6mm machine screw, 1 9VAC 0.67A plugpack (Altronics M9231) 1 universal infrared remote control (optional; Altronics A1012 suggested) 1 high capacity SD/SDHC/SDXC card (optional) Semiconductors 1 CS8416-CZZ digital audio receiver [TSSOP-28] (IC1) [element14 1023452, Digi-Key 598-1124-5-ND) 1 PCM2902E USB audio IC [SSOP-28] (IC2) [element14 8434700, Digi-Key 296-129865-ND) 1 CS4398CZZ 24-bit 192kHz DAC [TSSOP-28] (IC3) [element14 1023397, Digi-Key 598-10675-ND) 2 LM833N/LM833D dual op amps (IC4,IC6) 1 dsPIC33FJ128GP306-I/PT microcontroller programmed with 0110213B.hex [TQFP-64 package] (IC5) [element14 1294837, Digi-Key*] 1 PLL1708DBQ clock generator [SSOP-20] (IC7) [Digi-Key 29614183-5-ND] 1 74LV74D dual flip-flop IC [SOIC14] (IC8) [element14 1085356, Digi-Key 568-7663-1-ND] 1 7815 15V 1A linear regulator (REG1) 1 7915 -15V 1A linear regulator (REG2) 1 7805 5V 1A linear regulator (REG3) shakeproof washer and nut. Do the nut up tightly and then solder and trim the three pins. Note that each regulator is a different type. You can now fit the small signal transistors, bending their leads with small pliers to fit the pads. There are three different types, so match them to the type numbers shown on the PCB and parts list. The 4-way DIP switch can now be fitted, with its “ON” marking matching that on the board. Once it’s in, install VR2 & VR3, followed by the MMC, ceramic disc and MKT capacitors. Large inductors You now need to wind the two output inductors (L8 & L9, near headphone socket CON8) onto the 10Ω 1W siliconchip.com.au 1 AP5002S switchmode regulator [SOIC-8] (REG4) [element14 1085356, Digi-Key*] 1 MIC39100-3.3WS or LM3940IMP-3.3 LDO regulator [SOT-223] (REG5) [element14 1556715/9779280, Digi-Key 5761172-ND/*] 5 Si4804CDY dual N-channel Mosfets [SOIC-8] (Q1,Q2,Q13, Q15,Q16) [element14 1779273, Digi-Key*] 3 BC559 PNP transistors (Q3,Q4, Q14) 4 BC337 NPN transistors (Q5,Q7, Q8,Q10) 3 BC327 PNP transistors (Q6,Q9, Q11) 1 IRF7309 dual N+P channel Mosfet [SOIC-8] (Q12) [element14 9102175, Digi-Key*] 8 1N4004 1A diodes (D1-D5, D7-D9) 1 1N5819 1A Schottky diode (D6) 4 18V 1W zener diodes (ZD1-ZD4) 4 10V 1W zener diodes (ZD5-ZD8) 6 blue 3mm LEDs with diffused lenses (LED1-3,LED5-7) [Seeed Studios] 1 yellow 3mm LED with diffused lens (LED4) 1 red 3mm LED with diffused lens (LED8) 4 green 3mm LEDs with diffused lenses (LED9-LED12) Capacitors 3 470µF 16V electrolytic 2 220µF 35V electrolytic 3 220µF 25V electrolytic resistors. To do this, cut a 1-metre length of 0.4mm-diameter enamelled copper wire and strip 3-4mm of the insulation from both ends using a sharp hobby knife or fine emery paper. That done, tin both ends, then solder one end to the lead of a 10Ω resistor, as close to the body as possible. It’s then just a matter of carefully winding the wire around the resistor body; they normally narrow in the middle which helps keep the windings in place. Wind the turns as close together and as neatly as possible. Once you have wound on as many turns as you can fit, start a new layer in the opposite direction and repeat until you’ve used up all the wire. The first couple of layers can be pretty neat but you will probably find siliconchip.com.au 2 220µF 10V electrolytic 2 100µF 25V electrolytic 12 100µF 16V electrolytic 2 47µF 16V electrolytic 2 22µF 25V SMD ceramic, size 4832/1812 10 10µF 50V electrolytic 1 10µF 6.3V SMD ceramic, size 3216/1206 6 1µF MMC 2 220nF MMC 2 100nF 50V X7R SMD ceramic, size 3216/1206 28 100nF MMC 2 100nF MKT 2 22nF MKT 10 10nF MMC 2 10nF MKT 2 6.8nF MKT 2 4.7nF MKT 2 1.5nF MKT 1 330pF ceramic disc 2 100pF ceramic disc 4 33pF ceramic disc Resistors (0.25W, 1%) 4 1MΩ 2 750Ω 18 100kΩ 7 470Ω 1 47kΩ 2 240Ω 6 22kΩ 1 180Ω 8 10kΩ 7 100Ω 7 4.7kΩ 3 75Ω 3 3kΩ 6 22Ω 6 1.5kΩ 1 10Ω 1 1.2kΩ 2 10Ω 1W 5% 6 1kΩ 2 2.2Ω 5% * Note: catalog number for DigiKey components listed are the same as the part type code that the last couple are a bit messy. This doesn’t matter; just keep the turns tight and make it as neat as you can. Once the winding is finished, solder the free end to the other lead of the resistor, again adjacent to the body. Finally, slip a 15mm length of 10mmdiameter heatshrink tubing over the inductor and shrink it down using a hot-air gun to hold the windings firmly in place. It should now be possible to bend the resistor’s leads down close to its body and solder the part in place. This process is then repeated for the other resistor/inductor. The orientation isn’t critical, although it’s preferable to install them with their windings going in the same direction. The two pre-wound bobbin induct­ Capacitor Codes Value 220nF 100nF 22nF 10nF 6.8nF 4.7nF 1.5nF 330pF 100pF 33pF µF Value 0.22µF 0.1µF 0.022µF 0.01µF .0068µF .0047µF .0015µF   NA   NA   NA IEC Code EIA Code 220n 224 100n 104   22n 223   10n 103   6n8 682   4n7 472   1n5 152 330p 331 100p 101   33p   33 Component Change Since publishing the circuit last month, we have decided to change the value of RF inductor L7 from 100μH to 22μH (bottom of Fig.4, p21, March 2013). That’s because the resulting lower DC resistance of the inductor improves the unit’s immunity to severe power supply noise which may be coupled in via the host PC’s power supply (whether it is a desktop or laptop). This change is reflected in the parts list and PCB overlay published this month. ors (L5 & L6) can also now be fitted. Note that while these may look identical, they have different values. If they aren’t marked and you don’t know which is which, the lower value inductor should have a substantially lower DC resistance as measured with a standard DMM. Install the four pin headers next, followed by the USB socket (CON5), the electrolytic capacitors, the DC power socket (CON9) and the TOSLINK receivers (RX1-RX3). The RCA sockets can then go in but make sure these sit flat against the PCB and are perpendicular to the adjacent edge before soldering their pins. In fact, it’s best to solder one pin of each RCA socket first, then carefully adjust each socket before soldering its remaining pins. Use black sockets for CON1, CON2 & CON3 (the S/PDIF inputs), a white socket for CON6 and a red socket for CON7. Finishing up The next step is to test fit headphone April 2013  41 Designing The CLASSiC DAC’s PCB W E DON’T normally go into much detail describing the PCB layout, other than to present the overlay diagram. But since this is one of the more complex and demanding PCBs we have designed, we thought it was worth going over it quickly to explain the salient points of the design. We decided from the outset to place the power supply on the opposite side of the PCB to the analog circuitry to minimise the chance of rectifier buzz or other power supply noise from getting into the analog paths. As a result, the power supply is located at the left rear of the PCB, while the analog outputs are at right rear, with the digital audio circuitry in between. It was then immediately clear that with the power input socket at the left side of the rear panel and the analog outputs at right, the seven digital input sockets had to be spaced out between them. The logical place for the headphone socket and volume control was then on the right side of the front panel, to minimise the distance that the analog signal had to travel from the DAC chip to the headphone amplifier. It’s preferable to have the power switch at one end of the front panel so this then had to go on the left, with the status LEDs and SD card socket in the middle. socket CON8 and potentiometer VR1 on the PCB. Push these parts down as far as they will go, then check that the headphone socket’s entry hole lines up with VR1’s shaft. Provided you’ve used the socket specified in the parts list, it should be correct. On the other hand, if the socket is too low, you will need to swap it for a slightly different type which has wide pins emerging from the bottom of the plastic housing which then narrow to go into the PCB holes. Assuming you do have the right socket, install it now, making sure that it sits flush against the PCB. Before soldering VR1 into place, first check the distance between its threaded screw section and the Dshaped profile of the shaft. If it’s like the one we used and there is only a 2mm cylindrical shaft section, then you can leave it as it is. However, if the 42  Silicon Chip With all the main component locations determined, that just left the routing of the tracks. In doing this, we used three overriding principles to guarantee good performance: (1) join the digital and analog grounds at one point only (ideally, in the power supply); (2) minimise any radiation from the digital tracks; and (3) if possible, use a star earth for the analog ground tracks. We ended up joining the two sets of ground tracks at two different points. However, they are both close together, near the TO-220 regulators, and due to the thickness of the ground planes and numerous vias joining the top and bottom layers, there is no measurable digital noise coupling into the analog grounds. EMI from the digital circuitry was minimised by filling all the spare areas around and under digital tracks with copper ground planes. These are studded with dozens of vias between the top and bottom ground planes, thereby minimising the impedance through which ground currents flow. By doing this, the loops through which digital currents flow are kept very small and so very little magnetic radiation is produced which could otherwise couple into nearby analog tracks. In addition, all the SMD ICs have a ground fill underneath them on both flat section stops much further from the screw thread, you will need to file the rear section of the shaft flat. This can be done by holding the shaft in a vice and then using the flat section as a guide to file the rest down. Now cut the shaft at exactly 10mm from the screw thread using a hacksaw and clean up the edges with a file. That done, file a small area of passivation off the very top of the pot body. Do not breathe the dust during this process (wear a face mask) and be sure to wash your hands after cleaning up, as it may be plated with cadmium which is a heavy metal. The pot can now be soldered to the PCB. Once it’s in, solder a length of tinned copper wire to the adjacent pad marked “GND”, then bend it over the top of the pot (ie, over the section you filed clean) and solder it to the pad marked “GND” on the other side. layers. For the critical ICs, such as the digital audio receiver, DAC, USB-toS/PDIF converter and clock generator, a zig-zag of vias is placed directly under the chip to provide even better earthing. This does the same job for the internals of the IC as the copper fill does for external tracks, ie, it minimises electromagnetic coupling into, out of and between the parts inside the chips. Ground planes are also used under some parts in the analog section but these have been carefully designed so that they provide a star earth. The left and right channels have separate ground fills between the line output sockets and they join near the copper fill under the DAC IC. The DAC’s ground is then connected straight to the ground pin of the headphone socket, where the headphone amplifier and volume control grounds are also joined. A single track then runs from this point back down the middle of the board (along with the other power supply tracks which must run its length) and then down to where the digital and analog earths are joined. To further reduce the chance of any switched loads generating EMI which could affect the analog section, the LED brightness is not controlled by PWM (except in standby mode when the DAC is off). Instead, multiple microcontroller outputs and current-limiting resistors are used to control the currents through the LEDs. Finally, solder this wire to the top of the pot as well, so that the metal body is grounded. Preparing the case It’s easier to install the remaining components with the board in the case but first, some of the plastic posts must be removed from the base. Begin by checking which way around the board goes – it only fits one way since the posts are not symmetrically arranged – then remove any posts which don’t correspond to a mounting hole on the PCB (ignore the four right next to the taller posts). You can remove the unwanted posts by filing them down or cutting/levering them off with side-cutters and pliers. There are two at the front to get rid of (ie, under the SD card socket and headphone socket) and three at the rear (the middle three along the edge). siliconchip.com.au siliconchip.com.au 01102132 That completes the main PCB assembly; now for the front panel board. Fig.12 shows the parts layout on this PCB. Starting by fitting the two rightangle polarised pin headers. These are surface-mounted so install them in a similar way to the SMDs; ie, place a generous amount of solder on one of the pads, heat this solder and then slide the header into place. Make sure it’s the right way around and sitting flat before soldering the remaining pins. The four green LEDs are also surface-mounted but first their leads must be bent to suit. Start by bending them through an angle of about 100° right This view shows the completed front-panel PCB. The square cut-out at the bottom provides access to the SD card, while the two large holes at left are for the volume control pot shaft and the headphone socket. LED12 LED11 LED9 LED10 k k A k A k A A Front panel assembly Fig.12: follow this diagram to mount the parts on the rear of the front-panel PCB; ie, four green LEDs, two polarised connectors (CON14 & CON15) and the illuminated power switch. Take care with the orientation of the connectors; they face in opposite directions. The front-panel PCB is connected to the main PCB via two cables. Sw k A Sw + 1 1 A K1 K2 CON14 K3 K4 Fitting the LEDs You can now fit the eight LEDs along the front of the main PCB. The first step is to bend their leads down through 90° about 1mm back from the lens. Do this so that the longer (anode) lead will go through the hole marked “A” on the layout diagram. Check each LED carefully, then trim both leads on each device to about 14mm. LED1 can now be fitted to its PCB pads and its lens slipped into its corresponding hole in the front panel. Make sure the lens is pushed all the way into its hole, then tack solder the leads to the pads on the top of the PCB. Repeat this process for the other seven LEDs, making sure that you use a yellow LED for LED4 and a red LED for LED8 (the others are all blue LEDs). Now do the same for the infrared receiver (IRD1), except that its leads should first be trimmed to protrude around 8mm below its plastic body. Once these parts are in, undo the mounting screws along the front edge of the main PCB, remove the assembly from the case and separate the two boards. You can now finish soldering the LEDs and IR receiver to the bottom of the PCB and trim any excess lead lengths. Sw k A Sw CON15 Now remove any nuts and washers from the pot and headphone socket and slip the front panel over their shafts. That done, lower the assembly into the case, guiding the front panel into its slots. Check that the main board sits flat on its mounting posts on the base, then secure the board in place by fitting self-tapping screws to the four mounting holes behind the front panel. Note: M3 x 6mm machine screws can also be used here, although you will have to use some force to cut a thread in the plastic posts the first time you install them. up against the rear of the lens, so that the longer (anode) lead is on the left as shown in Fig.12. That done, bend the leads back parallel to the rear of April 2013  43 This view shows the completed assembly from the rear with the top cover removed. The two audio output sockets are at left, then the three S/PDIF (coaxial) inputs, the USB audio input and the three TOSLINK (optical) inputs. The power socket (to connect a 9VAC plugpack) is at far right. the lens so that with the LED pushed through its hole in the front panel, the leads sit flush against the surface of the PCB. Finally, trim the LED leads to 6mm and solder each one to its pads. Be sure to push the rear of each lens firmly against the panel while soldering (eg, use a small screwdriver), so that each lens protrudes fully from its frontpanel hole. That just leaves the power switch. Mount it in position and tighten its nut so that the sides are roughly vertical, as shown in Fig.12. This is necessary so that when it is installed in the case, the IR receiver can fit next to it. It’s now just a matter of running four wires from the switch tabs to the their corresponding PCB pads. You can use ribbon cable or light-duty hook-up wire for this job. Note carefully the orientation of the “+” and “-” symbols at the rear of the switch. Making the two cables Fig.13 shows how to make the two cables that connect the PCBs together. These are made up using 4-way and 5-way lengths of ribbon cable. Note that while you should ideally crimp both the insulation and the bare wire as shown, you can get away with just soldering the wires here, as there will be little stress on the cables. It’s much easier than crimping but you will still need to at least partially crimp the insulation with small pliers so that the pins fit into the plastic blocks. It’s a good idea to hold the pins Final assembly Now for the final assembly. This first involves fitting the front and rear panels to the PCB, slipping the entire assembly into the case and installing the seven mounting screws. The assembly is then completed by plugging in the two cables, attaching the four rubber feet (supplied) to the bottom of the case, installing the washer and nut on the pot and finally, fitting the knob. 2150 More to come CS 8 1 CS 9 80mm 60mm flat Fig.13: here’s how to make the two short connecting cables. It’s a good idea to flow a little solder into the crimp joint which holds the bare copper strands to ensure they can’t be pulled out, even if the crimps fail. Check that the pins are flat and straight after soldering, then push them into the moulded plastic blocks using a small screwdriver until they click in place. 44  Silicon Chip in an alligator clip stand while you solder them. Make sure they are held rigidly so that they don’t move during soldering. Once you’ve attached the pins to the cable, use a small jeweller’s screwdriver to push them into the plastic blocks until they click into place. If you have to get one out again, use the same screwdriver to push in the small protruding tab through the hole in the plastic block while pulling it out. That’s all we have space for this month. Next month, we will go through the procedure for testing and setting up the CLASSiC DAC. We will also go over some of the more interesting and important points of the software (firmware) design. Finally, we will give details on how to use the unit, including the remote control set-up and the advanced conSC figuration set-up. siliconchip.com.au