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USB Part III: Construction by Phil Prosser Over the last two issues, we’ve introduced our new USB Sound Card, which we’ve dubbed the SuperCodec, and described its performance and operation in some detail. You would agree it offers extremely high recording and playback performance – so much so that our Audio Precision system can barely even measure its distortion! Now it’s time to put it all together, and get it up and running. I t’s best to build the SuperCodec in stages, checking after each stage is complete that everything you have just assembled is working properly. Before starting, check that the PCB slides neatly into the case. This board is specifically made to fit a Hammond 1455N2201 case, which is sold by both Altronics and Mouser, as stated in the parts list published previously. The part codes given are for the case with black end panels, as we have used, but note that Mouser stocks it in several other colours too. Now let’s move on to mounting the components on the PCB. Mounting the pre-regulators Loading this section is pretty straight forward, as it is all through-hole. The PCB has a section marked to indicate this part of the circuit. Referring to the PCB overlay diagram, Fig.17 and the photograph alongside (which you should do throughout the construction process), this section is at lower right. Start by fitting the six resistors in this section, in the positions shown in Fig.17. Follow with the three diodes, D1 (1N4004) and D2-D3 (1N5822). Note that they are not all aligned in the same direction. They have been oriented to minimise path length and radiation loops, so double-check that your diode cathode stripe is aligned as shown in the overlay diagram and on the PCB, before soldering each. The next job is to install the seven MKT capacitors, which are not polarised, followed by the DC input barrel connector and the fuse clips, marked F1. Then you can fit the eight electrolytic capacitors; these are polarised, so their longer (positive) leads need to go into the pads nearest the + marks on the PCB and in Fig.17. Oh no! I put an IC in the wrong way around! Everybody makes mistakes! So what to do if you got a part the wrong way around or in the wrong spot? For through-hole parts, there are two ways to proceed. For electrolytic capacitors, you are best off using a solder sucker to get as much of the solder from the holes as you can, then judiciously heating one pin and “pushing” the capacitor to lever up the component on the hole you have hot. Be careful and make sure that the leads are straight and will not tear the through-hole plating out as they go. 72 Silicon Chip For op amps, resistors and diodes, the easiest and safest way by far is to cut the component from its leads, then remove the leads individually from the board and clean up the holes. It sounds wasteful, but this could save you tearing a track from your PCB, a lot of frustration and many naughty words. Surface mount parts are much easier to remove with a hot air gun. Set it to about 300°C, heat the part until all the leads come loose and use tweezers to lift it free of the PCB before the solder solidifies – job done. If you don’t have one, you can alternatively Australia’s electronics magazine heat each side of the part until it comes loose. If it’s an IC, this is easiest to do if you join all the pins on each side with one big blob of solder. It’s easy enough to clean up afterwards. If you won’t pay what your local electronics shop is asking for a hot air station, look on eBay; there are ‘decent’ hot air guns available at giveaway prices. Search for “hot air SMD rework”; some are well under $100. These are brilliant for heatshrink work too. Note that it’s best to keep these switched off when not in use! siliconchip.com.au Make sure that the 2200µF 10V capacitor goes to the right, as shown by the smaller circle, with the larger 2200µF 25V type to its left. Also ensure that the two 470µF capacitors fitted in this section are rated at 25V; the 470µF 6.3V capacitor goes elsewhere. You can then solder LED2 in place. For now, mount it vertically, with the base of its lens 10mm above the top surface of the PCB. Make sure its longer anode lead goes into the pad marked “A”. It’s then time to solder switchmode regulators REG1 & REG2 in place. They have five pins; if yours are all in a row, crank them out with needle-nose pliers to fit the pad pattern on the PCB. They don’t need heatsinks. Now solder the inductors to the board. L1 and L3 are both bulky toroidal types while L2 and L4 are smaller bobbin types. Put a dab of RTV or neutral cure silicone sealant under each inductor to help hold it into place, and prevent vibration, as shown in the photo overleaf. Finally, add the 0Ω link; we used a length of 0.7mm tinned copper wire bent to form an Earth connection point, but you can also use a zero-ohm resistor as shown on the PCB overlay diagram. Testing the pre-regulators Connect a voltmeter from ground (eg, either end of the 0Ω link) to the near end of FB12’s pad. This is a convenient point to measure the -12V rail, as marked on the PCB. Connect your 12V DC plugpack to CON1. The specified plugpack is a switch mode unit capable of delivering at least 1.5A continuously. Switch on the power and look for the -12V rail coming up. Check that it is between -11 and -13V. Ours measured -11.5V. Then move the red probe to the near end of FB8 (another empty pad) and check that the +6.5V rail measures 6.0-7.5V. Ours was close to 7V. Finally, move the probe to the near end of FB11 and check that the +12V rail is OK. It will possibly be close to 11V due to the forward voltage drop of diode D1. You can then disconnect the plugpack and proceed with Soldering tips • Use a very fine tip on your soldering iron, the finest solder you have, with gel or liquid flux and a magnifying lens. • Stay calm. Remember that if you only solder down one pin of each device at the start, you can easily melt this and move things around to get it all aligned. • Then by soldering a second pin, you can lock the part in place. Go easy on the solder and remember you can reflow one pin if you need to nudge the part a bit. • Use less solder than you think you need. You will be surprised! the construction. If any of the readings are off, look for short circuits or bad solder joints. Also make sure that your plugpack has the current capacity to kick that negative regulator into operation. Mounting the linear regulators This section is in the middle of the board and includes regulators REG3, REG4, REG6-REG8 and the surrounding components. Start by loading all the ferrite beads in this section, FB8 through FB13. These can be any small ferrite that fits; they are there to offer a high impedance at high frequencies to keep the noise on the rails down. If your beads came loose (as they often do), feed component lead off-cuts from the previous section through each one before soldering, or sections of tinned copper wire cut to length. When soldering them, try to ensure they are held tightly to the board to prevent rattling. Dabs of RTV or neutral cure silicone under each one should help in that regard. Next, fit REG7, the sole SMD regulator, while there is plenty of room around it. Follow with the ten resistors in this section, each being near one of the regulators. Then fit 1N4004 diodes D22-D29. As before, watch their orientations. Then install the six MKT capacitors, followed by the 12 polarised electrolytics. As usual, make sure their longer The completed project, albeit upside down! The main SuperCodec PCB “hangs” off the rear panel with no connection at all to the front panel – even the power LED shines through a hole in the panel. The daughter board (at left of main pic and inset above) is the MCHStreamer USB to I2S interface which plugs into the two 12-pin sockets on the underside of the main PCB. siliconchip.com.au Australia’s electronics magazine October 2020  73 leads go into the pads marked +. Keep in mind that they are not all orientated the same way. Again, with the two 470µF capacitors, they must be 25V-rated types, not 6.3V. Finally, fit TO-220 package regulators REG3, REG4, REG6 and REG8. Three of these (REG3, REG4 & REG6) are mounted on small heatsinks. In each case, place a lockwasher over a 6-10mm M3 machine screw shaft, followed by a flat washer. Insert an insulating bush into the hole on the regulator tab, then feed the machine screw through this. Slide a TO-220 insulating washer over the screw shaft, then feed the screw into the tapped hole on the heatsink. Do the screw up loosely, then drop the regulator leads into the PCB pads, while also lining up the heatsink posts with their mounting holes. Make sure the heatsink is pushed down fully and solder its posts to their pads. You will need a hot iron to do this, and it also helps to add a little flux paste to the area around the bottom of the posts. Then hold the regulator vertical and do up the machine screw tightly before soldering and trimming the regulator leads. Note that if you are using the recommended NE5532 op amps, in theory, you could leave off the heatsinks for REG3 & REG4. But they would run hotter. We recommend that you fit all three, just to be safe. Testing the linear regulators Reconnect the plugpack and measure the voltage at either end of FB9, on the left side of the PCB. You should get a reading in the range of 3.2-3.4V. Ours measured a touch over 3.4V – this is OK since the rail is currently unloaded. Measure either end of FB7 for +5V; this should read between 4.75 and 5.25V. Then measure the voltage on the tab of REG6, which is the +2.5V rail. This should give a reading between 2.3V and 2.7V. Next, check the voltages on the right-hand pads for the two 10Ω resistors in the upper-right corner of the board. The pad nearest the top edge of the board should be -9V (-8V to -10.5V) while the one immediately below should be +9V (+8V to +10.5V). If there are any problems, check the plugpack output voltage – is it working OK, or has it overloaded and shut down? If it shut down, look for a short circuit on the board. If you have not used the specified Coming up: a balanced attenuator add-on Phil Prosser has designed an add-on board for this project which adds balanced inputs and a switched attenuator with settings of 0dB, 10dB, 20dB and 40dB. This add-on board greatly improves the flexibility of the SuperCodec when used as a measurement instrument, and only slightly degrades its performance. If you’d like to build this add-on board, go ahead and start building the SuperCodec but don’t fit the headers for the MCHStreamer just yet, and don’t drill the case end panels either, as both the MCHStreamer and the main PCB are mounted slightly differently to make room for the addon board. The article describing this add-on board will be published within the next few months. 74 Silicon Chip Fig.17: the PCB overlay for the SuperCodec shows all components in place. However, as discussed in the text, it’s best to assemble the board section-by-section, allowing you to test each on completion and if necessary, fix any errors as you go. This overlay does not show the MCH daughter board, which plugs into the two header sockets (bottom left) on the underside of the main board. Australia’s electronics magazine siliconchip.com.au plugpack, is that negative regulator overloading it on startup? Try a beefier supply. Also check that all the diodes and capacitors are the right way around and all solder joints are good. Once the power supplies are all up and running, you are well on the way. We can now mount the remaining SMDs without fear of damaging them. Galvanic isolator and ASRCs This section is in the lower left-hand corner of the board, referring to Fig.17. Start by loading all the surface-mount capacitors in this section, then all the SMD resistors. The capacitors will be unmarked; while the resistors will be marked with codes indicating their values, you will need a magnifier to read them. In all cases, it’s easiest to rely on what’s written on the packaging, and fit one set of values at a time. Adding a little flux paste (or liquid flux) on each SMD pad before placing the component will make soldering easier. With the capacitors and resistors in place, proceed to solder IC6, IC7 and IC12. Note that pin 1 faces towards the bottom of the board in each case. Check and doublecheck the pin 1 marking on top of the IC package before soldering them, as they are difficult to remove. Again, flux paste will make soldering these parts much easier. Given the proximity of the pins on these ICs, it’s best not to worry about bridging pins when soldering them. Instead, check carefully after soldering using a magnifier, and use a dab of flux paste and some solder wick to clean up any bridges which have formed. If you are lucky, you will have a microscope; if not, you can use a smartphone camera to zoom in close to the soldered pins and take a photo. This is a good way to check for hidden bridges between pins. Next, mount the 4N28 and associated through-hole resistors, plus transistor Q1 and reset chip IC13. Finally, install the headers for the MiniDSP MCHStreamer which go on the back of the board. These should be ESQT-106-03-F-D-360 elevated headers providing 10mm clearance, to ensure the MCHStreamer fits. Testing this section And here’s the matching PCB photo which should also help you assemble the board. There’s a mix of through-hole and SMD components to be soldered in – you shouldn’t have a great deal of drama with the resistors and capacitors but some of the SMD ICs have quite fine pin spacing so you’ll need to take your time with these. Any solder bridges between pins must, of course, be removed! siliconchip.com.au Follow the instructions in the text box below to install the driver and get the MCHStreamer running. Once you’ve connected it to your computer, check that it has been detected by clicking on the volume control and checking that it comes up with Speakers (MCHStreamer Multi Channels), as shown in that panel. Operating systems other than Windows will use a different method. Once you’ve verified that it has been detected, unplug it from the computer and then fit it into the two matching sockets on the underside of the PCB. It should seat firmly onto the connectors. Power the sound card back up and connect the USB socket to your computer. You then need to make sure that the MCHStreamer is selected as the current sound output. To do this in Windows 10, left-click on the sound icon, and you will get a pop-up window as shown in the panel. If the MCHStreamer is already selected, then you’re all set. Otherwise, left-click on the caret (“^”) to get a list of available sound devices. You can then switch to the MCHStreamer. Now play some music or another audio file. It does not Australia’s electronics magazine October 2020  75 You can solder fine-pitched SMDs with a standard iron . . . matter what you choose, as we just want data to come out of the MCHStreamer. Check that the collector of Q1 (the pin towards the bottom of the PCB) goes high. Check for fixed-frequency square waves on the test points labelled on the PCB: MCLK (25MHz), BCLK_DAC (12.5MHz) and LRCLK_DAC (195.3125kHz). If you have trouble, check the power supplies. Anything odd here needs to be tracked down. The individual power supplies will assist you in isolating power-related problems to a small group of components. Also check for solder bridges, bad solder joints (especially on SMD IC pins) and check those capacitors. If you are lucky enough to own one, a PCB microscope can help identify problems in soldering – or alternatively, confirm you’ve done a great job! If you don’t own one, you could try using the camera in your smartphone to take close-up shots which you could then enlarge via your photoediting software to help you spot any “oopses”. Don’t have photo-editing software? Try downloading GIMP (it’s free!). The seven op amps are next. They are all orientated with pin 1 towards the upper right-hand corner of the board. You can either solder sockets and then plug the ICs in, or solder the ICs directly to the board (which will give better reliability, but make it harder to swap them later). Follow with all the MKT and ceramic capacitors, then the electrolytics. As usual, be careful to insert the longer leads Loading the DAC and ADC sections These sections are in the top half of the board and include all the remaining components. Start by fitting all the remaining surface mount capacitors. Make sure that the two 2.7nF (2700pF) caps go where indicated as these are critical to good performance. There is also one SMD resistor remaining (220Ω) so install that now. Then solder the ADC and DAC chips, IC1 and IC2. Orientate both with pin 1 towards the top of the board, with the power supplies are at the bottom. Use lots of flux paste, thin solder wire and tack down one corner to allow you to align the IC before soldering the remaining pins. Check there are no missed SMDs now, as after we load the through-hole parts, it is harder to get the soldering iron in there. Now mount REG5, the LP2950-3.3V in a TO-92 package. Follow with the seven ferrite beads left, FB1-FB7, then all the rest of the through-hole resistors and diodes. The diodes left are all BAT85s, but they don’t all face in the same direction, so check the PCB overlay, Fig.17, to make sure they’re all installed with the correct orientation. 76 Silicon Chip Australia’s electronics magazine siliconchip.com.au A view of the board with the power supply sections completely assemble and nothing else. This way, we can check that all the supply rails are correct without risking any damage to the expensive chips they will be powering. of the latter into the pad nearest the + symbol, which varies in orientation for each capacitor. The 470µF capacitor below IC9 is the 6.3V-rated type, to allow it to be closer to the chip, while the four 22µF capacitors are non-polarised types. (You could use 47µF or 100µF NP capacitors, as we did in our prototype, although we didn’t find this to give any benefits.) Now fit LED1, again with its lens 10mm above the PCB and with its anode to the pad marked “A”. Then fit polarised headers CON4 and CON5, and the PCB assembly is complete. Testing this section Check that there are no missing parts on the board. If there are, look them up and fit them. Also check your sol- Fig.18 (opposite): drilling/cutting diagram for the rear and front panels (most holes are on the rear panel with only one LED hole requrired on the front). Above are the rear panels (yes, we made two prototypes!) with masking tape holding down the panels and also providing a handy means of marking out the holes required. siliconchip.com.au Test points are provided to help you verify correct operation. dering to make sure it’s all good, especially on the SMD ICs. It’s best to clean flux residue off so you can get a good look at the solder joints. Now apply power, without the sound card plugged into a PC. It is not even necessary that the MCHStreamer is plugged in, but this does not matter as it is isolated from the rest of the board! Connect the ground of your DVM to a convenient ground point. We soldered a PCB pin to a few of the larger GND vias; there is a convenient one just above the 3.3V regulator. But you can also just hold the black probe in one of those holes. Apply power and board and re-check the 3.3V rail, the +5VA rail, the +2.5V rail and the ±9V rails, as before. This is to make sure that you haven’t introduced any short circuits across any of the rails. Assuming these are OK, and there is no part emitting smoke or getting hot, we can proceed. If something is wrong, follow the usual checks for solder bridges, especially on the ADC and DAC where the pins are close to one another. Also check the component orientations. Now it is time to get into some of the fun tests. Switch the power off, plug the MCHStreamer into the sound card and the PC, then plug its outputs into some sort of amplifier. Power it back up and play some sound (eg, music). Then you can check that you get appropriate sounds from the amp! Alternatively, you may choose to put a scope on the output(s) and look for the audio. Assuming that works, connect a stereo RCA-RCA cable from the outputs to the inputs, play some audio and then simultaneously make a recording. Check that the recorded sound file matches the playback audio. If any of these tests fail, check the data paths from the MCHStreamer to the DAC and ADC chips. This is ideally done using a scope with its timebase set to 50ns/division. Check the MCLK, LRCLK, SDATA, BCLK and RESET lines. If the RESET line is not high, the MCHStreamer is probably not connected properly. Is its light on? Why not? Check the clock and data lines on the USB card side of the galvanic isolators – they should be there is they are on the PC side. If not, why not? Metalwork If you are using the recommended case, the Hammond 1455N2201, there is refreshingly little metalwork to do. Cut and drill the front and rear panels as shown in Fig.18. Australia’s electronics magazine October 2020  77 After crimping and/or soldering the crimp pins to the end of the wires, push them into the plastic housings and they will click into place. The front panel has a single hole for the ADC Clip LED. The rear panel has cutouts for the USB input, power input, power LED and four RCA connectors. Rectangular holes are always a nuisance to cut. As these are small, we recommend marking the outlines on the panel, then drilling a series of small holes around the inside perimeter with a 1.5-2.5mm drill bit. Keep the holes close together and err on the side of drilling well inside the marked square, rather than touching the outline. Once you have broken free the tab of aluminium from the middle of the hole, use a square or triangular file to neaten the hoes to the required square. Touch up the edges with black paint or at a pinch, a marker, to make this neat. To finish the front panel, stick a small rubber stopper on the front panel in a location that will ensure that the SuperCodec is held tightly against the rear panel. This will minimise strain on the MCHStreamer connectors when power is being plugged in and out. If you have foam tape, a thick layer of this along the edge of the PCB would also work fine. The SuperCodec slides into slots in the case and is held tight by the rubber stopper at the front, and the MiniDSP MCHStreamer, which is attached to the rear panel. Final assembly You need to make up some cables using the two polarised header plugs and matching pins, two 30cm lengths of figure-8 screened cable, the four RCA panel-mount connectors and some heatshrink tubing. The result is two cables, each with two RCA connectors at one end and a four-pin header plug at the other. At the header ends, start by separating the two channels of coax, then striping 25mm of the outer sheath of each, exposing the shield braid. Tease the inner conductor from the braid, and strip the end by 5mm. Twist the braid wires together into a neat bundle. Next, cut two 20mm lengths of heatshrink, one around 3mm diameter and one 5mm. Slide the 5mm piece over both the shield braid and central conductor. Do not shrink this yet. Slide the 3mm heatshrink over the braid; there ought to be 4-5mm of wire protruding. Shrink this down. Slide the 5mm heatshrink sleeve to cover about 3mm of the junction where the braid and inner core separate, then shrink it down. Present the bare wires to a crimp pin. You need to trim off excess braid wire, so that the strain relief crimp (at the back of the pin) will go over the braid, with about 3mm of wire in the main crimp as shown. Crimp the middle section using sharp-nosed pliers. Make sure the crimping doesn’t cause the pin to splay out so wide 78 Silicon Chip A 10nF capacitor between the input grounds and rear panel Earth lug minimises hum pickup. that it will no longer fit into the plastic block. Then add a tiny amount of solder to the crimp, being careful not to allow it to wick down to the connector spring. Then crimp the strain relief onto the heatshrink around the braid. Next, strip back 3mm from each of the inner conductors and crimp and solder to another pin as above. Now push the pins into the header plug. The shield braids go into the middle two pins, with the left and right signals on the outside. You will feel and/or hear a click when they seat properly. Then take the two pairs of RCA socket and mount them to the rear panel using the supplied plastic insulating washers, to isolate them entirely from the back panel. As before, separate the twin coax cables into left and right wires, and strip back 25mm of the outside insulation. Cut two more pieces of 5mm and 3mm heatshrink and twist the braids, insulate them and then shrink the braid and overall sleeving, as with the header end. You can then solder the input and output wires to the RCA connectors, as shown in the photo above. The two things to check for are that the input pair and output pair are wired to the same cables and that the left (white/black) and right (red) sockets are wired to the appropriate pins on those headers – see Fig.17. Check the orientation of your polarised headers to determine which pin will go the left signals on the board, and which goes to the right. You can make these checks most easily by plugging the cables into the sockets on the board and then using a DMM set to continuity mode. Probe from the centre of each RCA connector to the pins on the headers (through the slots in the plastic housing), to verify that each one goes where it should. Mounting the USBStreamer The USBStreamer needs to be isolated from the case of the SuperCodec. This optimises the effectiveness of the galvanic isolation and improves hum rejection. This is achieved by using TO-220 bushes on the M3 machine screws that attach the USBStreamer through the rear panel, and placing fibre washers on the inside of the rear panel, between it and the USB Streamer brackets. See the photo overleaf, where you can see the insulating washers under the screw heads on the rear panel. This is required to prevent ground noise from the USBStreamer card being conducted through the case and injecting itself into the very sensitive ADC stages. While you’re doing this, something to note is that the mounting lugs on our MCHStreamer board were not lined up properly. We reckon this was due to sloppiness on the Australia’s electronics magazine siliconchip.com.au The pre-assembled USB Streamer PCB plugs into the two 12-pin header sockets on the underside of the PCB. part of whoever (or whichever robot) soldered the threaded standoffs to the board. This can result in the MCHStreamer sockets looking crooked on the rear panel. If, after mounting your board to the panel, it is noticeably crooked, all you have to do is pack one of its mounting screws on the inside of the panel with an extra fibre washer or two. That should straighten it right up. It’s also very important that you stick a 7.5-8mm tall rub- A piece of insulating material such as Presspahn, located as shown here, will ensure the MCHStreamer is always isolated from the case. ber foot on the bottom of the MCHStreamer board as shown in our photos. As this board is only attached to the main board via headers, and it’s only mechanically mounted at one end (to the rear panel), it’s possible for its pins to lose contact due to shock or vibration. The rubber foot rests on the bottom of the case and holds the far end of the MCHStreamer up so that the headers can’t come out of their sockets. Getting the USB interface up and running First, you’ll need to install the driver on Windows or macOS. Log onto the MiniDSP website with the password you used to buy the MCHStreamer, and navigate to the download section. Download the driver for the MHCStreamer. Follow the instructions to install this from the MiniDSP Website, which in summary are: 1. Plug the MHCStreamer module in via its USB cable. It does not need to be plugged into the sound card PCB; it can just be on your workbench (but make sure it’s on a non-conductive surface). It is powered from the computer via the USB cable 2. Our Windows 10 PC popped up a window saying it was “setting up the MCH Streamer”, then a second window saying “the MCHStreamer was ready to go” 3. Extract the contents of the ZIP file you downloaded from their website 4. Navigate to the “Drv_DFU\WinDrv” subdirectory and doubleclick on the installer, which in our case was named “miniDSP_ UAC2_v4.67.0_2019-08-15_setup.exe” 5. When asked if you want to allow the App to make changes, click “Yes” 6. Follow the prompts in the installer, selecting defaults including file locations. The SuperCodec should now be up and running. To set the sampling rate, right-click the speaker icon in the taskbar, usually in the bottom-right corner of the screen. Select “Open Sound Settings” and check that the system has “Speakers (USBStreamer Multi Channels)” selected as the output device (see below). This should automatically be selected. If not, select it. Then click on “Device Properties” in blue, just below the device selection pulldown box. In the new window that appears, look for “Additional Device Properties”, again in blue. Click this. In the pop-up window, go across to “Advanced”. Here you can select your sampling rate, and also click a “Test” button. We recommend selecting “24 bit, 192,000 Hz (Studio Quality)”. Then click “Apply” down the bottom left. While the download package includes the firmware, the MCHStreamer is shipped with the firmware already installed. This does not need to be changed. If you have fiddled with this, you will need to install the I2S_TOSLINK firmware. To do this, follow the instructions in the manual. Several other configurations will work for us, as all we need are I2S channels 1 and 2 in and out on the header. Once the drivers are installed and the MCHStreamer is plugged into your PC via USB, it is set as the default output device automatically. If for some reason it isn’t, you can select it from the list of available audio output devices by clicking the caret on the right. siliconchip.com.au Australia’s electronics magazine October 2020  79 board. You should also have a 10mm M3 machine screw, three locking washers, a solder lug and an M3 nut, again specified in the parts list. Cut a 6mm length of 3mm diameter heatshrink, then mount the M3 machine screw through the hole in the rear panel with a locking washer either side. Place the solder lug on top, then the third locking washer and finally the M3 nut. Do it up tight. Put the 6mm heatshrink over the capacitor leg, and solder this to the solder lug. Then solder the other lead of the capacitor to one of the shield braid wires of the output connectors. Tip: if you envisage using this as a measurement system, put a solder lug on the outside of the case as well. This can use the same screw. As we found in our tests, access to the unit’s ground can be useful in some cases to minimise overall system noise. Adding this while building it will be a lot easier than adding it later. Slide everything into the case once it is all working, then mount the panels and you are set! If you envisage this device being moved around a lot or vibrated, then you might want to add a piece of Presspahn or Elephantide as shown above. This is optional. A section of Kapton tape on the USB socket ensures it can’t short to any components on the main PCB. When you slide the PCB into the case, the foot should press against the bottom and provide a little extra resistance to sliding the board in, but not so much that it becomes impossible. This is how you know that it’s providing enough force to hold the boards together. Grounding If you want to get the 50Hz hum down below -120dB, as we achieved in our prototype, Earthing is very important. To be honest, in testing this, we found that even the slightest change in the configuration can cause changes of 10dB or more. That just shows how difficult it is to achieve such performance. In most tests of amplifiers etc, you will need the galvanic isolation that the system provides to measure really low noise floors. Where super-low noise is critical, you might find with some system configurations that the Earth of the PC does need to be tied to the device under test to eliminate induced 50Hz signals being picked up. This will require experimentation with your overall setup. You should establish the noise floor with no signal to the unit under test before running any tests. You should have a 10nF MKT capacitor left over, which was specified in the parts list (in part one) but not used on the Using it If you want to use the SuperCodec for playback, you can use just about any audio software. But if you want to take advantage of its full capabilities, you will need high-resolution content such as 96kHz or 192kHz, 24-bit FLAC files along with a player that can properly decode such files. For recording, we suggest that you try the free software package called Audacity (www.audacityteam.org). It is available for Windows, macOS and Linux and can take advantage of the Card’s full capabilities. For audio analysis use, such as measuring distortion (THD+N or THD), signal-to-noise ratios (SNRs), frequency responses and so on, various packages are available. We use audioTester (www.audiotester.de). This is ‘shareware’ so you can download and install it for free, but you can only use it for a limited time without paying for it. It only costs €39 or about AU$65 for the full version. We recommend this software because it is easy to use and has many comprehensive features that are ideal for testing audio equipment. That includes a low-distortion sinewave generator, spectral analysis with automatic display and calculation of the signal level and total harmonic distortion (THD) and much more. SC Here’s what the back panel of your SuperCodec should look like when finished. Note the comments in the text re the grounding/ insulaton of the sockets to avoid ground loops. 80 Silicon Chip Australia’s electronics magazine siliconchip.com.au