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Balanced Input and Attenuator for the USB Part 2 – by Phil Prosser As described last month, this add-on board for the USB SuperCodec provides two balanced inputs with four attenuation options: 0dB, 10dB, 20dB and 40dB. It will fit in with the SuperCodec (in the same instrument case), resulting in a sleek all-in-one recording and measurement instrument. Now let’s get onto building it! A ll of the components shown and described in the circuit last month fit on a single PCB which is quite quick and straightforward to build. The wiring to connect the two boards isn’t too difficult to make up either, consisting of one stereo shielded cable and one three-wire DC supply lead. The case end panels need to be drilled differently than what was described for the original SuperCodec. The first step in assembly is to mount all the main components on the printed circuit board. Before assembling it, if you have an accurate resistance meter, you may wish to measure the 0.1% tolerance resistors and find the best matched sets among those you have purchased. However, that is optional. As long as they meet the specified tolerances, the performance of your unit should be close to that of our prototype; it might even exceed ours, if you’re lucky. Tip: if you can match resistors between the ‘hot’ and ‘cold’ legs of each channel, you will get a useful improvement in common-mode rejection but this may not be possible depending on the resistors you purchase and the accuracy of your ohmmeter. If your ferrite beads are the loose types, feed resistor lead off-cuts through them. Keep them tight on the board, and it’s a good idea to use dobs of neutral cure silicone sealant or similar glue to stop them from moving and rattling. Next, mount all the zener diodes and the 1N4148 signal diodes. Be careful to orient the diode cathode stripes as shown in Fig.9, as they don’t all face the same way. We have specified 3.9V zeners for ZD3 and ZD4, but any value from 3.3V to about 4.7V should work, as these just establish a protection voltage. PCB assembly We have made an effort to use only through-hole components for ease of construction and made room for relay switching of the attenuators rather than a rotary wafer switch. Before starting construction, you will need to determine your desired input impedance. Review last month’s article and then refer to the parts list to see which parts you will need for your selected option. The add-on board (coded 01106202, available from the PE PCB Service) measures 99.5 × 141.5mm. Refer to overlay diagram Fig.9 during construction. Start by loading the low-profile components: ferrite beads FB1-FB4 and all resistors. Make sure that the 0.1% types go in the specified locations. Because the board is a tight fit in the SuperCodec case, some electrolytics must be installed horizontally, as shown here. Indeed, in some cases, they lie horizontally spaced above other components. 32 Practical Electronics | January | 2022 Now install the electrolytic capacitors, which are also polarised; their longer leads must go into the pads marked with + symbols. The 10µF capacitors must be laid down flat as shown in the accompanying photograph, or selected as very low profile units. This is important, as we will be squeezing this board into the box with the USB interface, ADC and DAC. Next, fit the remaining capacitors (plastic film and ceramic). Remember to use 10µF plastic film capacitors for the coupling caps if you’ve chosen resistors for a 10kΩ input impedance, or 1 1µF for a 100kΩ input impedance. These too must be laid over on their sides to clear components on the other board. After that, solder the six NE5532 ICs and eight relays. The IC and relay orientations are critical. All the relays are orientated with pin 1 away from the input connectors, while all the op amps have pin 1 toward the inputs. You can mount the ICs on sockets, but we prefer not to as the contacts can oxidise over time, leading to poor connections. If using sockets, solder them with the orientations shown, then straighten the IC pins and carefully push them fully into the sockets. Mount the connectors next, followed by the input select switch. The two right-angle polarised headers can be soldered from the top side, but it’s a good idea to solder the pins on the bottom too. Follow with the two 6.35mm TRS sockets. Make sure these are the specified low-profile types and that they are fitted snug to the board. Your add-on board is now be finished. The three boards are connected by several cables, which we will now describe. Making the internal cables The output of the Balanced Input Attenuator board is connected to the USB Sound Card board by a 180mm length of shielded cable. To make this, cut a piece of figure8 shielded cable to 180mm, strip 18mm off the sheath at each end, twist the screen wires together and apply the 2.5mm diameter heatshrink to these. Then put the 5mm heatshrink over each coax line and shrink, as shown in the photograph below. Crimp pins on each end and insert them into the 4-way plug as shown. The middle two pins are earth while the outer two pins are for the signal wires. Preparing the SuperCodec board If you haven’t already built the USB Sound Card board, as per the series of articles in the last three issues, do that now. But note that there are two things you need to do slightly differently when building it: 1) Do not mount the two 6x2-pin 2.0mm pitch header sockets on the back of the board for the MCHStreamer. We will instead be soldering pigtailed connectors to these locations, to allow us to mount the MCHStreamer above the USB Sound Card board. 2) When building that board, make sure the voltage regulator that is not mounted on a heatsink is pushed right down onto the PCB, or it may foul the Balanced Input Attenuator board. The output cable should be 180mm of twin screened coaxial cable. The middle two pins are the shields. Practical Electronics | January | 2022 Having completed that board (minus the MCHStreamer connectors), the next step is to solder a power cable to it, which will plug into the Balanced Input Attenuator board and power it. To do this, take 100mm lengths of red, green and black medium-duty hookup wire and attach them to crimp pins, then push these into the power header, as shown in the photograph below. Red (positive) is at the right-hand end, ground (green) in the middle and black (negative) at the left. Power cable and header for the attenuator board. Sleeve the whole cable in a heatshrink tubing sheath, with around 3cm of each wire protruding, then strip the insulation back by about 5mm on each wire and tin the ends. These bare ends are then soldered to component pads on the SuperCodec PCB. The photo below shows where they go. Check you have the wires in the right spots! The black wire goes to the end of the corner-most 10Ω resistor that is closest to the board edge; the red wire goes to the same end of the adjacent 10Ω resistor; the green to the end of the adjacent 5.6kΩ resistor that is furthest from the board edge. Once you’ve done this, double-check that the wires go into the appropriate positions on the plastic block at the other end; otherwise, there will be trouble when you plug it in later. Connecting the MCHStreamer The next step is to connect the MCHStreamer to the SuperCodec board, but we are doing it differently than for the standalone USB SuperCodec. Otherwise, it is impossible to fit the Balanced Input Attenuator in the same case. Rather than plugging the MCHStreamer directly onto the SuperCodec board, it connects via two 12-way plugs that connect to the board via sets of 12 flying leads. We need to solder the power cable to the main PCB as shown. Try to hook the wires around the resistor leads and keep things tidy! The plugs with attached leads should have come with the MCHStreamer unit. To prepare them, measure and cut the pigtail wires to 50mm (5cm), as shown in the photo. The MCHStreamer is supplied with pre-wired headers. Trim the leads to 50mm as shown. We need to keep these as short as practicable. 33        10F  When plugged into the headers, the MCHStreamer ought to sit as shown above. A tight fit but without stressing parts. Cut all the attached wires to this length and strip, twist and neatly tin 5mm at the ends. Note that while the plugs supplied have black wires on one side and red on the other, they will plug in either way around, and while there is a ground pin on one side, most of the pins carry signals. So it isn’t critical which way around you solder them. The best approach to soldering these to the sets of twelve pads on the PCB is to stand the connector vertically and looking from above, solder the inside row of wires to the outside row of holes in the PCB. We will be plugging this to the top of the MCH Streamer, which will swap the inside and outside rows of wires, as shown in the following photos. With the two cables soldered in place, present the MCHStreamer to the pigtailed headers and fold them as shown in the photo. The result is somewhat tight, but does fit inside the box.                              10F                 Watch the orientations of IC1-IC6, RLY1RLY8 and all the electrolytic capacitors and diodes. The other parts either only go in one way, or it doesn’t matter. Make sure to trim all soldered leads close to the underside of the PCB to prevent them shorting against the case later.   10F Fig.9: use this PCB overlay diagram and the photo below as a guide during construction, to see where the components are mounted on the board.         10F  You may notice that diodes D5-D8 are missing from this photo – they were left off the prototype to verify that they had no effect on performance (they didn’t!) but were added later. Constructors should fit all eight diodes (D1-D8) as shown on the component overlay above. 34 Practical Electronics | January | 2022 Fig.10: this shows the sizes and shape of the front and rear panels (front panel at the bottom), and where to cut or drill holes in them. The 3mm hole below the 7.5mm hole only needs to go partway through the inside of the panel. The ventilation holes shown in red are optional, but do help to keep the internal components at a reasonable temperature in hotter environments, so are recommended. (You can download this diagram from the January 2022 page of the PE website.) Testing Before inserting everything in the case, it’s a good idea to make sure it’s all working. If you haven’t already tested the USB SuperCodec board in isolation, do it per the instructions in the third SuperCodec article. This will also involve installing the MCHStreamer drivers and getting it working on your computer. Power down the SuperCodec board and plug the power connector from the SuperCodec PCB into the three-pin header on the Balanced Input Attenuator board (CON3). Then use the length of shielded cable with plugs on either end you prepared earlier to connect the audio output of the Attenuator (CON4) to the audio input on the USB Sound Card (also CON4). For the outputs, make up a twin shielded cable with RCA chassis connectors on one end and a 4-pin polarised plug on the other, as per the final SuperCodec article (if you haven’t already). Plug this into CON5. Make sure the whole rig is resting on a non-conductive surface, and nothing can short to anything else before proceeding. Now would be a good time to check, using a continuity tester, that the +9V and –9V rails on the two boards are connected the right way around and not swapped. Check for 0V continuity between the boards at the same time. Then, with the MCHStreamer plugged into the USB Sound Card, plug in the 12V supply to power the whole assembly up. Assuming it passes the ‘smoke test’, verify that all the supply rail voltages are still correct. You would have tested these with the SuperCodec alone already, but a fault on the Balanced Input At this stage, it’s worth checking both PCBs to make sure that you trimmed all component leads neatly. If you’ve left any long, they could interfere with, and possibly short out against the case once inserted into it. There is adequate room below the USB Sound Card to accommodate normal lead lengths; you should not have any problems provided you are tidy. Fig.11: if you drilled the ventilation holes on the rear panel, you should also drill some holes towards the front of the bottom panel, as shown here. These allow cool air to be drawn in via convection, which flows along and cools the two boards before exiting through the holes at the top of the rear panel. Practical Electronics | January | 2022 When soldering the MCHStreamer connector to the board, the red and black rows of wires need to cross over as shown. 35 Assembly is tight, but with the cable lengths recommended it allows the Balanced Attenuator to slide out enough to allow the output and power connectors to be plugged in. Watch for the cables snagging on parts on the SuperCodec main board though. The MCHStreamer is fixed to the rear panel using an insulating bush kit. Don’t forget this! Attenuator board could cause them to be wrong now. Assuming they’re OK, check that the attenuator relays work; each time switch S1 is moved, it should generate a nice click from the relays. Then plug the whole device into your computer and repeat the output test that you carried out earlier. Check that the USB Sound Card generates a signal when you play sound or music. If this does not work, check that there are no faults on the Balanced Input Attenuator board and check the wiring thoroughly. We have not changed this part of the USB Sound Card, so it should still work fine. Now launch your recording or analysis software (Audacity will work for basic testing). Set the input attenuator to 0dB, apply an audio signal of no more than 1V RMS to one of the balanced inputs (eg, using a test oscillator) and check that it is received undistorted in the correct channel (left or right). If you don’t have a test oscillator, you can rig up some cables to loop the USB Sound Card’s outputs back to the balanced inputs and play a test tone. If you do this, remember to set the output level no higher than −8dB to avoid overloading the inputs. If that checks out, switch to the −10dB setting and verify that the input level drops appropriately. If your test oscillator level can go higher, increase it to a maximum of 3V RMS and A view with the bottom panel off during assembly. Next comes the Balanced Attenuator and base plate. 36 confirm that you get undistorted near-full-scale input signals. You can also check the −20dB and −40dB settings and verify that the input level drops appropriately, but the waveform shape remains undistorted. Drilling the front and rear panels As mentioned earlier, we are using the same case that was used for the basic USB Sound Card. However, because we’ve had to pack an extra board in, the boards mount to the front and rear panels differently. The revised drilling details are in Fig.10. You can copy/print this and use it as a template, or you can measure with a ruler and mark out the hole locations on the panels. If you have already drilled the panels for the basic USB Sound Card, it is not hard to cut and make new panels from an aluminium sheet of a suitable thickness. You can achieve a high-quality finish by sanding with 400 grit paper after making the holes, then spraying the panels with satin finish black paint. Cut and finish the metal panels as shown in Fig.10. The 3mm ‘hole’ below the switch hole on the front panel (7.5mm in diameter) does not need to be drilled through; it is there to hold the locking pin on the switch. Note the series of holes on the rear panel shown in red; these are for venting hot air and help to lower the operating temperature of the internal components by around 5°C. These are necessary due to the extra internal dissipation due to the Balanced Input Attenuator board. You could opt not to drill these if you are never going to operate the device at higher ambient temperatures (ie, if it will always be used in an air-conditioned room). But as they are on the rear panel, they are unobtrusive, and it’s generally better to keep the components as cool as possible. Similarly, we have prepared a bottom panel drilling diagram (Fig.11) which shows the location of some extra holes in that panel. Combined with the holes on the rear panel, these First extend the ‘hot, ‘cold’ and screen of the leads, then cover with two layers of heat-shrink to make a robust test lead. Practical Electronics | January | 2022 Modifying your prebuilt SuperCodec TIP (hot) RING (cold) SLEEVE [or BODY] (screen) Connnections to the 6.35mm stereo plug – we have used the ‘TRS’ naming standard, although you will often see ‘TRB’ used instead. It doesn’t matter: the sleeve IS the body! provide some convective cooling to drop that temperature. If you’re going to drill one set of holes, you should drill both, or they will not be effective. When finished, install the rubber foot on the front panel as shown in Fig.10 to ensure that the USB Sound Card is held snug against the rear panel. We cut the chamfer off the top of the foot to ensure that the rubber foot fully pushes the PCB back into the case. Then do a test assembly and make sure everything fits OK. Get used to the jiggling required to get things in. Final assembly Assembly is pretty straightforward. Slip the bottom panel off the case, and slide the USB Sound Card in the top slot with the components facing to the bottom panel. The MCHStreamer should already be plugged to the USB Sound Card. Attach the MCHStreamer to the rear panel using an M3 crinkle/star washer, TO-220 bush and fibre or plastic washer. The bush and insulating washer are to ensure that it is insulated from the rear panel, as described in the USB Sound Card article. Make sure the bezel is in place (omitted in photo). You can now put the four screws into the rear panel. Then mount the output connectors as described in the USB Sound Card article. Again, make sure they are insulated from the case. Attach the earth screw and solder tags as described in the USB Sound Card article, and solder the 10nF capacitor between the Earth tag and ground of the output connector. Plug the 18cm cable that goes between the USB Sound Card input and Balanced Attenuator output into CON4 on the SuperCodec board. Now slot the Balanced Attenuator into the bottom slot, with its components facing towards the USB Sound Card. As you slide it in, pull out the power cable and audio cable that run between the cards and plug them into the Balanced Input Attenuator power connector and output connector. You will need to jiggle things to make sure that the cables do not foul between the two boards. Trust us; it will fit! Put heatshrink over the alligator clip to cable transition to act as strain relief, then slide the rubber boot over the lead. Practical Electronics | January | 2022 If you already built the SuperCodec USB Sound Card and have soldered the headers to the back of the PCB, it is possible to still add the Balanced Input Attenuator, but it’s tricky. Removing the two through-hole headers is not as simple as it sounds. We did it on our prototype, but note that this procedure is for advanced builders wishing for a little excitement! You will need a hot air gun set to about 290°C, a pair of pliers and a steady hand. Set the USB Sound Card on edge and grip the first 12-pin header with the pliers. Heat the solder side of this connector with the air gun, from a distance of about 10mm, and gently wriggle the connector with the pliers. Observe the solder connections and adjust your heating until you see some, then all pins moving in the PCB. At this point, gently pull the connector out while continuing to heat, ensuring that all pins are free to come out. Do not use force! Then use a solder sucker to clean the holes up, ready for the MCHStreamer connector wires. Ensuring that the rubber foot is stuck to the front panel as shown in the drawing (Fig.10), push the front panel bezel into place. You then need to slide the bottom panel on. After that, push the 6.35mm sockets and switch through the front panel and screw these tight with the provided mounting kits. You can now put the four screws into the front panel. At this point, you should be ready to go! Making some test leads If you’re primarily building the Balanced Input Attenuator so that you can make recordings from equipment with balanced outputs, chances are you already have suitable cables. You may need to purchase (or make) some XLR to TRS adaptor cables, to allow you to plug XLR equipment into the inputs. These are readily available and usually not too expensive; for example, Altronics Cat P0750. For audio equipment and distortion testing, though, you will probably want a set of cables with alligator clips on one end and TRS jacks on the other. This provides you with maximum flexibility to connect to the ends of various components in audio gear, as needed. The process of building leads is open to your needs and imagination. We will show our approach, but this is by no means the only way. We used 90° ‘stereo’ TRS 6.35mm jacks to get the cables out of the way of the attenuation switch. Strip 25mm off the ends of the balanced (twin-core shielded) cable. Also, strip 10mm off each of the inner conductors. Strip 10mm off each end of short lengths of red, green and black hookup wires, and twist and solder these to the balanced cable as shown. Then slip a 20mm length of 3mm heatshrink over the solder joints and shrink them down. Our finished lead – when constructing the prototype we ran out of green clip covers – at least the lead is green! 37 Test programs for your PC We have used AudioTester 3.0 for testing a lot of different audio gear. This is available as shareware, and a paid subscription is available. It is good but not perfect. You need to select the ASIO interface for playback and record, and also 192kHz for the sampling rate. You can download it from here: www.audiotester.de/download.htm One problem we’ve noted with AudioTester is that its THD+N readings seem off, especially with test signals well below or above 1kHz. We prefer to use it to measure THD only, and SNR only, then compute the THD+N reading as the RMS sum of the two figures. It appears to do a good job of computing THD, but you need to be careful to use a test signal that isn’t too far below the maximum that the device can accommodate. Otherwise, the resulting harmonics can be so low that they are unmeasurable or severely quantised, and you get an artificially low distortion reading. One alternative that we have used, but not as much, is ARTA. Many people seem to like this software. You can get it from www.artalabs.hr Now take two 40mm lengths of 6-8mm-diameter heatshrink tubing and shrink these over the junction of the cables. We used thin cable; you may need to use larger diameter heatshrink here. Then take two 60mm lengths of tubing and put these over the top as a strain relief. This will give you a secure connection and minimise the likelihood of wire fatigue. The next step is to connect alligator clips of your preference to the red, black and green wires. Start by slipping the rubber covers over the wires first, so you don’t forget them! Then slip a 15mm length of 3mm heatshrink over the cable. GET T LATES HE T CO OF OU PY R TEACH -IN SE RIES A VAILA B NOW! LE Strip off an appropriate length of insulation; for the Jaycar clips, this is about 6mm. Solder and trim off any daggy bits, then crimp the metal strain-relief tabs, right at the end of the clip, over the wire. For extra protection, slip the heatshrink down the wire and over the metal strain relief and shrink. Slide the covers over the clips, and these are done! The 6.35mm jacks are similar, just much larger. Don’t forget to slip the covers onto the cable first! Follow with 30mm of 3mm diameter heatshrink as a final cover for the cable (we used thin cable, you may need to use larger diameter tubing). We stripped about 15mm of insulation off the cable, and applied about 8mm of 2mm heatshrink to the earth screen. Check the connections for the solder lugs to the tip, ring and sleeve. The tip is ‘hot’ (red), the ring is ‘cold’ (black) and the sleeve is ground (green). Solder these on. If you intend to use this for testing amplifiers, the connector and cable will see the full amplifier output voltage in some cases. Make sure that all connections are secure and that clearances of no less than 1mm are present and secure. Do not use these on mains voltage, in any circumstances! Final testing With the case all put together, power the unit back up, plug it back into your computer and verify that everything still works as before. If it doesn’t, you may have a short circuit somewhere, or forgot to plug something back in when you put it all in the case. If you are recording from a professional audio source, plug this in and set the attenuator level to 10dB, and you are all set. Reproduced by arrangement with SILICON CHIP magazine 2021. www.siliconchip.com.au Order direct from Electron Publishing PRICE £8.99 (includes P&P to UK if ordered direct from us) EE FR -ROM CD ELECTRONICS TEACH-IN 9 £8.99 FROM THE PUBLISHERS OF GET TESTING! Electronic test equipment and measuring techniques, plus eight projects to build FREE CD-ROM TWO TEACH -INs FOR THE PRICE OF ONE • Multimeters and a multimeter checker • Oscilloscopes plus a scope calibrator • AC Millivoltmeters with a range extender • Digital measurements plus a logic probe • Frequency measurements and a signal generator • Component measurements plus a semiconductor junction tester PIC n’ Mix Including Practical Digital Signal Processing PLUS... YOUR GUIDE TO THE BBC MICROBIT Teach-In 9 A LOW-COST ARM-BASED SINGLE-BOARD COMPUTER Get Testing Three Microchip PICkit 4 Debugger Guides Files for: PIC n’ Mix PLUS Teach-In 2 -Using PIC Microcontrollers. In PDF format © 2018 Wimborne Publishing Ltd. www.epemag.com Teach-In 9 – Get Testing! Teach In 9 Cover.indd 1 01/08/2018 19:56 This series of articles provides a broad-based introduction to choosing and using a wide range of test gear, how to get the best out of each item and the pitfalls to avoid. It provides hints and tips on using, and – just as importantly – interpreting the results that you get. The series deals with familiar test gear as well as equipment designed for more specialised applications. The articles have been designed to have the broadest possible appeal and are applicable to all branches of electronics. The series crosses the boundaries of analogue and digital electronics with applications that span the full range of electronics – from a single-stage transistor amplifier to the most sophisticated microcontroller system. There really is something for everyone! Each part includes a simple but useful practical test gear project that will build into a handy gadget that will either extend the features, ranges and usability of an existing item of test equipment or that will serve as a stand-alone instrument. We’ve kept the cost of these projects as low as possible, and most of them can be built for less than £10 (including components, enclosure and circuit board). PLUS! You will receive the software for the PIC n’ Mix series of articles and the full Teach-In 2 book – Using PIC Microcontrollers – A practical introduction – in PDF format. Also included are Microchip’s MPLAB ICD 4 In-Circuit Debugger User’s Guide; MPLAB PICkit 4 In-Circuit Debugger Quick Start Guide; and MPLAB PICkit4 Debugger User’s Guide. ORDER YOUR COPY TODAY: www.electronpublishing.com 38 Practical Electronics | January | 2022