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PART 2: BY NICHOLAS VINEN AND TIM BLYTHMAN TOUCHSCREEN AND REMOTE DIGITAL PREAMP WITH TONE CONTROLS Our new Digital Preamplifier, introduced last month, combines high audio fidelity with convenience. It provides input switching, volume adjustment, bass/mid/treble controls via remote control and a colour touchscreen. It can be built as a standalone unit or integrated into a power amplifier. Having explained how it works, now we’ll go through the construction and testing procedures. T his preamp brings analogue and digital circuitry together, giving the best aspects of both. It’s a relatively simple design with excellent audio quality thanks to its analogue roots, but it avoids the complexity of the multiple, expensive ICs that would be needed for a purely digital design. It also avoids using mechanical parts that can wear out, like a mostly analogue design using a motorised potentiometer. It has a good range of features, including a colour touchscreen interface, infrared remote support, a three-band tone control, a wide gain range and four stereo inputs. Last month’s article explained how all of this is achieved using a Micromite LCD BackPack, two quad low-distortion digital potentiometers and a handful of op amps. That article also included all the relevant performance data. Now that we’ve explained how it works, let’s start on the assembly. Construction The main PCB overlay for the Digital Preamp is shown in Fig.7. This board is coded 01103191, measures 206 x 53mm (shown rotated) and is available from the PE PCB Service (September 2022). As mentioned last month, we don’t think the bypass relay (RLY4) and its associated components are necessary, so we have shown them greyed out. Instead, we recommend that you fit two wire links, shown in red. These let the signal pass to the output without RLY4 being fitted. Assembly is pretty straightforward, with just two SMDs on the board (IC6 Practical Electronics | October | 2022 and IC7). Those parts are quite large, similar in size to a 14-pin DIP IC, and with widely spaced pins are not hard to solder. Start with those two parts. Find their pin 1 markings and make sure they are oriented correctly, then apply flux paste to all the pads, rest the IC on top and tack one pin down. Check that all the pins are correctly aligned over their pads, then solder them. With enough good-quality flux paste on the pads, you can just load your iron with solder and drag it across the pins, and good joints will form. Clean off the flux residue and carefully inspect the joins to ensure they have all formed properly (with the fillet touching both the pins and the pads) and that there are no bridges between adjacent pins. If you find bridges, apply more flux paste and use some solder wick and a fair bit of heat to remove the excess solder. Repeat the cleaning and inspection process to verify all is OK. Now move on to the resistors, but leave off the larger 1W resistors for now. Note that two of the 100W resistors need ferrite beads slipped onto their leads before soldering – see Fig.7. Check each batch with a DMM set to resistance mode before fitting them to the board, and you can then fit those two wire links shown in red using resistor lead off-cuts. Next, mount the diodes. All diodes are polarised, so check their cathode stripes against Fig.7 and the PCB silkscreen before soldering them in place. D1-D12 are BAT42 schottky types, while D13-D15 are standard 1N4148 signal diodes. Follow with zener diode ZD1. Bend down REG4’s leads by 90°, about 6mm from its body. Insert them into the PCB and then attach its tab to the mounting hole securely using a short machine screw, washer and nut. Once it’s solidly attached and square, solder and trim its leads. Now you can solder op amps IC1-IC5 to the board, ensuring they are oriented correctly. You can solder sockets if you prefer; they make swapping op amps easier but can lead to reliability problems long-term. Follow with bridge rectifier BR1, ensuring its + lead (usually longer) goes into the marked hole. Install the two trimpots (both 500W) and then the three relays in a row, RLY1RLY3. Ensure the stripes on the relays are positioned as shown, as it is possible to install them backwards. Next, mount all the TO-92 package devices. These are transistors Q1-Q3 and Q5-Q7, plus regulators REG1REG3. As there are five different device types in similar packages, be careful to check the markings so that you don’t get them mixed up. Now is a good time to fit all the ceramic capacitors (two different values) and MKT capacitors (five different values). Refer to Fig.7 and the PCB to ensure the right ones go in their correct locations. Then fit the headers for links LK1-LK3, but do not insert the shorting blocks yet. Follow with the DC socket (if you plan to use it) and the 18-pin header, plus the 3-way terminal block, with its wire entry holes facing the outside of the board. If you are going to fit LED1 onboard, do it now, with its longer anode lead 33 soldered to the pad marked ‘A’. Otherwise, you could mount a header in its place, or solder a twin lead later. Also install the two 10W 1W resistors now. Bend their leads so that they are suspended a few millimetres above the PCB surface to allow air to circulate, as they get pretty hot. As mentioned last month, you could opt to use 2W resistors, or perhaps four 4.7W 1W resistors arranged in pairs and mounted vertically to spread out the heat load. Next, fit all the electrolytic capacitors, with their longer positive leads to the pads marked with a + symbol. Note that the two 47μF caps need to have their leads splayed out to fit the pads provided. That just leaves the RCA sockets. The right-angle sockets will have plastic tabs that clip into the holes drilled into the PCB. Once you have pushed them down fully so they are flat on the PCB, solder their leads. You should also push the vertical connectors down fully before soldering the two tabs and central pin on each. Building the BackPack You have the option of using the Micromite BackPack V2 with a 2.8-inch colour touchscreen (PE, May 2018) or the Micromite BackPack V3 with a higher-resolution 3.5-inch touchscreen (PE, August 2020). The main advantages of the 2.8-inch version are lower power consumption and the fact that it will more easily fit into a slimmer case. The 2.8-inch screen module is 38.5mm tall, while the 3.5-inch screen is 56.5mm tall. A 1RU case is 44.5mm tall, so it would be difficult to fit the 2.8-inch version into one, while fitting the 3.5-inch version would be impossible. A 2RU case would fit either. Regardless, it’s up to you; build the display version you prefer based on the instructions published in those previous issues. Assembly is pretty straightforward. We can supply a PCB for either version – just go to the relevant month in the PE PCB Service. For all other Micromite/BackPack items we recommend using: micromite.org Whichever version you purchase, make sure to select the right software. If you’re programming the chip yourself (eg, you already have a BackPack), note that there are two versions of the software to suit the two different BackPack and screens. See the panel on loading the software below for details. Micromite Preamp Interface Adaptor Fig.8: this small adaptor circuit makes it easy to wire up the Preamp board to the Micromite LCD BackPack using a ribbon cable with standard IDC connectors crimped at each end. The IR receiver and its supply filter are only fitted to the board at the BackPack end. 34 Fig.7: rather than fitting RLY4, we suggest you solder short wire links (shown in red) and then omit the other components (in green) including Q4, Q8 and six resistors. This is the tone control bypass circuitry which we found didn’t improve the performance. Also, watch the orientation of the ICs, relays and diodes, especially IC6 and IC7, as they are hard to reverse if you get it wrong. They should have a dot or divot in the corner to indicate pin 1. Practical Electronics | October | 2022 Wiring it up Next, we need to make up the cable and adaptors that will connect the Micromite BackPack to the Preamplifier board. The one which attaches to the BackPack also hosts the infrared receiver (see Fig.8). The two adaptors use identical PCBs (coded 01103192, measuring 12.5 x 45.5mm – available from the PE PCB Service (September 2022)). Both are fitted with a SIL header socket strip and a box header, but only one has the resistor, capacitor and infrared receiver onboard. This is the one that plugs into the BackPack. Assemble them as shown in the photos and the overlay diagram, Fig.9. Next, you will need to crimp the IDC sockets onto the ribbon cable, as shown in Fig.10. Adjust the length of this cable to suit your installation. Ideally, you should use an IDC crimping tool to do this (for example, the Altronics Cat T1540). However, in a pinch, you can do it in a vice (pun intended) with pieces of timber on either side to protect the plastic. Note that some IDC connectors come in three pieces, as shown in our diagram, with a bar on top to clamp the strain relief loop and another part below to press the cable down onto the blades in the socket. But we’ve also seen two-piece connectors with no strain-relief bar, and if you have that type, omit the loops. Assembly needs to be done carefully, so note the following two points. First, don’t compress the plastic so much that you break the top-most part of the connector – it’s not hard to do. Second, make sure that pressure is applied evenly, and all the parts of the socket have been fully pressed together (listen for clicks). This is so that the blades all cut through the insulation and make contact with the copper inside. The main cause of failures in these ribbon cables is due to one or more of the blades failing to cut through the insulation fully, leading to open-circuit connections. For some installations, it might be better to crimp the IDC connectors onto opposite sides of the ribbon cable, rather than the same side, as shown in Fig.10. You can do it either way, as long as you make sure that the triangle moulded into the IDC socket indicating pin 1 points to the red-striped wire in the ribbon cable at either end. Fig.9: build one Adaptor board with all the components, as shown here (refer to our photos to see how we bent the IR receiver lead to reach the front panel), while the second Adaptor board should only have CON1 and CON2 fitted. Testing You can perform some basic tests on the main board before connecting the ribbon cable to it. Even if you plan to power the final device from a mains transformer, you can use a 12V AC plugpack or dual-tracking bench supply for testing. With links LK1-LK3 open and nothing connected to the board, apply power. Use a voltmeter to probe the pins on the headers for LK1-LK3 closest to the edge of the board, taking care not to accidentally short across the pairs of pins. A convenient ground point for the black probe of the DVM is the mounting screw for REG4. You should get a measurement close to +5.5V for LK1, +12V for LK2 and −12V for LK3. Adjust VR1 until the reading for LK1 is between 5.49V and 5.50V (or as close as you can get). If you can’t achieve that, or either of the other two readings is way off, remove the power and check for faults in the power supply area. Also check the +5V rail, which will power the BackPack by probing the right-most lead of REG4. It should be between 4.8V and 5.2V. We’ll assume that you have already loaded the software onto the BackPack; if not, unplug it and do so now, using the usual procedure. The panel titled How to Load the Preamp Software has some helpful hints. If you can apply 5V power to the BackPack (eg, using a USB cable with JP1 fitted for the V2 or V3 BackPacks), then you can check that the software loads up normally. Press the buttons and step through the screens. Everything should ‘work’; it just won’t do anything without the Preamplifier board connected. Assuming it all looks good, remove the power and wait a minute or so for the capacitors to discharge. Now place shorting blocks on LK1-LK3. Plug the ribbon cable firmly into the adaptor board without the IR receiver, then plug its SIL socket into CON8, oriented as shown in our photos. One of these Adaptor boards needs to be connected to the Micromite BackPack, as shown in the lead photo. Similarly, plug the ribbon cable into the other adaptor board and the BackPack’s I/O header, as in our photos. Now is a good time to verify continuity between pin 1, where the header mounts on the BackPack PCB, and on the Preamp PCB, right in the corner. This will verify that you haven’t reversed the connections anywhere. It’s a good idea to check all the pins for continuity between the two boards, as this can show up ribbon cable crimping problems or soldering problems. Once you’re satisfied, reapply power to the Preamp board and verify that the LCD screen comes alive, and you can switch between Presets 1-4 by pressing the buttons. By default, these select between inputs 1-4, and you should hear soft clicks coming from the relay(s) each time you switch inputs. Next, adjust VR2 to get exactly half the 5.5V rail voltage at pin 5 of IC4 (ie, very close to 2.75V if your 5.5V supply is spot on). Now it’s time to connect the Preamp’s outputs to an amplifier with its volume wound down, and one of the stereo inputs to a signal source such as a Blu-ray or MP3 player. Select that input by pressing the associated preset button on the screen. This should pass the signal through more-or-less unaltered, although it might be somewhat attenuated. ► Practical Electronics | October | 2022 Fig.10: here is one way to assemble the ribbon cable. You can also put the IDC connectors on opposite sides of the cable if it suits your installation better; just make sure that the pin 1 triangle marking on each socket points to the red striped wire in the cable. Also be careful to crimp the connectors properly (firmly) without doing it so hard that you shatter the plastic. 35 Start the signal source and slowly wind the amplifier volume up to confirm that you can hear the audio passing through the Preamp. Ensure it is not overly distorted and that both channels are present; otherwise, you probably have a circuit fault. If it seems OK, adjust the volume using the on-screen controls, and check that switching to another input effectively mutes the signal. How to Load the Preamp Software Loading the software As you might expect with the option to run the software on either a 2.8-inch or inch disp a , there are two di erent fi es The asic so tware is written to work with both but requires different display drivers. o ha e a ank chip, se a programmer or the on oard icro ridge to oad the appropriate fi e, as this is ess e ort than oading asic and then loading the program separately. None of these choices exclude you rom accessing and tweaking the asic program to c stomise it The fi e is named 0110319A Preamp 2.8in.hex for the 2.8-inch display or 0110319B Preamp 3.5in.hex for the 3.5-inch display. You can also now go into the EQ settings screen and try adding some bass/ mid/treble boost or cut, to verify that those sections of both channels are operating correctly. Remote control Now is also a good time to test out the remote control, if you plan to use one. The Jaycar XC3718 should ‘just work’ while the Altronics A1012A needs to be set to Aux preset 0776 (see its manual for details on how to do that). Loading the software from scratch If you are building the Preamp with the 2.8-inch display, you simply need to config re the icromite to work with that screen rom the conso e, enter the following commands: OPTION LCDPANEL ILI9341, LANDSCAPE, 2, 23, 6 and for the touch panel: OPTION TOUCH 7, 15 Then ca i rate the to ch pane fie sing the same parameters as we ha e in o r GUI CALIBRATE 0, 143, 293, 893, 685 If the above calibration is not accurate, you can simply run: GUI CALIBRATE ... to perform a full manual touch calibration. Using the 3.5-inch display ince the icromite firmware does not inc de a dri er or the 88 to ch contro er in the inch pane , a separate i rar fi e needs to e oaded to pro ide that eat re and acti ate it when the icromite starts p oad the ILI9488 Library.bas fi e onto the icromite sing dit or o r pre erred seria termina program nter the o owing commands at the icro mite prompt: LIBRARY SAVE CPU RESTART OPTION TOUCH 7, 15 GUI CALIBRATE 0, 3891, 3851, -1277, -860 Again, o can simp se the A AT command witho t parameters i o find o r ca i ration doesn t match o r hardware At this stage, o wi ha e a icromite oaded with an appropriate disp a dri er, which o can test with the T T A and the T T T commands The MMBasic file The asic fi e is designed to work with either disp a dri er indeed, an display with a similar or higher resolution to the 2.8-inch display should work, a tho gh we can t o ch or the sca ing on other screens that we ha en t tested This fi e a so contains an a ridged ersion o the a o e notes in comments near the start o the fi e Simply load the Digital Preamp.bas fi e sing dit or o r seria termina program and r n it rom the asic prompt o sho d e greeted the A screen and the so nd o re a s c icking as the igita reamp initia ises The initia condition has inp t one connected with nomina midpoint a eroes o me, preamp ifier and tone contro settings This corresponds to modest gain across a ands At this stage, the icromite is in the same state as i it were oaded with the fi e as descri ed ear ier 36 Practical Electronics | October | 2022 The updated Altronics A1012A Univeral Remote Altronics has recently updated their A1012 Universal remote control to a newer model, the A1012A (www.altronics.com.au). We have used the A1012 to control our projects for many years now (along with some contemporary Jaycar remotes). This new model has some minor changes compared to the earlier version, which affect how it works with the Digital Preamp. While the design, styling, and button layout have changed, many button functions are the same. The six device buttons near the top have changed too, with the CD and VCR buttons being replaced by DVD and HD buttons. The setup process for the A1012A is similar to the older A1012. You select one of the devices using its button near the top, press the SET button, then enter a code. The A1012A uses four-digit codes, while the A1012 used threedigit codes. A glance through the codes list for both devices shows at least a partial correspondence between the two units. For example, we often use AUX code 171 for Micromite projects. The testing we did a few years ago showed that this setting produces distinct codes that are consistently detected by MMBasic’s inbuilt IR decoder. In the A1012 code list, this code is shown as third in the list for several manufacturers. When we referred to the A1012A’s code list and tried the third code for the same manufacturers, we found that it gave the same codes and thus worked with our Digital Preamp. So AUX code 0776 on the A1012A is a good substitute for AUX code 171 on the A1012. We haven’t exhaustively tested all the buttons, but it certainly worked for all the functions we tried. Over the last few years, we’ve created a few projects that use A1012 TV codes 089 and 170. It appears that TV codes for the A1012A don’t correspond one-to-one to those of the A1012, presumably due to newer TVs evolving and having more features. Still, we found that the codes 0088, 0149 and 0169 were suggested for TVs on the A1012A’s code list that would have required TV codes 089 and 170 on the A1012. So we tested these codes with an Arduino board that we had equipped with an infrared receiver (see: Control your PC with an infrared remote, PE, August 2019 for the hardware we used). The codes that we received were all identical to those used for the A1012, so it appears that these remotes are mostly interchangeable, and probably only differ in the more obscure codes. Thus, if you are updating any Micromite projects from the A1012 to the A1012A, we suggest using AUX code 0776. Other projects we have published that make use of the A1012 include: • Currawong Valve Amplifier, PE, November 2015 to January 2016 • High Visibility GPS Clock, PE, January-February 2017 • Preamp and Input Selector, PE, April-June 2020 • Altronics MegaBox, PE, January 2019. All of these use the TV codes mentioned above, so they should work fine with the A1012A programmed with TV codes 0088, 0149 or 0169. Note that the Jaycar Cat AR1975 ‘Total Contol 4 Device TV Remote Control’ is similar to the Altronics A1012A in many ways, and we will likely use that in future projects where their small XC3718 remote cannot be used (eg, due to having just 21 buttons, which was enough for this Preamp). Remote control code map: A1012 AUX 171 TV 089 or TV170 A1012A AUX 0776 TV 0088, TV 0149 or TV 0169 Point the remote at the IR receiver and check the following functions: • Volume up/down should change the audio level, and you should get a large on-screen display to show you the new volume level (see Screen 9); the popup only shows on the MAIN screen Practical Electronics | October | 2022 • The mute button should toggle the mute function; since the Jaycar remote lacks a mute button, the play/pause button operates this function • The CH UP and CH DN keys can be used to tweak the band settings after first selecting a band using Screen 9: we showed photos of most of the screen displays last month, but here’s one we didn’t show: the large volume number shown when you adjust the volume via the remote control. It’s large enough to see across a room. Each step equates to about one-third of a decibel. In this screen, you can adjust the tone control and preamplification settings and see how the Preamp's frequency response will be affected. buttons 7 (bass), 8 (mid-range) or 9 (treble) • Number keys 1-6 should select one of the six presets Final wiring After mounting the unit in the case, all that’s left is to wire up the power supply – assuming you aren’t using the onboard barrel socket. If you have a transformer with a single secondary, wire it between either pins 1 and 2, or 2 and 3 of CON6. If it has twin secondaries, connect them in series in-phase and then wire the junction to pin 2 of CON6 and the other ends to pins 1 and 3, either way around. Similarly, if it’s a centre-tapped secondary, connect the tap to pin 2 and the other wires to pins 1 and 3. If you have a source of ±15V DC, wire the rails to pins 1 and 3 of CON6 either way around, with GND to pin 2. If you are building the Preamp into a full amplifier, connect RCA plug leads to the amp module inputs and plug them into the vertical outputs (CON4 and CON5) on the board. You should be ready to rock’n’roll – or whatever takes your fancy! Reproduced by arrangement with SILICON CHIP magazine 2022. www.siliconchip.com.au 37