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PART 3: PHIL PROSSER Digital Preamplifier and Crossover Last month, we assembled and tested the three PCBs used in our new Digital Preamplifier, which uses digital signal processing (DSP) rather than analog techniques. It isn’t just a preamp – it can also perform as an active crossover. Now we’ll describe how to prepare the case, mount the modules, wire it up and use it. O nce you have assembled & tested the three Digital Preamplifier PCBs, it is time to prepare the case so we can fit everything into it and wire the boards up. The first job is to drill and cut the holes in the rear panel so that you can get the main PCB into position, as many of its connectors go through that panel. We have provided a drilling guide for this panel in Fig.17. This is a view of the rear panel from the outside, rotated 90° and shown at half scale (50%) so it will fit in the magazine. Ensure you make the measurements from the outside of the panel. We applied a layer of masking tape over the whole rear panel and used a fine felt-tip pen to make a very precise transcription of the cutting and drilling guide. Another option is to print Fig.17 at exactly twice its original size (200% scale), line it up and stick it onto the 82 Silicon Chip panel, then mark the hole positions with a punch or pilot drill. Since the panel artwork at 200% size won’t fit on an A4 sheet of paper, if you don’t have an A3 printer, you could print it across a couple of sheets with some overlap so you can line them up. Centre punch the holes, as you want the drilled holes to be located as accurately as possible. Having marked the hole positions, it’s a good idea to line up the PCB as a sanity check, to make sure they are all correct before drilling. You don’t want to end up with holes in the wrong places! We started drilling with a 1.5mm pilot drill bit to get the initial holes exactly centred, then increased the hole sizes in a series of steps to ensure the centres were accurately located. If you go straight from punching to using a large bit, it will have a tendency to wander initially, so even if you punched the exact centre, the large hole may be offset. Slowly increasing Australia's electronics magazine the size with a series of larger bits mostly eliminates that error. It is important that the screw retention holes for the dual RCA connectors are in the right spots; by comparison, the larger 11mm holes really just need to clear the metal ground connection on the RCA socket, and they can be filed a little larger if needs be. Also make sure that you get the placement on the rear panel at the specified height. This has been selected to ensure clearance between the screw holes in the bottom panel. If the board is mounted too low, you will have possible interference with the PCB. As you test-fit the board, check these clearances on the back of the PCB. As specified, there is about 3mm clearance between the pins on the bottom of the PCB and threaded inserts on the bottom of the case. The IEC and two other square holes are easily made by drilling many small siliconchip.com.au holes and filing these to shape. The panel is soft aluminium, so it won’t take a lot of filing to get right. Check the fuse holder and IEC socket fit as you file, as it is easy to take more off than you want. We countersunk the RCA screw holes using an 8mm drill bit to reduce the protrusion of these screws near the RCA sockets. Once you’ve made all these holes, check that the connectors on the main PCB will fit through those holes without binding. Front panel preparation Now is also a good time to drill and cut the holes in the front panel, as per Fig.18. This is a view from the outside of the front panel. The procedure is much the same, although it should be quite a bit easier this time, as there are just six holes to drill, plus one rectangular cut-out to make. Like with the rear panel, we covered the whole front panel with masking tape and marked the hole locations before drilling. The presence of masking tape has a benefit of reducing the chance of scratching the panel while working it. The front panel drilling is complicated by the need to mount the LCD screen. We have specified a 3D-printed bezel for the LCD that we have used on a couple of projects now. This is in an STL file named “Altronics Z7018 LCD Bezel v8.stl”, which you can download from siliconchip.au/Shop/11/2917 Our cut-out matches this, but if you want to take a different approach to mounting the LCD, you need to consider the actual LCD and how it mounts. The 3D-printed bezel also ensures electrical isolation of the LCD from the chassis. One challenge we faced is that the space between the extrusions on the inside of the front panel leaves only 36mm. This is not a lot of space for a display, which limited our choice. You can use the ‘drill and file’ method for making odd-shaped holes; it is tedious, but the results are generally good. In this case, we used our Dremel rotary cutting tool with a thin metal cutoff disc. We found that being careful with our siliconchip.com.au rotary tool, we were able to cut the vast majority of the LCD hole neatly, and only had to file a little in the corners to get them just right. Remember that the bezel will cover this cutout, so if there is a little roughness around the edges, that will be hidden by the bezel. Mounting the main PCB The locations of the holes that need to be drilled in the base panel are shown in Fig.19; ignore the grid of blue-outlined holes on the left-hand side for now and just make the black outlined holes. These include four for mounting the main PCB (the four holes marked “A” that run down the middle from the top). However, we recommend that once you have drilled the rear panel, you dry-fit the main PCB to it and mark the locations on the base for the four mounting holes on the opposite edge of the PCB. This will ensure that they are in the right positions, so that the board is not stressed once installed. To mount the Digital Preamplifier PCB to the rear panel, jiggle the RCA sockets into the holes drilled for them and secure using a couple of 6mm-long, 4GA self-tapping screws (we used Jiffy box screws from Altronics). Now see where the holes line up with the base of the chassis. We stuck masking tape to the chassis and used this to mark where to drill the mounting holes. Once drilled, secure the PCB to the base using tapped spacers, 6mm M3 machine screws and shakeproof washers. With this in place, you have completed the trickiest part of the assembly process. Power supply assembly Mount the power supply board also using tapped spacers, machine screws and shakeproof washers. It is attached via four mounting holes in a roughly rectangular pattern on the right-hand side of Fig.19. Orientate it so the heatsinks are roughly centred in the case, with the terminal blocks near the main PCB. This requires four 10mm standoffs, 6mm panhead machine screws and crinkle washers. If in doubt, refer to Australia's electronics magazine ◀ Figs.17 & 18: the rear panel drilling details are on the lefthand side of this diagram. This is shown from the outside of the case; note the orientation labels and that this is at 50% scale. Try to position the holes as accurately as you can (there is some leeway, but not a lot). The holes required in the front panel are presented in the righthand side of this diagram. The edges of the large rectangular cut-out will be covered by the 3D-printed LCD bezel. Fig.19: the case bottom panel drilling details. All the smaller holes are 3.5mm in diameter. While we have provided their locations, the four holes for the PCB standoffs for the edge of the main board should be marked and drilled with the board fitted to the rear panel. The blue-outlined holes in the left are the ventilation pattern for the lid, not the base, and are shown rotated 180° compared to the rest to avoid interfering with PCB mounting holes. Fig.21. This board is Earthed through the chassis via an exposed pad under one of the mounting screws. This connection is made through one of the spacers that attaches the Power Supply board to the case and Earths the board through exposed metal under the screw head. Therefore, you should scrape away the paint under the screw head that attaches this spacer to the case to ensure it is Earthed properly. After mounting the Power Supply board, use a multimeter to measure the resistance from the power supply PCB ground to chassis Earth and ensure the reading is under 1W. Before going any further, you will need to make the safety panel from a sheet of 1mm-thick aluminium, Presspahn or similar. This panel will be placed between the mains section and the low-­voltage section. We made ours from a 46 × 225mm piece of 1mm-thick aluminium. The intent of this part is to ensure physical separation of these sections in the case. Cut it to the dimensions shown in Fig.20 and drill the five holes. Next, fold the two tabs up by 90°. Start by scoring along the edge; if it’s made of Presspahn, it should be easy to fold using a ruler. For aluminium, you can clamp the tabs between two pieces of straight timber tightly in a vice, with the bend line right at the top, then carefully bend the part projecting out over until it hits the top of the timber. The folding is most easily done with a little encouragement from a hammer, using a piece of timber against the aluminium to avoid dents. We intentionally cut out a section of the 10mm lip to avoid it running under the transformer. Ensure you cut this out. Australia's electronics magazine siliconchip.com.au 84 Silicon Chip The main PCB is only attached to the bottom of the case along one edge. At the other end, it attaches to the rear panel via numerous screws into the RCA socket housings. This version is using the DSP board and you can see some of the ribbon cables connecting the LCD screen and front panel controls. We used 3mm rivnuts on this section to make assembly easier (they’re basically internally threaded rivets). If you don’t have a rivnut tool, you will need to use nuts, bolts and lock washers to install this. Insert a 9.5mm hole grommet in the large hole to ensure there are no sharp edges that might cut the insulation of the transformer wires running through to the power supply PCB. You will note that we painted ours satin black; this is purely for aesthetics. Make sure this panel is installed prior to the transformer. The mains wiring is made with 7.5A mains-rated medium-duty wire, referring to the wiring diagram, Fig.21: 1. Mount the front panel to the case and install and tighten the power switch, ready for wiring. 2. Also mount and install the IEC connector and fuse holder to the rear panel. 3. Solder a length of green/yellow-­ striped mains-rated wire to a solder lug, then attach the other end to the Earth terminal of the IEC mains input connector. Scrape away any paint on the case around the Earth lug mounting hole, then attach the solder lug with a 16mm M3 machine screw, hex nut and crinkle washers. Tighten it and verify that there is a low resistance from the mains input Earth pin and the chassis’s exposed metal work. 4. Using blue wire, run a Neutral connection from the IEC connector terminal marked “N” through to one of the upper terminals on the DPDT mains switch (not the common terminal). Insulate both these connections to ensure there is no possibility of accidentally touching any live parts (Neutral and Active can sometimes be swapped in house wiring or extension cords). siliconchip.com.au 5. Using brown wire, connect the “A” terminal of the IEC connector to the end connector of the fuse holder. Again, insulate this well. We added a short length of 13mm diameter heatshrink tubing, which we ultimately used to cover the whole rear of the fuse holder. 6. Continue with brown wire from the other terminal of the fuse holder to the switch terminal next to the one you’ve already connected, at the top on the other side. Again, insulate this well. 7. Connect the blue wire from the transformer to the middle terminal of the switch below the existing neutral wire. Similarly, connect the brown wire from the transformer to the terminal below the existing brown wire connection. Insulate both well. 8. Add insulation to the unused terminals on the switch ‘just in case’. Wiring up the power supply Next, loosely mount the transformer, making sure to include two rubber washers underneath it (with one between the transformer and the dish that holds it in place). Sleeve the four secondary wires together with heatshrink tubing after cutting them to a length that will allow them to comfortably reach the inputs of the power supply PCB, without too much slack. Ensure that the two wires that go to the middle two “GND” positions of CON1/CON4 on the power supply board are from opposite ends of the two windings (ie, start/finish and not start/start or finish/finish). You can verify this once the unit is powered up by measuring the AC voltage between the outermost two terminals of those blocks. You should measure close to 24V AC. If you get close to 0V AC, they are not phased correctly, or there is a bad connection. Fig.20: the safety panel shown at actual size. This can be made from aluminium, Presspahn or another thick insulating material. Cut the rectangle to size, remove the notch, drill the holes, then score and bend the two tabs up by 90°. Australia's electronics magazine December 2025  85 N Australia's electronics magazine 470mF 4148 1 + + ~ KBL404 BR1 ~ 1 1 1 470mF 2200mF + 2200mF + F2 1A F1 1A 470mF 10mF 10mF 10mF 100nF 470mF 100nF 100nF 47mF 100kW CON13 47mH 100nF 100kW 100kW J1 J2 + 470mF 470mF + 220mF 4.7kW 10kW J3 10kW 4.7kW 1 miniDSP MCHStreamer + 100nF 100nF 2200mF 100nF 100nF 100nF 470mF 2200mF 100nF 4.7kW 100nF 100nF 10nF 100nF + 10mF 470mF 100nF 100nF 2200mF 2200mF 100mF 10mF 47mH 100mF 47mH 100mF 4004 10mF 4004 220W 10mF 100nF + 10kW 10kW 1mF + + COIL 10nF Microcontroller 10mF 100nF 100mF 100mF 220mF 10kW 10kW 4148 100pF 100kW 100pF 22mF 100pF 4148 COIL 100kW 100nF 100pF 4148 10kW 10kW 10kW 10kW 470pF 22mF 47mF COIL 100kW 100nF DOWN 22nF S2 2025-03-24 5.6nF CLIP 100nF 100nF 150pF 10kW 1kW 100pF 22mF GND UP 22nF S2 + 100mF 100nF CON1 TGM Was Here Mar 2024 1 180W 180W 2.7nF 100nF 22nF RE1 22nF 2.7nF 100nF 2.7nF DAC Ch4 1 100nF 27nF + 200W 1 ITSOP4136 TP1 B A CK S1 22nF 2.7nF IRD1 22nF IR R X CON2 100nF Q14.4 4148 2.7nF 100nF + 200W 100nF 10W 200W 10W 200W 180W 180W 100nF 8.2nF 2.7nF 100nF 27nF + 100mF 2.7nF DAC Ch1 1 2.7nF 100nF 27nF 2.7nF COIL CON8.4 OUT1 100W 100nF RLY6.4 8.2nF 100mF + 100nF 8.2nF 8.2nF 8.2nF 8.2nF 100nF 27nF 2.7nF DAC Ch2 1 100nF CON8.3 OUT2 100mF 10W 200W 10W 200W 180W 180W 27nF + COIL RLY6.3 2.7nF 100nF 2.7nF 100nF 2.7nF 100mF + 200W 100nF 8.2nF 8.2nF Q14.3 4148 8.2nF 100nF 2.7nF 8.2nF 27nF 100mF 10W 200W 10W 200W 180W 180W 100nF Mar 2025 Digital Preamp V2.3a TGM Was Here 2025 100nF COIL RLY6.2 DAC Ch3 2.7nF 100nF 27nF + 100mF 100nF 8.2nF 8.2nF Q14.2 4148 8.2nF 100nF 8.2nF 100mF 10W 200W 10W 2.7nF 100nF COIL RLY6.1 200W 27nF + FOR PCM1794A 200W TO 270W 8.2nF TO 2.7nF 180W TO 0W 8.2nF 8.2nF Q14.1 BC547 4148 FOR PCM1794A 220W TO 560W OMIT 27nF Digital Preamp Controls v1.1 DSP CORE 100nF + + 100mF FOR PCM1794A 2.7nF TO 2.2nF 820W TO 750W 100mF 47mF 10mF BAT85 100mF 100nF 100mF + 100nF ADC 47mF 47mF 47mF 10W 10W 10W 10W 100W 100W 47mF CON8.2 OUT3 5 Note: as with any anodised aluminium, the rack enclosure will not necessarily have the rear, side, front and top panels Earthed due to the anodising providing an insulating layer between the panels. Each panel should be checked for electrical continuity to the bottom Earthed plate. That also applies to the internal safety panel if it is made from metal. Use separate Earth wiring between panels that don’t become Earthed when assembled using Earth wire and crimp eyelets and ensuring the anodising is scraped off at the mounting positions. LCD MODULE 10mF 1.5kW 100nF 10mF 10mF 100nF 100nF 1.5kW 23 34 220W DVDD3.3 12 1 10mF 10mF 4148 COIL 100pF 22mF 22mF INPUT SWITCHING 100pF 100kW 680W 100nF 220W 4004 PIC32MX270F256D-50I/PT GND 47mH 4148 COIL 75W 75W DIGITAL I/O 4.7kW 10kW Power + Supply 4.7kW 4148 4.7kW 4148 4148 + 4.7kW 4004 CON14 5819 22mF 1 2 3 4.7kW 10kW + 22mF 4.7kW 22mF 100kW 4.7kW BAT85 BAT85 BAT85 BAT85 100nF + 100kW + 4004 4004 47kW + + E + A + T1 12V+12V 30VA + 4004 BAT85 100kW 4.7kW 100kW 4.7kW 5.6W 91W 91W BAT85 100pF 100kW 1kW 680W BAT85 BAT85 BAT85 BAT85 680W 680W 100nF 680W 470pF 470pF 470pF CON7 1kW 91W 91W 10kW 1kW 47kW 100nF 47kW BAT85 47kW 100W 4.7kW 10kW 100W 47kW 180W 200W 220W 100nF 22nF 820W 220W 10kW 1kW 10kW 220W 220W 820W 820W 10kW CON8.1 OUT4 10kW CON6 MONITOR OUT 10kW 180W 47kW 100W 4.7kW BC547 220W 820W 820W 220W 220W 100W 47kW 220W 100W 47kW 180W 180W 200W 220W 180W 180W 200W 820W 820W 220W 47kW 100W 4.7kW BC547 220W 820W 820W 220W 220W 820W 820W 220W 47kW 100W 4.7kW BC547 220W 820W 820W 47kW 180W 180W 200W 200W 220W Silicon Chip 820W 86 820W 820W 3 Fig.21: this shows all the low-voltage and mains wiring, which are separated by the safety panel. Make sure you follow the mains wiring details carefully and insulate all exposed terminals, except for the Earth terminals, which may be left uninsulated. Don’t forget to tie the wires down with cable ties. siliconchip.com.au Above: a neat trick to getting the PCB front mounting holes in exactly the right spot on the base of the case is to mount the rear panel to the case, and put some masking tape gently under the standoffs. Push them down and they will magically mark the exact locations to drill. Right: a close-up of the mains wiring. Between the insulation & separating panel, none of the wires can come loose and contact any of the low-voltage circuitry. Now use zip ties to secure all the mains wiring to the base plate using the holes provided, as shown in Fig.21. Ensure there is only a little slack in all the mains wires. Mounting the control board The control PCB is secured to the front panel using the rotary controller’s threaded bush and the supplied nut. The three pushbutton switches need to be jiggled to get their shanks to fit into the holes we drilled. If any were soldered in slightly askew, you will need to adjust them. Having these a snug fit ensures the front panel controls are all solid and steady in use. Once it is mounted, install the washer and tighten the rotary encoder shaft screw. As noted earlier, adding epoxy or silicone sealant around the pushbuttons on the inside of the case will provide increased tolerance to rough treatment. Fit the LCD bezel into the front panel and, once it is neat, glue it in place with a few dabs of neutral-cure silicone sealant. Allow this to set, ready to install the LCD once it has its cable plugged in. Finishing the wiring Next, we need to connect the +5V DC and ±10V DC outputs of the power supply board to the main Digital Preamp board. Use lightweight hookup wire, and select colours that will avoid you making errors in the connection. The connections required are shown in Fig.21, although the exact routing shown is not necessarily ideal; refer to the photos to see how we ran the wires. We ran heatshrink over the groups of wires to ensure things remained tidy, and ran the wires under the Digital The final power supply PCB uses two terminal blocks for the transformer connection, making connecting the AC inputs easier. Also note that in the final boards, we have moved the DC input on the Digital Preamp board in from the edge of the board, so making connections to it is easier. siliconchip.com.au Australia's electronics magazine December 2025  87 Preamp PCB. It is a tight fit to get the wires up and out between the safety screen and the Digital preamplifier PCB – there is a 5mm gap, which is just wide enough to fish them out. In the final design, we have shifted the power connectors inboard from their original position on the Digital Preamplifier board to give you a little more room to get the wires into the screw terminals. We ran the ribbon cables under the power supply PCB as this makes things neat. Fold the ribbon cable so that the connector mates up to the control board and LCD neatly. Ensure that the red stripe of both wires connects to pin 1 at both ends. Fig.21 shows an alphanumeric LCD with a SIL header soldered to an adaptor board to allow the IDC connector to plug in. That is a valid way to connect it, but you will see from our photos that we used an LCD screen with a DIL header that the IDC connector plugged into directly. Whichever way you do it, make sure the wiring is correct such that the GNDs of the two boards are joined. Now attach rubber feet to the bottom of the case to stop the screws in the chassis scratching up your workbench. Lid preparation If the Digital Preamplifier is to be used in a very warm environment, it is a good idea to augment the ventilation in the case lid over the power supply heatsinks. The Altronics H0625 heatsinks specified are dissipating in the region of 2.5W each. They have a thermal resistance to ambient of 10°C/W, which in free air would mean a 25°C rise in temperature above ambient. However, the case compromises this, because the internal temperature will rise above the ambient temperature of the surrounding air. Many of our tests were carried out in a hot room (35°C) and the Digital Preamplifier case was measured to be 45°C. This results in the LM317/LM337 devices sitting at around 67°C. This is well in specification for them, but higher than we would prefer. Using a larger heatsink for these devices didn’t help a lot. The right thing to do is to get the heat out of the case by increasing the ventilation over these heatsinks. We did this by marking and drilling an array of 5.5mm holes, which does not take that long to do. 88 Silicon Chip The process we used was to first mark the locations (see Fig.19), centre punch them all and drill 2mm pilot holes, then the 5.5mm holes, finishing with an 8mm drill bit to deburr the holes. A coat of satin black paint on the lid tidies this up nicely. Labels You can now install the labels for the rear and front panels. We thought about getting the front and real panels engraved, but the logistics and cost put us off. So we came up with some simple labels that we 3D printed. The approach was to print a 1.5mm-thick base layer using black filament, then we have extruded the text for our labels to be 0.8mm higher than this base. When printing these, we wait until the base layers are printed, then pause the printer and change the filament to a contrasting colour. One preamp got gold labelling, and another red. We then used a couple of tiny dabs (a dab is less than a drop) of superglue to affix the labels to the panels. We used a run of masking tape to define the mounting line so all are affixed level and even. We think this is not a bad way to label the Digital Preamplifier. You might come up with another method. Perhaps you’re keen to dust off the old Dymo and give it some of those oh-so-stylish green labels! Using the Digital Preamplifier The Preamplifier works just like any preamplifier, except it has many more features. The first time you use it, make Once your unit has been fully assembled, it should look something like this. You don’t need the extra bits of aluminium we added to the heatsinks. Double-check all the mains wiring and insulation before powering it up. very sure that you set your crossovers to appropriate bands for your drivers, and that you turn the volume down to a sensible level before you power on any connected amplifiers. If you don’t, you might be very surprised by some loud noises, and could damage your expensive speakers. The best way to set up the crossover will come down to what test equipment you have and how you are using it. Some of our active systems are only a subwoofer channel added to a good pair of speakers. It is entirely possible to set up such systems pretty well by ear. On the other hand, if you want to time align your speakers and implement a full multi-channel crossover, you will need some measurement equipment. The first thing to set up is the time alignment of your speakers; after that, the crossovers will behave much as you would expect. We have set limits on the volume control that allow you to turn the volume up to 11 (in fact, the maximum gain is 12dB). Be aware that turning the volume up on this preamplifier will generate no noise unless your input is noisy – so treat that control with respect. The good news is: no more pots that get scratchy over time! Remember that pushing the control knob in saves your settings; this includes the selected input and current volume. So the next time you turn it on everything will load up and the volume will be where you left it. Past this, we hope you enjoy your truly digital preamplifier. Here’s a quick run through the user interface. In the idle/normal state: ● The rotary encoder simply changes the volume. ● Channel selection is via the buttons to the left. ● The ‘back’ button to the right changes the interface into the Function Select state. ● Push on the main encoder saves the current state to EEPROM. In the Function Select state: ● The rotary encoder allows selection of: > Volume > Channel Setup > EQ Setup > Save > Load ● Pushing the encoder selects that function. ● Pressing ‘back’ exits. The Channel setup menu: ● Allows selection of channel 1-4. ● For each channel in turn, you can make live adjustments of: > Low-frequency crossover slope (6, 12, 24 or 48dB per octave) > Low-frequency crossover point: 5Hz to 15kHz > High-frequency crossover slope (6, 12, 24 or 48dB per octave) > High frequency crossover point: 5Hz to 15kHz > Channel attenuation: 0-20dB in 1dB steps > Channel delay: in 1.7mm steps (rounded on the display) > Mono selection: for channel 1, allowing you to mono a subwoofer output ● Invert selection: allowing you to invert the audio output on individual channels The Equaliser Setup: ● Allows selection of Common EQ1-3 and Channel 1-4 Equalisers 1-3 each (15 in total) ● For each of these: > EQ: off or parametric > Centre Frequency: adjustable > Q: from 0.1 to 10 > Gain: -10dB to +10dB (range can be increased by modifying software but this should be enough) Load and save loads or saves the selected set of parameters to EEPROM. Conclusion This is the first fully digital preamplifier we’ve published, and one of the most complex circuits we have described. While it will take some time to assemble, it isn’t an especially difficult job overall, and we think the SC result will be well worth it! December 2025  89