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PART 2: PHIL PROSSER Digital Preamplifier and Crossover This advanced preamplifier uses digital processing to provide unprecedented flexibility. It has three digital inputs, including high-fidelity USB, four analog stereo inputs, four stereo outputs, two digital outputs (including USB) and a stereo monitor channel. Having described how it works, let’s get into the assembly process, starting with the circuit boards. T he Digital Preamp is housed in a slimline 1U (44.5mm-tall) rack-mounting case, although it can just as easily sit on a shelf. Specifically, we used the Altronics H5031 vented black aluminium case. Since rack cases have a standard height and width, the only real variable is the depth. In this case, it is 255mm, which is on the low end for rack cases. So most 1U vented rack cases should be suitable for this build, but we think the H5031 is an excellent choice unless you have a particular reason for wanting to use another. The result is very neat, and the required metalwork is not hard – although there is a fair bit of drilling to do on the rear panel. It houses the IEC C14 mains input connector, mains fuse holder, holes for the USB input, S/PDIF input/output and 10 dual RCA connectors for analog inputs and outputs. Before we get to preparing the case, though, let’s assemble the PCBs. It is not an overly difficult process, but there are a lot of parts to fit onto three boards, so it will take a while. Power Supply PCB assembly Build the Power Supply board as shown in its overlay diagram, Fig.14. Assembling this board is straightforward, and a quick job compared to the main board. Features & Specifications Four stereo analog inputs (1V RMS maximum) Frequency response: 7Hz to 43kHz <at> -3dB (with PCM1798 DACs) One analog input can be configured to handle 2V RMS+ S/PDIF coaxial and TOSLINK digital audio inputs Monitor output for analog inputs Four independent stereo output channels, 2V RMS full scale High sampling rate/bit depth USB audio stereo input and output Programmable equalisation, crossovers, relative attenuation & delay for each output Memory for four different configurations Attenuation at 20Hz: 0.3dB; Attenuation at 20kHz: 0.0dB Volume control: +12dB gain to -128dB attenuation in 0.5dB steps Total harmonic distortion plus noise (THD+N): 0.003% across the audio band (largely unchanged to >40dB attenuation) 68 Silicon Chip Australia's electronics magazine siliconchip.com.au siliconchip.com.au The completed Power Supply PCB. We have used a small amount of silicone sealant on the heatsinks and inductor to keep them stable. D6 100nF 470mF 47mH L3 100mF 12V AC ~ 2200mF 100mF + + 2200mF D4 LM337 4004 L2 2200mF D5 4004 + 2200mF BR1 KBL404 10 m F 10mF 100nF 100nF D1 100nF + + + 2200mF 2025-02-16 v2.1 Digital Crossover Power Supply 47mH L1 ~ REG1 LM317 220W 1.5kW 4004 F2 1A F1 1A 10mF D2 4004 + CON4 10mF CON2 100mF L4 330µH GN D CON1 +10V GND -10V REG3 5819 + 100nF CON3 12V AC LM2575T-5 + +5V GND + Start by fitting the resistors. There are only two different values; the 220W resistors will have two red stripes at one end, while the 1.5kW resistors will start with brown and green stripes. Follow with the diodes; these all have the cathode stripes either to the right or upward. Make sure the schottky diode (D6) goes in the correct position, near REG3. With these parts in, you can fit the 100nF MKT and higher-value electrolytic capacitors. We have arranged these so that, in each case, their longer “+” lead goes towards the top of the board when the silkscreen is the right way up. Next, mount the inductors. There are three bobbin-style inductors and one toroidal type. The three bobbin inductors are all the same value; they must have current ratings of at least 500mA. Put a dab of neutral-cure silicone sealant under the toroidal inductor to keep it stable and avoid stress on the solder joints. Follow with the connectors (with the terminal block wire entries going towards the nearest edge of the board), fuse holders (retaining clips outwards), fuse and bridge rectifier. Make sure the bridge’s positive terminal goes nearest to the terminal blocks as shown in Fig.14. Next, install the LM2575-5 switchmode regulator. Make sure this is the 5V version, and that you install it with its heatsink tab facing the edge of the PCB. The PCB footprint is right for the bent lead version of this device; if you get the version with leads all in a row, gently bend the first, third and fifth leads out to suit the PCB pad arrangement. Next, mount the LM317 and LM337 linear regulators to their heatsinks (a folded piece of aluminium similar to the dimensions of the Altronics H0625 will do) using insulating washers and bushes. The heatsinks must be no more than 26mm tall, so that the power supply board will fit inside the case later. If using the specified heatsinks, mount them flush to the PCB; this is required for it to fit in the case. Add a dab of neutral-cure silicone sealant to the base of each heatsink to ensure it is stable and does not move around in use. When soldering the devices to the board, make sure you don’t get REG1 (LM317) and REG2 (LM337) mixed up. 47mH 220W 1.5kW REG2 10nF 2200mF 10 m F GND 10mF 100nF Fig.14: the power supply board assembly is straightforward. The main thing to watch is the orientation of all the electrolytic capacitors and bridge rectifier. Make sure the terminal block wire entries are accessible and the fuse holder retaining clips are on the outside. Finally, don’t forget the heatsinks for REG1 & REG2 – they are required! Testing the power supply With everything mounted, connect a DC power supply set to anything between 15-25V, with its negative output to ground, and positive output to either of the AC inputs. Check the +5V output. This should measure 4.9-5.1V. If there is no output, verify you have the fuses in and that the 1N5819 diode is the right way around. Also check that you have the LM2575 (REG3) the right way around. On one prototype, we bent the leads the wrong way, and can attest to the fact that the device doesn’t work when it is back-to-front! Australia's electronics magazine Check the voltage on the positive DC output connector, CON2. You should measure 9.7-10.3V on its left-most terminal. If not, check around the LM317 device (REG1), especially the 220W and 1.5kW resistors and the orientation of its protection diodes. Now connect the positive of your power supply to the ground input terminal, and the negative to either of the AC inputs. Repeat the above check on CON2, but this time look for a negative voltage with a magnitude of 9.7-10.3V on the right-hand terminal. That verifies the power supply is November 2025  69 470mF 470mF + + D1 Make no mistake, this is a big board. It measures 331 × 150mm with 553 parts – see Fig.15. Plan to assemble this in stages, and mount groups of parts in batches so you don’t lose track of where you are at. We find it very helpful to make a copy of the parts list and to install groups of components one at a time, then cross them off the list. Our strategy is to get the onboard power supply working first, then the input and output switching, then the microcontroller (so we can see the LCD working), then the rest. This strategy does need to consider mounting the SMD parts first, as that is easier with some ‘elbow room’. First, install 10mm standoffs on all CON16 CONTROLS GND 18pF 18pF 23 12 GND X2 CLATCH 8MHz CDATA CCLK COUT 1 IC17 25AA256 1 100nF Digital Preamplifier assembly Silicon Chip IC15 PIC32 100nF 100nF A working, so it’s time to move onto the main Digital Preamplifier board. 70 10mF 470W 4.7kW 10 m F 34 100nF FB16 1kW 10kW CON19 LCD JP1 1 CON21 LCD BIAS CON8 1 DVDD3.3 LCD - REVERSE MOUNT mounting holes. The four at the front of the board remain there for installation, while the two at the rear should be removed when you install the board to the rear panel of the case. A few things to consider before we get stuck in. If you are using the ADAU1467 Core Board, do not load anything inside the area marked DSP CORE or DSP ADAU1467. Also, if you are using PCM1794A DAC ICs instead of PCM1798s, you must use the alternative resistor and capacitor values, which are marked on the PCB. A trick we use for through-hole parts is to insert several, then place a sheet of paper over them, allowing us to flip the board over without them falling out. The general loading order is then: 1. Fit all the surface-mounting capacitors and resistors, which are mostly in M2012 packages, except for 100pF CLIP BAT85 BAT85 1kW 100 m F 47mF + IC6 NE5532 1kW 100nF 91W 91W IC8 NE5532 47 m F 10kW 10kW + 100 10W 47mF BA D16 10W 470pF 100nF D11 100nF 100 m F 10W 10kW 10kW 100nF 47kW COIL 100nF 22mF 100kW 100kW BC547 100nF D13 4.7kW IC7 NE5532 470pF 47 m F RLY5 4148 D18 100nF 10W 1 ADC 47mF 100nF GND FOR P 2.7nF 820W DSP CORE 10mF 100nF 100nF 100nF 100nF 1kW 10mF 10mF 100nF 100nF IC18 ADAU1467 100nF 100nF 100nF 10mF Microcontroller VR44 20kW 1k W 10kW 22 m F 470pF 100kW 4.7kW D12 D19 D23 D22 BAT85 BAT85 BAT85 BAT85 4148 D17 10kW D20 BAT85 33mF D21 BAT85 D25 BAT85 680W D24 BAT85 91W 2.7nF 10nF 2.7nF 1 CON9 ADC TEST CON17 (ICSP) 100nF Australia's electronics magazine COIL RLY4 CON7 220W 1mF 1 Q8 IC9 CS5381 220pF 100nF 100pF 100W 100W 150pF 4.3kW 100nF 5.6nF 10mF FB1 X1 100nF CON12 D3 470mF 1 LED2 LD1117V33 REG1 -10V + 100nF FB4 100nF 100nF GND Power + Supply 100nF 4004 +10V LD1117V33 REG2 FB14 100nF FB2 4.7kW PIC32MX270F256D-50I/PT + + 100nF 100nF 10nF 10nF 100nF + + 10mF 10nF 10nF FB15 100kW 100nF 100nF 100nF 10mF 100nF Q1 1 + D10 4148 47mF Q4 BC547 CON11 BC557 D7 L5 470mF 470mF +5V 47mH GND FB6 470mF 100nF 100nF 470mF FB7 10mF 100nF FB3 100nF 10mF 680W 100nF 220W 100nF + 100 m F 100 m F 33mF 220 m F 4004 REG3 D4 Q12 BC547 10kW MCLR V+ GND PGED PGEC BC547 10kW + D6 10mF AVDD_3.3 100kW 100kW Q13 47kW 220 m F BC557 100kW 100kW Q2 10kW 4.7kW 4.7kW 10kW 4148 4.7kW 4148 4148 Q9 BC547 4.7kW 1 10kW 5V_DAC LRCLK 4.7kW IC16 MAX22345SAAP+ Q10 BC547 D5 COIL RLY3 10kW 22 m F 100kW Q7 GND MCLK LRCLK BCLK SDATA 100nF CON13 10mF DIGITAL I/O 100nF 100W 1 J2 Q6 COIL FB12 100pF BC547 BC547 RLY2 4148 4148 D9 INPUT SWITCHING D14 10nF 4.7kW Q5 LM317 (100nF) 100nF J1 TOSLINK TX 4004 1 100kW 4.7kW COIL RLY1 4148 D8 22m F 100pF 100nF J3 100kW 100pF Q3 BC547 5.6W 10kW 10kW 1 (OPT2) 100kW IC13 74LVC244 miniDSP MCHStreamer 4.7kW 1 75W 75W FB10 470pF * 22m F FB13 BC547 FB8 FB11 BT 100pF 680W 91W * 22mF CON14 * 100pF Q1 NJT4030P FB9 1 2 3 CON10 OUT 100nF S/PDIF * ATTENUATION 100nF RESISTORS 22 m F * IN OPT1 TOSLINK RX CON5 680W TUNER 680W CON4 AUX1 100pF CON3 AUX2 2 2m F CON2 100nF 12.288MHz 18pF 100W 18pF 2025-03-24 the capacitors in the μF range, which will be larger. The numbers in square brackets (“[]”) are for when you are using the ADAU board. There are: T 1[0] × NJT4030P transistor in an SOT-223 package T 4 × 47μF tantalum capacitors T 2 × 33μF tantalum capacitors T 22[17] × 10μF tantalum/ceramic capacitors T 42[29] × 100nF ceramic capacitors T 5 × 10nF ceramic capacitors T 2 × 2.7nF ceramic capacitors T 5 × 220pF ceramic capacitors T 4[2] × 18pF ceramic capacitors T 10 × 10kW resistors T 1[0] × 4.3kW resistor T 2[1] × 1kW resistors T 1 × 470W resistor T 5 × 220W resistors T 2[1] × 100W resistors T 5 × 22W resistors siliconchip.com.au 100nF 100nF 12.288MHz IC4.1 100nF 100nF 1 PCM1798 SDATA 1 10 m F BCLK LRCLK 10kW MCLK GND CON1.1 10mF W 18pF 74LVC244 22W 22W 22W 22W 22W 47kW 180W 180W 200W 200W 100 m F + 100 m F 2.7nF 100nF 2.7nF 2.7nF 10mF 100nF 47mF 100nF 10kW 220W 27nF 100nF 2.7nF 10mF 10mF IC4.4 100nF 100nF PCM1798 SDATA 1 1 10mF BCLK LRCLK 10kW MCLK GND CON1.4 DAC Ch2 Mar 2025 Digital Preamp V2.3a TGM Was Here 2025 T 16{0} × 820W resistors T 0{32} × 750W resistors T 17{1} × 220W resistors T 16 × 200W{270W} resistors T 16 × 180W{0W} resistors (wire links can be used as 0W resistors) T 1 × 5.6W resistor 3. Fit the 15 [14] ferrite beads by inserting resistor/diode lead off-cuts or tinned copper wire through the beads and then soldering them to the board. If you need the AUX1 input to handle more than 1V RMS, swap FB8 & FB9 for resistors and then install the attenuator resistors to make dividers (see the red text in Fig.15). This approach can be used to make the other inputs handle high voltage if needed. 4. Fit all the MKT polyester and through-hole ceramic capacitors: T 49 × 100nF T 8{0} × 27nF Australia's electronics magazine 820W 100nF IC3.4 NE5532 220W 220W 27nF 2.7nF 100nF 47mF 100nF 10mF 10kW 10mF 100 W IC1.4 NE5532 + 820W 820W 220W 220W 200W 10W 10W 220W 220W IC1.3 NE5532 47kW 100W 4.7kW BC547 180W 180W 200W 200W + 100nF 2.7nF 820W 820W 220W 10mF 100W 47kW 47kW 100W 4.7kW BC547 180W 180W 200W 2.7nF 820W 820W IC2.2 NE5532 2.7nF 100nF 180W 27nF 2.7nF DAC Ch3 2. With those all in place, install all the diodes and through-hole resistors. We recommend doing these now as you can still flip the board and solder things flush to the PCB without too much fiddling. Keep the lead off-cuts as you will need them later for the ferrite beads. Numbers/values in braces (“{}”) are for PCM1794A DAC ICs: T 3 × 1N4004 diodes T 13 × 1N4148 (or 1N914) diodes T 12 × BAT85 diodes T 12 × 100kW resistors T 11 × 47kW resistors T 13 × 10kW resistors T 17 × 4.7kW resistors T 5 × 1kW resistors T 5 × 680W resistors T 10 × 100W resistors T 4 × 91W resistors T 2 × 75W resistors T 12 × 10W resistors siliconchip.com.au 27nF IC4.2 IC4.3 100nF 100nF 100nF 100nF PCM1798 1 PCM1798 SDATA 1 1 SDATA 1 BCLK 10mF BCLK 10mF LRCLK LRCLK 10kW MCLK 10kW MCLK GND GND CON1.3 CON1.2 DAC Ch4 100nF IC10 100mF 180W 200W 100mF 8.2nF 820W 820W 10mF 100nF 10mF 47mF 100nF 10kW 10mF 8.2nF 8.2nF 8.2nF 100nF 10mF 2.7nF 8.2nF COIL 100nF RLY6.4 8.2nF IC2.4 NE5532 100nF 2.7nF + 100nF 100nF Q14.4 4148 220W 220pF 100nF 47mF 100nF 10kW 100nF 27nF 100nF 200W 10W 10W IC2.3 NE5532 IC1.2 NE5532 2.7nF 100nF 10mF 100W 47kW 47kW 100W 4.7kW BC547 220W 2.7nF 820W 820W 220W 220W 2.7nF 100nF 200W + CON8.4 OUT1 100nF 180W 200W 100 m F 27nF 180W 220W 100W 47kW 180W 200W + 100nF 100nF 100nF IC18 ADAU1467 100nF + 8.2nF 220W 220pF 100nF 100 m F 200W 10 W 10 W 220W 220pF DSP CORE 200W 27nF 820W 820W ADC GND 220W 220pF nF 180W COIL RLY6.3 8.2nF 8.2nF 8.2nF IC3.2 NE5532 2.7nF 180W 220W 220W 2.7nF 100nF 820W 820W 100nF FOR PCM1794A 2.7nF TO 2.2nF 820W TO 750W 8.2nF Q14.3 4148 100nF 100nF 27nF COIL RLY6.2 8.2nF 8.2nF 200W 100mF + 100mF 100nF 7m F 200W 10W 10W + 100 m F FOR PCM1794A 220W TO 560W OMIT 27nF 8.2nF 200W 820W 47mF + 1kW 1kW 100 m F IC6 NE5532 1kW 100nF 10W Q14.2 4148 180W 220W + 100nF 180W 100nF + 100mF 10W mF 47kW D16 10W mF FOR PCM1794A 200W TO 270W 8.2nF TO 2.7nF 180W TO 0W 1kW 10W BAT85 8.2nF D15 100mF BAT85 IC5 NE5532 BAT85 BAT85 47kW 8 D11 100nF IC1.1 NE5532 8.2nF 100nF D13 COIL RLY6.1 180W 100kW Q14.1 BC547 4148 220W 47m F 100nF 47m F IC3.1 NE5532 10mF IC2.1 NE5532 22mF 47kW 100W 4.7kW 100pF 100W 100W CON8.3 OUT2 CON8.2 OUT3 IC3.3 NE5532 CON8.1 OUT4 CON6 MONITOR OUT DAC Ch1 Fig.15: building this board will take a while, so make sure you’re organised. It’s best to break it up into several sessions, and follow our suggested order of assembly. The most important thing is to get all the SMD ICs orientated correctly, make sure the solder flows onto all the pins and pads, and fix up any solder bridges that form. Clean off the flux residue so you can inspect all the joints properly. T 1 × 10nF T 16 × 8.2nF{2.7nF} T 16 × 2.7nF{2.2nF} T 4 × 470pF T 1 × 150pF At this point, you have fitted all the low-profile parts other than ICs. Now we can complete the onboard power supply section so we can test it. Load everything else in the section of the board marked Power Supply, at lower left. Use insulator kits and jiggle the pins of the heatsinks into the holes in the PCB to secure them. While finishing the power supply, it is ideal to fit the following across the whole board: T 14 × BC547 NPN transistors T 2 × BC557 PNP transistors T 8 × 10μF electrolytic capacitors T 5 × 47μF electrolytic capacitors T 14 × 100μF electrolytic capacitors November 2025  71 ◀ This Digital Preamplifier was built using the discrete ADAU1467 chip. We have gone to a fair bit of bother to get all the capacitors facing the same way; check yours as you go. Remember that the + indicates the side where the longer lead is inserted (the stripe on the can indicates the opposite, negative side). Power supply testing You can now apply 5V DC to the digital power input, CON11. This should draw only a nominal current as there is no load. Measure the voltage on the DVDD3.3 test point, which is next to the LCD header, CON8, and close to the bottom edge of the PCB. You should measure 3.2-3.4V. If not, verify your applied voltage, check for anything getting hot and ensure you have all the capacitors in the right way around. Next, measure the voltage on the AVDD3.3 test point, which is just to the left of diode D6, below the DIGITAL I/O section. You should again measure 3.2-3.4V. If not, find what is wrong, most likely a capacitor or regulator back-to-front. Now apply ±10V to the analog power input, CON12. You can use the previously assembled and tested power supply board for this, feeding in low-voltage AC (eg, from a 12V AC plugpack). This should also draw only nominal power. Measure the voltage at the 5V_DAC test point, which is near the AVDD_3.3 test point you checked earlier. This should be 4.85-5.15V. If those are all correct, power it down as it’s time to move onto the next section of the board. Filling the I/O sections With the power supply rails working, we can move onto the next stage and get the inputs and outputs working. This means fitting the remaining parts in both the DIGITAL I/O and INPUT SWITCHING sections, in the upper-left and upper-mid parts of the board. Fit the following: T 8 × 100pF ceramic capacitors T 8 × 22μF bipolar electrolytic capacitors (they are not polarised) T 9 × 5V telecom relays; ensure they go in the right way around T 10 × 2-way RCA sockets; make sure these are neat and align with one another The best way to test the board now is to connect it to the power supply board and use that to power everything. Connect the 5V DC, grounds 72 Silicon Chip Australia's electronics magazine siliconchip.com.au siliconchip.com.au Before we make the LCD cable, we need to discuss how it will connect to the LCD screen itself. The screen will have a space for a a 14-way (7×2) DIL header. We need to use this type of screen, rather than the more common type with a 16-pin SIL header, because those latter types are too large to fit in the limited space available in a 1U rack case. The LCD module will need a 7×2 header, and you will need to extend the wires through to the backlight. We used an 8×2 header and cut the spare pins off, then running light duty hookup wire to the backlight pads. This allowed us to plug the 8×2 IDC header in. Double-check the power supply pins on your module; the Altronics module should be a straight plug-in (with the IDC socket orientated correctly), but the other ones specified may have swapped power and GND pins! If so, you will have to swap them in your cable. With that in mind, cut a 250mm length of 16-way cable and install the IDC connector(s), making sure that it will be able to go from the LCD connector on the main board (most likely CON8) to the rear of the LCD panel once installed. Make sure the IDC connectors are fulled crimped on both cables. If they aren’t compressed adequately, some wires may be open circuit, and the TGM Was Here Mar 2024 22nF BACK S1 22nF 10kW 22nF IR RX CON2 10kW Digital Preamp Controls v1.1 1 22nF UP 22nF S2 10kW CON1 DOWN 22nF S3 10kW Fig.16: compared to the other two, the control board is a doddle. Make sure all the controls are square and fully pushed down onto the board before soldering them, though. 10kW Now we really start to bring the Digital Preamplifier to life. Load all the single-row pin headers. These can be snipped or snapped off 40-way header strips. These are: T 6 × 5-way pieces for the ADC, DAC and SPI test points. These are not essential, but can be really handy for debugging. T 1 × 2-way section for the microcontroller reset capacitor enable. Remove the jumper on this if you need to reprogram the micro. T 1 × 6-way section for the programming header, CON17. There are also some DIL headers to fit: T Solder a 5×2 section for the controls (CON16). T Only one LCD header is needed. If you plan to mount the LCD with a 90° header soldered to the rear of the LCD (ie, on the inside of the case), fit CON8. If you have an arrangement where you 10kW Microcontroller section can actually solder the header to the front of the LCD, use CON19 (although we can’t see how this can be done). Next, mount the 20kW trimpot, then solder in the 8MHz crystal, 25AA256 EEPROM IC and the PIC microcontroller. Soldering surface-mount parts has been described in many articles so I won’t go into great detail. The main thing is to ensure the parts are aligned with their pads and, critically, orientated correctly before soldering more than one pin. Use plenty of flux paste and do not be scared to add too much solder, then use wick to remove solder bridges. A bit more flux paste will make the wick extremely effective. Always inspect every pin on the devices after soldering them using a loupe or microscope. Another good trick is to use a phone with macro photograph capability; the pictures on page 77 of the ADAU chip were taken with an iPhone 15. To test this section of the circuit, we’ll need cables to connect the LCD panel and control board. You’ll also need to assemble the control board, as per Fig.16. There aren’t too many components on it, so fit them starting with the lowest profile parts, moving to the tallest. To connect the LCD panel and control board to the main board, you need lengths of 16-way and 10-way ribbon cable. Cut a 300mm length of 10-way cable and use a vise (or proper tool if you have one) to crimp 10-way IDC sockets onto both ends. Orientate the connectors so that, once installed, the cable will exit the main PCB in the direction of the front panel control board. Make sure that the pin 1 marker at each end goes to the same edge of the ribbon. 10kW and ±10V rails. You can power the whole lot from a ±15V power supply connected to the AC inputs, or a 12V AC 1A plugpack (but only short-term). On powering it up, you should find: ● The voltages at CON11 & CON12 are as expected, and the AVDD_3.3, DVDD_3.3 and 5V_DAC rails/test points are good. ● After a few seconds, the output relays should click on. If this doesn’t happen: > Check that the emitter of Q9 goes from 0V up to more than 3V a few seconds after power on. If not, there is something wrong with what is driving this. Are the BC547 and BC557s in the right spots? > Check that the anode of D10 goes high a few seconds after power-on; this is just below pin 1 on CON13 for the MiniDSP. > There are two pairs of resistors in the upper-left corner of the power supply section, 10kW/10kW and 4.7kW/4.7kW. Check that their junctions settle to about the same voltage; if they don’t, something is awry. Check the part values and orientations in this section. ● Finally, check your relay driver transistors and the back-EMF diodes, and make sure the relays are not backto-front. If the relays click on after a few seconds, everything is looking good, so we can move on. 22nF TP1 RE1 ITSOP4136 IRD1 73 following tests won’t go too well. But you don’t want to crush the connectors to the point that they fracture. Parts List – Digital Preamplifier & Crossover Now connect the LCD screen and Control PCB to the main PCB using your new cables. Make sure you have the headers the right way around, and pin 1 on the PCBs aligns with pin 1 on the cables. To verify this, use a DMM set on continuity mode to check for GND continuity between all three boards once they are connected. If you can’t find continuity, check the cables and connectors. Power the Digital Preamplifier from its Power Supply PCB, as before. Check that the current draw is less than 200mA DC or 500mA AC and nothing gets hot. You will then need to adjust the LCD bias by turning VR44, the sole trimpot on the main board. Adjust this up and down until you get either clear text or squares on the display. If you have not programmed the PIC yet, now is the time to do so. If you purchased your PIC microcontroller from the Silicon Chip Online Shop, it will come pre-programmed, so you won’t need to program it. Remove the jumper from JP1 if one is inserted, and use a PICkit or Snap programmer connected to CON17 and the Microchip MPLAB X IPE to load the 0110725A.HEX file into the PIC. We have always used the Digital Preamplifier’s power supply during programming. Once the chip is programmed, you should see a boot screen on the LCD, then the Digital Preamplifier should go into the idle volume set mode. Rotate the rotary encoder; in this mode, it acts as a volume control, so you should see the Attenuation level go up and down. Next, get a Philips RC5 compatible TV remote control (eg, a universal remote set for a Philips TV) and check this also controls the volume. You may need to try a few different Philips TV codes until you find one that works. Then press the channel up and down buttons on the remote. You should hear the relays click. If any of these don’t work, and especially if the display doesn’t work: ● Check that the 8MHz crystal has a waveform at 8MHz using an oscilloscope. ● Check that the LCD_RS, LCD_E and LCD_RW lines, as well as LCD_D4 through LCD_D7, have signals on them 1 1U black aluminium 19-inch rack-mount case [Altronics H5031] 1 16×2 wide-angle blue LED backlit alphanumeric LCD [Altronics Z7018] ♦ 1 four-layer PCB coded 01107251, 331.5 × 150.5mm 1 12V+12V 30VA toroidal mains transformer [Altronics M4912C] 15 small ferrite beads (FB1-FB4, FB6-FB16) [Altronics L4710A] 1 47μH 0.5A high-frequency inductor/choke (L5) [Altronics L6217] 1 TOSLINK fibre optic receiver (OPT1) [Altronics Z1604] 1 TOSLINK fibre optic transmitter (OPT2) [Altronics Z1603] 9 5V DC coil 2A DPDT telecom relays (RLY1-RLY5, RLY6 × 4) [Altronics S4128B] 1 3A 250V AC DPDT switch [Altronics S1050] 1 20kW top-adjust miniature trimpot (VR44) 1 12.288MHz crystal, HC-49 (X1) 1 8MHz crystal, HC-49 (X2) 2 16 × 22mm PCB-mounting heatsinks for TO-220 devices (for REG1 & REG3) [Altronics H0650] ♦ Mouser 758-162KCCBC3LP can be substituted but the power & ground pins may be swapped Hardware 2 TO-220 insulator kits [Altronics H7210] 1 225 × 46mm piece of 1-1.5mm thick aluminium, Presspahn or similar material 10 4G × 6mm self-tapping screws [Altronics H1145] 4 M3 × 10mm tapped spacers 8 M3 × 16mm panhead machine screws 12 M3 × 6mm panhead machine screws 20 M3 shakeproof metal washers 4 M3 flat metal washers 10 M3 hex nuts 5 100mm cable ties 4 large adhesive rubber feet [Altronics H0950] 1 rubber boot for the mains input socket [Altronics H1474] 5 9.5mm rubber grommets [Altronics H1456] 1 3D-printed LCD bezel (details to come) Wire & cable 3 1m length of 7.5A mains-rated blue wire 1 1m length of 7.5A mains-rated brown wire 1 1m length of 7.5A mains-rated green/yellow striped wire 1 1m length of 16-way ribbon cable 1 250mm length of 13mm diameter clear heatshrink tubing 1 1m length of 5mm diameter clear heatshrink tubing Connectors 6 5-way pin headers, 2.54mm pitch (CON1 × 4, CON9, CON21) 10 2-way vertical PCB-mounting red/white RCA sockets (CON2-CON6, CON8 × 4, CON10) [Altronics P0212] 1 2-way polarised header with matching plug and pins (CON7) 2 2×8-pin headers, 2.54mm pitch (CON8, CON19) 1 2-way miniature terminal block, 5/5.08mm pitch (CON11) 1 3-way miniature terminal block, 5/5.08mm pitch (CON12) 1 2×5-pin header, 2.54mm pitch (CON16) 1 6-way pin header, 2.54mm pitch (CON17) 1 2-way pin header, 2.54mm pitch, plus jumper (JP1) 1 chassis-mounting IEC C14 10A mains input socket [Altronics P8320B] 1 panel-mounting M205 safety fuse holder [Altronics S5992] 2 16-way IDC crimp connectors [Altronics P5316] 2 10-way IDC crimp connectors [Altronics P5310] Semiconductors 16 NE5532(A) dual low-noise op amps, DIP-8 (IC1.1-IC3.4, IC5-IC8) 4 PCM1798 or PCM1794A DAC ICs, SSOP-28 (IC4.1-IC4.4) 2 74LVC244APW,118 octal buffers/line drivers, TSSOP-20 (IC10, IC13) 1 CS5381 ADC IC, TSSOP-24 (IC9) 1 PIC32MX270F256D-50I/PT 32-bit microcontroller, TQFP-44 (IC15, 0110725A.HEX) 74 Australia's electronics magazine Testing the microcontroller Silicon Chip siliconchip.com.au Additional Parts for the Preamp 1 25AA256-I/SN 32kB serial EEPROM, SOIC-8 (IC17) 1 Analog Devices ADAU1467WBCPZ300 digital signal processor, LFCSP-88 (IC18) 2 LD1117V33 3.3V low-dropout regulators, TO-220 (REG1, REG2) 1 LM317T adjustable linear regulator, TO-220 (REG3) 1 NJT4030P 40V 3A PNP transistor, SOT-223 (Q1) 2 BC557 45V 100mA PNP transistors, TO-92 (Q2, Q11) 14 BC547 45V 100mA NPN transistors, TO-92 (Q3-Q10, Q12-Q13, Q14.1-Q14.4) 1 5mm red LED (LED2) 3 1N4004 400V 1A power diodes (D1, D3, D6) 13 1N4148/1N914 75V 200mA signal diodes (D4-D5, D7-D10, D14, D17-D18, D26.1-D26.4) 12 BAT85 30V 200mA schottky diodes (D11-D13, D15-D16, D19-D25) Through-hole capacitors 7 470μF 25V low-ESR radial electrolytic 2 220μF 25V radial electrolytic 14 100μF 25V low-ESR radial electrolytic 2 47μF 50V bipolar radial electrolytic 5 47μF 25V low-ESR radial electrolytic 8 22μF 50V bipolar radial electrolytic 8 10μF 50V 105°C radial electrolytic 1 1μF 63V radial electrolytic 49 100nF 63V/100V MKT 8 27nF 63V/100V MKT 1 10nF 63V/100V MKT 16 8.2nF 63V/100V MKT 1 5.6nF 63/100V MKT 16 2.7nF 63V/100V MKT 4 470pF 100V C0G/NP0 ceramic [Kemet C317C471J1G5TA] 1 150pF 50V C0G/NP0 or SL ceramic 8 100pF 50V C0G/NP0 or SL ceramic SMD capacitors (SMD 0805 size 50V X7R ceramic unless noted) 4 47μF 16V tantalum, SMC case [Kyocera AVX TAJC476K016RNJ] 2 33μF 16V tantalum, SMC case [Kyocera AVX TPSC336K016R0150] 22 10μF 10V tantalum, SMA [Kyocera AVX TPSA106K010R0900] 41 100nF 5 10nF 2 2.7nF ±5% C0G/NP0 5 220pF C0G/NP0 4 18pF C0G/NP0 Through-hole resistors (all ¼W ±1% metal film unless noted) 12 100kW 16 820W 10 100W 11 47kW 5 680W 4 91W 13 10kW 17 220W 2 75W 16 4.7kW 16 200W 12 10W 5 1kW 16 180W 1 5.6W SMD resistors (all M2012/0805 size ±1% unless noted) 10 10kW 2 1kW 5 220W 5 22W 1 4.3kW 1 470W 2 100W siliconchip.com.au Optional parts for MCHStreamer USB interface 1 miniDSP MCHStreamer or MCHStreamer Lite kit 1 MAX22345SAAP+ four-channel (3+1) digital isolator, SSOP-20 (IC16) 2 2×6-pin headers, 2.0mm pitch (CON13, CON14) [Mouser SAMTEC 200-SQW10601LD] 2 100nF 0805 50V X7R ceramic capacitors Alternative parts to ADAU1467 chip 1 ADAU1467 Core board 2 2×18-pin female headers Control board parts 1 double-sided PCB coded 01107252, 108.5 × 24mm 1 TSOP4136 infrared receiver (IRD1) 1 90° PCB-mounting rotary encoder with integral switch (RE1) [Altronics S3352] 3 SPDT momentary 90° PCB-mounting subminiature pushbutton switches (S1-S3) [Altronics S1498] 1 2×5-pin header, 2.54mm pitch (CON1) 1 3-pin polarised header (CON2; optional) 7 22nF radial MKT or ceramic capacitors 7 10kW axial ¼W resistors Power supply parts 1 double-sided PCB coded 01107253, 127 × 76mm 3 2-way miniature terminal blocks, 5/5.08mm pitch (CON1, CON3-CON4) 1 3-way miniature terminal block, 5/5.08mm pitch (CON2) 4 M205 PCB-mounting fuse clips (F1, F2) 2 M205 1A fast-blow fuses (F1, F2) 3 47μH 0.5A high-frequency inductors/chokes (L1-L3) [Altronics L6217] 1 330μH 3A high-frequency vertical-mounting toroidal inductor (L4) [Altronics L6527] 2 Mini-U flag heatsinks [Altronics H0625] 2 TO-220 insulator kits [Altronics H7210] 3 M3 × 16mm bare metal panhead machine screws 8 M3 × 6mm bare metal panhead machine screws 4 M3 × 10mm metal tapped spacers 12 M3 metal shakeproof washers 4 M3 flat washers 4 M3 hex nuts 1 3.2mm solder lug [Altronics H1503] Semiconductors 1 LM317T adjustable linear regulator, TO-220 (REG1) 1 LM337T adjustable linear regulator, TO-220 (REG2) 1 LM2575T 5V buck regulator, TO-220-5 (REG3) 1 KBL404 400V 4A SIL bridge rectifier (BR1) [Altronics Z0076A] 4 1N4004 400V 1A power diodes (D1-D2, D4-D5) 1 1N5819 40V 1A schottky diode (D6) Capacitors 6 2200μF 25V low-ESR electrolytic 1 470μF 25V low-ESR electrolytic 3 100μF 50V low-ESR electrolytic 6 10μF 50V 105°C electrolytic 6 100nF 63V/100V MKT 1 10nF X2 Resistors (all axial ¼W ±1% metal film unless noted) 2 1.5kW 2 220W Australia's electronics magazine November 2025  75 when booting and when you rotate the encoder after booting. If not, are the plugs the right way around? Have you used the right 16-way header? Is the LCD contrast on pin 3 of CON19 adjustable from 3.3V down to about -1.8V or so? ● Check the soldering of the PIC; are there any dry joints, or bridges or pins where solder has not adhered to the pad? The microcontroller soldering is by far the most likely problem in this part of the circuit. With the microcontroller up and running, check the LCD backlight; modules are wildly inconsistent in how these are wired and set up. We found that some modules needed the 100W series resistor reduced or linked out to get decent backlighting brightness. Next, familiarise yourself with the user interface: ● The buttons to the left of the Control knob simply control the channels. ● The button to the right of the Control knob is an exit/back button. ● The Control knob can be pushed as an Enter button. Go through the following steps: 1. Push the exit button. You can now rotate through “Save”, “Load”, “Channel Setup”, “EQ Setup” and “Exit to Idle”. 2. Select “Channel Setup” and push in the Control knob. 3. You can set the following: Low Crossover (XO) frequency, Low XO slope, High XO frequency, High XO slope, channel attenuation, channel invert, channel delay in millimetres (1mm ≈ 2.9us) and mono output for channel 1. 4. Make sure these are set to sensible value, and for testing, set Low XO Slope and High XO slope to “none”. This disables the crossover for that band for now, which is useful during testing, as every channel will simply reproduce the input signal. 5. Exit to the Idle screen & click the Control knob. This will save the configuration data to EEPROM. If the system hangs on this, you have a connection problem to the EEPROM (IC17); check the soldering of the EEPROM & associated PIC microcontroller pins. 6. Go into the EQ setup menu. 7. Go through all 15 EQ settings and select “none” for the EQ type. ◀ This photo shows the PCBs & LCD connected so that they could be tested before wiring it up in the enclosure. 76 Silicon Chip siliconchip.com.au An example of a dodgy solder joint on the DSP chip. This is visible as an absence of the clean solder fillet on the third pin in from the left, and possibly the pin next to it. We apply a generous dollop of flux gel (from an Altronics syringe); don’t be stingy and definitely don’t bother trying to reflow the pins without adding flux. Look how much better the joint looked after reflowing! This is the same photo that was shown in the panel last month. 8. Go back to the main menu & click the Control knob to save this state. 9. Double-check the volume control works on the remote. 10. Click up and down channels; the input relays should click to change the input selected. on this and you will have a lot of dry joints on your pins. A thin layer of solder on this is sufficient. 3. Now tin the pads on the chip itself – both the outer pins and its ground tab. Again, ensure all are well tinned but that the central tab has only a thin layer of solder. 4. If you have too much solder on either the PCB or DSP heat spreader tab, use solder wick to remove some. 5. Put flux gel all over the PCB footprint. Be generous; this is essential. 6. Align the chip’s pin 1 with the marking on the PCB. Don’t worry too much about exact alignment, as the chip will be floating around soon. 7. Set your hot air gun/wand to 350°C or so, with a medium airflow rate. 8. Holding your hot air gun in one hand, and your tweezers in your other hand, start heating the board in the DSP area. Keep those tweezers handy to allow you to poke the hot chip around in the air stream. 9. Starting slowly, and from 10cm or so, work your way in as it heats up. Watch the capacitors around the DSP as they are smaller and will show signs of the solder flowing before the DSP does. 10. Bring the hot air gun in closer, to 5cm or so. You might see the DSP chip move around. Try not to make this occur too much. Use your tweezers to poke it back to about where it belongs. 11. You will see some capacitors reflow when you are close to the right temperature. Then the DSP chip solder will melt. There will be a visible change from the DSP chip sitting in the flux, to the solder melting and wetting between the chip and PCB. This will create surface tension, which will pull the chip onto the ground pad. If close to the correct alignment, the pins will pull it into place. 12. Keep the heat on for a little while, and if the chip has pulled itself onto the wrong pads (and of course it will), use your tweezers to poke it into alignment. Once in about the right place, it will snap into place with the surface tension of the solder. 13. We found that gentle and small pushes of the chip got it properly aligned in a few seconds. Work gently and stay calm; the surface tension will help you. Your job is to get the DSP square and in about the right spot. 14. Gently remove your hot air gun and admire your work. 15. While the board is still warm, inspect the solder joints. If any are not pristine, you need to address those now (refer to the photos above). Make sure all the connections are cleanly soldered and show that visible fillet of solder. 16. If there are a lot of dodgy joints, don’t despair; resolder them as described above. One of our chips required quite a lot of touching up, but it worked perfectly in the end. The above procedure might sound scary, but we went through it quite a few times, including removing chips and resoldering them to other boards, with success. We are not in any way expert, so it can’t be that hard. Using the ADAU1467 DSP If you are using the ADAU1467 Core Board, there should be no parts inside the area labelled DSP CORE or DSP ADAU1467. If there are, remove them. Next, load the 36-way DIL sockets. We cut ours from 40-way sockets from Altronics. The best way to mount the sockets is to mate them to your core board, then install the sockets to the PCB. This way, when you solder them to your Digital Preamplifier PCB, they will be perfectly aligned to your module. The EEPROM boot switch on our Core Board was set to ON. It seemed to work fine when set there. You can now plug in the core board. Make sure you get it the right way around; the 10-way header goes at the top. If you are loading the ADAU1467 chip by hand, you will need a hot air gun/wand, a soldering iron with a fine tip, flux gel/paste, fine-point tweezers, a magnifying glass or microscope and, ideally, a camera or phone with a good macro mode. There are many good videos on the internet for this, but essentially, the steps are: 1. If you have no experience in soldering SMD parts, buy the core board. 2. Tin the pads on the PCB. Don’t put so much solder that there are bridges, but make sure all the small pads are well tinned. Do not overdo the central heat spreader tab, or the chip will float siliconchip.com.au Australia's electronics magazine Testing the ADAU1467 DSP 1. Apply power and check that no smoke comes out. The DSP draws a fair current when running all the inputs and outputs, but in this configuration, it drew less than 300mA from our ±15V DC supply. So a 12V AC 1A plugpack should be OK (for now). November 2025  77 2. Use a DVM to measure the voltage on the collector (tab) of the NJT4090P; you should see 1.1-1.3V DC. If you don’t, it is very likely that you have a dry joint on the DSP chip. Check this, especially around pin 3 and its power pins until you get that 1.2V rail up. 3. Wait until the microcontroller is booted. Then, using an oscilloscope, look for a 12.288MHz sinewave on the 100W resistor just below the 12.288MHz crystal. If you don’t see this, it is likely that there is no communication between the DSP chip and the PIC. To debug this: a. Monitor the signals on CON21. This is the SPI interface from the PIC to the DSP chip. Look for activity on CLATCH, CDATA and CCLK on boot and when you change volume. b. If there is no data on any one of these three lines, you have a soldering problem at the PIC microcontroller. Check these pins on the PIC and fix the problem. c. If there is data on all of these lines (noting that COUT is data from the DSP and normally not active), you have a soldering problem on your DSP chip. These lines are on the side of the chip next to the crystal; find the dodgy connection and reflow it. 4. With this interface working, look at the LRCLK lines of channels 1-4. You should see a 192kHz waveform. Similarly, you should see a BCLK signal at 12.288MHz and MCLK at 24.56MHz. If any are absent, hunt down the dodgy solder joint and fix it. Now you have the DSP talking to the PIC and running. Fitting the ADCs & DACs We are almost there; it’s time to mount the ADC and DAC chips. You don’t need to install all channels. As mentioned previously, you have a choice of two different ADC chips and two DAC chips. Make sure you have the right resistors and capacitors installed for the DAC you selected, or else the gain and filter will be wrong. 1. Fit the CS5361/81 ADC chip and associated 1μF and 220μF throughhole capacitors. 2. Install the clipping LED header (CON7). We have not run this to any LED on the front panel on our prototypes, but you can if you wish. 3. Mount the four PCM1794/98 DACs ICs. These are a little fiddly, but not too bad. Be sure to check your soldering on each with a magnifier. 78 Silicon Chip 4. Fit all 16 NE5532(A) dual operational amplifiers. You can use sockets, if you wish; it will make swapping them easier, but they can oxidise over time and eventually lead to problems. 5. Load the last two 47μF bipolar capacitors (either way around). At this point, you should have everything on the board except the TOSLINK transceivers, MiniDSP headers and digital isolator. We have also seen a short circuit between the Iref pin and the adjacent ground, which changed the output amplitude. Ensure that all channels generate the same output levels. If any channel is missing or lower in amplitude (most likely half), check the soldering of the DAC output lines. The output is balanced, and if one pin has a dry joint, you will see a half-­ amplitude output. Further testing MiniDSP & TOSLINK interfaces 1. Apply power. You should see a lot more current draw; ours drew about 0.4A on the positive rail with a ±15V supply. This is all those NE5532s and the DSP having data to work on. At this point, powering it from a plugpack is becoming difficult (unless you have a particularly beefy one, eg, >1.5A). So you're best off using a dual bench supply, or two floating bench supplies connected in series. 2. The power supply heatsinks should get quite warm to touch, but not ‘burning hot’. 3. Use a ‘scope to look for data on the ADC Test Header (CON9). You should see data on the SDATA line, and if you trigger your ‘scope using the LRCLK line, you will see the data aligned with the LRCLK. This is currently noise being measured by the ADC. If you don’t see this data, check that the LRCLK, BCLK and MCLK signals are present. If they are, look for soldering problems on the ADC. Otherwise, examine the DSP chip and fix any bad solder joints you find. Now look at the same SDATA lines for each of the output DAC channels. Turn the volume right up to +12dB. There should be data on all output channel data lines. Again, if not, check the MCLK, BCLK and LRCLK lines, and make sure they are present. If not, fix the DSP chip soldering. You should now be able to feed audio into an input, select that input using the controls and see it on all the outputs, given we disabled all crossovers and equalisers right at the start of testing. Present a 1V 1kHz sinewave to the Bluetooth input, select it using the controls on the front panel, and look at each of the channel outputs. With all channel filters disabled and the gains set to zero, the output signals should all have the same amplitudes. If they are not all the same, check for solder bridges on the outputs of the DAC chips. Australia's electronics magazine Now fit all the remaining parts, which should be: T The MAX22345 isolator (IC16) T The TOSLINK receiver (OPT1); the transmitter (OPT2) is not used and is experimental only. T The two 12-way pigtail headers that come with the MiniDSP MCHStreamer. These are wired pin-to-pin with pin 1 aligned and the pigtails standing straight up. These plug onto J1 and J3 of the MCHStreamer (not J2). With those in place, we can do more testing: 1. Plug the MiniDSP into your PC, Mac or Linux box. 2. Install the ASIO drivers for the MiniDSP onto your PC, or on Mac/ Linux, simply select the MiniDSP as the current audio device. 3. Play some audio on the computer. Use an oscilloscope to look for a signal on the LRCLK test point that we have added on the production PCB, just to the right of the MAX22345, labelled LRCLK. This needs to be present for the Digital Preamplifier to receive the audio. 4. Select the MiniDSP interface using the controls. This should allow you to stream data from your PC to your Digital Preamp. Check that the output signals are as expected. 5. In the Monitor menu, you can also select which channel is sent back to your computer; the Digital Preamp can route this audio to the MiniDSP while doing everything else. At this point, you should have a set of fully loaded PCBs that are operational and ready to install in the case! Next month We still have a fair bit left to do, but we’ll pick this up in the next issue. That final article will have the case drilling and cutting details, final assembly instructions, wiring, final SC testing and usage guide. siliconchip.com.au