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High-end Part 2: By Phil Prosser Active Monitor Speakers With Subwoofer The Active Crossover Amplifier fits in a clean-looking black metal case and contains everything you need to drive the Active Monitor Speakers presented last month. It could also be used to power a different two-way bi-amplified speaker system with or without a subwoofer. Y ou will have heard us discuss active speakers and their benefits before. One of the problems with them is that if you use standalone parts, you end up with a stack of boxes containing preamplifiers, crossovers, power amplifiers and speaker protectors. The result can deliver excellent performance but can also be an unruly mess. This article will describe how you can fit all the required electronics into a svelte two-rack-unit (2RU) high case, offering 50W per channel for each midrange/woofer and tweeter, with line level outputs for an active subwoofer or two. A high-quality matching subwoofer will be described next month that can deliver substantial, clean bass down to almost 20Hz. An output power of 50W for the midrange/woofers and treble drivers might seem modest, but there is also a 180W amplifier in the subwoofer, giving a total system power of 380W. 50W is actually an enormous amount of power for the other drivers as these amplifiers do not need to handle the large voltage swings required to deliver the bass (any signals below ~85Hz). This article brings together several previous projects; in terms of electronics, we are only adding a very simple power supply board. I have worked to keep metalwork to a modest level of complexity, though some drilling and filing will be necessary. I built it in a high-quality Altronics H5038 case as this avoids the hassle What is needed to build a stereo Active Monitor Amplifier system 4 x Hummingbird Amplifier Modules – December 2021; siliconchip.au/Article/15126 3-Way Active Crossover – October-November 2021; siliconchip.au/Series/371 Multi-Channel Speaker Protector (4-CH) – January 2022; siliconchip.au/Article/15171 Active Monitor Speakers Power Supply – described in this article 2RU rack case, heatsink and other miscellaneous parts of fabricating the enclosure and provides enough space to fit all the parts. To start building it, gather or make all the required sub-assemblies, as shown in the panel at lower left. The input to the Active Crossover Amplifier is the stereo output from your preamplifier, with line level outputs to your active subwoofer and speaker level to the midrange/woofers and tweeters. The Active Crossover Amplifier is the heart of the High-End Speaker System, as shown in Fig.1 from last month. A full description of each subsystem is provided in the referenced articles. I suggest you read them as they provide good background information that I won’t repeat here. The metalwork and subsystem integration forms the majority of this project. Let’s start with building the case, as once that is done, the modules drop in, ready for wiring. You can see the overall arrangement in the adjacent panel and Photo 9 overleaf. Chassis and metalwork Start by marking and drilling the base of the chassis as shown in Fig.17. Also drill and file the front and rear panels as shown in Figs.18 & 19. Testfit the connectors and other items to ensure you won’t need to rework anything. On the front panel, be careful to check the height of your PCB standoffs, as these determine the location of the holes for the crossover controls. For the front panel, you will be best off installing the 35mm standoffs at both the front and rear locations of the 62 Silicon Chip Australia's electronics magazine siliconchip.com.au Features & Specifications ∎ Stereo three-way active crossover with 24dB/octave Linkwitz-Riley roll-off ∎ Four 50W high-fidelity amplifier channels ∎ Line-level subwoofer outputs (left and right or dual mono) ∎ Speaker protection and de-thumping on all outputs ∎ Baffle step correction implemented at line level ∎ Fits in a high-quality, two-rack-unit (88mm high) case ∎ Silent operation with passive cooling are not that many holes, and the holes line up with the gaps in the fins. If you cannot tap these holes, it is possible to run long M3 machine screws or bolts through the heatsink, but I found that tapping the holes was easy enough and took less than half an hour. To tap the holes, drill to 2.5mm diameter and use an M3 × 0.5mm tap with plenty of lubricant (light oil). Amplifier construction crossover board and sliding it forward to verify the drill holes match up with the height of the potentiometer shafts on the crossover. There are small locating pins at the bottom of the potentiometer mounting threads. The best thing to do is use a 3mm drill and drill a ‘blind hole’ into the rear of the front panel deep enough to accommodate the pin without going right through the panel. This is not as hard as it sounds, but if you are concerned, filing, cutting or snapping these off is a cheeky alternative. Slide the crossover in and check the alignment with the holes in the base. Mine were very close. If there is a minor misalignment, it is fine to drill the mounting holes in the base out to 4 or 4.5mm, which will give you wiggle room with the standoffs. I did not install the front standoffs on the Active Crossover board as they interfere with the lip on the front panel. Now is a great time to drill and tap the heatsink, as shown in Fig.20. There If you haven’t already, assemble four Hummingbird amplifier modules as described in the December 2021 issue (siliconchip.au/Article/15126). It is important that you attach the wiring before mounting them on the heatsink; once they have been installed on the heatsink, you will not be able to get a screwdriver in to tighten the terminals. I used 300mm lengths of heavy-duty (7.5A rated) red, green and black wire and a 500mm length of white wire (for the positive, ground, negative and Fig.17: mark & drill the base of the Altronics 2RU case as shown. Drill the holes to 3.5mm for mounting locations; if you need extra wriggle room, you can drill or file them to 4mm. If using a different case, you will have to make adjustments. siliconchip.com.au Australia's electronics magazine December 2022  63 Photo 9: When you have built all the modules, installed them in the case and wired everything up, it should look like this. I put a fair bit of effort into keeping all the wiring neat as it helps with the performance. In particular, keep those AC loops tight and away from the Crossover. output of the modules, respectively). These will be slightly too long, but we can trim them to be the perfect length when we connect them to the other modules (mainly the power supply). If you did not fully test them when you built them, you need to do that now. Once installed, it would be a real bother to strip everything apart to fix a silly mistake. To do this, strip the ends of the pigtail leads on each module and power each amplifier up. You can run functionality tests without the heatsink if there is no bias. If a module draws a lot of current, switch it off immediately and sort the problem out! The most likely cause is that the pot is adjusted the wrong way, and you have maximum bias. The most basic functionality check is to power the amplifier up and check for DC on the output. If the output is within 50mV of 0V, it is very likely that the amplifier is working, as this shows the DC feedback loop is operating. If available, check the output with a scope to verify that it is not oscillating. For bonus points, run a sinewave through the amplifier module and check that the output waveform is clean. You can run this last test using an AC voltmeter provided you use a test signal of 400Hz at 100mV RMS; you should get about 2.8V at the output. Once the modules are all working, mount them and adjust their bias. First, mount the module at the back Photo 10: the four Hummingbird modules mounted to the heatsink with pigtails. Some prototype V2 Hummingbirds were used, along with a variety of spare transistors! Australia's electronics magazine of the heatsink. Do not forget to use insulators and insulating bushes on the screws. Otherwise the power supply will be shorted out via the collector tabs and heatsink! Also use flat and shakeproof washers on each screw so that they don’t back out. Power up the first module using a bench supply and adjust the bias current until it is 50mA, either by measuring across a resistor in the fuse holder (in place of the fuse) or across the emitter resistors. Anything that can supply at least ±15V DC at 1A or more is sufficient to power the module for this test. Let the module sit for a while; the current will eventually settle down (it will change as the transistors warm up). During development, I tested the impact of changes in the bias current. I determined that minor misadjustments only marginally impact performance; the amplifier gives well under 0.01% distortion when it is close to correct bias. As you finish one module, mount the next and make all adjustments. Rinse and repeat until you have all modules mounted. You will end up with an assembly like that shown in Photo 10. siliconchip.com.au Fig.18: drilling details for the front panels. These are outside views. If drilling a different case, you can use the same general pattern, but you might need to adjust the overall position of the template. Fig.19: the amplifier rear panel drilling details – note that this is an inside view. Fig.20: each set of three holes on the heatsink is for mounting one Hummingbird amplifier, with two more holes for the thermal cut-out. Drill and tap at least two holes in the bottom of the main section to mount it to the base of the case. siliconchip.com.au Australia's electronics magazine December 2022  65 Twist the wires together to ensure you know which ones go where and also to make tidy bundles. This has the added benefit of keeping magnetic field radiation from the power wiring to a minimum. Tie wrap the power leads as shown in the photos. You will achieve pretty good mechanical rigidity by tying the bundles between adjacent modules. If you plan to use this as a portable amplifier or for road use, you will need to install bracing between the Hummingbird amplifiers and the chassis base. For example, angle brackets secured to the mounting holes in the Hummingbird amplifier boards. Next, mount a 70°C normally-closed thermal switch via the two remaining holes on the heatsink, with flat and shakeproof washers on each screw. I have included this as a safety measure – if the heatsink gets too hot, it will switch off. I have never managed to get to that point with mine, but I am happier with that protection in place. Power supply assembly The power supply is very simple, comprising a 300VA transformer, Photo 11: space the wirewound resistors off the PCB to help with heat. bridge rectifier and filter PCB. Its circuit is shown in Fig.21. As this is supposed to be a ‘highend’ design, I decided to provide maximum scope for constructors to ‘go the extra yard’ [extra metre? - Editor]. My original power supply accepted 10,000μF capacitors. I tweaked this to fit 35mm diameter capacitors, and as seen in the final pictures, that allows me to fit three 15,000μF capacitors in parallel for each rail. I doubt that will make a big difference, but it makes me feel happy. I recommend a minimum of three 6,800μF capacitors, with 10,000μF being the ‘sweet spot’. The limiting factor on capacitor size is the 10A fuses at the input to the power supply. If your capacitors are too large, these fuses will become unreliable on power-up due to the massive inrush current. I have included a one-second delay on the Speaker Protector power supply. This is arguably unnecessary given that there is also a switch-on delay built into the Speaker Protector. There is also a 100W resistor in series with the power supply to the Speaker Protector. This drops about 10V, thus reducing dissipation in the Speaker Protector regulator. PCB assembly is straightforward – use the overlay diagram, Fig.22, as a guide. The power supply is built on a double-sided PCB coded 01112221 that measures 147 × 60mm. Start by fitting the screw terminals, then the fuse clips and fuses. I put a fuse in the clips and soldered the assembly in from the top, ensuring everything aligned and fitted. Load in the components in the delay section next, making sure not to swap the PNP and NPN transistors. The BD139 must go in with the metal surface facing the edge of the PCB. Fig.21: the upper part of the power supply circuit is a capacitor bank with multiple terminals to connect the amplifier modules, fuses for protection and LEDs to indicate when power is present and act as bleeders. The lower part is a delay circuit that applies power to the Speaker Protector after roughly one second. 66 Silicon Chip Australia's electronics magazine siliconchip.com.au The 3.3kW resistors have a maximum dissipation of 380mW with the nominally ±35V supply rails, so 1W resistors are OK, provided you space them at least 5mm off the PCB. The 100W 5W resistor for the speaker protector runs quite warm to the touch, dissipating about 1W. The 82W 5W resistors for powering the Active Crossover drop about 10V and dissipate 1.5W. This might be much less than their 5W rating, but they still get very warm. Stand all these resistors off the PCB by 10mm, as shown in Photo 11. Mount the power supply board in the case using tapped spacers and machine screws with flat and shakeproof washers. When doing the wiring, do not place plastic insulation wiring against these resistors. The final power supply board, as presented here, moves these resistors away from the power amplifier boards to make that easier. Powering the Active Crossover At this point, it will save you a lot of fiddling to connect 500mm of twisted red, green and black light-duty hookup Photo 12: the four Hummingbird amps are now wired up to the power supply, and the output wires running under it are ready to attach to the Speaker Protector. Note that this is not the final power supply board design. Fig.22: use this PCB overlay diagram as a guide to fit the components on the power supply board. Be very sure to get the electrolytic capacitor polarities right, or it could fail spectacularly! siliconchip.com.au Australia's electronics magazine December 2022  67 wire to the Active Crossover header on the power supply board and leave this for later. If you forget this now, it can be installed later, but you will need needle-nosed pliers to get the wires into the terminals. Now mount the power supply PCB in the case. It should be about 5mm clear of the Hummingbird modules horizontally, with DC input close to the transformer and rectifier. Next, we need to connect the amplifier wiring to the power supply. The DC supply and mains wiring details are shown in Fig.23. I chose to run one pair of amplifiers from each side of the power supply PCB. Note that there are output headers for up to six modules, but we only need four in this application. It does not matter which terminals you use as they all connect to large low-­ impedance copper fills on the PCB. I kept track of the amplifier modules, numbering them 1 through 4 from front to rear of the heatsink. I used tape on the twisted bundles and for the outputs, ran amplifiers 1 through 4 left to right, looking from the rear of the amplifier case – see Fig.26. For the wiring, cut the positive, negative and ground wires so that they are a neat fit to the connectors on the Power Supply board, ensuring there is sufficient slack that you can remove PCBs later if necessary. Do not cut the speaker output wire; this goes right through to the Speaker Protector, twisted with the extra ground wire we are about to add from the power supply. Route it with the power loom to minimise the output current loop area. That also minimises distortion by reducing the coupling of these fields into the amplifier front end and input circuitry. Connect 450mm lengths of heavyduty green wire from the second ground screw terminal on each output from the power supply. These go to the speaker terminals, following the speaker output wiring through the Speaker Protector. These will finally be trimmed to length when you connect these to your speaker terminals. The Speaker Protector We are using the four-channel version of the Multi-Channel Speaker Protector (January 2022; siliconchip.au/ Article/15171). However, I had some spare six-channel versions left over from the development of that project, and it seemed a terrible waste not to use them. There is no need for more than four channels, though. FOLD UP V1.2 2021-09-17 POS GND 25-40VDC + + + + + + _ + _ _ COIL COIL 914 914 914 + + + + + 27V HEATSHRINK SLEEVES OVER ALL CONNECTIONS Multichannel Speaker Protector PRESSPAHN SHIELD COIL NO NC NO NC NO NC CHANNEL 4 AMPLIFIER CH 1 SPKR AMP SPKR AMP CH 2 CH 3 CH 4 CH 5 CH 6 SPKR AMP SPKR AMP GROUND COM COM COM SPEAKER PROTECTOR MODULE CABLE TIES TGM SPKR AMP SPKR AMP T1 + ACTIVE MONITOR SPEAKERS POWER SUPPLY 2022-04-15 01112221 CHANNEL 3 AMPLIFIER 3.3kW 1W *82W 5W N EG LED2 PREAMP POWER CON8 CHANNEL 2 AMPLIFIER 1 PO S LED1 *82W 5W CON4 10A FUSE 1 + + BR1 + – + ~ + ~ + + ACTIVE MONITOR SPEAKERS POWER SUPPLY 3.3kW 1W M5525C 10A F U SE 2 12V CON9 CHANNEL 1 AMPLIFIER *100W 5W SPKR PROT + + + PRESSPAHN L G G R COIL CABLE TIES 4148 + 150nF 150nF 150nF + 12kW + LK2 COIL L G G R 150nF 12kW LK1 4148 V– 150nF 150nF 150nF ACTIVE CROSSOVER MODULE 12kW 12kW 150nF 12kW 12kW + COIL L G G R 2.7kW 1 2.7kW HIGH + 22nF 22nF 22nF 22nF 2.7kW 2.7kW MID 2.7kW SUB LK4 GND 22nF + 22nF CON3 2x 12V DC/AC or 24V DC 2.7kW 22nF 22nF 2.7kW 2.7kW + + + + V+ HEATSHRINK SLEEVES + POWER SWITCH + CABLE TIES THERMAL BREAKER 47 m F LK3 + + + + Install for Mono Sub Fig.23: the mains and DC supply wiring. The signal and amplifier output wiring is shown separately, in Fig.26. Read the text for important information on safely running and insulating the mains wiring. 68 Silicon Chip Australia's electronics magazine siliconchip.com.au To test these, power them from a bench supply. As described in the original article, apply positive and negative DC voltages to the AMP inputs one by one, and check the relevant relay ‘clicks’ out. With this working, your speaker protection is good to go. Mount the module to the chassis using tapped spacers and machine screws with flat and shakeproof washers. Wiring the protectors into the system is easiest with the rear panel removed. Wire up the inputs as shown in Photo 13. Note the following: 1 - The ground wire from the power supply to the speaker terminals runs straight underneath the Speaker Protector PCB. 2 - I twisted the output wires with the ground, as shown in the photo. This keeps things neat and again minimises current loops. 3 - I marked the wires to be soldered to the output terminals with a small piece of heatshrink tubing to ensure I did not confuse them with the amplifier outputs, then connected these to the “SPKR” terminals. I ran channels 1-4 left-to-right across the protector – although the critical thing to get right is the pairing of the amplifier and speaker terminals. 4 - These connections are definitely the fiddliest bit of this project. Use needle-nosed pliers, and don’t cut the leads too short. Now cut a 600mm of white lightduty hookup wire plus two 300mm lengths (white & red) for the speaker protector power and ground connections. Twist them together and secure with heatshrink tubing, referring to Fig.23 for the required layout. Run the wires between the Speaker Protector power terminal, under the Power Supply PCB to the power output for the Speaker Protector, with the GND side going via the thermal switch. The connections to the thermal switch are made using 6.3mm spade lugs. The recommended 100W 5W resistor on the Power Supply PCB is the correct value for a 25V AC transformer. If your transformer voltage is below 20V AC or above 30V AC, check this resistor once it is operational and adjust as needed. Top tip: connect this wire before you screw the rear panel on unless you have three arms! Transformer and rectifier Now is the time to install the transformer. The recommended transformer is a 25+25V AC 300VA toroidal type. A lower power unit would work but should only be used if you will either reduce the supply voltage or don't plan on ever driving the amplifier hard. Suppose you really want more than 50W output per driver and will only ever connect this to 8W speakers or our Active Monitor Speakers. In that case, you could use a 30V AC transformer instead, provided you check the voltage ratings of all the power supply capacitors. That will Photo 13: the wiring to the Speaker Protector is easier to do before you have fully mounted it in the chassis. Note the removal of the rear panel to gain some extra space while doing this. siliconchip.com.au Australia's electronics magazine give you close to 70W per output. I have specified a 35A bridge rectifier; this is especially necessary if you use high-value capacitors on the Power Supply board. The 35A bridge rectifier should be mounted to the base of the chassis with a 25mm-long M3 panhead machine screw with a flat washer and shakeproof washer. Put a dab of thermal paste under the bridge rectifier to ensure it stays cool even if the amp is driven hard for extended periods. Secure the power transformer with the flying leads toward the bridge rectifier. We are trying to minimise high current paths near the crossover here. Transformers are typically supplied with two rubber washers for the top and bottom, plus an M6 bolt and dished plate. Do the bolt up moderately tight, but not so tight that you crush the windings. Using the colour codes for the Altronics transformer: 1 - Connect the white and black secondary wires directly to the middle two GND terminals on the power supply PCB. If necessary, scrape the enamel insulation off to expose bare copper. Also check that the tinning on these wires does not extend back under the PVC sleeve, as that can be a shorting hazard. 2 - Next, connect the orange and red wires to the AC terminals on the bridge rectifier. Usually, the positive terminal and one AC terminal are marked on rectifiers. The other AC terminal will be diagonally opposite the marked one, and the negative terminal will be diagonally opposite the positive terminal. 3 - You will need to cut these leads to a sensible length, but too long is better than too short. These wires have very high current pulses, and we don’t want big loops to generate magnetic fields. Depending on the type of wire used, you might need to scrape off the enamel coating after cutting them. 4 - Tie wrap the leads from the Power Supply as shown in the photos. It’s now time to install an eight terminal length of the terminal strip. These come in various sizes; 57mm spacing is good for the recommended part. If you are using an alternative, check the mounting hole placement. Cut a 70 × 80mm piece of insulating card such as Presspahn and fit it under the terminal strip. Our terminal strip is laid out as shown in Fig.23. December 2022  69 Fig.24: these minor modifications to the Active Crossover midrange/ woofer output implement ‘baffle step correction’ below 250Hz. The 2.2kW resistors and 330nF capacitors are added to the existing PCB, while the existing 100W resistors change to 1kW. regulators cool during operation. Compensation for baffle diffraction Single Rail, jumper JP1 & JP2 across pins 2-3 ual Rail, jumper JP1 & JP2 across pins 1-2 requires a Dslight boost to the bass/mid ZD1 3x BC547 output below about 250Hz. This com47kW Q3 pensates for diffraction100from the edges kW 100kW Q5 of the loudspeaker for the particular REG2 LM337 enclosure. The following changes suit the Active Monitor Speakers; for100othnF ers, you will need to change the values: JP1 JP2 1 - Instead of 100W at the output 10kW 22kW of the midrange/bass section, use 1kW 100nF 36kW 7.5kW (these are next to RLY2). 5.6kW 7.5kW V+ HS1 D8 5V1 10kW + 4148 220nF 220nF 220nF 5.6kW 220n F 220n F 36kW 36kW IC16 NE5532 100nF + 100nF 36kW 5.6kW 36kW 220n F 220n F 150nF 10kW SUBSONIC FILTER 2.7kW IN: Link pins 2 & 3 of both JP6 & JP7 OUT: Link pins 1&2 330W 330W 100nF JP6 JP7 330n F 47mF 47mF 2.2kW 1kW 1kW 4.7kW 4.7kW + 4.7kW RLY1 COIL 100W RLY2 12V DPDT SIGNAL RELAY 100W COIL LOW OUT CON4 MID OUT L G G R CON2 L G G R 150nF IC6 NE5532 47mF 150nF 12kW C ON 5 100nF 47mF RLY3 12V DPDT SIGNAL RELAY 150nF 150nF 12kW IC5 NE5532 12kW 2.2kW + 330n F 47mF 4.7kW IC4 NE5532 IC17 NE5532 + IC3 N E5 5 3 2 33kW 100nF 220nF 2.7kW 10kW 150nF 47mF 150nF 100W 5.6kW 22kW 12kW + + + + + + L G G R 1kW 4.7kW 100nF 12kW IC2 NE5532 150nF 4.7kW 1 2 V D PD T SIGNAL RELAY HIGH OUT 22nF IC15 NE5532 47mF + + + + + + LOW – MID Resistor, R1 LOW – MID Capacitor, C1 22kW COIL 470mF D3 100nF 47mF IC1 NE5532 36kW 10kW JP5 100W 47mF 2.7kW 22nF Install for Mono Sub 1kW 4.7kW 22n F 2.7kW LK1 2.7kW 2.7kW 22nF IC14 N E5 5 3 2 SUB VR3 10kW LOG 100nF 7.5kW 36kW 5.6kW 100nF 22kW 7.5kW 47mF 5.6kW 33kW 47mF 100nF IC13 NE5532 47mF 22kW 10kW 2x BC557 4.7kW 100kW D4 D7 12kW BEAD 100pF 22nF IC12 N E5 5 3 2 47mF 22kW 4.7kW Q1 Q2 4148 47mF 100nF 12kW NP 47kW 47mF + 47mF 22nF 2.7kW MID VR2 10kW LOG 22kW 7.5kW 1kW 4004 5.6kW 47kW CON1 12kW 36kW IC8 NE5532 33kW Configuration for 2 or 3 way crossover 2 Way: Jumpers on JP3 & JP5 across pins 1-2 3 Way: Jumpers on JP3 & JP5 across pins 2-3 7.5kW 1kW 22kW 100nF 100nF 100nF D9 JP3 47mF 22n F 2.7kW 22kW BEAD 100pF NP 2.7kW IC11 NE5532 2.7kW 22nF 47mF 1kW 100nF 5.6kW HIGH VR1 10kW LOG 100nF 100nF 1kW 100nF IC10 NE5532 47mF 33kW 4004 R2 1kW 1kW 100nF 22kW 1.6kW 10mF 270W D1 10mF 4004 D2 100nF 4004 R1* 1000mF 270W When building the Active Cross100nF over, install Altronics H0655 heatsinks (or equivalent) in place of the 100nF suggested Altronics H0650. These 22kW 7.5kW are twice the size7.5kand will keep the W 22kW 4.7kW Q4 10mF 1000mF 10mF REG1 LM317 220mF V– CON3 4004 Active Crossover + 4004 D11 4004 HS2 Single rail R1 = 3.6kW Dual rail R1 = 1.6kW POWER > 1 2x 12V DC/AC or 24V DC POS GND NEG /AC /AC 4004 Silicon Chip D10 70 input and speaker terminals using a 10mm machine screw, flat washer, shakeproof washer and nut as shown in Figs.23 & 26, just touching the bot100nF tom of the lid. D5 Now do the mains wiring as follows, using Fig.23 as a guide: 1 - Attach the IEC socket to the case using 10mm M3 panhead screws, nuts and shakeproof washers. The nuts need to make connection to the chassis by scraping away any paint or anodising. Connect the IEC Active pin through the fuse to the terminal strip using brown mains-rated wire. 2 - Connect the active from the terminal strip through the power switch and back to the terminal strip (making the front panel easy to remove). Ensure that the active input wire goes to the power switch's switched (NO) pin, with the output from the common terminal (so the spare pin is not connected to Active when power is off). Insulate the pins on the switch, including any unused ones. 3 - Connect the Active wire from the front panel switch to one side of the transformer primary. 4 - Connect a wire to the IEC Neutral pin running alongside the Active run to the front panel, then to the terminal strip using blue mains-rated wire. 5 - From here, connect to the other side of the transformer primary. 6 - Connect the Earth pin of the IEC connector to the chassis Earth lug using a 3.2-4mm solder lug or (even better) crimp eye terminal screwed down securely to an M3 machine screw to the chassis. Make sure that the paint on the chassis is scraped back to bare metal and that you have a star washer to cut through to the chassis under the bolt. Use green/yellow striped wire for this. 7 - Score and fold the 120 x 40mm sheet of Presspahn to form an L-shape 90mm tall, 30mm wide and 40mm deep. Mount it between the mains MID – HIGH Resistor, R2 MID – HIGH Capacitor, C2 BAFFLE STEP CORRECTION 100W changed to 1kW and add 2 x 2.2kW and 2 x 330nF Fig.25: the annotations show the components whose values determine the crossover frequencies, plus the changed parts for the ‘baffle step correction’. The full overlay for the Active Crossover PCB is shown in the October 2021 issue. Australia's electronics magazine siliconchip.com.au 2 - Connect a 2.2kW resistor in series with a 330nF MKT capacitor and connect this network from the junction of the 1kW resistor & relay to ground. The modified Active Crossover circuit is shown in Fig.24, while PCB changes are shown in Fig.25. Also, when building the Active Crossover, set it up for dual rail operation and set the jumpers as described in the original article. It’s a good idea to do a quick bench test to check its operation after construction. Feeding it with ±15V DC will allow you to check that the regulators are generating the correct output voltages, and that the de-thump relays click out after a couple of seconds. The jumpers on the Active Crossover need to be set as follows: ■ Three-way operation is achieved with JP3 and JP5 set to pins 2-3. ■ JP1 and JP2 set to pins 1-2 for dual-rail operation. ■ I left the 20Hz subsonic filter in, but note that the active subwoofer will generate useful output below that! To do this, set JP6 and JP7 set to pins 2-3. Other choices you need to make when building the Active Crossover are whether it should be a two-way or three-way crossover and what the crossover frequencies should be. We will configure it as a three-way crossover (with the lowest output for the subwoofer) and crossover frequencies of 88Hz for Low-Mid and 2.7kHz for Mid-High. However, if you are not planning on using the system with a subwoofer, you will need to change it to a two-way crossover at 2.7kHz. The required component values were given in Table 1 on page 48 of the October 2021 issue. They are 12kW/150nF for 88Hz (Low-Mid) and 2.7kW/22nF for 2.7kHz (Mid-High). MKT capacitors are readily available in both values in either 5% tolerance (preferable) or 10%. Use 1% metal film resistors for the best precision. The locations for all these components are also shown in Fig.25. Now install the Active Crossover in the case. The front panel should have been drilled to suit it already. Power wiring for the Crossover should have been connected to the power supply already; route and trim this to connect to the power connector at the right front corner of the Active Crossover. Doing the input and output wiring for the Active Crossover involves siliconchip.com.au Parts List – Active Monitor Amplifier / Crossover 1 430mm wide, 330mm deep 2RU black rack-mount case [Altronics H5038] 4 assembled Hummingbird amplifier modules (Silicon Chip, December 2021) 1 assembled 4-way Speaker Protector with larger heatsink (see text) (January 2022) 1 assembled Stereo Active Crossover with modifications as per text (October 2021) 1 300mm wide, 75mm tall diecast aluminium heatsink, 10mm fin spacing, 0.37°C/W [Altronics H0545 or two Jaycar HH8555 joined with hole position adjustments] 1 300VA 25-0-25 toroidal mains transformer [Altronics M5525C] 1 double-sided PCB coded 01112221, 146.5 × 108.5mm 1 250V 3A+ SPST power switch (toggle, rocker etc) 1 normally-closed thermal switch/breaker, 250V AC 10A, 70°C [Jaycar ST3823] 8 TO-3P insulating kits [Altronics H7220] 4 TO-126 insulating kits [Altronics H7120] 1 small tube of thermal paste 1 3.2-4mm solder lug or crimp eyelet connector Connectors & fuses 1 chassis-mounting IEC mains input socket [Altronics P8320B] 4 chassis-mounting dual red/black binding posts [Altronics P9257A] 1 red chassis-mounting insulated gold RCA socket [Altronics P0218] 1 black chassis-mounting insulated gold RCA socket [Altronics P0220] 2 yellow chassis-mounting insulated gold RCA sockets [Altronics P0219] 1 8-way 17.5A terminal block strip [Altronics P2135A] 6 4-way 5mm terminal blocks (CON1-2, 4, 6-8) [Altronics P2026A] 1 2-way 5mm terminal block (CON9) [Altronics P2034A] 4 2-way polarised header plugs with pins [Altronics P5472 × 4 + P5470A × 8] 1 M205 10A chassis-mount safety fuse holder [Altronics S5992 or Jaycar SZ2028] 1 M205 5A fast-blow fuse 4 M205 PCB-mount fuse clips 2 M205 250V 10A ceramic fuses Hardware 1 M3 × 25mm..... 9 M3 × 16mm......... 9 M3 × 10mm......... 19 M3 × 6mm panhead screws 35 M3 shakeproof washers 32 M3 flat washers 7 M3 hex nuts 8 M3 × 10mm tapped spacers 40 100mm cable ties 2 sheets of Presspahn or similar insulating material, 80mm × 70mm & 120 × 40mm sheets Wire & cable 1 2m length of each colour (red, black, green & white) heavy-duty (10A+) hookup wire 1 2m length of 7.5A mains-rated brown wire 1 1m length of 7.5A mains-rated blue wire 1 10cm length of 7.5A mains-rated green/yellow striped wire 1 150cm length of each colour (green & white) light-duty hookup wire 1 50cm length of red light-duty hookup wire 1 3m length of figure-8 screened cable [Altronics W2995 or W3022] 1 10cm length of each diameter (3mm, 5mm & 10mm) heatshrink tubing Semiconductors 1 BC556 80V 100mA PNP transistor (Q1) 1 BD139 80V 1A NPN transistor (Q2) 1 BC546 80V 100mA NPN transistor (Q3) 2 5mm LEDs, any colour (LED1, LED2) 1 12V 400mW zener diode (ZD1) [eg, 1N963] 1 400V+ 35A chassis-mount bridge rectifier with spade terminals (BR1) 1 1N4148 75V 200mA signal diode (D1) Capacitors 6 10,000μF 50V electrolytic, 10mm lead spacing (6800μF-15,000μF acceptable) 1 47μF 50V low-ESR radial electrolytic 2 330nF 63V MKT 1 ● Resistors (all 5% 5W wirewound unless otherwise stated) 3 22kW 1% 0.6W metal film 2 3.3kW 1W 2 2.2kW 1% 1/4W metal film ● ● 2 1kW 1% 1/4W metal film 1 100W 2 82W ● for the baffle step correction (see Fig.25) Australia's electronics magazine December 2022  71 making four flying leads of 800mm length using figure-8 shielded cable, plus two at 350mm long. To make the cables, you need the following parts (also in the parts lists): ■ 4 × four-way 2.54mm polarised header plugs with matching pins ■ 4 × two-way 2.54mm polarised header plug with matching pins ■ 2 × 80cm lengths and 2 × 35cm lengths of figure-8 screened cable ■ 3mm and 5mm heatshrink tubing Photo 14 shows what the header ends of these cables should look like. To make them: 1 - Start by separating the two coax channels, then strip 25mm of the outer sheath from each, exposing the braid. 2 - Tease the inner conductor from the braid and strip the end by 5mm. 3 - Twist the braids together into a neat bundle. 4 - Cut two 20mm lengths of 3mm heatshrink, such that when put on the braid, it will leave enough exposed copper to crimp to. 5 - Slide a 10mm-long, 5mm diameter piece of heatshrink over both the braid and central conductor but do not shrink it yet. 6 - Slide the 3mm heatshrink over the braid; there should be 4-5mm of wire protruding. Shrink this down. 7 - Slide the 5mm heatshrink to cover about 3mm of the junction where the braid and inner core separate and shrink it down. 8 - Present the braid to the crimp connector. You need to trim off excess braid wire so that the strain relief LEFT TWEETER RIGHT TWEETER crimp will go over it, and there is about 3mm of braid wire in the electrical crimp section. 9 - Take one of the pins and, using sharp-nosed pliers, crimp the end of the braid conductor. Carefully add a tiny amount of solder to the crimped part, careful not to let it wick down to the spring section. 10 - Strip back 3mm from each of the inner conductors and crimp and solder as above. I was dissatisfied with the strain relief crimp missing the plastic and added a small piece of heatshrink, but that is optional. 11 - Now push the pins into the header plug, with the braids in the middle and left and right conductors on the outside. You will feel and/or hear a click when they seat properly. LEFT MID SPEAKER RIGHT MIDRANGE SPEAKER AUDIO IN LEFT & RIGHT SUBWOOFERS OUT Multichannel Speaker Protector V1.2 2021-09-17 POS GND 25-40VDC + + + + + + + + _ + _ _ COIL CO I L 914 914 914 + + 27V + CO I L CHANNEL 4 AMPLIFIER NO NC NO NC NO NC CH 1 SPKR AMP SPKR AMP CH 2 CH 3 CH 4 CH 5 CH 6 SPKR AMP SPKR AMP GROUND COM COM COM SPEAKER PROTECTOR MODULE (RIGHT TWEETER) TGM SPKR AMP SPKR AMP T1 CHANNEL 3 AMPLIFIER + ACTIVE MONITOR SPEAKERS POWER SUPPLY 2022-04-15 01112221 *82W 5W NEG LED2 PREAMP P OWE R CO N 8 1 PO S (RIGHT MIDRANGE) LED1 *82W 5W 10A + + FUSE 1 – + ~ CHANNEL 2 AMPLIFIER CHANNEL 1 AMPLIFIER 3.3kW 1W CO N 4 ACTIVE MONITOR SPEAKERS POWER SUPPLY + BR1 + + + ~ 3.3kW 1W (LEFT TWEETER) 10A M5525C FUSE 2 12V CO N 9 *100W 5W (LEFT MIDRANGE) SPKR PROT L G G R COIL + + + PRESSPAHN 4148 + L G G R COIL 150nF 150nF + LK2 150nF 12kW + ACTIVE CROSSOVER MODULE 150nF 12kW LK1 4148 V– 150nF 150nF 12kW 150nF 12kW 150nF 12kW 12kW 47mF + LK3 COIL L G G R 2.7kW 1 2.7kW HIGH + 22nF 22nF 22nF 22nF 2.7kW 2.7kW MID 2.7kW SUB LK4 GND 22nF + 22nF CON3 2x 12V DC/AC or 24V DC 2.7kW 22nF 22nF 2.7kW 2.7kW + + + + V+ HEATSHRINK SLEEVES + POWER SWITCH + CABLE TIES + + + + Install for Mono Sub Fig.26: the signal wiring for the Active Monitor Speakers. While the wires from the Active Crossover board to the Hummingbird amplifier modules are shown separately for clarity, they should be run using figure-8 shielded cable to avoid hum and buzz pickup. 72 Silicon Chip Australia's electronics magazine siliconchip.com.au Now that you’ve made the cables, you can complete the signal wiring as in Fig.26. The input and subwoofer output connections go to the rear panel, while the midrange/woofer and tweeter outputs go to the amplifier modules. I opted to use modules 1 and 2 (the two most forward in our case) for the midrange/woofer and modules 3 and 4 (rearmost) for the tweeters. The final configuration is shown in Photo 15. Testing By this stage, you should have verified that the amplifier modules, Speaker Protector and Active Crossover function correctly. The next steps are a few safety checks: 1 - Using a DMM, check that there is no continuity between the chassis and the power supply ground (or, for that matter, the main positive and negative DC rails). The aim here is to check the integrity of the insulation bushes. If your meter registers a resistance on its 20MW range, you need to find and fix the conductive path. 2 - Using a DMM, check that there is a solid connection between the Earth pin of the mains socket and all chassis panels. You should get a reading under 1W in each case. If not, find the problem and, if necessary, add Earth jumpers from the affected panels to the base panel or main Earth lug. 3 - Using a DMM, check that there is no continuity from the Active/ Neutral wiring to the amplifier's chassis and the power supply 0V point. If your meter registers a resistance on the 20MW range, you need to find and fix the conductive path. Assuming that all checks out, insert the 5A mains fuse in the chassis holder and, while monitoring the voltage across the main supply rails, briefly switch on mains power. As you need to do this with the lid open, ensure you stay clear of the mains wiring while it’s switched on. Use two DMMs with alligator clip leads attached so you can do it hands-off. If you don’t have two DMMs or enough clip leads, connect a DVM between the main positive and negative rails. The rails should very quickly rise to close to ±35V or 70V total. They could be a few volts higher or lower than that. If you don’t get the correct reading(s), switch off quickly and check the following: ■ Carefully check all of the mains wiring. ■ If the voltage is zero: is the fuse blown? Is the switch on? ■ Is there mains voltage across the transformer primary? You can check this by probing the terminal strip. ■ Is there AC at the input to the bridge rectifier? ■ Is there pulsating DC at the power supply input terminals? The voltage across each pair of amplifier module outputs should be under ±50mV. If that all checks out, apply an AC signal (or music) to the inputs and check that the sub, midrange/ woofer and tweeter outputs behave as expected. If not: ■ Check the wiring from the Active Crossover to the amplifier modules. ■ Check that the amplifier modules have a reasonable output; this can be measured on the top of the emitter resistor using an oscilloscope probe or AC voltmeter. ■ Check that the amplifier outputs go to the correct Speaker Protector terminals and, subsequently, the rear panel connector. ■ Check that the Speaker Protector is working properly. At this point, you should have a functioning Active Crossover Amplifier. The levels need to be set to match your speakers. The process for doing that was at the end of the article on the Active Monitor Speakers published last month, so refer back to that. If you’re using the Active Crossover Amplifier with different speakers, you’ll have to tweak the crossover frequencies and levels to suit. Next month The final article in this series will describe the High-­Performance Subwoofer that can optionally be paired with the Active Crossover Speakers. It connects to the subwoofer output on the Active Crossover Amplifier and extends the bass of the system almost down to 20Hz. We highly recommend that this Subwoofer be built as part of the system, although you can still enjoy the Active SC Monitor Speakers without it. Photo 14: this is how each of the four stereo shielded cables should look once terminated to the polarised plugs, ready to connect to the Active Crossover board. Photo 15: a close-up shot showing the details of the complete low-voltage DC and signal wiring. siliconchip.com.au Australia's electronics magazine December 2022  73