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PART 2: BY PHIL PROSSER Low-cost Two- or Three-Way Active Crossover This Active Crossover, described last month, is very flexible. It can be configured as a two-way or three-way crossover, runs from AC or DC supplies, has adjustable levels for all the outputs and has an optional subsonic filter. It’s ideal for building two-way or three-way speakers with each driver powered by a separate amplifier, or it can be used as a signal conditioner for the Tapped Horn Subwoofer described in the September issue. I n the introductory article last month, we explained why you might need an active crossover, how they are used and how this design works. We also showed some performance graphs, indicating that it is suitable for use in a hifi system, plus listed the parts you need to buy in order to build it. Picking up where we left off then, we’ll go over the PCB assembly process, followed by information on how to set up and use it. There is also a troubleshooting section at the end of the article, in case you run into difficulties. There are a few different ways to build the Active Crossover; we’ll explain which parts can be left off in some cases, and how to set up the jumpers for your particular application. coded 01109211 that measures 176 x 117.5mm. The assembly process is pretty straightforward. First, work out where it will be mounted and powered. If you can slip it into its own metal box with an internal power supply, that is ideal. Determine how you will power it and thus the parts you need. Refer also to the panel below on power supply options. Second, select your crossover frequencies. Check the panel describing how to do this from last month. That will affect some of the resistor and MKT capacitor values needed. If you are not sure about the crossover frequencies you require, you could fit PC pins to those component pads and solder the resistors and capacitors to these, to make it easier to change them later. If you only need a two-way crossover, none of the components in the high-frequency section are required (outlined with a red dashed line). Top tip for soldering the resistors and capacitors Fig.15 is the PCB overlay diagram, which should help you during construction. The Active Crossover is built on a double-sided PCB If you envisage yourself significantly ‘tweaking’ the crossover frequencies, we suggest that you select a resistor/capacitor (R and C) combination that is about right for your application and then mount the capacitors on the board. These are more expensive than resistors and do not need to change. Then fit PC pins for all the resistor pads marked “R” and solder your resistors onto these, on the top side of the board. This will allow you to easily shunt them or change them later. Remember that you can use E24-series or parallel resistor combinations to get the exact frequency that you want. 78 Australia’s electronics magazine Construction Silicon Chip siliconchip.com.au The components marked in red should be changed in value to match the chosen crossover frequency – see Table 1 in last month’s issue. Fig.15: use this PCB overlay diagram to help you build the Active Crossover. Note how the design is split into boxes, with the subsonic filter at the top, high-frequency filter section at the bottom, low/mid filter in the middle and power supply/ de-thumping on the left. If you don’t need the subsonic filter, you can omit all the components in the blue outlined area, and if you are building a two-way Active Filter, you can leave off the components in the red outlined area. Single supply applications don’t require the parts in the green outlined area. Similarly, if you don’t need the subsonic filter, you can leave out the components in the blue dashed area. Once you have figured out the component values needed and gathered them all, start by fitting all the resistors. Don’t forget to change R1 to 3.6kW if you will be using a single DC supply rail. In that case, you can also leave off REG2 and its associated components. With the resistors in place, fit the ferrite beads after inserting resistor lead off-cuts through them. Ensure they’re tight on the PCB before soldering them in place (a dob of neutral-cure silicone will help stop them from rattling). After that, fit all the diodes, ensuring they are orientated as shown. Don’t get the three different types mixed up. Now you can install the op amps, either by soldering them directly to siliconchip.com.au the board (the most reliable method) or by soldering sockets, making it much easier to change them later. Regardless of which approach you use, be careful to make sure that they are all orientated correctly. Follow with all the MKT and ceramic capacitors, then the relays. The stripes on the relays must face as shown in Fig.15; note that if you’re building a two-way crossover, you can leave off RLY3. Next, fit the headers. You can place the polarised headers either way around, although our recommended orientations are as shown in Fig.15. After that, solder the single terminal block in place with its wire entry holes towards the nearest edge of the PCB. Now it’s time to solder in all the electrolytic capacitors, starting with Australia’s electronics magazine the smallest ones and working your way up. There are two non-polarised (bipolar) devices at the input, on either side of CON1, but all the rest are polarised. So the longer leads should go to the holes marked + on the overlay diagram. With those in place, fit the three potentiometers, or two (excluding VR1, “high”) if building a two-way crossover. Then attach the regulators to the heatsinks using the insulation kits and solder the heatsink pins to the PCB, followed by the regulator pins. Don’t get the two devices mixed up. Jumper setup Fit shorting blocks (‘jumpers’) to the headers for JP1-JP6 and LK1 now. There are instructions printed on the PCB, but in case they aren’t clear: November 2021  79 The single-rail powered version of this project is suitable for use at 24-30V DC 1. For single-rail DC operation, place the blocks between pins 2 & 3 of JP1 & JP2. For dual-rail (split rail) or AC operation, place them between pins 1 & 2. 2. For two-way operation, fit the blocks between pins 1 & 2 of JP3 & JP4. For three-way operation, place them across pins 2 & 3 instead. 3. If you want to use the recommended subsonic filter, bridge pins 2 & 3 of JP5 & JP6. Otherwise, bridge pins 1 & 2. 4. If you want the ‘low’ (woofer/subwoofer) outputs to be in mono, insert a shorting block on LK1. Otherwise, leave it open. Making the connections We have used pluggable, polarised pin headers for all inputs and outputs. This allows you to make cables that suit your installation. Being able to unplug the board keeps things neat and makes testing/fixing it much easier. You have the option of soldering cables directly to the header pads if you never expect to service the device, but you will achieve a much more professional result if you invest the time in making plugs. Setup and testing Having set your jumpers as per above, connect your power supply to CON3. For a centre-tapped transformer 80 Silicon Chip (18-24V AC), the tap goes to the middle pin and the ends of the windings to the other two (it doesn’t matter which way around). For a transformer with a single secondary (9-12V AC), connect one end to the middle pin and the other end to either of the outer pins. For single-rail DC (around 24-30V DC), connect ground to the centre pin and the positive output to either outer pin. For dual-rail DC (±12-15V), connect 0V to the centre pin and the two supply rails to the outer pins, either way around. If you’re socketing the op amps, you could leave them out during testing. Now apply power and check the output voltage of REG1. You can use the central screw of terminal block CON3 as the ground reference and probe the test point labelled “+9Volts” near REG1. The reading should be +17-19V for single-supply operation or +8-10V otherwise. For a dual-rail or AC supply, check the output of REG2; there is a “-9Volts” test point near REG2 that you can use. Expect a reading between -8V and -10V. If you are using a single-rail DC supply, check that the half-supply voltage is correct by dividing your REG1 output reading in half, then probing the left-hand end of the two 1kW resistors to the left of the leftmost potentiometer. You should get Australia’s electronics magazine a reading very close to the predicted value. If using a power supply with current metering (or connect an ammeter in series with your supply), check for a current draw of around 150mA with the op amps installed or less than 50mA without them. If it’s significantly higher than this (say >250mA and >100mA respectively), then you have a problem, possibly a short circuit somewhere. The final test is to check that each output produces the correct range of frequencies and that you can adjust the level with the appropriate potentiometer. While you can do this with the aid of a swept sinewave generator and oscilloscope, it’s easy enough to check without either of those instruments. Simply connect a wide-range signal source to the device’s inputs (such as rock or pop music) and connect each pair of outputs to an audio amplifier in turn (make sure it’s turned down initially!). Check that you get mostly bass from the low outputs, mid-range signals (eg, human voice) from the mid outputs and treble (cymbals etc) from the high outputs. Also check that the sound is clean and that the potentiometers correctly adjust the output levels. The only aspect you can’t really test using your ears is the operation of the subsonic filter, as it is intended to remove signals that you can’t hear. For that, you will need a sinewave siliconchip.com.au generator set to a low frequency (eg, 10Hz) and a scope to verify that the signal is heavily attenuated. It should be 6dB down at 20Hz and much more than that (less than 1/10th its original amplitude) at 10Hz. If any of the above checks fail, switch off the power and see the troubleshooting section below. Final setup & usage The setup of an active crossover comes down to setting the appropriate attenuation values for each channel. If you are building a complicated speaker system, you will need to make many measurements and tweaks to get the crossover frequencies and levels right. That is beyond the scope of this article. You will notice that we haven’t gone into many details of how to house or wire up the Active Crossover. You could build it as a standalone unit, integrated into a preamplifier, integrated into a power amplifier or as part of a complete preamp/crossover/ amp system. For standalone use, we have specified some shielded cable and chassismounting RCA sockets in the parts list. Mount these RCA sockets on the box with one pair as the inputs and two or three pairs of outputs, then wire them up to polarised header plugs using the shielded cable. The ground shields go to the middle two pins, with the left/right signals to the outer two (it doesn’t matter which as long as you are consistent). That would just leave the power supply wiring, which could go to a chassis-mounted barrel socket for operation from a DC or AC plugpack. Alternatively, you will need a case large enough to fit a mains transformer. However, do not take that approach unless you are experienced in building mains-powered equipment and know how to do it safely. That includes Earthing the metal case and transformer frame. If integrating it with a preamp, the input connection can go directly to the preamp’s output rather than to sockets (or you could fit sockets and bridge them externally for maximum flexibility). Similarly, if building the amplifiers into the same chassis (most likely with a mains power supply), the output headers can be wired directly to the amplifier module inputs, or via sets of pre-out/pre-in sockets. siliconchip.com.au While the Active Crossover does not need to be built into its own separate case, you can do so as shown above. The example provided uses an Altronics H0480F, which is 200mm wide, 155m deep and 65mm tall. A 12V AC plugpack (Altronics M9267A) was used to supply power, but a 0.5A version will work fine. Table 2 – resistor colour codes Australia’s electronics magazine November 2021  81 Power supply changes between single & dual-rail modes For single-rail DC operation, we want the positive rail to be about 18V (17.9V actual). The virtual ground splitter then generates a +9V signal ground, allowing the op amps to operate from virtual ±9V supply rails. For this, the LM317 (REG1) reference resistor, R1, needs to be 3.6kW, as shown on the circuit and overlay diagram. In this case, there is no need to fit the negative rail components (LM337 and associated parts). For dual-rail operation (including any AC supply), we want the LM317 voltage to be about 9V (8.7V actual). For this, LM317 (REG1) reference resistor, R1, needs to be 1.6kW instead of 3.6kW. The LM337 produces -9V (-8.9V actual) by default. Regardless, this unit draws less than 150mA (our prototypes drew 120mA), so any 24V DC or ±15V supply capable of delivering 150-200mA should be fine. Keep the supply voltage below 35V DC; if necessary, use a 5W resistor to drop excess voltage. A 100W 5W resistor will drop about 12V. Note that while you could power the Active Crossover from the preamp power supply in an integrated system, this does introduce a risk of Earth loops and hum injection. Using an independent power supply avoids the potential for such problems. If you experience hum, the first thing to try is powering the Active Crossover from an independent source. Using it with the Tapped Horn Subwoofer While this is a flexible design suiting many applications, its design was in part kicked off by my Tapped Horn Subwoofer design published in the September 2021 issue (siliconchip. com.au/Article/15028). That subwoofer needs a bandpass filter as it has a very uneven frequency response above about 80Hz, and can easily be damaged by subsonic signals. This Active Crossover is ideal for driving it; the two-way configuration is fine, although the three-way configuration will also work. Leave the lowpass filter for the LF output at 80Hz and make sure to enable the subsonic filter. You can then feed your regular hifi system from the MF outputs. The LF output level control will let you set the subwoofer level to be appropriate for your room. Troubleshooting A comparison shot showing what parts are omitted in the single-rail version (shown above) compared to the dual-rail version below. 82 Silicon Chip Australia’s electronics magazine If you can’t get it to work, first check that you have set all the jumpers correctly. Next, examine the board carefully. Look for dry or incompletely formed solder joints, short circuits (eg, solder blobs connecting pads that should not be connected), reversed components, swapped components and so on. Check that all the resistor and capacitor values match those shown in Fig.15. Fix up any problems you find, then start the tests over again. If it still doesn’t work, verify that the power supply input voltage(s) are correct and that the onboard power supply is working. For single-rail versions, check that the virtual ground is about half the overall voltage rail, as described above, plus or minus 200mV. Also check that the overall voltage between pins 4 and 8 of each op amp is double this, ie, 17.8V±200mV. For dual-rail versions, check that the positive and negative rail voltage amplitudes are within ±200mV. siliconchip.com.au You should hear a click from the relays about five seconds after you apply power. It is not loud but should be discernible. If not, that suggests a problem with this part of the circuit, a supply rail imbalance or a shortcircuit on the virtual Earth, causing it to fail to release. If the relays do not click in, check the voltages around Q1 and Q2 in the power detection circuit. After a few seconds, the base-emitter voltages of Q1 and Q2 should be less than 400mV (ours settled to about -30mV). If you are reading 0.6V or so, check the resistor values in this part of the circuit. Check that Q4 is off after a few seconds. You can check this by verifying that its base-emitter voltage falls close to zero. Check that the base-emitter voltage of Q5 is about 0.6V after things settle. This will switch this Darlington pair on, and thus the relays. Verify that you have the correct relays installed and they are the right way around. If the supply rails are too low, the voltage regulators will stop functioning. The de-thump circuit will then detect the ripple on the supply rails and disconnect the output. With the power supplies working and “turn off muting” working, apply an input signal and trace it through the circuit. Are there output signals from the state variable filters that go to the potentiometers? Are the potentiometers appropriately set? Remember that 24dB/octave filters are pretty steep, so if you apply a 1kHz signal with the filter values in the article, you will see nothing on the low outputs and only a small signal at the high outputs. Final thoughts The maximum allowable input voltage to the active crossover is 35V DC, at which point you will find the heatsinks become quite warm. Check this in your installation, and if they are warmer than you like, insert a 5W resistor in series with the power source to drop the voltage. Start with 100W and work from there. With a current draw of 120mA, that will drop 12V and dissipate 1.5W. With this all up and running, now it is time to connect your speakers. We recommend that you connect the tweeters through high-value bipolar electrolytic capacitors to protect them from any DC or low-frequency siliconchip.com.au Building it into its own case While we expect many constructors to build the Active Crossover into another piece of equipment, it can certainly be housed in its own case, as shown in the lead photo. Putting it into a case is quite simple. For our application, we chose to power it using a 12V AC plugpack containing a small mains transformer with a single 12V AC secondary. In this configuration, the onboard rectifier diodes act as a half-wave voltage doubler, producing the +12V and -12V DC (approximately) rails to power the onboard +9V and -9V linear regulators. In this case, each filter capacitor is ‘recharged’ at 50Hz rather than 100Hz, as would be the case with a transformer having dual 12V secondaries or a centre-tapped 24V secondary. This is convenient, and analysis shows that it works just fine and doesn’t affect performance. This configuration worked perfectly, with no noise at switch on or off. Even with the Horn-loaded Subwoofer with an efficiency close to 110dB <at> 1W/1m, there was no hum or noise evident (that was the application for this particular unit; see the article in the September 2021 issue for details on how to build it). The extra parts used to build the Active Crossover into a case are listed below. Note that some of these parts are suggested in the main parts list, but this is more comprehensive. First, mount the PCB in the bottom of the case using the tapped spacers and machine screws, the connectors to the rear panel and the switch to the front panel. Then it’s just a matter of soldering all the wires to those connectors and switches and connecting the other ends to the appropriate points on the board. Part list – for building into a separate case 1 ABS plastic instrument case, 200 x 155 x 65mm [Altronics H0480F, Jaycar HB5912] 1 12V AC 500mA plugpack [Altronics M9265A, Jaycar MP3058] 1 panel-mount barrel socket, 2.1mm inner pin diameter [Altronics P0628, Jaycar PS0522] 4 M3 tapped, 20mm-long spacers [Altronics H1250, Jaycar HP0907 25mm] 8 M3 x 6mm panhead machine screws [Altronics H3110A, Jaycar HP0400] 4 gold-plated red panel-mount RCA sockets [Altronics P0152, Jaycar PS0259] 4 gold-plated white panel-mount RCA sockets [Altronics P0151, Jaycar PS0261] 1 small knob to suit 18T spline shaft [Altronics H6510, Jaycar HK7734] 1 panel-mount power switch [eg, Altronics S1040, Jaycar ST0581] 1 1m length of red light-duty hookup wire [Altronics W2250, Jaycar WH3010] 1 1m length of black light-duty hookup wire [Altronics W2251, Jaycar WH3011] 1 2m length of shielded figure-8 audio cable [Altronics W2995, Jaycar WB1506] 1 100mm length of 5mm diameter heatshrink tubing [Altronics W0913A, Jaycar WH5533] 1 100mm length of 1.5mm diameter heatshrink tubing [Altronics W0910A, Jaycar WH5530] transients your amplifier may put out. A 100μF 50V non-polarised capacitor such as Altronics Cat R6590A will work well. This device has a ripple current rating of 900mA, more than enough for a tweeter. At 2kHz, this will have an impedance of 0.8W and introduce a loss of about 0.8dB. If you want to reduce that, you can double up the capacitor. Usually, your volume control would remain in your preamp, which drives Australia’s electronics magazine the input to the active crossover. Ideally, you will use test instruments to set the Crossover levels. If you don’t have much in the way of fancy test equipment, an FM receiver set between stations gives pretty good white noise. Use this to set the three level controls to get apparently equal volumes from the speaker drivers. That is a pretty good starting point from which to fine-tune the levels. SC November 2021  83