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Building the 3-Way, Fully Adjustable Stereo Active Crossover for Loudspeakers Part 2 – by John Clarke Last month we described the circuitry and operation of our new 3-Way Adjustable Active Crossover for Loudspeakers. Now we continue with its construction – building the PCB, testing it, then putting it in its Acrylic case for a truly professional finish. It looks so good and works so well your friends won’t believe you built it! T his Active Crossover has true hifi performance, as shown in the specification panel and accompanying plots. Harmonic distortion is well below 0.001% across most of the audible frequency range, rising to only about 0.0015% at 20kHz. The combined frequency response of the three outputs is almost completely flat from 20Hz to 20kHz. As you might expect, distortion is much higher when the bass limiter is actively limiting, at around 2% but this is much lower than the distortion you would otherwise experience with a woofer driven into clipping, which is what the limiter is designed to prevent. Channel separation is around -50dB and note that most of the crosstalk is due to the simple balance control and so this will not lead to any noticeable distortion. Tracking of the high-pass and low-pass filter pairs is very good, as you can see from the relevant frequency response plots (Figs.17 & 18). Overall, this Active Crossover will have insignificant effect on the signals passing through it and so will not “colour” or degrade the audio signals. Ultimately, that means that the sound quality you get depends entirely on the amplifiers and speakers used. The project itself is constructed using a single PCB, coded 01108171 and measuring 284 x 77.5mm. It comprises a mixture of both through-hole and surface-mount components. Most are mounted on the top of the PCB but a few resistors and capacitors mount underneath. The PCB and panels are designed to fit into a stand-alone case made from front and rear panel PCBs along with pre-cut Resplendent in its laser-cut acrylic case and highgloss black screen-printed front and rear panels, the Adjustable Active Crossover would look perfect in any hifi or home theatre setup. Of course, you could also build it into existing equipment (sans case) if you preferred that approach. 66 Silicon Chip Celebrating 30 Years siliconchip.com.au Woofer with bass limiter Low-pass (Woofer+Mid) Tweeter Mid-range Woofer Mid-range Woofer Low-pass (Woofer+Mid) Tweeter Fig.11: distortion plotted against frequency, with all four outputs measured independently. The dotted sections are where the amplitude of that output is dropping off, resulting in the distortion level appearing higher, due to diminishing signal-to-noise ratio. As you can see, at the frequencies where each output carries the majority of the signal, harmonic distortion is very low. Fig.12: a plot of total harmonic distortion (actually THD + noise) against signal level for each output, demonstrating that almost all the distortion present is actually just noise. The dark blue curve demonstrates the operation of the bass limiter; the input signal was swept up to 2V with the unit set for unity gain, however, once the signal exceeds 0.72V RMS, the woofer output voltage barely rises further. 3mm black Acrylic panels. Alternatively, you could fit the PCB in a 1U rack case but then you would need to come up with your own mounting and panel arrangements. And it’s pounds to peanuts that it won’t look as good as the Acrylic case! align and solder the 100nF supply bypass capacitors (code 104) for each of these ICs. Check for a short circuit between each side of the 100nF capacitor after soldering each one as this can save a lot of time tracking down a short across the supplies later on. The surface mount resistors can now be now be soldered in place. These are coded with a 4-digit number: the first three digits representing the value and the last digit representing the number of extra zeroes. For example, a 1kΩ resistor (1000Ω) is labelled 1001: 100 plus one extra zero. For 100kΩ, (100,000Ω) the value is 100 with three extra zeroes. So it is labelled as 1003. Install all the surface mount resistors on the top and bottom of the PCB. The remaining surface mount capacitors can now be fitted to the underside of the PCB. Soldering SMDs You will need a fine tipped soldering iron bit, 0.71mm diameter solder, a good light and a magnifying glass or spectacles to be able to solder the surface mount components in place. Begin by mounting the surface mount ICs, all LM833 dual op amps. Each IC must be oriented correctly – note that the chamfered side is the pin 1-4 side of the IC. The technique for soldering these in place is the same for all: locate the IC in position over its PCB pads and solder one corner pin. Check alignment and remelt the solder if the IC needs realignment. When the IC is aligned correctly, solder the remaining pins. If you end up bridging adjacent pins, these can be cleared using solder wick. Once all 25 ICs are soldered in, then Through-hole components Once all the surface mount components are installed, the through-hole components can be mounted. Start with the resistors first but don’t throw out all the lead off-cuts. The two inductors (L1 and L2) are simply wire links which pass through ferrite beads. Here’s where you use a couple of those resistor lead off-cuts! The diodes also can be mounted, taking care with Specifications Measurement conditions: .......................................2V RMS in, 1.5V out, 20Hz-20kHz bandwidth Signal-to-noise ratio:..............................................100dB+ (100dB for tweeter, 105dB for midrange and 108dB for woofer) Frequency response, 20Hz-20kHz: .........................+0,-0.25dB (see Fig.14) Total harmonic distortion plus noise: .....................<0.002%, 20Hz-20kHz (see Fig.11) Distortion with bass limiter active: .........................~0.005% before limiting; ~2% while limiting (see Fig.12) Output gain range: .................................................zero (full attenuation) up to 3.8 times gain Balance adjustment range: .....................................±7.5dB Bass/midrange crossover frequency (-6dB): ..........85-900Hz (see Fig.18) Midrange/tweeter crossover frequency (-6dB): ......465Hz-5kHz (see Fig.17) Channel separation:................................................>46dB, 20Hz-20kHz (see Fig.16) Input signal handling:.............................................up to 2.6V RMS siliconchip.com.au Celebrating 30 Years October 2017 67 Fig.13: most of the components mount on the top side of the PCB, although there are quite a few SMD resistors and a few capacitors mounted on the underside (see overleaf). Use this component layout diagram along with the same-size photo below to assist you in construction. The full parts list was printed in part 1 of the 3-Way Active Crossover, published last month. The PCB is double sided, hence the number of apparently empty holes on the board which are “vias” going through to the opposite side. 68 Silicon Chip Celebrating 30 Years siliconchip.com.au Low-pass (Woofer+Mid) Woofer Tweeter Mid-range Fig.14: extended frequency response of each of the four outputs, showing that the -3dB points are well below 10Hz and above 100kHz respectively, making for a very flat summed response over the audible range (20Hz-20kHz). This demonstrates how the Tweeter and Low-pass outputs can be used as a two-way crossover, if necessary. Think you’ll have difficulty with SMDs? You need a very fine-tipped iron, a good magnifying glass and a steady hand to solder them in. For all the tips, refer to the article “How to hand-solder very small surface-mount ICs,” back in our October 2009 issue (siliconchip.com.au/Article/1590). orientation (the striped end is the cathode [K]). Now install the MKT polyester capacitors – there are 20 120nF and 20 22nF (these should be clearly labelled as such – see capacitor codes panel). Electrolytic capacitors are mounted now. There are 35 in total – 25 are polarised and must be soldered in the right way around. The ten NP (Non Polarised) 22µF capacitors are not polarised. Potentiometers Check that the pins on the potentiometers are all straight before insertion – if necessary, straighten them using flat nose pliers. Double check that each pin has entered its hole before soldering in place. The 8-ganged pots are best inserted by placing in the back row of leads first (ie angle the potentiometer slightly) and then progressively insert the remaining pins as the pot is lowered onto the PCB. Be careful with VR1, VR2 and VR7-VR10 as these have the same value (10kΩ) but VR1 is a log type (marked “A”), while the remaining are linear (marked “B”). VR11, the bass limiter threshold preset, is mounted with the screw adjustment to the left. Power supply Next to go in are the power supply components. All of these are polarised so be careful with orientation. First is the bridge rectifier, followed by the four filter capacitors (two 470µF and two 10µF), the Schottky diode and the two 15V regulators (again, note that they are different!). Both regulators should have their heatsinks attached via M3 screws and nuts before soldering in. Seat the regulators as far down on the PCB as their heatsinks will allow. LED1 needs to mount with the correct orientation (longer lead is the anode) and to allow it to poke through the front panel, is bent over at 90°, at 6mm back from the rear of the LED body. Provision is made for a single 16VAC supply via CON4 or a 15V-0-15VAC supply via CON5. You will only need siliconchip.com.au Celebrating 30 Years October 2017 69 Fig.15: the component overlay and matching photo for the reverse (or under) side of the PCB shows the large number of SMD resistors and capacitors to be placed. The eight 100nF capacitors in the photo are only there because at the time, we’d run out of 120nF MKT capacitors (normally mounted on the top side of the board!) Similarly, the diode shown tacked across the board in this prototype has been replaced with one mounted on the top side in the final version of the PCB. one of these. If using CON5 (a 3-way screw terminal) it is mounted with the opening toward the PCB edge. LDR and LED pairs LDR1/LED1 and LDR2/LED2 need to be made into two separate lightproof assemblies. Each assembly allows light from the LED to directly shine onto the face of an LDR. We used 6mm diameter black heatshrink tubing cut to 25mm in length to cover and secure the LED and LDR to70 Silicon Chip gether and with a small bead of Blu-Tack (or similar) at the rear of each LED and LDR to prevent light entering from outside of the tubing. Orient the leads of the LED to the same plane as the LDR before shrinking the tubing with a hot air gun. When installing onto the PCB, ensure that the LEDs are oriented correctly with the longer lead (the anode) inserted into the “A” marked position. We inserted the LED directly onto the PCB with the LDR leads bent over to insert into Celebrating 30 Years siliconchip.com.au Mid-range right-to-left Tweeter right-to-left Tweeter left-to-right Mid-range left-to-right Bass left-to-right Bass right-to-left Fig.16: a plot of cross-talk between channels for the three primary outputs. As you would expect, cross-talk is highest within the frequency range that the output retains. Most of the cross-talk is due to the shared signal paths in the balance circuitry, with only a slight hint of capacitive cross-talk at higher frequencies (this effect is reduced at higher mid-range/tweeter crossover frequency settings). struction for correct parts placement or for shorts on the power supply rails. Setting it up the LDR allocated holes. The LEDs are polarised but the LDR leads can be oriented either way in the PCB. See the photo at right for more detail. That should have completed construction of the PCB but before putting it in its case, we need to test it and set up VR11. The input sockets can be connected either to the output of a preamplifier or directly to a line-level signal source such as a CD/DVD/Blu-ray player, MP3 player or mobile phone (thanks to the onboard volume control). For driving a pair of 3-way loudspeakers, the woofer, mid-range and tweeter outputs should be connected to three stereo amplifiers, ie, one to power the woofers, one the mid-range drivers and one the tweeters. It’s common practice to use lower power amplifiers for the mid-range drivers than woofers, and again for the tweeters than the mid-range drivers. Note though that some (fairly unusual) program material may overload the amplifiers in such a configuration. Rock/pop music is normally safe in this sort of configuration as it is usually quite bass-heavy and so will overload the (larger) woofer amplifier first. You will then need to determine the correct crossover frequencies, based on the specifications of your drivers and the cabinets they are mounted in and adjust the unit accordingly. Making the adjustments The easiest way to set the crossover frequencies is with an adjustable signal generator and AC millivoltmeter. You Initial testing Apply power (either 16VAC via CON4 or 15-0-15VAC via CON5) to test for voltage at the op amps. Switch on S1 and the power LED should light. Now measure voltage between pin 4 and pin 8 of one of the op amps. This should be close to 30V (ie, +15 to -15V). If this is not correct, switch off power and check consiliconchip.com.au Celebrating 30 Years LDR & LED pic This close-up shows the two LED/LDR assemblies, arranged so the light from the LEDs shine directly into their LDRs. Black heatshrink makes them lightproof. October 2017 71 Fig.17: simultaneous frequency response plots of the woofer+mid and tweeter outputs with five different crossover frequency settings. This demonstrates the adjustment range and filter tracking and also shows how the unit can be used as a two-way crossover. In three-way mode, the effect is the same but the mid-range response will be hump-shaped, rather than extending all the way down to 20Hz. Fig.18: simultaneous frequency response plots of the woofer and mid-range outputs with four different crossover frequency settings. This demonstrates the adjustment range and filter tracking. With the woofer/mid crossover set to 900Hz, this is close enough to the mid/tweeter crossover frequency that the peak output level is below 0dB. Otherwise, it would produce a peak in the summed frequency response. will need a signal generator that has a stable amplitude earthed). Adjust the balance control until the millivoltacross a wide range of frequencies (eg, 30Hz to 10kHz or meter reads zero, indicating that the channels are correctwider, if possible) and an AC millivoltmeter which can ly balanced. measure up to about 1V RMS and is accurate across the Then connect the millivoltmeter normally to measure same frequency range. the left channel woofer output level. Adjust the volume If you don’t have such tools, you could purchase them or control to get a reading of 1V RMS. alternatively, build our Digital Audio Millivoltmeter project Next, set your signal generator frequency to be your defrom March 2009 (www.siliconchip.com.au/Article/1372) sired woofer/mid-range crossover frequency and then adand/or the Touchscreen DDS Signal Generator from the just the left channel lower crossover frequency potentioApril 2017 issue (www.siliconchip.com.au/Article/10616). meter until you get a reading of 500mV RMS. This is 1V Set the signal generator output to 30Hz RMS minus 6dB. Small Capacitor Codes and around 1V RMS and set all four levThen connect your millivoltmeter el controls on the Active Crossover to to the right channel woofer output and No. Value SMD EIA IEC maximum. adjust the right channel lower crosso 20 120nF MKT 124 120n ver frequency to get the same result. Hook up the signal generator to the inputs and the millivoltmeter across the 25 100nF (1206) A5 The procedure for adjusting the up223 22n centre pins of the two woofer outputs 20 22nF MKT per crossover threshold is the same (we’re assuming it has a battery or float- 11 100pF (1206) A2 except that you start with a 10kHz 2 100pF ceramic 101 100p signal and adjust the tweeter output ing mains supply, ie, its ground is not Resistor Through-Hole Colour Codes and SMD Codes 72 No. 2 7 8 2 26 1 8 2 2 37 2 8 1 Value 100kΩ 100kΩ 22kΩ 10kΩ 10kΩ 5.6kΩ 2.2kΩ 2.2kΩ 1kΩ 1kΩ 620Ω 150Ω 100Ω Silicon Chip 4-Band Code (1%) brown black yellow brown 1206 SMD – code 104 (or 1003 in E24) red red orange brown brown black orange brown 1206 SMD – code 103 (or 1002 in E24) green blue red brown red red red brown 1206 SMD – code 222 (or 2201 in E24) brown black red brown 1206 SMD – code 102 (or 1001 in E24) blue red brown brown brown green brown brown brown black brown brown Celebrating 30 Years 5-Band Code (1%) brown black black orange brown red red black red brown brown black black red brown green blue black brown brown red red black brown brown brown black black brown brown blue red black black brown brown green black black brown brown black black black brown siliconchip.com.au The completed PCB placed inside its Acrylic case (before top attached), with matching black PCB front and back panels. You’d have to agree, it looks brilliant! The only thing you can’t experience here is just how brilliant it makes your speakers sound – and you’ll have to build it to hear that! level control to get 1V RMS, then set the signal frequency to your desired crossover frequency and adjust both upper crossover frequency adjustment pots until you read 500mV at both tweeter outputs. You can then set the generator to a frequency in the middle of your mid-range band and adjust the midrange level output to get a reading of 1V RMS. Adjusting the output level for each pair of drivers At this point, you have set the crossover frequencies and the output amplitudes are all set to be identical, giving you a flat summed response. However, chances are your drivers do not have identical sensitivities. Also, your individual amplifiers may not have the same gain. So you will need to change the relative levels of the outputs so that the drivers are producing identical sound levels at the crossover point(s). Start by determining the sensitivities of each driver. These are normally specified by the manufacturer or supplier and are in units of decibels (sound pressure level) per watt at one metre (dB[SPL]/W <at> 1m). In order to better explain the procedure, we’ll use a hypothetical example of a three-way speaker system with drivers as shown in Table 1. In this example, each driver has a different sensitivity figure and the woofer’s impedance is different from the other two. The stereo amplifiers used to drive each pair also have different gains, as indicated. Impedance has an effect because this determines the signal amplitude required to deliver one watt to the driver. To determine the required voltage, take the square root of the impedance. So for a 4-ohm driver, you need 2V RMS (P = V2÷R); for an 8-ohm driver, you need 2.828V RMS; and for a 6-ohm driver, you need 2.45V RMS. Now divide the required signal level by the amplifier gain to determine the signal that you need to feed into the siliconchip.com.au amplifier to get 1W out of the driver. If you only have a dB gain figure, use the formula 10^(dB÷20) to determine the linear gain factor. If your amplifier has a volume knob, the gain will depend on its setting; unless you plan on running it at maximum gain (and you already know what that is), you will have to feed a signal into the amplifier, measure the input and output amplitude and divide the output voltage by the input to determine the gain. We suggest you do this before wiring up the outputs since otherwise it may be very loud and depending on the signal level you inject, you could damage the driver. This may result in a slightly higher reading (due to the outputs being unloaded) but the difference is unlikely to be significant. So, in the case of our tweeter, we can compute the required amplifier input signal for 1W as 282.8mV RMS (2.828V÷10). For the mid-range driver, it’s 188.6mV (2.828V ÷15) and for the woofer it’s 100mV RMS (2V÷20). Now we convert these figures to dB(V) using the formula dB(V) = 20log10(VRMS). If your calculator doesn’t have a base-10 log function, you can take the base-e (natural) log and then divide by the natural log of 10, ie, log10(x) = loge(x) ÷ loge(10). This gives us figures of -11dBV for the tweeter, -14.5dBV for the mid-range driver and -20dBV for the woofer. Subtract the sensitivity figures from these values to get the required signal level to produce 1dB(SPL). These are shown in Table 1. This reveals that the mid-range driver requires the highest signal level, followed by the tweeter and then the woofer. Sensitivity Impedance Amplifier Input level gain for 1dB(SPL) Tweeter 96dB/W<at>1m 8Ω 10x (20dB) -107dBV Mid-range 89dB/W<at>1m 8Ω 15x (30dB) -103.5dBV Woofer 92dB/W<at>1m 4Ω 20x (40dB) -112dBV Table 1 – example of speaker system level adjustment Celebrating 30 Years October 2017 73 The first step to make the adjustments then is to set the output level for the mid-range driver to its maximum setting, feed a reference signal into the Active Crossover in the middle of the mid-range driver’s frequency band (ie, between the two crossover points) and then adjust the input volume control until we get a reference level of 1V RMS at the mid-range output sockets. Based on the figures we’ve just computed, we can determine that the tweeter output should be 3.5dB lower than this reference level. Using the formula 10^(dBV÷20) we can determine that the tweeter output voltage needs to be adjusted to 10^(3.5÷20) = 0.668V or 668mV. Use a similar procedure, injecting a signal of the same amplitude as before but in the tweeter’s frequency range (say, 10kHz) and then adjust the tweeter output to this level. Similarly, we can compute the woofer output for the same amplitude input signal, at an appropriate frequency, should give an output of 10^(-8.5÷20) = 376mV RMS (-8.5 = [-112] - [-103.5]). If you’re using the unit as a two-way crossover, the procedure is essentially the same except that you set either the Tweeter or Low-pass (Woofer+Mid) output to 1V RMS and then adjust the other once you’ve computed the difference in level required. Tweaking it In a perfect world, the above procedure should give you a nearly flat response from your loudspeakers. However, there are a number of factors which can throw a spanner in the works. For example, the fact that the drivers you purchase may not have exactly the sensitivity or frequency response the manufacturer specified. They may not even be identical to each other! Then you also have effects of the enclosure on the performance of the drivers, the fact that their impedance will not be exactly the nominal value and will vary with frequency and so on. All this means that that the setting you made above will only be approximately correct. It may well be good enough, but unless you make further measurements and do tweaking, you won’t know if it can be improved upon. The most scientific way to finish adjusting the Active Crossover to give the best results is using a device which can actually measure the frequency response of the loudspeaker, allowing you to calculate (or at least estimate) any further adjustments which need to be made to improve it. You don’t need particularly expensive equipment to do this. See our article titled “How to do your own loudspeaker measurements” in the December 2011 issue (www. siliconchip.com.au/Article/1248), which describes how to use the low-cost Champ and Prechamp amplifier boards, with an electret microphone, a PC and a few other bits and pieces to measure loudspeaker frequency response. Assuming you go to the trouble of building such a rig, once you have measured the response, it’s then just a matter of determining whether you need to slightly increase or decrease the level to one driver in order to even out the speaker’s overall response. If you do, you will normally notice a “shelving” effect in the response curve. You can then re-measure to verify that your change is an improvement. As we said earlier, various factors such as driver variances and enclosure design can also affect a driver’s frequency response and thus you may find that there are dips or peaks near the crossover frequencies. If so, this suggests that you may be able to flatten the response by adjusting the crossover frequency itself. You will need to make small adjustments and re-measure the loudspeaker to verify that your change led to an improvement (if not, reverse it). This is an iterative process and you may need to make a number of adjustments before you are happy with the overall response. If you don’t have the equipment to do this and you have well-calibrated ears and a good variety of source material, which you are familiar with (ideally, having listened to it multiple times on speakers or headphones with a flat response), you might trust yourself to tweak the crossover “by ear”. There is no guarantee that you will get the best result with this method, though! Limiter adjustment The signal level at which the bass limiter becomes active (when switched on via S3) can be adjusted using trimpot VR11. Typically, you would set the limiter to restrict the signal level so that the amplifier/woofer combination you are using does not run into clipping. The signal level at which clipping occurs depends on the amplifier power rating, its gain, the woofer power rating and its impedance. So you will need to calculate the signal level at which clipping will occur to set the limiter correctly. You could adjust it experimen- An “exploded” view of the laser-cut Acrylic case designed especially for the Active Crossover. 74 Silicon Chip Celebrating 30 Years siliconchip.com.au tally, however you risk causing damage using that method. Briefly, take the lower of the two power ratings (amplifier or woofer, taking into account the woofer’s nominal impedance) and then calculate the RMS voltage required to be delivered to the woofer’s impedance to achieve that power level using the formula V = √P x R. Then divide this by the amplifier’s gain to determine the maximum signal level at the amplifier’s input. You can then multiply this RMS voltage by 1.414 to calculate the maximum peak signal voltage before clipping occurs. The limiter level can be monitored between TPG and TP1 for the positive peak level and TPG and TP2 for the negative level. You should get a similar reading in both cases (with opposite polarity). Adjust VR11 until the voltages at TP1 and TP2 are just below the peak voltage level you computed above. Acrylic case The case is formed from four pieces which slot together, forming the top, bottom and ends. The front and back of the case are high-gloss, screen printed PCBs with drilled holes for the controls, connectors and LED. The whole lot is held together with eight screws and twelve tapped spacers, along with tabs and slots joining the panels to each other. The first step is to loosely fit the front and rear panels to the main PCB. The rear panel slips on over the 10 RCA connectors and is held in place with three short black 4GA self-tapping screws which go into the middle of the two 4-way RCA sockets and to the side of the 2-way RCA socket. Before fitting the front panel, you will need to remove the nuts and washers from all the potentiometers. It’s then just a matter of slipping the panel over the pot shafts and loosely re-attaching the washers and nuts while guiding LED1 into its hole. Now remove the protective film from the base panel. This is the largest acrylic panel, with two extra slots compared to the top. Do this carefully since the two long slots are near the edges of the panel, making it relatively weak – don’t hold it by these edges or press on them. You can orientate the acrylic panels so that the outside (visible) faces are either matte or gloss black; we prefer matte, since it gives better resistance to fingerprints and hides scratches. Feed the four 32mm machine screws up through the bottom and screw a 9mm tapped Nylon spacer onto each shaft until the screw is held firmly in place. Now remove the protective coating from the two side panels and push the onto the sides of the front and rear panels, so that the tabs in those panels go through the slots on the side panels. You can then lower the PCB onto the bottom panel, lining up the screws with its mounting holes. Screw four 15mm M3 spacers fully onto the screw shafts to hold the PCB in place, then screw the other four 15mm spacers on top. Now you can remove the protective coating from the top panel and lower it into place. You may need to cajole the front and rear panels to fit into the slots. Use four black M3 x 8mm machine screws to attach it to the top of the four spacers, then tighten up all the potentiometer nuts and push the knobs onto the pot shafts. Stick on some rubber feet and the case is complete. SC SAD HAPPY Because you can't find that difficult-to-get special project part at your normal parts supplier. . . Or perhaps they've discontinued the kit you really want to build. . . To discover that the elusive bit that you want is stocked in the Silicon Chip ONLINE SHOP! There's a great range of semis, other active and passive components, BIG LEDs, PCBs, SMDs, cases, panels, programmed micros AND MUCH MORE that you may find hard to get elsewhere! If it's been published in a recent Silicon Chip project and your normal supplier doesn't stock it, chances are the SILICON CHIP ONLINE SHOP does! YES! We also stock most Silicon Chip project PCBs from 2010 and even earlier! Don't forget: Silicon Chip Subscribers qualify for a 10% discount on all shop items!* Log on now: www.siliconchip.com.au/shop * Excluding subscriptions siliconchip.com.au Celebrating 30 Years October 2017 75