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PART 1: BY PHIL PROSSER Low-cost Two- or Three-Way Active Crossover We are frequently asked for active crossover designs because they can provide significant benefits for driving loudspeakers compared to passive crossovers. They allow you to use a separate amplifier for each driver, avoid the need for large power-carrying inductors and capacitors and provide much closer to ideal performance. This Crossover also suits the Tapped Horn Subwoofer we presented last month. W hen building a really serious speaker system, an active crossover and independent amplifiers for bass, mid and high frequencies should be front and centre in your consideration. The general configuration of a three-way loudspeaker system with an active crossover is shown in Fig.1. While excellent results can be achieved with a conventional passively crossed-over system, passive crossovers significantly limit your driver choices and cabinet design. A versatile, active solution is the best way to get the most out of those expensive drivers. One major advantage of active crossovers is that even when the subwoofer or woofer is driven into clipping, which they often are, the mid and high channels remain unclipped and clean. Another benefit is the ability to use a 24dB per octave crossover on the mid-range driver, reducing the amount of low-frequency signal it must handle below the crossover 42 Silicon Chip point, consequently minimising midrange cone excursion. This is often observable by the mid-range sounding ‘cleaner’. We have published several active crossovers in the past, both simple and complicated. There is often a trade-off between cost and versatility, which this project seeks to address. This project makes no compromise with sound quality and includes new features such as turn-on muting to de-thump the output and a subsonic filter to protect your expensive subwoofer. Our last two published designs are a 3-Way Active Crossover in the September & October 2017 issues (siliconchip. com.au/Series/318) and a DSP Active Crossover and Parametric Equaliser in the May-July 2019 issues (siliconchip. com.au/Series/335). Both are excellent designs but cost significantly more to build than this one, and the DSP version is also quite a bit trickier to build. This version eschews the adjustability of those two designs to keep the Australia’s electronics magazine cost and complexity down. You can still set the crossover points where you need them, but that’s done by selecting resistor and capacitor values, so you can’t change them on the fly. In a domestic setting, a typical subwoofer, mid-range driver and tweeter configuration might use crossover frequencies at say 90Hz and 3kHz. This system might use a subwoofer amplifier of 100W plus mid-range and high-frequency amplifiers of 50W each (per channel). Many readers would have these amplifiers already. Of course, using higher power amplifiers is fine. The mid-range and tweeter channels will be delivering only a few watts of continuous power, but having the headroom of a 50W or 100W amplifier means that massive dynamics can be delivered. We plan to follow this article up with a compact, low-cost amplifier of which you can build five or six into a single housing along with a shared siliconchip.com.au heatsink and power supply. So if you don’t already have the amplifiers but want to build a system with an active crossover, keep an eye out over the next couple of issues! Features The outstanding features of this design are: A multi-way active crossover Because every project is different, you can use the same board to make a two-way or three-way crossover by fitting the parts required and setting a few jumpers. Versatile power supply Excellent results can be achieved using low-cost class-D amplifiers available on the internet, but these mostly require a single DC supply rail. A higher-power Class-AB amplifier can be used for the best results, such as our Ultra-LD series, which provides split rails (±15V DC) for the preamplifier. This Active Crossover can run from either supply type, again by varying a few components and two jumper selections. Crossover frequencies set by passive parts To make the crossover frequency adjustable using a potentiometer would require four-ganged potentiometers, which are expensive and results in a much larger PCB. Using fixed resistors and capacitors reduces cost significantly and avoids the potential of someone turning a dial that they really should not touch! Mono/stereo subwoofer output This gives you a fair bit of flexibility. Even if you have two subwoofer channels, if your crossover frequency is set below 100Hz, you might want to use the mono option (ie, drive both with the same signal). Subsonic Filtering Many subwoofer/bass enclosures use vented, bandpass and sometimes hornloaded arrangements. These systems require frequencies below their range of operation to be filtered out. Failure to do this can lead to over-excursion and/or overheating and failure of the driver. All professional sound systems include this. Turn-on/off delay An active crossover is connected directly to a power amplifier and your expensive speaker drivers. Especially when operating from a single-rail, the crossover must not generate a ‘thump’ siliconchip.com.au Features & Specifications ● Two-way or three-way stereo active crossover ● Can be powered from 24-30V DC, split rail DC (±12-15V) or low-voltage AC (9-12V or 18-24V CT) ● Muting to eliminate switch-on and switch-off transients ● Subsonic filter to protect vented subwoofers and remove unneeded subsonic signals ● Low noise and low distortion; <0.0022% THD+N, 20Hz-20kHz ● Low-cost design using available parts; cheaper than building pairs of passive crossovers. ● Mono or stereo subwoofer output. ● Level controls for all three bands. ● Modest power demands; typically draws around 150mA. AUDIO SIGNAL SOURCES Tuner, Phono, CD, DVD etc. Fig.1: the basic configuration of a hifi system using a three-way active crossover (only one channel shown). Each individual driver in the cabinet has its own amplifier, with the signal being split into three to feed these, each containing signal components over a different range of frequencies to suit the drivers. HIGH FREQUENCY POWER AMPLIFIERS PREAMP WITH SOURCE SELECT & VOLUME CONTROL ACTIVE CROSSOVER MIDRANGE POWER AMPLIFIERS HIGH MIDRANGE LOW FREQUENCY POWER AMPLIFIERS LOW/SUBWOOFER Fig.2: plots of total harmonic distortion plus noise against frequency for each output, with the test frequencies chosen to be well within the bandpass of each. The actual harmonic distortion is extremely low, virtually unmeasurable with our equipment. These readings are basically noise; the subsonic filter adds more noise, hence higher readings with it enabled (note that LF noise is not very audible). Australia’s electronics magazine October 2021  43 Fig.3: the solid coloured lines show the left-to-right channel coupling within each band, while the dashed coloured lines show the right-to-left coupling (it’s basically the same, so the solid lines tend to hide the dashed ones). The thin black lines show the worst-case inter-band coupling. A single-rail DC supply gives slightly worse results for the LF outputs. at power on and off. We have included relays to disconnect the outputs both at switch-on (until it stabilises) and switch-off. Performance We measured the performance of the Active Crossover to characterise distortion, crosstalk (channel separation) and the operation of the output muting. One trick when measuring the performance of a crossover is that the test signals need to be within the passband of each filter, unlike a preamp, where we can do most of our tests at 1kHz. The measurements were made with crossover points at 90Hz and 2.7kHz, so our test frequencies are within each band (ie, not too close to 90Hz Fig.4: the same plot as Fig.3 but with a split rail DC supply (using an AC supply gives the same result). As you can see, this improves the LF results greatly, and the MF results somewhat. However, even with the single DC supply rail, crosstalk is hardly a concern given that it is less than -55dB in the worst case. or 2.7kHz). The results are shown in Figs.2-4. The distortion/noise performance does not vary depending on the supply configuration, but the crosstalk does, so that is plotted in two separate graphs, Figs.3 & 4. The solid coloured lines show the left-to-right channel crosstalk, the dashed lines the right-to-left channel crosstalk (which is generally the same, so mostly hidden under the solid lines). The thin black lines show the worst-case inter-band crosstalk for signals fed into that band (ie, how much of it bleeds into the other band outputs). When powered with a single supply rail, the low frequency-cross talk is not as good as the dual-rail configuration. This is because some of the signal leaks into the virtual ground (described below), which has a higher impedance at low frequencies in the single-supply configuration. That said, the worst-case crosstalk of -60dB at low frequencies, improving to -80dB to -90dB at higher frequencies, is as good as many amplifiers. So it probably doesn’t matter that much, but a dual-rail or AC supply configuration is preferred for optimal performance. Figs.5 & 6 show the Active Crossover in action. In Fig.5, the frequency response of the LF output is shown in green with the subsonic filter bypassed and in blue with it active. The red curve is the MF output and the mauve curve is the HF output. Similarly, Fig.6 shows the LF and Fig.5: frequency response plots for the LF (blue & green), MF Fig.6: similar plots to Fig.5 but with the Crossover (red) and HF (mauve) outputs showing how they cross over. configured for two-way use without the subsonic filter. The green curve is with the subsonic filter bypassed, while the blue curve shows the effect when it is active, rolling off the output steeply below 20Hz. 44 Silicon Chip Australia’s electronics magazine siliconchip.com.au What is a Linkwitz-Riley filter and why use it? A Linkwitz-Riley filter is a fourth-order low-pass or high-pass filter (-24dB/ octave), comprising two second-order (-12dB/octave) Butterworth filters connected in series (hence the alternative name ‘Butterworth-squared’). This is different from a fourth-order Butterworth filter. The corner frequency of a filter is generally defined as the -3dB point. Cascading two filters down by 3dB at the corner frequency gives -6dB at this frequency, rather than the -3dB you would get with a fourth-order filter. The Butterworth configuration gives a perfectly flat passband (assuming ideal components). Consider that the sound from a pair of in-phase speaker drivers (eg, tweeter and mid or mid and woofer) combines via constructive interference. This follows different rules from power summing, with two -6dB signals constructively interfering to give a 0dB result. The roll-off characteristics of the Butterworth filter, combined with the -6dB figure at the crossover frequency, gives a flat summed response across the entire frequency range covered by both drivers (assuming ideal drivers, ideal sound radiation patterns etc). Of course, various factors combine to cause the response to be less than perfectly flat in the real world. But using a Linkwitz-Riley crossover filter arrangement is usually a great starting point and gives excellent results, assuming the drivers are well-matched. MF output frequency responses in blue and red respectively, with the unit configured as a two-way crossover with the subsonic filter bypassed. Operational overview Fig.7 is the block diagram of the Active Crossover. We’ll start by describing how it works as a 3-way crossover, then discuss the 2-way option. The stereo input signals are fed into a pair of filter blocks (blue) which separate out the high frequencies. The treble signals from these blocks go to the level control & buffering section at upper right (blue), then via the de-thump relay to the treble (HF) output connectors at upper right. The mid/low signals from the LOW OUTs of those two blocks are fed to another pair of virtually identical IN Mid/Low range Linkwitz Riley Filter LOW OUT IN HIGH OUT Turn your attention now to the whole circuit, which is spread across Figs.8-10, as it is quite large. Note that there are two ground symbols used LIN HIGH OUT High Frequency Level Controls & Buffers RIN De-thump Relay L LOUT G ROUT G R LOW OUT MF OUTPUTS (LEFT CHANNEL) (LEFT CHANNEL) INPUTS Circuit details HF OUTPUTS 3 or 2 way SELECT High/Mid range Linkwitz Riley Filter filter blocks (green) via two 3-way links. The high-frequency outputs of these blocks are the mid-frequency signals (as the treble has already been removed), and these go to another level control & buffer block and then, via a second relay, to the mid-frequency (MF) outputs. The low-frequency outputs of these green filter blocks contain only the bass signal. This goes through the final level control/buffer section, then optionally to the subsonic high-pass filter to remove any signals below 20Hz (which can be bypassed via the two three-way links at the bottom). Either way, it goes to the LF outputs via the third de-thumping relay. The power supply circuitry provides appropriate regulated DC supply rails to run the rest of the circuitry, plus some discrete logic to control the three de-thumping relays. This is so they disconnect the outputs for the first few seconds of operation and also switch off immediately when power is removed, before the supply rails can decay enough to affect the output signals. L LIN G RIN Mid Frequency Level Controls & Buffers De-thump Relay L LOUT G ROUT G R G R High/Mid range Linkwitz Riley Filter IN Mid/Low range Linkwitz Riley Filter HIGH OUT IN LOW OUT M O NO SUBW HIGH OUT +IN/AC G ND GND –/AC –IN/AC Low Frequency Level Controls & Buffers LOUT ROUT (RIGHT CHANNEL) 3 or 2 way SELECT +/AC RIN LOW OUT (RIGHT CHANNEL) POWER IN (DC or AC) LIN Power Supply & Switch-On/ Off Detection SINGLE/DUAL RAIL JUMPERS +9V or +18V V+ LEFT SUB FILTER OUT/IN Subsonic High Pass Filters 0V or +9V Signal ground –9V or 0V V– Relay drive RIN LIN (CF = 20Hz) LF OUTPUTS De-thump Relay L LOUT G ROUT G RIGHT SUB FILTER IN / O U T R Fig.7: a block diagram showing how the Active Crossover works. The blue-shaded boxes are bypassed for two-way operation, and the two lower links can bypass the red-shaded subsonic filter. The Crossover is based on several fourth-order state variable filters plus a fourth-order Sallen-Key filter. We split off the high-frequency signals first, so they have minimum processing and additional noise, as your ears are very sensitive to this. All outputs include level control and buffering. siliconchip.com.au Australia’s electronics magazine October 2021  45 Fig.8: the main part of the Active Crossover circuit. It looks pretty complicated, but if you refer back to the block diagram (Fig.7), you will see that it consists of repeating patterns (filter blocks etc). Each state variable filter consists of four cascaded op amp stages with feedback from the last to the first. This has the somewhat unusual characteristic that it acts as a low-pass and high-pass filter simultaneously. 46 Silicon Chip Australia’s electronics magazine siliconchip.com.au Changing the subsonic filter frequency The project as presented gives a 20Hz subsonic cutoff, and we recommend that you stick with it. This means 220nF capacitors in Fig.9 (eight arranged in pairs across the centre top of the PCB) and 36kW resistors (eight again, surrounding those capacitors). To change the subsonic filter cutoff frequency to 30Hz, for high-power and PA work, stick with the 220nF capacitors but change those eight 36kW resistors to 24kW. For a 15Hz subsonic cutoff (for the young and brave only!), leave the 220nF capacitors alone but change the eight 36kW resistors to 47kW. siliconchip.com.au Australia’s electronics magazine October 2021  47 throughout. The symbol with three horizontal lines is the power supply ground and is tied to the 0V supply input. The triangular symbol is the signal ground, and it’s tied to power ground for AC or split DC supplies. However, when a single-ended DC supply is used, this triangular symbol connects to a generated half-supply rail (ie, 12V for a 24V DC supply). The input and output signals are AC-coupled to allow for this signal voltage offset throughout the filter chains, regardless of the supply configuration; all that changes is the signal ground voltage. The PCB has stereo inputs, each of which has a 47kW pull down, feeding through a DC blocking capacitor (if you are using a single-rail power supply, you can use polarised electrolytics with “+” toward the level controls for all capacitors). This feeds through a ferrite bead and is bypassed to ground with a 100pF capacitor to reduce susceptibility to RF interference. All operational amplifiers (op amps) are NE5532 dual low-noise types. These have been selected as they deliver excellent performance at a modest cost and are available from many sources. The selection of resistances in the circuit has been made to minimise noise. This has influenced the R and C selections for the filters, with higher resistances only being used for very low frequencies. The crossovers are based on a fourth-order state variable filter configured with a Q of 0.5, forming a Linkwitz-Riley (Butterworth-squared) alignment. The state variable filter is slightly more complicated than the more common Sallen-Key filter. Still, it has the benefit that the crossover frequency is easily calculated and set by four equal resistor and capacitor values. The filter also separates both the high and low-frequency components of the input. Hence, an error in resistor or capacitor values simply results in a shift of the crossover point without otherwise affecting how they combine later. The component values shown are for a low-frequency crossover at about 88Hz and a high-frequency crossover at about 2.7kHz. For the low-frequency point, we have used 12kW and 150nF for R and C. This choice was made as 150nF is a practical maximum size for an MKT film capacitor, and a 12kW is 48 Silicon Chip Table 1 – R & C values for a range of crossover frequencies Desired frequency R Ideal C value Actual C value Actual frequency (nominal) 80Hz 13kW 153nF 150nF 82Hz 88Hz 12kW 151nF 150nF 88Hz 100Hz 11kW 145nF 150nF 96Hz 110Hz 12kW 121nF 120nF 111Hz 120Hz 9.1kW 146nF 150nF 117Hz 150Hz 10kW 106nF 100nF 159Hz 360Hz 4.3kW 103nF 100nF 370Hz 400Hz 4.7kW 85nF 82nF 413Hz 440Hz 4.3kW 84nF 82nF 450Hz 500Hz 4.7kW 68nF 68nF 498Hz 1kHz 4.7kW 34nF 33nF 1026Hz 1.5kHz 4.7kW 23nF 22nF 1539Hz 2kHz 4.3kW 19nF 18nF 2056Hz 2.5kHz 4.3kW 15nF 15nF 2468Hz 2.7kHz 2.7kW 22nF 22nF 2679Hz 3kHz 2.4kW 22nF 22nF 3014Hz 3.3kHz 2.7kW 18nF 18nF 3275Hz How does a state variable filter work? A state variable filter essentially consists of a series of cascaded integrators (similar to high-pass filters) with the output of each feeding back to one of the inputs of the first. In this case, each filter uses four cascaded integrators. A state variable filter has three useful outputs that can be picked off at various points: a low-pass output, high-pass output and bandpass output. The main advantage of a state variable filter (besides providing those various output signals) is that its Q can be precisely controlled via resistance values. As described in Wikipedia, “Its derivation comes from rearranging a high-pass filter’s transfer function, which is the ratio of two quadratic functions. The rearrangement reveals that one signal is the sum of integrated copies of another... By using different states as outputs, different kinds of filters can be produced.” For more details, including the mathematical derivation, see https://w. wiki/3e6K not such a high resistance value that it will compromise noise performance. For the high-frequency section, we have used 2.7kW and 22nF as R and C. The reasoning here is that 2.7kW is low enough to minimise noise, but not so low as to adversely load the op amps, and 22nF is a standard capacitor value. Of course, you will have specific frequencies at which you want to cross your speakers over. Table 1 provides component values for a range of useful frequencies, or you can use the following formula: f = 1 ÷ (2 × π × R × C). We’ll have some tips on how best to assemble the board if you envisage Australia’s electronics magazine fine-tuning your crossover frequency after construction. The final part of the circuit is the subsonic filter. This pair of conventional Sallen-Key filters in series provides a 24dB per octave high-pass filter. We have used these rather than state variable filters as there is no need for both high and low pass outputs, so this approach is simpler and cheaper. We have kept all resistors and capacitors the same value to simplify the parts list and construction procedure. This requires the filter to have a gain of 3.8dB per stage, or a total of 7.7dB. We have reduced this with an input siliconchip.com.au Fig.9: the LF output buffering and level control circuitry (at centre) is the same as for the other two outputs, but the LF output also has the optional subsonic high-pass filter circuitry. JP6 & JP7 select whether the LF output connector gets its signal from before or after the subsonic filters, which also provide some gain. LK1, if jumpered, mixes the L & R signals and sends the resulting mono signal to both LF output channels. attenuator to 6dB, as our experience is that having a bit of extra output available for the sub is handy. If the subsonic filter is bypassed, this gain is not available. We have set a cutoff frequency of 20Hz for this, which is low enough for any sensible purpose. If you really want, you can set this to a lower frequency or bypass it entirely, but if you have anything other than a sealed sub, we strongly advise against this. siliconchip.com.au Suppose you plan to use this crossover in a high-powered system or for PA applications. In that case, we recommend increasing the subsonic filter cutoff frequency to 30Hz, as PA subs almost always roll off at 30Hz or higher. See the panel titled “Changing the subsonic filter frequency” which explains how to do this. The mono function introduces two 1kW resistors in the audio path before the subwoofer level control. Australia’s electronics magazine This allows a jumper to be inserted to convert the LF output to mono. This means that the maximum level on the subwoofer output drops by slightly less than 1dB. This has been taken into account in the subsonic filter and associated attenuator. Power supply The power supply is pretty well standard, although a little complicated as you can configure it in a few October 2021  49 Price Changes For Silicon Chip Magazine From October 31st 2021, the price of Silicon Chip Subscriptions will change as follows: Online (Worldwide) Current Price New Price 6 Months $45 $50 12 Months $85 $95 24 Months $164 $185 Print Only (AUS) Current Price New Price 6 Months $57 $65 12 Months $105 $120 24 Months $202 $230 Print + Online (AUS) Current Price New Price 6 Months $69 $75 12 Months $125 $140 24 Months $240 $265 Print Only (NZ) Current Price New Price 6 Months $61 $80 12 Months $109 $145 24 Months $215 $275 Print + Online (NZ) Current Price New Price 6 Months $73 $90 12 Months $129 $165 24 Months $253 $310 Print Only (RoW) Current Price New Price 6 Months $90 $100 12 Months $160 $195 24 Months $300 $380 Print + Online (RoW) Current Price New Price 6 Months $100 $110 12 Months $180 $215 24 Months $330 $415 All prices are in Australian Dollars The cover price of the October issue onwards will be $11.50 in Australia. The New Zealand cover price will remain the same at $12.90. SILICON CHIP 50 Silicon Chip Parts List – 2/3-Way Active Crossover 1 double-sided PCB coded 01109211, 176 x 117.5mm 1 case (ideally metal; plastic OK if plugpack is used) 1 transformer or plugpack (see text) 3 10kW dual gang 9mm log potentiometers (VR1-VR3) 3 2A 12V DC coil telecom relays (RLY1-RLY3) [eg, Altronics S4130B or S4130C] 4 4-way polarised headers (CON1, CON2, CON4, CON5) 1 3-way mini horizontal terminal block (CON3) 6 3-pin headers with shorting blocks (JP1-JP3, JP5-JP7) 1 2-pin header with shorting block (LK1) 4 4-way polarised header plugs with pins (for CON1, CON2, CON4 & CON5) [Altronics P5474+P5470A, Jaycar HM3404] 2 4mm ferrite beads (L1, L2) 2 16 x 22mm TO-220 PCB-mount heatsinks [eg, Altronics H0650] 2 TO-220 insulation kits (insulating pads & bushes) 15 8-pin DIL sockets (optional, for the op amps) 4 M3-tapped spacers, length to suit # 8 6mm panhead machine screws & shakeproof washers # 1 1m length of twin-core shielded cable # 8 chassis-mount RCA connectors # (eg, four red, four white) 1 AC/DC power connector # (depends on supply used) # parts to suit a typical standalone application; different parts may be required depending on your case, power supply and whether you plan to integrate the Active Crossover with other modules. Semiconductors 15 NE5532 dual low-noise op amps, DIP-8 (IC1-IC6, IC8, IC10-IC17) 1 LM317T adjustable positive linear regulator, TO-220 (REG1) 1 LM337T adjustable negative linear regulator, TO-220 (REG2) 2 BC557 100mA PNP transistors, TO-92 (Q1, Q2) 3 BC547 100mA NPN transistors, TO-92 (Q3-Q5) 1 5.1V 400mW zener diode (ZD1) 8 1N4004 400V 1A diodes (D1, D2, D5, D7-D11) 2 1N4148 signal diodes (D3, D4) Capacitors 2 1000μF 50V electrolytic (16mm diameter) 1 470μF 25V low-ESR electrolytic (10mm diameter) 1 220μF 25V electrolytic (8mm diameter) 12 47μF 50V low-ESR electrolytic (8mm diameter) 2 47μF 50V non-polarised electrolytic (8mm diameter) [eg, Jaycar RY6820] 5 47μF 35V electrolytic (5mm diameter) 4 10μF 35V electrolytic (5mm diameter) 8 220nF 63V MKT 8 150nF 63V MKT ★ 25 100nF 63V MKT 8 22nF 63V MKT ★ 2 100pF 50V C0G/NP0 ceramic disc Resistors (all 1/4W 1% metal film) 3 100kW 10 4.7kW 3 47kW 1 3.6kW (R1 for single-rail operation) 8 36kW ★ 10 2.7kW ★ (only 8 of the 2.7kW change) 4 33kW 2 1.6kW (R1, R2) 12 22kW 8 1kW 8 12kW ★ 2 330W 6 10kW 2 270W 8 7.5kW 6 100W 8 5.6kW ★ change these values to alter the crossover frequencies (90Hz & 2.7kHz with the values given) Australia’s electronics magazine siliconchip.com.au Fig.10: the power supply section at top is the usual rectifier/filter/regulator arrangement to produce split rails from an AC (or dual rail DC) supply. JP1 & JP2 control how the outputs of this section are fed to the rest of the circuitry. This allows a single-rail DC supply of approximately 24V to be fed into CON3 and the circuit will still operate normally (with slightly reduced channel separation). The transistors at bottom switch on the de-thumping output isolation relays a few seconds after power-on, when everything has settled, and switch them off immediately when the supply rails start to collapse. different ways. Diodes D5, D8, D10 & D11 act as a bridge rectifier for an AC input at CON3 or reverse polarity protection for DC. If using AC, preferably a centre-tapped transformer (or two windings in series) should be used, although using a transformer with a single secondary is possible. Two 1000μF capacitors are used for storage/smoothing, and these feed positive and negative adjustable regulators, REG1 and REG2, set up to deliver ±9V. With an AC or split DC supply where both these rails are present, the two grounds mentioned earlier are jumpered together via a shorting block across pins 1 & 2 of JP2. In this case, the -9V rail is the negative rail, with pins 1 & 2 of JP1 shorted. If DC is applied, only the positive siliconchip.com.au regulator section is powered, and resistor R1 is changed to 3.6kW to double the output voltage to 18V. This gives the op amps the same effective supply voltage as with AC or split DC supplies. A virtual ground half-supply rail (ie, about 9V) is generated by a pair of 4.7kW resistors and bypassed with 470μF and 100nF capacitors, and this is connected to all the signal ground points (it’s shorted to power ground by the jumper for AC operation). There are capacitors between the input ground and virtual ground spread through the PCB to ensure it has a low AC impedance to ground at all points. De-thumping The switch-on/off detect circuit Australia’s electronics magazine does two things. First, it provides a startup delay of about five seconds to allow the virtual ground to settle before connecting the outputs. Until this time, the relays short the outputs to ground. This circuit also monitors the virtual ground, and if it deviates more than 0.6V from half of the positive and negative rail, it switches the output off. Note that this requires your supply rails to be within a couple of hundred millivolts of each other in a dual-rail setup. As long as you use 1% resistors to set up adjustable regulators, that should be the case. Otherwise, you will need to shunt one or the other to get a good match. PNP transistors Q1 and Q2 compare the voltage between two equal October 2021  51 Fig.11: without the de-thumping relays, the unit’s outputs produce a large excursion at switch-on. Fig.12: here is the switch-off pulse without the de-thumping relays; pretty bad at 5V swing! This is what the finished Active Crossover PCB looks like if you are building the dual-rail version with the optional subsonic filter. Fig.13: with the de-thumping relays in place, there is no longer a noticeable excursion at switch-on. Fig.14: it is now also similarly wellbehaved at switch-off with the relays added. 52 Silicon Chip voltage dividers, but one has a long time constant created by the 220μF bypass capacitor. These transistors have their collectors joined, creating a single logic output that drives NPN transistor Q4 to discharge a 47μF delay capacitor, thus disabling the output relays at switch-on and switch-off. The specified relays have 12V DC coils. 5.1V zener diode ZD1 performs two functions. Firstly, it sets a reference voltage for Q3/Q5 so the 47μF delay capacitor must charge to about 6V before the relays switch on. Its second function is to drop the 18V total supply voltage to 12V for driving the relays (with a modest drop across NPN driver transistor Q3). To illustrate the need for muting, Australia’s electronics magazine Figs.11 & 12 show the subwoofer output for the single-rail version at switch-on and switch-off (lower trace) without the muting relays. Those excursions would cause massive thumps, possibly damaging the driver! Figs.13 & 14 shows the same measurements with the relays operating. There is still an excursion of a few millivolts, but nothing significant and certainly no hazard to your speaker drivers. That’s all we have space for this month. The following article in our next issue will have all the PCB construction details along with instructions to set up and test the unit, some tips on how to use it and a troubleshooting section. SC siliconchip.com.au