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Bass Extender Mk2 By NICHOLAS VINEN . . . gives a big bass improvement for little outlay Want to get deeper bass out of your loudspeakers? Who doesn’t? But what if you could get more bass without spending much money? Even better – this Bass Extender can give as much as an octave more bass from your speakers! So with good quality tower speakers, you could get extended bass response down to 20Hz or below! A LL LOUDSPEAKERS have a rapidly falling response below a certain frequency which is a basic limitation of their design. This is firstly a function of the cabinet design but is also controlled by the bass driver characteristics, such as free-air cone resonance and their Thiele-Small parameters such as Vas. Typically, a large good-quality hifi loudspeaker system will be almost flat down to somewhere between 30Hz and 50Hz (the -3dB point or corner frequency) and will slope off below that at 20  Silicon Chip 24dB per octave for a vented enclosure (ie, a bass reflex system) and 12dB per octave for a sealed enclosure. Smaller speakers will have a somewhat higher corner frequency of, say, 60-80Hz. While a frequency response extending down to around 40Hz may seem like a good figure, this means that you will miss out on the lowest bass octave. So you will miss out on the lowest fundamental notes from pianos, pipe organs, double bass, timpani, tuba – the list goes on. In fact, life probably won’t be worth living. Of course, you will still hear the harmonics of these notes when they are played but that will be but a thin shadow of what could have been. Seriously though, for a lot of music which really does explore the lower music registers, the widest possible bass response is highly desirable. As a point of reference, the lowest note on an 88-key piano (A0) tuned for A4 at 440Hz has its fundamental at just 27.5Hz – below the -3dB point of all but the biggest and most expensive speakers available! siliconchip.com.au Bass Extender response, sealed speaker cabinet, -3dB = 30Hz Simulated speaker response beyond cut-off (dB) +6dB Filter response (dB) Overall response (dB) Phase shift (°) +3dB Bass Extender response, vented speaker cabinet, -3dB = 30Hz Simulated speaker response beyond cut-off (dB) +6dB Filter response (dB) Overall response (dB) Phase shift (°) +3dB 0dB 0dB -3dB -3dB -6dB -6dB -9dB 180° -9dB 180° -12dB 120° -12dB 120° -15dB 60° -15dB 60° 0° -18dB 10Hz -18dB 10Hz 20Hz 30Hz 40Hz 60Hz 80Hz Fig.1: this plot shows the simulated vented loudspeaker response near its 30Hz -3dB point (green), the frequency response of the Bass Extender when it’s active (red) and the combination of these two (blue). This shows that with the Bass Extender in place and correctly set up, the speaker’s frequency response becomes much flatter in the deep bass region; in this case, from about 50Hz and below. The phase shift introduced is about 60° at 20Hz. Now we know that few of our readers can afford such grandiose loudspeakers so our Bass Extender is a very worthwhile accessory and by the way, since it has a volume control, it can also double as a very wide range lowdistortion stereo preamplifier. Principle of operation The basic idea is simple – if we know the characteristics of the lowfrequency roll-off of the speakers, we can design a filter which increases signal amplitude right at the frequency where the speaker’s response is dropping off, thus compensating for this loss in sensitivity at low frequencies. That assumes that your amplifier has sufficient headroom to deliver more power at these lower frequencies. Generally, this will be the case unless some combination of the following is true: your speakers are very inefficient, your amplifier has a very low power output or you play music very loudly. If you typically run your amplifier with its volume control below the half-way point on the volume knob, it’s likely that you have sufficient headroom for the Bass Extender. There is also the issue of “frequency doubling”. As we increase the drive level to a speaker operating at or siliconchip.com.au 20Hz 30Hz 40Hz 60Hz 80Hz 0° Fig.2: same plot as Fig.1 but with a simulated response for a loudspeaker in a sealed cabinet, also with a -3dB point of 30Hz. Its roll-off is not as steep and so the Bass Extender boost is more gentle. The achieved response is slightly flatter than for a vented enclosure (assuming the filter is correctly tuned) but doesn’t extend to quite as low a frequency. The phase shift is slightly higher and is nearly 90° at 20Hz. below resonance (which is usually a frequency near the -3dB point), its distortion increases and ultimately, the second harmonic can overwhelm the fundamental, leading to an apparent doubling in the frequency being reproduced. But unless you have small speakers and drive them very hard, this is unlikely to be a severe problem. Ultimately, of course, the Bass Extender cannot turn your little bookshelf loudspeakers into giant high-fidelity monsters with 15-inch bass drivers but it can certainly give you a worthwhile improvement in bass response. By the way, because the Bass Extender can run from a 12V DC supply, it’s also suitable for use in car sound systems. Mind you, you would then have to very careful about how much extra bass boost you apply! Flattening the response Fortunately, the bass roll-off of a loudspeaker can be modelled pretty accurately, based on two parameters: its construction (sealed or vented) and its -3dB point. As Neville Thiele pointed out in his paper “Loudspeakers In Vented Boxes”, Proceedings of the IRE Australia, 1961 (reprinted in Journal of the AES, May & June 1961), sealed enclosures tend have an ulti- mate -12dB per octave roll-off while vented (bass reflex) speakers have an ultimate slope of -24dB per octave. Most modern hifi speakers are vented because these tend to have a more extended bass response. If you are unsure what type of system you have, check the front and back of the speaker for ports; front ports may be hidden behind a grille. If your speakers have ports, then they are the vented type. A -12dB roll-off is the same response as a second-order low-pass filter while -24dB matches a fourth-order lowpass filter. These are shown in green on Figs.1 & 2, with the -3dB point at 30Hz in both instances, typical of a medium-sized tower speaker. These figures also show the response of the filter in the Bass Extender Mk2 (red) when it is tuned to compensate for these specific speaker responses. The blue curves show the compensated response of the speakers. As you can see, the improvement in flatness is considerable, especially for vented enclosures, and is now almost completely flat to 20Hz – the small dip in the 30-50Hz region being just 0.25dB. This is inevitable, as the Bass Extender’s boost does not perfectly cancel out the speaker’s roll-off but it comes pretty close. January 2014  21 22  Silicon Chip siliconchip.com.au CON1a FERRITE BEAD 220pF 6.8k 10k 10k = SIG GND 220pF 6.8k LOG VR1b 10k LOG VR1a 10k 47k 10 µF VOLUME 47k 10 µF A K A K C 6.8k* 6.8k Q2 BC337 * E B B C BASS EXTENDER MK2 D2 * 1N4004 D1 1N4004 E Q1 BC327 S2c S2b S2d S2a POWER * THESE COMPONENTS ARE NOT INSTALLED FOR DC SUPPLY COMPONENT VALUES IN RED BRACKETS ARE FOR DC SUPPLY 47k 10 µF FERRITE BEAD = PWR GND 47k 10 µF 6 5 2 3 –15V 7 1 *220 µF 25V 47k 220 µF 25V IC1b IC1: LM833 –15V 4 IC1a 8 S1c 470nF S1b 470nF BYPASS 470nF BYPASS 470nF OUT GND GND OUT 100 µF * 25V 100 µF * 25V Y VR3 2k 10k X Y VR2 2k X X 10k X A K A K LED1 IS INSIDE S1 BUTTON LED2 IS INSIDE S2 BUTTON REG2 79L15 * IN IN REG1 78L15 * LINK ONLY FOR DC SUPPLY 100nF 6 5 2 3 (LINK) D4 1N4004 D3 * 1N4004 Y Y 8 POWER A K 1N4004 K LED2 λ (S2e) A 7 3.3k (3.0k) 2.2k (1k) IC2b 1 3.3k (3.0k) IC2: LM833 –15V 4 IC2a S1d 2.2k 2.2k 10 µF 10 µF B C –15V –Vout BC 32 7 , BC337 K BYPASS –Vin 100Ω 47k +15V S1a LED1 λ (S1e) A 22k (10k) 220pF E 47k 100Ω LEFT OUTPUT CON2a COM 79L15 0Ω (10k) IN GND OUT 78L1 5 (100 µF 25V) NOT INSTALLED NOT INSTALLED (10k) RIGHT OUTPUT CON2b RESISTOR VALUES IN GREEN ARE FOR SEALED ENCLOSURE SPEAKERS (USE DEFAULT VALUES FOR VENTED ENCLOSURES) 220pF –15V 100nF Fig.3: the circuit includes two identical channels, each consisting of an input buffer stage (IC1a & IC1b) followed by an equal component Sallen-Key filter based on two 470nF capacitors, resistors X & Y and op amps IC2a & IC2b. Trimpots VR2 & VR3 allow the filter frequency to be adjusted. SC 20 1 4 CON3 15–17V AC (12–24V DC) RIGHT INPUT CON1b LEFT INPUT +15V Similarly, for the sealed enclosure, the response is now down by less than 2dB at 20Hz and is virtually flat down to about 23Hz. Either way, this is a major improvement for very little investment. The reason the cancellation isn’t quite so good for sealed enclosures is that we have to reduce the amount of boost we apply to better match their more gentle roll-off. Note that in both cases, the +3dB point of the filter in the Bass Extender is actually slightly below the -3dB point of the speaker response. This gives optimal flatness and is taken into account in the formulas we give below. The Bass Extender Mk2 can be set up to suit sealed or vented speakers with virtually any corner frequency simply by selecting a few key resistor values, as described later. By the way, simply turning up the bass on the tone controls on an amplifier won’t do the same job since that usually involves boosting frequencies well above the -3dB point of the speakers, resulting in a ‘lumpy’ response. Revised circuit This design is based on a similar circuit we published in the April 2005 issue but there are several important improvements. First, the audio performance is a lot better, partly due to the use of superior op amps but also due to more carefully chosen component values. Distortion and noise are an order of magnitude lower and the high-frequency distortion especially has been reduced. We’ve also fitted it into a more attractive case and provided some externally accessible controls – a volume knob and a bypass switch. The volume knob means you can use it with an amplifier that lacks a volume control without needing a separate preamp. The bypass switch makes it easier to determine just how much effect the Bass Extender Mk2 has, as you can easily compare the sound with and without bass boost. We have also greatly improved the headroom. With the original unit running off 12V DC and giving a 10dB peak boost, it could only just handle a 2V RMS signal without clipping. However, some CD/DVD/Blu-ray players or DACs will put out more signal than that. By contrast, this new unit runs off a nominal 15-17V AC plugpack which siliconchip.com.au Bass Extender Mk.2 Parts List 1 double-sided PCB, code 01112131, 148 x 80mm 1 ABS plastic instrument case, 155 x 86 x 30mm (Altronics H0377) 1 15-17VAC plugpack rated <at> 100mA or more 1 set front and rear panel labels 1 10kΩ dual-gang logarithmic 9mm horizontal potentiometer (VR1) 2 2kΩ mini horizontal trimpots (VR2,VR3) 1 small knob to suit VR1 1 4PDT yellow illuminated latching pushbutton switch (Altronics S1452) (S1) 1 4PDT green illuminated latching pushbutton switch (Altronics S1451) (S2) 2 8-pin DIL IC sockets (optional) 2 stereo side-by-side PCB-mount RCA sockets (Altronics P0213) 1 PCB-mount DC socket (CON3) 2 ferrite beads 4 No.4 x 6mm self-tapping screws Semiconductors 2 LM833 dual op amps (IC1,IC2) 1 78L15 +15V regulator (REG1) means better performance and more headroom – although we have retained the option to run it off 12-24V DC, which might be required if you want to use it in a car, truck, caravan or boat. Adjustment trimpots have been added which allow the boost frequency to be fine-tuned to match the speakers; the previous version required resistors to be changed and this wasn’t much fun if you had to make multiple adjustments until you got the right effect. Overall then, this new Bass Extender Mk2 is a much better proposition and can be added to a hifi system without degrading the sound quality. Circuit description The complete circuit diagram for the new Bass Extender is shown in Fig.3. CON1 is the stereo RCA input connector. The two halves of the circuit, for the left and right channel signals, are identical so we’ll describe the left channel only. A 47kΩ resistor provides ground bias/loading for the driving equipment, in case this is necessary. The 1 79L15 -15V regulator (REG2) 1 BC327 PNP transistor (Q1) 1 BC337 NPN transistor (Q2) 4 1N4004 1A diodes (D1-D4) Capacitors 2 220μF 25V electrolytic 2 100μF 25V electrolytic 2 10μF 50V electrolytic 4 10μF 50V non-polarised (NP) electrolytic 4 470nF MKT 2 100nF multilayer ceramic 4 220pF ceramic disc Resistors (0.25W, 1%) 7 47kΩ 2 3.3kΩ 1 22kΩ 3 2.2kΩ 4 10kΩ 2 100Ω 4 6.8kΩ 1 0Ω Plus 8 resistors (X, Y) selected to suit speaker roll-off; see text & Table 2 Changes For DC Power Supply (1) Add 3 x 10kΩ and 1 x 1kΩ 0.25W 1% resistors (2) Add 1 x 100μF 25V capacitor (2) Delete REG1, REG2, D2-D4 and several passive components (see Fig.3) signal is then AC-coupled via a 10µF non-polarised (NP) capacitor to an RC filter consisting of a 6.8kΩ series resistor with a ferrite bead slipped over one of its leads and a 220pF ceramic capacitor to ground. This forms a lowpass filter with a -3dB point of just over 100kHz so that any high-frequency signals above that do not pass on to the following stages. Note that the 10µF AC-coupling capacitor also forms a high-pass filter in combination with its load resistance of around 10kΩ, which gives a -3dB point of 1.6Hz at the low end. Following the RF filter, the signal passes to volume control potentiometer VR1 which is shunted by a 10kΩ resistor. The reason for this is that a following filter stage (describe later) has a gain of 2.5 and this way, with the volume control set to maximum, the overall gain through the unit is 1. That’s because the 6.8kΩ RF filter resistor forms a divider in combination with the 10kΩ pot and its parallel resistor (ie, 5kΩ) and that gives a gain of approximately 0.42. January 2014  23 Filter Resistor Selection To select the appropriate filter resistor values, first you need to know the -3dB low-frequency roll-off point for your speakers. Usually, this will be in the specifications (if you can find them!) but you need to be careful as the quoted frequency response isn’t always measured at the -3dB points; in some cases, manufacturers use the -6dB points. If you don’t have this information, you will either have to measure it or guess. To make this measurement, you will need an adjustable-frequency sinewave generator, an amplifier and an accurate sound level meter (or a calibrated microphone and AC millivoltmeter). This type of measurement was explained in the “How To Do Your Own Loudspeaker Measurements” article in the December 2011 issue of SILICON CHIP. Basically, what you need to do is measure the sound level at a fixed point in front of the woofer with a relatively high frequency signal being fed into the amplifier (eg, 200Hz), then reduce the frequency until you get a reading that is 3dB lower. You can then use that frequency (Fc) in the formula listed below. If you have (or can generate) an impedance plot for the speaker, you can also use this to estimate the -3dB point. For the most common (vented) type, there will usually be two impedance peaks in the bass region. The -3dB point will be at the lowest point (dip) between these peaks. For sealed speakers, there will be a single peak (the resonance frequency) and the -3dB point will be about 10% below this. Failing that, use the figures in Table 1 as a rough guide. But we must emphasise that this is only a guide and the actual -3dB point will depend heavily on the driver and cabinet design and may also vary slightly between different samples of the same speaker. Having determined the -3dB point (Fc), use the following formula to determine the required total resistance (R) for resistors X & Y: R = T ÷ Fc where T = 585kΩ for vented enclosures and T = 510kΩ for sealed enclosures. It’s then just a matter of determining which two series X & Y resistors add up to give a value that’s close to R. For example, if Fc = 40Hz then R = 585kΩ ÷ 40 = 14.6kΩ. In this case, you can select Y = 12kΩ and X = 2.7kΩ (close enough). Or you can use Y = 10kΩ and X = 4.7kΩ. Note though that typical 470nF capacitors have a tolerance of ±5% at best so as long as the total is within a few hundred ohms, that’s good enough. Because this is a stereo unit, you will need eight resistors in all, four of each selected value. To save time, we have included Table 2 which shows the best resistor values to use for common -3dB points. If in doubt as to which values to use, err slightly on the side of a higher corner frequency as you can later adjust it down slightly using trimpots VR2 & VR3. From there, the signal on the wiper of the volume pot is AC-coupled to the input of op amp IC1a (LM833) via another 10µF capacitor. This prevents IC1a’s input bias current from flowing through the pot and causing a DC voltage to appear across it. While this voltage would be small, it could be enough to cause noise or ‘crackling’ as the pot is rotated. IC1a buffers the signal and provides a low driving impedance for the following filter network which consists of two 470nF capacitors, two identical pairs of resistors (X & Y) and an adjust24  Silicon Chip ment trimpot. These resistor pairs have been used to overcome the limited range of values available in a single resistor. As far as the circuit operation is concerned, you can consider each series pair as a single resistor. The signal at the ‘output’ end of the filter is now fed to pin 3 of IC2a via switch S1c, which is shown in its normal operating position. IC2a is set up as a non-inverting gain stage, with a gain of 2.5 as mentioned earlier. Its output goes both to output connector CON2a and to the junction of the two 470nF capacitors at the output of IC1a via one of the XY resistor pairs. This is what gives the filter its characteristic hump shape (see Fig.1 & Fig.2). Trimpot VR2 allows the filter response to be tweaked without having to change component values. This is necessary because the -3dB point of the speakers you are using is unlikely to be exactly the same as the manufacturer’s specification. Turning VR2 clockwise increases its resistance and shifts the filter peak lower in frequency while increasing its amplitude. IC2a’s 3.3kΩ feedback resistor is shunted with a 220pF capacitor which rolls off its frequency response well above 20kHz. This lowers its output noise and improves stability without impacting on the overall frequency response in the audio band. The output signal is AC-coupled to CON2a via a 10µF capacitor to remove any DC offset picked up in the filter. This capacitor also ensures that no damage will occur if the output is shorted to a DC supply rail. Finally, the output is DC biased to ground using a 47kΩ resistor (in case it’s floating), while a 100Ω series resistor provides some short-circuit protection for the op amp and also isolates IC2a’s output from the load capacitance (eg, cable capacitance) to prevent instability. Note that while we have specified low-noise, low-distortion LM833 op amps, others such as the NE5532 and OPA2134 are also suitable. Bypass function When S1c is in its alternative position, IC1’a output is fed directly to IC2a’s pin 3 input, bypassing the filter network entirely. This switch thus provides a bypass function and effectively allows the Bass Extender Mk2 to be disabled so that you can check whether it is having any audible effect. S1b provides the bypass function for the righthand channel. Note that switch S1 is a 4PDT type – its other two poles (S1a & S1d) switch on its integral LED (LED1) when the bypass function is engaged. This LED is driven via a 22kΩ current limiting resistor at around 1.3mA (ie, 28V ÷ 22kΩ). The 10kΩ resistors connected across switches S1b & S1c are shorted out during normal operation. These ensure that input pins 3 & 5 of IC2 do not go open circuit when S1 is switched, preventing loud clicks or pops from being injected into the audio signal siliconchip.com.au and making A/B comparisons with the Bass Extender enabled and disabled much easier. A note about the use of electrolytic capacitors in the signal chain – we don’t think that this is a problem and this is backed up by our measurements. However, we have made provision on the PCB for 1µF MKT capacitors to be installed instead of the 10µF electrolytics for those people who really want to do so. Besides being more linear, MKT capacitors also have a longer lifespan than electros but they cost more and are harder to get. Power supply The recommended power supply is a 15-17VAC plugpack. The unit can also be run off 12-24V DC with reduced headroom but the circuit is shown configured for an AC supply. Diodes D1 & D2 form a half-wave rectifier and charge the 220µF filter capacitors to around ±20V via transistor switches Q1 & Q2. These ±20V rails are then regulated to ±15V by 3-terminal regulators REG1 and REG2, to power the op amps. The power switching arrangement is rather unusual and consists of PNP transistor Q1, NPN transistor Q2, switches S2a/S2d & S2b/S2c and two 6.8kΩ base resistors. Basically, the two transistors are there to carry the supply current and the reason for doing it this way is that we are using a second 4PDT illuminated switch as the power switch but these are only rated to carry 50mA per contact. While that’s sufficient current to run the unit, the switch-on surge current is much higher at over 1A while the input filter capacitors charge. Paralleling the switch contacts doesn’t help since inevitably, one will make contact before the others and carry the full surge current. So Q1 & Q2 do the actual switching and this arrangement also limits the inrush current to around 450mA. The circuit works as follows: when S2 is switched to the on position (as shown in Fig.3), it connects the 6.8kΩ base resistors for Q1 & Q2 to ground. Thus, when D1 is forward-biased, Q1’s emitter is at the full positive supply voltage and its base is pulled towards ground due to the current flowing through its 6.8kΩ resistor. As a result, PNP transistor Q1 switches on and so current flows from D1 to the 220µF filter capacitor across the input of REG1. siliconchip.com.au Table 1: Typical Loudspeaker Bass Roll-Off Frequencies Woofer Size (Approx.) Cabinet Style Typical -3dB Point (Approx.) >30cm (>12-inch) Tower 25-30Hz 30cm (12-inch) or 2 x 25cm (10-inch) Tower 28-35Hz 25cm (10-inch) or 2 x 20cm (8-inch) Tower 35-40Hz 20cm (8-inch) or 2 x 16cm (6.5-inch) Tower 40-45Hz 18cm (7-inch) or 2 x 13cm (5-inch) Tower 45-55Hz 16cm (6.5-inch) or 2 x 13cm (5-inch) Bookshelf 50-70Hz 13cm (5-inch) or 2 x 10cm (4-inch) Bookshelf 55-80Hz 10cm (4-inch) Bookshelf 60-100Hz Table 2: Resistor Values For Typical -3dB Points -3db point Vented Y Vented X Sealed Y Sealed X 28Hz 18kΩ 3kΩ 18kΩ 220Ω 30Hz 18kΩ 1.5kΩ 15kΩ 1.8kΩ 35Hz 15kΩ 1.8kΩ 12kΩ 2.7kΩ 40Hz 12kΩ 2.7kΩ 12kΩ 680Ω 45Hz 12kΩ 1kΩ 10kΩ 1.2kΩ 50Hz 10kΩ 1.8kΩ 10kΩ 220Ω 55Hz 10kΩ 560Ω 8.2kΩ 1kΩ 60Hz 8.2kΩ 1.5Ω 8.2kΩ 270Ω 70Hz 8.2kΩ 150Ω 6.8kΩ 470Ω 80Hz 6.8kΩ 470Ω 4.7kΩ 1.5kΩ 90Hz 4.7kΩ 1.8kΩ 4.7kΩ 1kΩ 100Hz 4.7kΩ 1.2kΩ 4.7kΩ 330Ω Q2 operates in similar fashion. When D2 is forward biased, Q2’s emitter is pulled to the negative supply rail and so this NPN transistor turns on and current now flows through D2’s anode and charges the 220µF filter capacitor at the input of REG2. In operation, the 6.8kΩ resistors limit the transistor base currents to around 20V ÷ 6.8kΩ = 3mA. Since a BC327/337 has a current gain (hFE) of about 150 under that condition, this means that the collector currents are limited to around 3mA x 150 = 450mA. If power switch S2 is in the alternative position (ie, off), each 6.8kΩ base resistor is connected back to the emitter of its respective transistor. This effectively ‘shorts’ out the base/ emitter junctions and switches both transistors off. In this condition, the only current drawn from the supply is the leakage current through Q1 & Q2 which is very low (typically <100nA). The 47kΩ resistor between the two regulator inputs provides a discharge path and prevents this leakage current from (very slowly) charging the input capacitors. The power LED (LED2, inside S2) is connected in series with a 2.2kΩ current-limiting resistor across the regulator outputs. Diodes D3 & D4 prevent the regulator outputs from becoming reverse-biased by more than about 0.5V during power-up or power-down. Note that while you may be able to get away with using a cheaper and smaller 12VAC plugpack rather than the 15-17VAC plugpack specified, it’s a bit marginal. If using a 12VAC plugpack, you would want to check that its actual output under light load is at least 13VAC (and ideally higher) in order to prevent the regulators from entering drop-out. Having said that, even if they do, the performance should still be quite acceptable. DC supply Note that one 10kΩ resistor and one January 2014  25 15–17V AC SUPPLY VERSION VR1 100 µF S1 Bypass IC2 LM833 2.2k 3.3k 220pF 220pF VR2 47k 47k VR3 2k 100nF 10 µF NP L R CON1 Input 47k R 6.8k S2 Power Q2 D1 D2 Bass Extender Mk.2 C 2013 SILICON CHIP 100Ω 47k 100Ω L A K 337 2.2k 10 µF NP REG2 79L15 220 µF 47k 327 6.8k 4004 Y 220 µF 4004 Y D4 REG1 78L15 4004 470nF 470nF Y 2k 220pF D3 4004 X 100nF X Q1 100 µF 22k A K X Y 47k 47k NP 10 µF IC1 LM833 6. 8k 10k 6. 8k BEAD 10k X 10k 470nF + BEAD 10k 470nF 10 µF + NP + + 10 µF 10 µF + 220pF + 3.3k 2.2k 10k log 01112131 CON2 Output CON3 Power Fig.4: follow this layout diagram to build the PCB if you are going to power the unit from a 15-17V AC plugpack. Note that there’s provision for the PCB to accept 1μF MKT capacitors instead of the 10μF electrolytics if you don't want to use electrolytics in the signal chain (and different MKT capacitor sizes are catered for). Resistors X & Y are selected from Table 2, as described in the “Filter Resistor Selection” panel. 12–24V DC SUPPLY VERSION VR1 A K X X Y 220pF 2.2k 3.3k 220pF 10k 10k VR3 2k 100nF 3.3k 2.2k 2k S1 Bypass 47k CON1 Input 10 µF NP 47k R 100 µF 47k 1k D1 SILICON CHIP 100Ω 47k L 100Ω NP S2 Power Bass Extender Mk.2 C 2013 10 µF R A K 4004 Y 470nF 470nF Y 47k L 327 6.8k X Q1 220 µF 100nF IC1 LM833 220pF 47k 47k Y VR2 10 µF 10k 470nF 10k X NP 6. 8k BEAD 10k 470nF 10 µF 10k 6. 8k BEAD 10k NP + + 10 µF 10 µF + 220pF + IC2 LM833 10k log 01112131 CON2 Output CON3 Power Fig.5: this is the layout diagram to follow if you intend running the unit from a 12-24V DC supply. The differences between this and the AC-supply version of Fig.4 mainly involve the power supply components at top right plus the current limiting resistors for LED1 & LED2. 26  Silicon Chip siliconchip.com.au 100µF capacitor (ie, across the 15V rail) are not installed when using an AC plugpack. In addition, one resistor is specified as 0Ω. For operation from a 12-24V DC plugpack (higher being better), the components marked in red on the circuit must be changed. First, D4 is replaced with a wire link, so that the negative supply rail of the op amps is now connected to the power supply ground. Second, the LED currentlimiting resistors are reduced to give sufficient brightness with the lower operating voltage. And third, we need to adjust the DC input bias for all four op amps so that it will be at half supply; eg, with an 18V supply, it must be at 9V. That’s done by fitting two 10kΩ resistors across the supply rail as a voltage divider, along with a 100µF filter capacitor to filter any supply noise. This capacitor is critical because without it, any ripple or noise from the plugpack supply would get into the audio path. Modern plugpacks are switchmode devices, so there are often audible harmonics present. Finally, when operating from DC, REG1 is linked out since this gives the op amps maximum headroom and they should have sufficient supply ripple rejection to run from an unregulated DC rail. Construction All the parts for the Bass Extender Mk2 are mounted on a PCB coded 01112131 and measuring 148 x 80mm. This fits neatly into an ABS plastic instrument case measuring 155 x 86 x 30mm and is secured to the integral stand-offs in the case using selftapping screws. Figs.4 & 5 show the parts layout on the PCB. Follow Fig.4 if you are building the AC-powered version. Alternatively, follow Fig.5 if building the DC-powered version. Table 3: Resistor Colour Codes o o o o o o o o o siliconchip.com.au No.   7   1   4   4   2   3   2   1 Value 47kΩ 22kΩ 10kΩ 6.8kΩ 3.3kΩ 2.2kΩ 100Ω 0Ω 4-Band Code (1%) yellow violet orange brown brown black orange brown brown black orange brown blue grey red brown orange orange red brown red red red brown brown black brown brown single black stripe Start the assembly by fitting all the resistors, including the eight selected for X and Y (see “Filter Resistor Selection” panel and Table 2). Note that the ferrite beads should be slipped over the two 6.8kΩ resistor leads before soldering them in place. It’s a good idea to check each resistor value using a DMM before fitting it. Follow with the diode(s) and then the two IC sockets. Alternatively, if you aren’t using sockets, solder the op amp ICs directly to the PCB. Note that in either case, the notches/dots at one end of the sockets/ICs must go towards the top of the PCB. The two trimpots can go in next, followed by the transistors and regulators (if fitted). Note that the transistors and regulators all look the same so be sure to check their type numbers carefully before installing them. You may need to crank their leads out slightly to fit the PCB pads. The six ceramic capacitors are next on the list, followed by the two slide switches (S1 & S2). Check that the latter are sitting flush against the PCB before soldering their pins (it’s a good idea to re-check this after lightly soldering the first two pins). The DC socket, MKT capacitors and electrolytic capacitors can then all go in. Take care to ensure that the polarised electros are all orientated correctly. The four non-polarised (NP) 10µF electrolytics can go in either way around. The PCB assembly can now be completed by fitting the volume pot and the stereo RCA sockets. Before fitting the latter, they need to be modified. As supplied, they are too tall to fit Table 4: Capacitor Codes Value µF Value IEC Code EIA Code 470nF 0.47µF 470n 474 100nF 0.1µF 100n 104 220pF NA 220p 221 5-Band Code (1%) yellow violet black red brown brown black black red brown brown black black red brown blue grey black brown brown orange orange black brown brown red red black brown brown brown black black black brown single black stripe January 2014  27 Left: the PCB assembly is mounted on the lid of the case and is secured to integral stand-offs using four self-tapping screws. Right: rear view of the completed unit. It’s installed between the preamp and the amplifier. into the case so it’s necessary to cut off the upper projection with the central screw hole. This can be done using a rotary cutting tool or a hacksaw but make sure you don’t damage the lower plastic housing. You can then fit the sockets in place, ensuring that they are pushed all the way down and are parallel with the edge of the PCB. The plastic tabs on either side of the sockets fit into matching holes in the PCB to help hold them in place. Having done that, separate the case halves by removing the front and rear panels, then fasten the PCB assembly to the top half using four No.4 x 6mm self-tapping screws. This means that when the case is later assembled, the PCB hangs upside down off the lid. first, make sure that both switches are off (push them until they pop out), then connect the plugpack supply and switch on. Next, use a DMM to check the DC voltage between pins 4 & 8 of one of the IC sockets (or between pin 4 & 8 of one of the ICs); you should get a reading that’s very close to 0V (ie, with power switch S2 off). Now push the power switch and check that both LEDs illuminate. The Testing If you fitted IC sockets, leave the op amps out of circuit for the time being. Now for some initial checks: SILICON CHIP BASS EXTENDER MK.2 www.siliconchip.com.au 15VAC 28  Silicon Chip www.siliconchip.com.au R Output L R Input L Fig.6: these two artworks can be copied and used as drilling templates for the front & rear panels. They can also be downloaded as a PDF file from the SILICON CHIP website. siliconchip.com.au Finishing off Once you are happy with the results, you can prepare the front and rear pansiliconchip.com.au 0.01 Bass Extender THD+N vs Frequency, 2V RMS in/out 29/11/13 08:57:54 Left channel, 20Hz-80kHz bandwidth Right channel, 20Hz-80kHz bandwidth Left channel, 20Hz-22kHz bandwidth Right channel, 20Hz-22kHz bandwidth 0.005 Total Harmonic Distortion + Noise (%) voltage between pins 4 & 8 of the IC socket (or IC) should now measure 30V DC for an AC supply, or about the same as the plugpack output voltage if you are using a DC supply. Next, if using an AC supply, connect the black probe to pin 3 of IC1 and measure the voltage at pin 8 (+15V) and pin 4 (-15V). For a DC supply, check the voltage between pins 3 & 4 of IC1; you should get a reading almost exactly half that between pins 8 & 4. Assuming this all checks out OK, press S1 (Bypass) in and check that its LED switches off, then switch off and install the two op amp ICs in their sockets. That done, connect the unit between a signal source and your amplifier (turn the volume down first), switch on and verify that undistorted audio is passing through the unit, for both channels. You can now check whether the unit is doing its job, ie, extending the bass response without introducing any dips or peaks in that region. If you have a speaker measurement set-up, as described in the December 2011 issue, then you can run this to plot the frequency response in the 20-200Hz region to check this. If not, you will have to do it by ear but that’s far less precise. The simplest test would be to run a sinewave frequency sweep and listen for any obvious peaks or dips. If there is a peak present that goes away with the Bass Extender’s defeat switch activated, that suggests that you have the roll-off frequency set too high. In that case, you can turn trimpots VR2 & VR3 slightly clockwise to decrease the frequency of the bass boost and re-test the set-up. If necessary, you can repeat this procedure until the peak disappears. If adjusting VR2/VR3 fails to remove the peak (even when they are set fully clockwise), then you will have to change resistors Y and/or X in order to decrease the roll-over frequency further. Note that it’s easier to do this adjustment one channel at a time. Conversely, if there’s a dip in the response, that suggests that the rollover frequency is too low and if VR2/ VR3 are set at minimum (fully anticlockwise), the only option then is to change resistors Y and/or X. 0.002 0.001 0.0005 0.0002 0.0001 20 50 100 200 500 1k 2k 5k 10k 20k Frequency (Hz) Fig.7: total harmonic distortion across the audible frequency range with the unit operating in typical conditions. This shows that the unit is suitable for use in a hifi system, with distortion below 0.001% over virtually the entire frequency range (even with a bandwidth of 80kHz, which is used to show the slight rise of distortion with frequency). With the bandwidth limited to a more realistic 20kHz, distortion never rises above 0.00065%. Features & Specifications Power supply ............................................................ 15-17VAC or 12-24V DC <at> <100mA Signal handling ............................................................3.88V RMS (+14dBu) (AC supply) Frequency response (bypass mode) .......................................... 20Hz-20kHz, +0,-0.06dB Boost corner frequency .........................................................................+3dB at 20-100Hz Peak boost ........................................................................................approximately +10dB Suitable speaker types ........................................................................ bass reflex, sealed Gain adjustment ...........................................................................................................0-1 Total harmonic distortion .............................................. <0.0005% up to 1kHz (see Fig.7) Signal-to-noise ratio ......................................................................... -113dB, unweighted Note: measurements taken with 20Hz-20kHz bandwidth, 2V RMS signal and gain = 1. els. The labels shown in Fig.6 can be copied and used as drilling templates (or they can be downloaded in PDF format from www.siliconchip.com.au – free to online subscribers). It’s simply a matter of accurately drilling the three front panel holes and the five rear panel holes. That’s best done by first drilling small holes with a pilot drill (say 2-3mm) and then carefully enlarging them with a tapered reamer. Once drilling is complete, de-burr the holes and then attach the panel labels (the labels can be printed onto photographic paper and attached using silicone). The case lid can then be fitted in position, the front and rear panels snapped on and the knob pushed onto the pot shaft. You can now hook the unit up permanently. It should ideally go between your input selector/preamplifier and power amplifier. If you have an all-inone unit, check if it has preamp-out/ preamp-in connections and if so, use those. Otherwise you will need to connect it between your most commonly used signal source (eg, CD player) and the amplifier. Finally, it may be possible to build the unit permanently into your stereo amplifier and run it from a 15-17VAC secondary tap on the mains transSC former. January 2014  29