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Senator 10-inch Bass Reflex Loudspeakers Part II Last month we presented the Senator 10-inch bass reflex loudspeaker system and gave the details of its cabinet construction, based on a Bunnings Caboodle kit. This month we conclude the description with details of the crossover network PCB and enclosure wiring. Pt.2 By Leo Simpson B efore discussing the cross-over network PCB, we should respond to a number of reader comments on last month’s article. One reader noted that the prototype enclosure evidently had a circular hole in the baffle for the horn tweeter whereas the cabinet diagrams showed a rectangular cutout. It is certainly true that the prototype cabinets did have circular tweeter mounting holes but we judged that this removed too much of the timber behind the horn and that it would be better and easier to make rectangular cutouts instead, ie, 140mm wide by 100mm high. After all, the more surface area in the baffle in the region behind the tweeter, the greater will be its rigidity and it will also lead to a better seal for the tweeter mounting. Still on the subject of speaker mounting, it is desirable to provide a 78  Silicon Chip seal of draft-exclusion tape around the periphery of the woofer’s chassis. This was shown in the rear photograph of the woofer on page 35 of the September 2015 issue. Another reader commented that both side panels of the finished Kaboodle cabinets jut out by 8mm in relation to the top, front and back panels. He would prefer to have the side panels line up flush by reducing the width of all the panels associated with the Kaboodle 450mm Wall Cabinet kit. That would require the 300mm carcase width to be reduced by 16mm to 284mm. To do this you would need a bench saw which cuts perfectly square. Now the side panels of the prototype enclosures do jut out and this was done to simplify construction and minimise the need to cut any of the finished Kaboodle panels. As it stands, only the top finished panel needs to be cut. This means that the assembly procedure is more straightforward and also, in the opinion of all who have seen the finished cabinets, “in the wood” they look very good. However, if you prefer, you could reduce the enclosed volume of the Kaboodle carcase (or if building cabinets from scratch) as suggested, and this should have little effect on the overall performance. There were further queries from a reader concerning how the frequency response was measured and how the system power rating of 250W was determined. We put the first question to the designer, Allan Linton-Smith. He used an average of six near-field sweeps with two Bruel & Kjaer 4134 calibrated microphones with two Bruel & Kjaer 4669 preamplifiers feeding into an HP/Agilent35670a Audio siliconchip.com.au Analyser (with UK4 mic option power supply) and then into an Audio Precision System 2222 analyser. We will come to the power handling of the tweeter later in this article. We have also been informed by the local agent for Celestion loudspeaker, Electric Factory Pty Ltd, (www.elfa. com.au) that they only have limited supplies of the specified Celestion NTR10-2520D 10-inch woofer as it has been discontinued by the English manufacturer. Fortunately, there is a suitable replacement, very close in specifications, the model NTR10-2520E and this has been extensively tested by Allan Linton-Smith so we can recommend it for the Senators. The chassis is exactly the same but there is a slight difference in the cone profile. You will need one of these crossovers for each Senator speaker box. The jumper set (bottom right) is for adjusting the profile – its use is explained in the text. Crossover network PCB The crossover network used in the Senators is virtually identical to that in the Majestic loudspeaker system featured in the June 2014 issue, with only a slight component value change in the tweeter attenuator resistors. The modified circuit is shown in Fig.1. The crossover uses a low-pass filter, comprising a sole 2.7mH air-cored inductor, to drive the woofer and roll off the signal at a rate about 6dB/octave above 2kHz. Similarly, the tweeter is fed by a high-pass filter, mainly involving a 4.7µF capacitor (C1) to roll off frequencies below 2kHz. There is also an optional treble boost circuit which can be switched in and out to compensate for tweeter roll-off at high frequencies. After extensive listening tests, our preference was to leave the boost circuit in permanently and so we did not install the switches on the rear of the enclosures. The crossover network also needs to match the efficiencies of the tweeter and the woofer, so that the overall frequency response is as flat as possible. As in the Majestic loudspeaker system, the horn-loaded tweeter is extremely efficient, at around 109dB/W<at>1m, far more efficient than the 10-inch Celestion woofer which is itself 95dB/W<at>1m – and this is a very efficient driver. You might think that we could attenuate the signal to the tweeter using a single resistor. For example, by connecting an 8Ω resistor in series with an 8-ohm driver to halve the voltage level and thus provide 6dB of attenuation. siliconchip.com.au However, this would also increase the source impedance “seen” by the driver by 8Ω (from the very low figure provided by the power amplifier) and performance would be significantly impacted due to poor damping. Instead, we are using a divider with a low resistance, including two paralleled 3.3Ω 5W resistors across the tweeter, to keep its effective source impedance low. The horn tweeter is therefore better damped to keep distortion low. The parallel combination of the two 3.3Ω resistors and the tweeter is fed by two paralleled 12Ω 10W resistors. This resistive divider provides more precise attenuation as it swamps the effect of the tweeter’s impedance which rises with increasing frequency. The overall attenuation provided by this resistor divider is -14.6dB (assuming a tweeter impedance of 8Ω) and this matches the sensitivity of the tweeter to the woofer. As a consequence of this amount of signal attenuation, the power dissipation in this resistive divider is a significant issue. In effect, we are throwing away about 66% of the power which would otherwise be fed to the tweeter! With that in mind, some readers may question the relatively modest power rating of the resistors we have specified. At a peak program power of 250W, that seems like a lot of power to be dissipated! But there are a couple of reasons why we can manage the situation with much lower-rated resistors. Even when driving the speaker system at a peak of 250W, the average program level will typically be only a small fraction of this; perhaps around 10W, at most. Secondly, a good deal of that program power will be going to the woofer. With a typical recording, the energy in each octave is about half that of the octave below. So even though we using resistors with a total power rating of 30W, for home (hifi) use, these should be more than adequate. Construction Fig.2 shows the PCB layout. Start with the spade lugs; we used the PCB-mounting type however chassismounting spade lugs can also be pressed into service. For the PCBmount type, there are various ways they can be fitted as there are four holes per position but we aligned them with the board edges and placed them as close to the edge as possible. Solder them in place with a highpower iron. Start with the pins on the bottom side but it’s also a good idea to ensure that there are solder fillets from the top side pad to the sides of the spade connectors too. If using chassis-mounting spade lugs, use either the single-lug type or cut off one lug from a double-lug connector. Install each one by first feeding an M4 x 6mm machine screw up through the hole in the bottom of the board, then fit a shake-proof washer, then the connector, then another shake-proof washer and finally the nut. Tighten the nut with the lug projecting October 2015  79 3.3F 1 5W * * * OPTIONAL 12 10W HF PROFILE S1 * CON3 (R1) 12 10W (C1) + (R2) 3.3 5W 3.3 5W TWEETER CON4 4.7F L1 2.7mH CON1 – CON5 INPUT – + + WOOFER CON6 CON2 – First-ORDER Loudspeaker CROSSOVER Fig.1: SC the crossover circuit is quite simple, consisting primarily of inductor L1 2014 to act as a low-pass filter for the woofer and a 4.7μF capacitor as the highpass filter for the tweeter. Resistor pairs R1 and R2 attenuate the tweeter signal so that its output level is matched L1to2.7mH the L1 2.7mH L1 2.7mH woofer. The + + +    remaining 10 components form 22F INPUT INPUT INPUT a switchable treble 8.2F boost – circuit. – – (C) LC FILTER S1 * S1 5W 1 J * + 5W 3R3 J 10W 12 J 10W 12 J L1 2.7mH TO TWEETER – 2-Way Crossover 3.3F K 250V * 4.7F K 250V Fig.2: follow this PCB layout diagram to assemble the crossover. It includes provision for treble boost via S1; however we don’t believe it will normally be required with the tweeter specified. (B) LR FILTER WITH IMPEDANCE EQUALISATION 5W 3R3 J (A) LR FILTER * = OPTIONAL + + FROM INPUT TERMINALS – TO WOOFER – 80  Silicon Chip out from the edge of the PCB. The capacitor(s) go in next. Bend the leads to fit the pads and push them down so they sit flat on the PCB before soldering them in place. Note that we have provided multiple pads in case you prefer to use radial types (eg, X2style polypropylene capacitors). Polyester capacitors are not ideal as they are less linear but would probably work OK. The 4.7µF capacitor next to next L1 must be fitted. The other is optional depending on whether you want the treble boost feature. Solder the capacitor leads on both sides of the board, assuming you’re using the specified axial capacitors. We’ve provided pads so that the wirewound resistors can be supported by sections of stiff tinned copper wire, so that if they are exposed to shock or vibration, their primary solder joints are not the only means of support. You don’t have to fit these support “trusses”; they are optional. The wirewound resistors should be spaced off the PCB by a few millimetres, to help power dissipation. That’s done by pushing each resistor down onto a 2 or 3mm-thick spacer. You then turn the PCB over and solder the leads, on both sides of the PCB. The 1Ω resistor can be pushed all the way down onto the PCB if desired, as it handles relatively little power. The next step is to fit a pin header to connect S1, if you are using it. Once it’s in, install inductor L1. The inductors used in our prototypes came from Jaycar (Cat LF-1330) but these have been discontinued as a stock line and are presently being run out at a discount price. Get yours while you can. We hope to arrange for an alternative source for these inductors or we will provide instructions on how to make your own bobbins and wind the inductors, in a future issue. The leads of the inductor should have the enamel coating scraped off its two ends; while they are supplied pre-tinned, the tinned sections are too far from the bobbin to allow it to be soldered to the PCB. You will have to scrape them back to the point where they exit from the bobbin, then tin those sections. Mount the inductor using a Nylon, brass or stainless steel M4 machine screw and nut. It is most important that you do not use a steel screw and nut because that would increase the inductance quite substantially as well siliconchip.com.au as making it a non-linear component. Then solder and trim the leads. Mounting the crossover PCB While the crossover PCBs in the prototype Senators were installed behind the internal sloping panel inside the cabinet, we do not recommend this position as it would be virtually impossible to remove the PCB if a fault subsequently developed. Instead, we recommend mounting the crossover PCB in front of the sloping panel, on the floor of the cabinet, using four self-tapping screws. To connect the PCB, you need to crimp 6.3mm yellow female spade connectors onto the ends of the wires from the woofer and tweeter and plug these into the appropriate connectors on the PCB. We also need some 400mm-long spade-lug to spade-lug cables using spare speaker wire off-cuts to connect the input terminals on the PCB to the binding posts mounted on the rear panel of the speaker. If using the treble peaking switch, drill a hole through the rear panel and wire the switch up across one of the pairs of terminals marked on the PCB (ie, the middle pin and one of the upper pins). Alternatively, use a jumper shunt instead, shorting out the indicated pins to enable the treble peaking or placing it across the lower pins to disable peaking. Finishing off Your Senator speaker box(es) are now complete and almost ready for use. However, we do not recommend using them “flat on the floor” as this will tend to make the bass “boomy”. Raising them by, say, 100mm or so will virtually eliminate this problem and as a bonus, will raise the tweeters up to a level which is more in line with a typical listening position. Fortunately, Bunnings have an ideal solution to the problem, again intended for kitchen cabinets. We bought sets of their “leggz” 100mm cabinet furniture legs, as seen below. Each pack contains four legs so is suitable for one speaker box. Once fitted, they have the added advantage of being height-adjustable so can help fix any minor discrepancies in floor levels. You simply screw the legs to the outer corners of your speakers, in (say) 100mm from the sides and front. Sit back, relax with your favourite music SC . . . and enjoy! Bunnings’ “leggz” are intended for furniture use so are ideal for the Senator speakers. siliconchip.com.au Parts List – Senator Crossover (one required per speaker box) 1 PCB, code 01205141, 107 x 120mm (available from   www.siliconchip.com.au/shop) 1 2.7mH air-cored inductor; (Jaycar LF1330; see text) 1 M4 x 10mm machine screw and nut (Nylon, brass   or stainless steel) 6 PCB-mount 6.3mm spade connectors, 5mm pitch   (Altronics H2094) (CON1-CON6) OR 6 chassis-mount 6.3mm spade lugs plus M4 machine screws,     shake-proof washers and nuts 1 3-pin header, 2.54mm pitch (CON7)* 1 jumper shunt* 1 SPST or SPDT toggle switch* 1 2-way cable terminated with female header plug* 4 No.4 x 12mm self-tapping wood screws   * optional component for treble peaking network – see text Capacitors 1 4.7µF polypropylene capacitor (Jaycar RY6954) 1 3.3µF polypropylene capacitor (Jaycar RY6953)   (optional, for treble boost) Resistors 2 12Ω 10W 5%   2 3.3Ω 5W 5%   1 1Ω* 5W 5% Additional Parts For Speaker Connections 1 pair long binding posts, red & black (Altronics P2004/P2005) 8 yellow 6.3mm female crimp spade “quick” connectors   (Jaycar PT4707, Altronics H1842) 1 2m length heavy duty figure-8 speaker cable (eg, Jaycar WB1732, Altronics W2130) LOOKING FOR PROJECT PCBS? PCBs for most* recent (>2010) SILICON CHIP projects are available from the SILICON CHIP On-Line Shop – see the On-Line Shop pages in each issue or log onto siliconchip.com.au/shop You’ll also find some of the hard-to-get components to complete your SILICON CHIP project, plus back issues, software, panels, binders, books, DVDs and much more! Please note: the SILICON CHIP OnLine Shop does not sell complete kits; for these, please refer to kit suppliers’ adverts in each issue. * PCBs for some contributed projects or those where copyright has been retained by the designer may not be available from the SILICON CHIP On-Line Shop October 2015  81