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Add extra bass to your system with this Subwoofer Controller Adding a subwoofer to your home theatre or hifi system is the easiest way to extend the bass response. A relatively small speaker system driven by a big amplifier can give heaps of bass while not taking up a lot of space. This new Subwoofer Controller has all the features you could want, including low and high pass filters, parametric equaliser and auto-turn on. T he previous (and only) subwoofer controller described in SILICON CHIP was featured in the December 1995 issue. Since then we have had quite a lot of input from readers and this completely new design is our response to our readers’ comprehensive wish list. Adding a subwoofer to a home theatre or hifi system can achieve a dramatic improvement in listening siliconchip.com.au enjoyment, by extending the response of the system down into the low bass frequencies. But this improvement can only be fully realised if various basic conditions are met: 1. The crossover between your main system speakers and the subwoofer is smooth, with no obvious peak or dip in overall frequency response By JIM ROWE during the transition; otherwise the system will sound either boomy or weak in bass. 2. The subwoofer level is correctly balanced or matched with the level from the rest of the system speakers. 3. The response of the subwoofer itself is smooth (ie, without pronounced peaks or dips) over its operating frequency range. 4. Very low (subsonic) frequencies August 2007  57 cations ifi c e p S r e ll o tr n o C Subwoofer uts 47kW e level and LFE signal inp Lin r . .... .... .... .... e:. nc da pe with an 11:1 mixing divide Input im Speaker line inputs 10kW .... -8dB to +8dB, variable Gain:.................................. z between 41Hz and 200H Corner frequency variable 12dB/octave rolloff slope 12V DC supply from either a battery or a regulated mains plugpack. The current drain is modest too – less than 60mA when active. How it works You can get a good overview of the way the controller works from Low pass filter: the block diagram in Fig.1. As you can see, the source select switch z Hz and 200H 30 n ee tw be ble ria is right at the inputs, allowing va cy ....... Centre frequen at centre frequency B 2d ±1 n ee Parametric equaliser:.... you to choose between the left tw be ble ria Cut/boost va and right channel line outputs 5 ly Q approximate of your main amplifier if it has e tav /oc dB -18 them, from the speaker outputs if pe slo f rner frequency 15Hz, rollof Co . . . er: filt ic) it doesn’t, or from the LFE output on bs (su ss High pa of your surround sound decoder ut, inp S RM 1V dB unweighted relative to -80 .... or DVD player. .... .... io: rat ise no to Signal The line and speaker level 2V RMS output stereo inputs are each mixed together to produce a mono ..... 2.4V RMS Maximum output signal:.. signal for the Subwoofer Controller but the LFE signal is ts) tpu ...... 1kW (both ou Output impedance:.......... already mono so mixing isn’t ls required. na sig of d en er aft ....... Approx 11 minutes The signals are selected Amplifier hold-on time:.. by switch S1, then passed m a battery or regulated fro , DC V 12 through an input buffer stage m fro tes era .... Op Power requirements:........ which allows you to adjust ly. pp plugpack su , less de their level (and hence the mo by nd sta in mA 45 Current consumption subwoofer volume) for tonal . balancing. than 60mA in active mode The input buffer uses a feedback-type level control, which can either atselect between three possible sources tenuate or amplify by up are prevented from reaching the for the subwoofer signal: line level to 8dB either way, giving a 16dB subwoofer, as these can cause its outputs from your main amplifier; adjustment range which will be cone to ‘flap around’ – which can speaker level outputs or the ‘LFE’ more than adequate. cause unwanted noises and possible (low frequency effects) channel output Next the signals move to the low damage to the subwoofer. from your DVD player or surround pass filter stage which can be adjusted The Subwoofer Controller unit sound decoder; finally there are norbetween 41Hz and 200Hz. This alwe’re describing here caters for all mal and inverted subwoofer output lows you to ‘fine tune’ the crossover these conditions. It provides: signals, so you can easily use a stereo frequency where the subwoofer takes • A convenient adjustment of subamp to drive the subwoofer in bridge over from the main system speakers, woofer upper frequency rolloff, mode. to achieve the smoothest transition. so you can achieve the smoothest It also has an auto turn-on circuit to (This filter is not needed when you possible crossover transition. switch on your subwoofer’s amplifier are using the LFE signal to drive the • Easy adjustment of subwoofer level, automatically as soon as it detects subwoofer, so in this case you just for optimum overall tonal balance. the presence of audio signals. Then it set the low pass filter frequency to • A parametric equaliser circuit holds the amplifier’s power on while maximum, where it will have miniwhich allows you to compensate for ever audio signals are being fed to the mal effect.) any response peaks or dips which controller and only turns it off again Next is the parametric equaliser the subwoofer may have in its opafter waiting about 11-12 minutes from stage, which allows you to compensate erating range, to achieve a smoother when they are no longer detected. for any peaks or troughs (dips) in the response. So you no longer have to worry subwoofer’s own frequency response. • There’s also a built-in subsonic about remembering to turn on the It does this by allowing you to produce high pass filter, which rolls off the power to the subwoofer amplifier or a counteracting trough or peak at any response steeply below 15Hz to off again afterwards. frequency in the range from 30Hz to protect the subwoofer from damage. All of the controller’s circuitry fits 200Hz, and with an amplitude of up Three signal sources inside a compact low-profile instruto 12dB either way. ment case and operates from a single This should smooth out most In addition, there is the ability to 58  Silicon Chip siliconchip.com.au INVERTER LINE INPUTS L OUTPUT 2 R SPEAKER INPUTS L SOURCE SELECT INPUT BUFFER LOW-PASS FILTER PARAMETRIC EQUALISER BOOST HIGH-PASS FILTER (CF=15Hz) Q2 FREQ OUTPUT 1 R CUT LFE INPUT LEVEL Fig.1: block diagram of the Subwoofer Controller. likely peaks or dips in the subwoofer’s performance - provided that it’s in a reasonably damped enclosure. Following the equaliser, the signals pass through the subsonic high pass filter. This effectively blocks any rumble or other ultra-low frequency components which could cause trouble for the subwoofer, allowing only ‘genuine’ (above 15Hz) sub-bass signals to pass through unchanged. The output from the subsonic high pass filter becomes the main controller output signal for driving the subwoofer amplifier, while a simple unity gain phase inverter stage is used to provide the second ‘opposite phase’ signal (Output 2). As mentioned earlier this allows the use of a stereo amplifier to drive the subwoofer in bridge mode. A separate panel in this article explains the concept of amplifier “bridging”. The remaining sections of the con- ADJUST FREQ ADJ BOOST/CUT AUTO TURN-ON CIRCUIT Q3 ADJUST FREQ AUTO HOLD-ON & MUTING troller are used to perform automatic turn-on of the subwoofer amplifier when signals are detected at the output of the input buffer, and also to keep the subwoofer amp switched on until no signals have been detected for the ‘hold on’ time (about 11-12‑ minutes). The signal outputs from the controller are muted (by MOSFET switches Q2 and Q3) whenever the controller decides that there are no signals to be passed to the subwoofer. In more detail For more insight on how the various sections of the controller work, refer to the main circuit diagram of Fig.2. Again, you’ll find the input source selector switch S1 at upper left, with the selected signal passing through a 10mF non-polarised capacitor into the non-inverting (pin 3) input of op amp IC1a, the input buffer stage. The buffer is configured in a slightly SUBWOOFER AMPLIFIER POWER SWITCH unusual way, to allow its gain to be varied both above and below unity by potentiometer VR1. VR1 varies both the ratio of the input signal divider at pin 3 of IC1a and the gain of the buffer itself, by varying the negative feedback ratio. It does these actions in inverse fashion, to achieve the desired +/-8dB adjustment range. When the wiper of VR1 is in the centre position, the resistor ratios give the buffer an overall gain of unity (0dB). When the wiper is fully clockwise (ie, back toward IC1a’s pin 2), the input divider ratio is reduced while the negative feedback ratio is increased, giving an overall gain of approximately 2.5 (+8dB). Conversely, when the wiper of VR1 is turned fully anticlockwise (in contact with IC1a’s pin 3), the input divider ratio is increased while the negative feedback ratio is reduced, lowering the overall gain to 0.4 (-8dB). From the output of IC1a (pin 1) the signals pass to the adjustable low pass filter stage, based on IC1b. This is a standard Sallen & Key active low-pass filter with unity gain in the passband and a cutoff slope of 12dB per octave. Its corner frequency can be varied between 41Hz and 200Hz using VR2a and VR2b, two sections of a dual-ganged 50kW pot. Parametric equaliser Here’s the back end of the prototype Subwoofer Controller showing the inputs and outputs. It’s a bit different to the final version (the mains out lead is moved, for example) but this shows the basic arrangement. siliconchip.com.au The output signal from pin 7 of IC1b then passes to the parametric equaliser stage, based on IC2, a TL074 quad op amp. This is a ‘state variable’ filter circuit, using multiple feedback paths around two single-pole filter stages based on IC2b and IC2a. The use of both positive and negaAugust 2007  59 LINE INPUTS +12V 47k L IC1: LM833 CON2 R 10k L SPKR IN+ SOURCE SELECT S1 10 F 47k LPF CORNER FREQUENCY VR2a VR2b INPUT BUFFER 3 NP 2 L SPKR GND VR1 10k 100 IC1a 12k 12k 1 50k 50k 82pF 15k 1k R SPKR GND 100 CON1 4 LOW-PASS FILTER 100nF 10 F 100k LFE IN 7 IC1b 6 47nF 100nF 22k 8 5 LEVEL 10k R SPKR IN+ 100nF 47k +12V IC3: LM358 47k 100nF AUTO TURN-ON CIRCUIT 6 +6V 7 IC3b 5 10 F LL C B E G 47k 2 100k 3 10 F LL D1 2N7000 D K K 4 PN100 D2 A 8 IC3a 1 1M A S 220k 3.3M 1k 10k +6V LEDS 10 F IC6: LM358 K +12V A 6.8k ZD1 A 15 K 7 IC6b SC 2007 2 IC6a 1 4 100 F SUPPLY RAIL SPLITTERS K SUBWOOFER CONTROLLER Fig.2: the complete circuit of our new Subwoofer Controller. As well as providing the appropriate output for the subwoofer amplifer, it’s also capable of switching it off in the absence of audio signal. tive feedback results in a bandpass filter characteristic at the output of IC2b (pin 7) and this signal is mixed with the original signal from IC1b in IC2c, an inverting mixer stage with a gain of -1. Because the input of IC2d (pin 13) 60  Silicon Chip 6 15 8 K D1–D5: 1N4148 A 3 6.8k 1N4004 A 5 Using it in a car We designed the Subwoofer Control to run from a 12V supply so that it can be used in a car. However, it will not be able to switch the DC power to the subwoofer amplifier. If you intend to only use it in a car, you can leave out the solid-state relay (SSR1) and all the 240VAC mains wiring. is fed from the wiper of pot VR3 and the two ends of this pot are connected to the output of IC1b and IC2c respectively, the phase of the signal sent to IC2d is varied over a 180-degree range as VR3 is turned from one extreme to the other. As a result, the bandpass signal fed to IC2c can be made to either add to or subtract from, the original signal coming from IC1b. This results in boost or attenuation of the frequencies in the bandpass range, as desired. Dual ganged pot VR4a/VR4b is used siliconchip.com.au 10 +12V K 2200 F 82nF VR4b 50k IC2: TL074 4 22k 1 CUT/ BOOST 10 F VR3 10k 9.1k 150pF 82nF 22k 13 NP 12 9.1k 14 IC2d 11 +6V 3.3k +6V +6V VR4a 50k 47k CON4 22k 3 EQUALISER FREQUENCY 22k A 2 IC2a +12V IN ZD1 16V 1W 6 22k 7 IC2b 5 9 +6V 47k 8 IC2c 10 10 F EQUALISER D6 1N4004 +12V +6V A K 150pF 10 F 100 F 22k 220k 22k 220nF 220nF 220nF 33k 8 3 IC5a 2 12k 1 6 IC5b 5 IC5: LM833 NP 100 PHASE INVERTER +6V 4 OUTPUT 2 D G Q2 2N7000 10 F HIGH-PASS FILTER MUTING 1k 10 F 7 S CON3 OUTPUT 1 1k NP 100 D G Q3 2N7000 S +12V 1k D3 K 10k A 1M D4 K A 7 6 A STANDBY 100nF  LED1 K 8 4 2 10k 1 400 F LL (4 x 100 F) 10nF 100 K A 5 Q1 PN100 B 1k 10k E SSR1 ACTIVE LED2 E LOAD1 E A LOAD2  AUTO HOLD-ON & MUTING siliconchip.com.au +  C K to vary the centre frequency of the equaliser, while VR3 is used to vary the degree of boost or cut. When VR3 is in its centre position, the equaliser circuit provides unity gain at all frequencies. From the parametric equaliser stage N A D5 3 IC4 555 IEC MALE SOCKET 2200 F 10k 1.5M the signals pass to the high pass filter stage, based around IC5a. This is again a standard three-pole Sallen & Key active high pass filter configuration, with capacitor and resistor values chosen to give a corner frequency of 15Hz. This stage has unity gain in the passband – SUBWOOFER AMP POWER SWITCH A N 3-PIN MAINS SOCKET (ON CAPTIVE CORD) but with a rolloff slope of -18dB per octave, for signals below 15Hz. The controller’s main output signal is fed from the output of IC5a (pin 1) to the OUTPUT 1 connector via a non polarised (NP) 10mF capacitor and a series 1kW resistor, while IC5b is conAugust 2007  61 nected as a simple unity-gain inverter to produce the opposite phase output signal fed to the OUTPUT 2 connector – again via a 10mF/1kW series combination. As you can see, both output signals are effectively switched on or off (muted) by transistors Q2 & Q3. These are 2N7000 Mosfets which are controlled by transistor Q1 and in turn by the auto turn-on/hold-on circuitry based around IC3 and IC4. the inverting input (pin 6) of IC3b, via a 100nF coupling capacitor and series 100kW resistor (which forms a 2:1 voltage divider with the second 100kW resistor from pin 6 to the +6V bias line). IC3b also has positive feedback applied to it via the 3.3MW resistor connected from output pin 7 to Auto turn-on & muting The two sections of IC3 perform the signal detection and auto turn-on functions. The output signal from pin 1 of input buffer IC1a is fed to LFE IN CON1 SOURCE SELECT 47k S1 1x3 LINE INPUTS SPKRS 47k LFE 47k LINE INPUTS 10k CON2 RIGHT SPKR+ (GND BELOW) 1k 10k ROTOR 100 100 10k LIN VR1 82pF LS+ LS GND ECNARAELC LANIMRET 15k 17080110 7002 C REFOOWBUS RELLORTNOC + 10 F LEVEL RS GND REKAEPS ROF TUOTUC RS+ 10 F NP LEFT SPKR+ (GND BELOW) 22k 22k 10k LIN 82nF 1k 10k 12V DC INPUT 10 16V 4004 ZD1 2200 F 2200 F 9.1k 100 F LL 100 F LL SY-4089 10k SSR1 100 F LL 4148 IC4 555 1M 10k 4148 4148 1.5M 47k 220k D3 10k 4148 4148 IC3 LM358 100k 100k 6.8k 15 15 D2 D1 VR4 50k x2 1k 3.3M 1M + 100 F LL D5 IEC MALE SOCKET 100 Ain niA LL + + 100nF 100nF + 10 F ACTIVE Aout 10nF + 1k 100 F tuoA LED2 + LL 10 F AMP PWR FIT HEATSHRINK SLEEVES OVER IEC PLUG CONNECTIONS + IC6 LM358 10 F 6.8k NYLON P-CLAMP D4 9.1k 47k 1k STBY LED1 CON4 V21+ 82nF CON3 Q1 PN100 10k D6 V21+ VR3 150pF TL074 + FREQUENCY 10 F NP 22k V6+ EQUALISER IC5 LM833 47k IC2 10 F NP V6+ 22k 22k 22k CUT/BOOST 100nF OUTPUTS 150pF + 10 F 10 F 22k 220k 3.3k 10 F NP 12k 100 33k 100nF V6+ FREQUENCY VR2 50k x2 12k LPF CORNER Q3 Q2 2N7000 2N7000 220nF 220nF 220nF 100nF 47nF 100 1k IC1 LM833 12k 22k POT CASE EARTHING WIRE Fig.3: same-size PC board component layout to help you build the Subwoofer Controller. Note that the photo at right is similar but has extra holes for a compressor stage (we decided it was unsatisfactory) and the amp power lead is moved. 62  Silicon Chip siliconchip.com.au non-inverting input pin 5 and so it acts as a Schmitt trigger - producing a square wave version of the audio signal coming from IC1a as soon as that signal’s amplitude reaches its triggering level. The squared-up audio signal from IC3b is then fed to a simple rectifier circuit using diodes D1 and D2, which siliconchip.com.au effectively convert it into a ‘signal detected’ DC voltage across the 10mF capacitor connected from pin 2 of IC3a to ground. The 47kW resistor in series with D2 is used to set the ‘attack time’ of this control voltage (ie, how quickly it rises after the start of audio signals) to about 200ms (1/5 of a second). On the other hand, the 1MW resistor across the 10mF capacitor sets the signal detector’s ‘decay time’ – how long the control voltage remains high after the audio signals end, approximately 10 seconds. This is long enough to ensure that the control voltage stays high during short pauses in the audio. The DC control voltage developed across the 10mF capacitor and 1MW August 2007  63 resistor is fed directly to IC3a, again configured as a Schmitt trigger, because of the positive feedback from pin 1 to pin 3 via the 220kW resistor. So IC3a’s output pin 1, which remains at very close to the +12V level when no audio signal has been detected, suddenly switches to 0V as soon as a signal is detected. And it remains at 0V while ever the audio signals are present, only switching back to the +12V level about 10 seconds after they end. In short, the voltage level at output pin 1 of IC3a is high when there are no audio signals entering the controller but switches low as soon as signals are present. This output voltage from IC3a is used to trigger IC4, a 555 timer chip configured as a monostable, which controls the subwoofer amp power switching via solid state relay SSR1 and also muting transistors Q2 and Q3, via switching transistor Q1. Here’s how it works: Solid-state relay In the absence of audio signals and with the output of IC3a therefore staying high, IC4 is in its ‘off’ or reset state with output pin 3 held low and its internal discharge transistor (connected to pin 7) conducting, which keeps the 400mF of capacitance (4 x 100mF) connected between pins 6 and 7 and ground in the discharged state. Because pin 3 of IC4 is low, transistor Q1 is turned off and its collector voltage rests at about +6V (set by the two 10kW resistors). As a result both Q2 and Q3 are turned on, clamping both of the controller’s audio outputs to ground and hence keeping them muted. At the same time because pin 3 of IC4 is low, no current can flow through diode D5 and its series 100W resistor to activate solid-state relay SSR1. SSR1 therefore remains off, preventing the subwoofer amplifier from being powered up via the external 3-pin cord socket. When audio signals do arrive at the controller input, this results in the output pin of IC3a soon switching low. This sudden drop is coupled to the pin 2 trigger input of IC4 via diode D3 and the 100nF capacitor, with the result that IC4 immediately switches into its ‘on’ or set state. Output pin 3 rises to approximately +12V, which turns on both Q1 and SSR1. 64  Silicon Chip Muting transistors Q2 & Q3 are turned off, removing the muting from the controller’s audio outputs, while SSR1 switches on the power to your subwoofer amplifier. At the same time the discharge transistor at pin 7 of IC4, which has been holding the 400mF capacitance discharged, is now turned off. But the capacitance is not able to begin charging at this stage, because the voltage at pins 6 and 7 of IC4 is still held at a fairly low level (about +1.2V) by diodes D3 and D4, connected back to output pin 1 of IC3a - which is now held at ground potential. But when the audio signals do eventually cease (or strictly, about 10 seconds after this) and the output of IC3a switches back up to +12V, both D3 and D4 become reverse biased and stop conducting. This allows the 400mF of capacitance between pins 6 and 7 of IC4 and ground to begin charging, via the 1.5MW transistor connected to the +12V line. The charging is fairly slow due to the long time constant (T = RC = 400mF x 1.5MW = 600 seconds) but after about 11 minutes the voltage at IC4’s second threshold sensing pin 6 reaches its triggering level. IC4 then switches back to its reset state, with its output pin 3 going low. This turns off SSR1, switching off the subwoofer amplifier, and also turns off Q1 so muting transistors Q2 and Q3 are turned back on again to mute the controller outputs. There are two indicator LEDs in the circuit. LED1 is green, connected between the controller’s +12V line and ground via a 1kW series resistor so it lights whenever +12V is applied to the controller – becoming the ‘Standby’ LED. Red LED2 is connected across the output of IC4, again via a 1kW series resistor, so it only lights when the auto turn-on circuitry detects the presence of audio signals, and turns the muting off and the subwoofer amplifier on. So LED2 indicates when the controller and subwoofer are in the ‘Active’ state. Construction As shown in the photos, almost all of the controller circuitry and components are mounted directly on a single PC board which measures 200 x 156mm and is coded 01108071. The board mounts snugly inside a standard low-profile ABS instrument box which measures 225mm wide by 165mm deep by 40mm high. By the way, please note that the controller shown in the photos is our prototype which originally included a compressor stage. We subsequently omitted this because its noise and distortion were unsatisfactory. Hence the output circuit sections have since been moved nearer the centre of the board, as you can see from the board overlay and wiring diagram. You Inside the Subwoofer controller from the front, showing the mains wiring in particular. Note that this was modified in the final design (see the component layout) with a change to the output mains lead position in particular. siliconchip.com.au should use the wiring diagram as the main reference for component placement then, rather than the internal photos. The board wiring diagram is shown in Fig.3. First fit the fixed resistors. These all have their leads bent down to mate with PC board holes spaced 0.4” (10.2mm) apart, with the single exception of the 47kW resistor alongside IC2 - which mates with holes spaced 0.5” or 12.5mm apart. Don’t throw the resistor lead offcuts away because you can use them to fit the seven ~10mm wire links on the board. Follow these with the sockets for the various ICs, if you’re using them, and then the input and output connectors which are mounted along the rear of the board: CON1 and CON2, CON3 and CON4, then fit trimpot VR5, but not the main control pots at this stage. Next fit the various small nonpolarised capacitors, followed by the electrolytics. Begin with the 10mF caps because there are actually three different kinds of these used in the project: five of the standard polarised 10mF RB caps, two of the low-leakage polarised 10mF RBLL caps and five of the nonpolarised 10mF NP RB caps. So make sure you fit each type in the correct positions, marked on the wiring diagram with either a simple polarity ‘+’, a ‘+’ and an ‘LL’ or an ‘NP’ as the case may be. All of the remaining electrolytic and tag tantalum capacitors are polarised and must be orientated correctly, as shown in Fig.3. Once the passive parts are all in place, you can add the five 1N4148 diodes D1-D5, making sure to orientate them correctly, followed by 1N4004 diode D6 and zener diode ZD1, followed by transistor Q1 and finally Mosfets Q2 & Q3. Then you can fit the solid-state relay SSR1, which will only fit one way around. Now is the time to cut the shafts of control pots VR1-VR4 to about 10mm long, smoothing off any burrs so they’re ready to accept their knobs. Do the same with the shaft for switch S1, and while you’re doing this it would be a good idea to check S1’s stop washer position so it’s correctly set for only three switch positions. When the spindles of VR1-VR4 have been cut to length they can all be mounted in position along the front of the board. Note that VR1 and VR3 are both single 10kW linear pots, while VR2 and VR4 are dual-ganged 50kW units. Next you need to prepare your front and rear panels by drilling and cutting the various holes in them for the controls and connectors, if the kit supplier hasn’t already done this for you. Then or otherwise you can fit switch S1 to the front panel, at the left-hand end. After it’s in place you can solder the ends of four 30mm lengths of insulated hookup wire to the rotor lug and those for the first three positions, ready to Similarly, the view from the back panel. The exposed mains (ANE) wires should be as short as possible and anchored to the PC board, as shown in the component overlay, just in case the worst happens and one or more pull loose. siliconchip.com.au make the connections to the board when the panels and board have been assembled together. Now fit the IEC mains plug to the right-hand end of the rear panel, using two 10mm long countersink head M3 machine screws with star lockwashers and M3 nuts. Also fit the cable gland for the mains output cord into the next hole in the rear panel, with its large mounting nut on the inside. After this you can complete the rear panel assembly by fitting the four screw terminals used for the speaker level input connections. The two red terminals should go in the upper holes, while the black terminals go in the lower holes. When all four terminals are in place, carefully solder the ends of two 20mm lengths of tinned copper wire (or resistor lead offcuts) to the rear ends of the two lower terminals, and the ends of two 30mm lengths of insulated copper wire to the rear ends of the two upper terminals. These wires will be used to connect the terminals to the PC board when the rear panel is assembled to it. The next step is to attach the front panel to the board. This is done by removing the mounting nuts from the threaded ferrules of pots VR1-VR4 (but leaving on the flat washers), then offering up the panel until the spindles and threaded ferrules of pots VR1-VR4 pass through their matching holes. The nuts are then re-applied to the pot ferrules, and screwed up until they are finger tight. This will hold the panel and board assembly together while you make the connections from switch S1 to the board, using the four wires already soldered to the switch lugs. This is also a convenient time to solder a length of tinned copper wire to the top of the metal case for each of the four control pots VR1-VR4, with the end of the wire passing down through the hole in the PC board midway between VR1 and VR2, about 6mm from the board’s front edge. The wire is then soldered to the copper underneath, to make sure all four pots are earthed for minimum hum pickup and to prevent any hand capacitance effects. At this stage you can also fit the two LEDs at the right-hand end of the front panel, dressing their leads so they pass down through the board holes without strain. Make sure you have both LEDs oriented with their longer anode leads towards the left (i.e., towards VR4). August 2007  65 Parts List – Subwoofer Controller 1 1 1 1 2 1 1 2 2 5 5 1 1 1 3 1 1 1 2 Low profile ABS instrument case, 225 x 165 x 40mm PC board, code 01108071, 200 x 156mm Single pole 3 position rotary switch (S1) RCA socket, PC board mounting (CON1) Dual RCA sockets, PC-mounting (CON2, CON3) 2.5mm concentric DC socket (CON4) panel-mounting IEC male mains socket Screw terminal, red Screw terminal, black 16mm diameter knobs, black aluminium 8-pin DIL sockets 14-pin DIL socket Cable entry gland (for 3-6.5mm cable diameter) Plastic cable clamp (Jaycar HP-0754) 20mm lengths of 6mm OD heatshrink sleeving Solid state relay, 250V 2A zero voltage switching Length of 3-core mains flex with 240V outlet socket 150mm length of 0.25mm or 0.3mm tinned coper wire 10mm long countersunk head M3 machine screws with M3 nuts and star lockwashers 8 6mm long small self-tapping screws Semiconductors 2 LM833 dual low noise op amps (IC1,IC5) 1 TL074 quad op amp (IC2) 2 LM358 dual op amps (IC3,IC6) 1 555 timer (IC4) 1 PN100 NPN transistor (Q1) 2 2N7000 Mosfets (Q2,Q3) 1 16V 1W zener diode (ZD1) 1 3mm green LED (LED1) 1 3mm red LED (LED2) 5 1N4148 silicon diodes (D1-D5) 1 1N4004 1A silicon diode (D6) Capacitors 2 2200mF 16V RB electrolytics 2 100mF 16V RB electrolytics 4 100mF 25V RBLL low leakage electrolytics 4 10mF 16V RB electrolytics 2 10mF 50V RBLL low leakage electrolytics 4 10mF 50V RBNP non-polarised electrolytics 3 220nF 100V MKT metallised polyester 3 100nF 100V MKT metallised polyester 2 100nF 50V multilayer monolithic ceramic 2 82nF 100V MKT metallised polyester 1 47nF 100V MKT metallised polyester 1 10nF 50V multilayer monolithic ceramic 2 150pF disc ceramic 1 82pF disc ceramic Resistors (0.25W 1% unless specified) 1 3.3MW 1 1.5MW 2 1MW 2 220kW 2 100kW 6 47kW 1 33kW 8 22kW 1 15kW 3 12kW 7 10kW 2 9.1kW 2 6.8kW 1 3.3kW 6 1kW 5 100W 2 15W 1 10W 2 10kW linear pot, 16mm PC -mounting (VR1,VR4) 2 50kW x2 ganged linear pot, 16mm PC-mounting (VR2,VR3) 66  Silicon Chip When both LEDs are in place, invert the assembly and solder their leads to the board pads. Next you can offer up the rear panel assembly to the rear of the PC board, so the outer sleeves of RCA connectors CON1, CON2 and CON3 pass through their matching holes in the panel. You can then fasten the two together using two 6mm long self-tapping screws, through the centre mounting holes for CON2 and CON3. Once this has been done, you can connect the four wires from the rear of the speaker line input terminals to the board underneath. The shorter tinned copper wires from the lower terminals pass down through the rearmost holes, while the ends of the longer insulated wires from the upper terminals pass down through the holes nearer the front of the board (marked LS+ and RS+ in the wiring diagram). Then the assembly is inverted again and the wires soldered to the PC board tracks. The last wiring to be done is that for the subwoofer amp’s mains switching, just behind SSR1. This is not difficult to do, but you need to do the wiring carefully to eliminate the risk of electric shock. Begin by preparing the cable for the mains output cable. If this doesn’t have a cord-type 3-pin socket already attached to one end, fit the socket carefully in the manner recommended by the manufacturer. Make sure that the green/yellow wire connects to the Earth contact screw, and the brown and blue wires to the Active and Neutral screws respectively. Then slide the socket’s outer sleeve over the screws and click it into place to make sure it’s safe again. Next cut off a 50mm length from the other end of the mains cable, and carefully remove the outer sleeving so you can extract the 50mm length of wire with brown insulation. The other two lengths of wire can be discarded, but the brown wire should have 5mm of insulation removed at each end because you’ll be using it shortly to make the connection between the IEC plug’s Active lug and the hole in the PC board underneath. Now remove a further 45mm of outer sleeving from the other end of the mains cable, doing this carefully to avoid damaging the insulation on the three wires inside. Then remove about 5mm of insulation from the ends siliconchip.com.au of all three wires, tinning them lightly with the soldering iron so the strands are soldered into a compact group. The outer sleeve of the cable gland can now be unscrewed from the gland on the rear panel, and slipped over the free end of the cable. This end of the cable can then be pushed through the gland’s inner hole (from the outside, of course), far enough to allow you to make the cable connections. But before you make the connections, slide a 20mm length of 6mm heatshrink sleeving down over the blue and green/yellow wires, pushing them down as far as they’ll go. You’ll then be able to solder the blue wire to the Neutral (N) lug on the IEC plug, the green/yellow wire to the centre earth (G) lug on the plug, and the brown wire to the PC board - in this case by passing it down through the ‘Aout’ hole near the end of SSR1. Then when all three wires have been soldered (and the joints have cooled down), slide the heatshrink sleeves on the blue and green/yellow wires up and over the solder joints on the IEC plug and in fact over all exposed metal of the lugs, and then apply heat from your soldering iron or a hot air gun so the sleeves shrink down to atsiliconchip.com.au tach them in place. One end of your 50mm length of brown insulated wire can now be soldered to the remaining Active (A) lug on the rear of the IEC plug. After the solder cools, you can then slide your remaining 20mm length of heatshrink sleeving up from the free end of the wire and over the solder joint and any remaining exposed lug. Then heat the sleeving as before, to shrink it down around them and prevent accidental contact. Finally the free end of this wire can be passed down through the ‘Ain’ hole in the PC board below, and soldered carefully to the pad underneath. To complete this assembly stage, pull the mains output cord back through the cable gland until there’s only just enough cable inside the gland to avoid any strain on the soldered connections. Then slide the outer sleeve of the gland up the cable and thread it on the threaded ferrule, until the gland contracts enough to clamp the cable quite firmly. Do not forget to add the plastic cable clamp to provide further anchorage of the mains cord. It might seem superfluous but it is there to anchor the cord in case the soldered connections subsequently fail; this might cause the Active conductor to come into contact with the signal circuitry. Your board and panels assembly will now be complete and ready to lower into the lower half of the box, although before this is done you’ll need to cut off three of the spigots moulded into the box lower half, to clear some of the soldered joints under the board. The three spigots to be cut off are the one at centre front, to clear the joints under VR2; the one at centre rear, to clear the joints under CON3; and the one in the left rear corner, to clear the joints under CON1. All other spigots can be left intact. Once the three spigots are cut short, the board and panel assembly can be lowered into the bottom of the box with the ends of the panels sliding down in the slots provided for them. Then the assembly can be fastened in place using six 6mm long self-tapping screws, passing through the four mounting holes along the front of the board and the two along the back, all of which align with matching support spigots. Now you should be able to plug all of the ICs into their sockets, doing this carefully so that each one is orientated August 2007  67 correctly and none of their pins is bent out of shape. All that remains is to tighten up the mounting nuts for S1 and the four control pots on the front panel so they won’t work loose, and then fit the five control knobs. Your Subwoofer Controller should then be functionally complete and ready for a quick checkout. Checkout & adjustment To prepare it for checkout, first switch S1 to select the speaker terminal inputs, and also set pots VR1 (Level) and VR3 (Cut/Boost) to their midrange positions. Then connect the Controller’s DC input socket CON4 to a suitable source of 12V DC, such as a battery or a regulated 12V plug pack. Make sure that the plug mating with CON4 is wired so that the centre pin becomes positive. As soon as the power is applied, the only activity you should see is that Standby LED1 lights up. If it doesn’t light, you make have fitted it to the board with reversed polarity, so switch off and check this -- remedying it if necessary. The only other likely reason for LED1 not lighting up is that you’ve managed to connect the 12V supply with reversed polarity. If this is the case, there will probably be another sign: a small cloud of smoke arising from the 10W resistor just behind CON4, because this resistor will be dissipating about 10 watts of power and burning up. Obviously the thing to do in this event is switch off immediately, and reverse the DC input plug connections. If you do this quickly enough, the resistor may not need to be replaced. Assuming that all is well so far, you may want to use your multimeter to measure the current being drawn from the 12V supply. It should measure 45mA or less, with the Controller in its ‘standby’ state. If that’s what you find, your Controller’s circuitry is probably functioning normally. Now connect a source of line level (i.e., 250mV - 1V RMS) audio signals to the Controller’s LFE input socket CON1. The signals can be from an audio generator if you have one (set to say 200Hz), or otherwise the audio from a CD/DVD player or a radio tuner. Then switch S1 to its centre ‘LFE’ position, and within about half a second LED2 should light up to indicate that the auto power-on circuitry has detected the incoming signals, and switched the controller into its ‘active’ state. (If you are still monitoring the Controller’s battery current with your multimeter, this will show the current has increased to about 55-60mA.) If you then switch S1 back to the original Speaker Inputs position, LED2 should remain alight for about 11-12 minutes, showing that the auto hold- Resistor Colour Codes o o o o o o o o o o o o o o o o o o No. 1 1 2 2 2 6 1 8 1 3 7 2 2 1 6 2 5 1 Value 3.3MW 1.5MW 1MW 220kW 100kW 47kW 33kW 22kW 15kW 12kW 10kW 9.1kW 6.8kW 3.3kW 1kW 150W 100W 10W 68  Silicon Chip 4-Band Code (1%) orange orange green brown brown green green brown brown black green brown red red yellow brown brown black yellow brown yellow violet orange brown orange orange orange brown red red orange brown brown green orange brown brown red orange brown brown black orange brown white brown red brown blue grey red brown orange orange red brown brown black red brown brown green brown brown brown black brown brown brown black black brown 5-Band Code (1%) orange orange black yellow brown brown green black yellow brown brown black black yellow brown red red black orange brown brown black black orange brown yellow violet black red brown orange orange black red brown red red black red brown brown green black red brown brown red black red brown brown black black red brown white brown black brown brown blue grey black brown brown orange orange black brown brown brown black black brown brown brown green black black brown brown black black black brown brown black black gold brown on circuitry is also working correctly. But then it should turn off again, as the Controller switches back into Standby mode. Putting it to use The Subwoofer Controller is intended to connect into your audio system just ahead of the amplifier that you’re using to drive the subwoofer. If this amplifier is a mono one, you only need to feed its input from the Controller’s upper socket of CON3: Output1. However if you’re using both channels of a stereo amplifier to drive the subwoofer in bridge mode (for extra power), you’ll need to feed its two inputs from both of the Controller outputs. The input signals for the Controller will normally be derived from either the LFE output from your DVD player or surround sound decoder or ideally, from line level outputs on your main amplifier - assuming it has some. In this case you simply use a stereo RCA-RCA lead to connect the amp’s line level outputs to the two Controller inputs of CON2. In most cases it’s not really feasible to use an amplifier’s recording outputs by the way, because these generally provide signals tapped off before the main volume controls (so they are uncontrolled and ‘full bore’ all the time). You need line level outputs that are controlled by the main volume controls, so the balance you set between the main speakers and the subwoofer is not upset as soon as you adjust the main volume. If you don’t have controlled line level outputs available from the main amplifier, the alternative is to derive the signals for the Controller from the main amp’s speaker connections. This is again quite easy, involving a couple of lengths of light duty figure-8 flex connecting the main speaker terminals of the amplifier to the terminals on the rear of the controller. Just make sure you don’t reverse the connections at Capacitor Codes Value 220nF 100nF 82nF 47nF 10nF 150pF 82pF mF code IEC Code    EIA Code 0.22mF 220n 224 0.1mF 100n 104 .082mF 82n 823 .047mF 47n 473 .01mF 10n 103 NA 150p 150 NA 82p 82 siliconchip.com.au either end, or the bass components in the two signals (right and left) will subtract and cancel rather than add together. By the way the connections to the main amp speaker terminals won’t disturb the operation of the main speakers, because these Controller inputs are high in impedance – over 10kW – much higher than the speakers. There are also small resistors connected in series with the two ‘negative’ speaker inputs of the Controller, so connecting them both doesn’t create any significant earth loop. The only other connections required for the Controller are to allow it to control the power to the subwoofer amplifier. All that’s needed for this is to connect the amp’s power lead to the 3-pin socket on the end of the Controller’s mains cord, and then to supply power to the Controller’s IEC mains plug using a standard 3-pin plug to cable IEC socket cable, as used for most computers, peripherals and many other modern appliances. Note that the mains power supplied to the Controller’s IEC plug is used purely for running the subwoofer siliconchip.com.au amplifier. The Controller itself operates entirely from the external 12V DC source. Once everything is connected up, setting up the Controller doesn’t really involve a lot of fancy test instruments - although you could of course use instruments like a sound level meter if you have them. In most cases it will be quite sufficient to find the correct control settings by ear, using a suitable music CD or DVD movie soundtrack. The procedure is quite straightforward. First select the input signal source you’re using, via S1. Then set both of the Controller’s pots VR1 and VR3 to their midrange positions, and also both VR2 and VR4 to their midrange positions. You should then be able to hear the audio signal’s low bass components emerging from the subwoofer. If they’re either not audible or too low in volume, try turning VR1 clockwise until they do rise to a level which matches the higher frequency components from the main speakers. On the other hand if the low bass is already too high in level and tending to ‘boom’, turn VR1 anticlockwise until the subwoofer level comes down to match that of the main speakers. Should you find that the low bass is still too low in level when VR1 is turned fully clockwise, you will need to turn up the volume control on the subwoofer amp itself. But don’t turn it up any further than is absolutely necessary, because this may increase the risk of subwoofer overload on sudden bass peaks. Once you have the subwoofer’s overall level balanced fairly well against the main speakers, listen carefully to see if you can detect any ‘peak’ or ‘trough’ in the overall system response, in the transition region where the response of the main speakers is tapering off and the subwoofer is taking over. A peak will make itself evident as some residual ‘boom’ or over-loud sound, especially in the frequency range from 100Hz to 200Hz. On the other hand a dip will cause the bass to sound weak, especially in the same region of frequencies. If you believe you do have a response ‘peak’ in the transition region, try turning the LP corner frequency control VR2 slowly anticlockwise. This lowers the frequency where the August 2007  69 Amplifier bridging explained SUBWOOFER PROCESSOR LEFT INPUT OUTPUT 2 OUTPUT 1 SUBWOOFER (CONNECTED BETWEEN +VE L & R SPEAKER TERMINALS) STEREO AMPLIFIER LEFT AMP (STANDARD RCA-RCA STEREO CABLES) RIGHT INPUT RIGHT AMP + – – + + – Many readers wonder about the principle of amplifier bridging and how to do it. In effect, it allows the two channels of a stereo amplifier to drive one loudspeaker and thereby deliver maximum power. To do so, the loudspeaker must be connected to the two active (+) speaker outputs on the stereo amplifier, leaving the earth (–) outputs unconnected. We then feed the same mono signal to the amplifier inputs but the phase of one signal reversed. So if we have a 100W per channel amplifier (into 8W loads), the maximum undistorted signal available from each channel output will be 28.28V RMS or 80V peak-peak. However, if we consider that with our phase reversed input signal to one channel, the total voltage available across the loudspeaker will now be 56.56V RMS or 160V peak-peak. With an 8W loudspeaker, this equates to a maximum undistorted power of 400 watts RMS. We have illustrated the principle with the above scope screen shot. The two upper traces show the out-of-phase signals. The red trace is produced by the MATH function of the scope, with one signal subtracted from the other to give a resultant doubling in the peak-to-peak voltage. Mind you, amplifier bridging does not work quite this well in the real world. Few amplifiers can deliver four times their rated single channel power in bridge mode. Nor can few amplifiers deliver twice their 8W power into a 4W load from each channel which is exactly the situation here. While an 8W loudspeaker was suggested, the load “seen” by each amplifier channel will be 4W. This means that any amplifier to be used for bridging must be capable of driving half the loudspeaker’s nominal impedance from each channel. 70  Silicon Chip subwoofer begins to take over, which should reduce the peak. So stop turning VR2 as soon as the over-loud bass in the 100Hz-200Hz region seems to have gone. Conversely, VR2 is turned slowly in the clockwise direction if you seem to have weak bass, caused by a dip in the transition region. This increases the frequency where the subwoofer begins to take over, and hence allows it to ‘fill in the dip’. But again it’s a good idea to stop turning VR2 as soon as the dip seems to have gone, or you may well begin to create a bump. The Controller’s two remaining controls, VR3 and VR4, are mainly provided to allow you to compensate for any unevenness in the subwoofer’s own response. For example if it has an unpleasant response peak at a particular frequency – say 80Hz – you can use VR4 to tune the equaliser’s centre frequency to match the peak, and then turn VR3 anticlockwise to reduce the signal level at that frequency to smooth the overall response by cancelling the peak. Conversely if the subwoofer has a response dip at a particular frequency, you can use VR4 to tune the equaliser to that frequency and then turn VR3 slowly clockwise to boost the signal level at that frequency and again smooth the response by ‘filling in the dip’. This is the main purpose for the Controller’s parametric equaliser controls, then: lopping peaks or filling in dips in the subwoofer’s own response. However if they are really not needed for this, because you have a subwoofer with a particularly smooth response, the controls can instead be used for carefully extending the subwoofer’s low bass response a little. The idea here is to turn VR4 anticlockwise (to the 30Hz end), and then slowly turn VR3 clockwise to boost these very low frequencies relative to those above the subwoofer’s own cutoff frequency. You shouldn’t expect to achieve a dramatic extension in low bass response this way but if your subwoofer is already pretty good, you may be able to make it sound even better. Don’t overdo this extra sub-bass boost though, because the subwoofer might end up being overdriven and damaged. That could be very expensive. SC siliconchip.com.au