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Subwoofer controller with signal limiting & automatic power-on Have you ever wanted to really feel those extra bass notes from your favourite CD? Ever felt you missed out when watching that “Top Gun” video? Well here is a project that will control an amplifier/speaker combination to give that extra grunt in the bass department! The Subwoofer Controller presented here will not only control the level and frequency passed through to the sub bass amplifier, it also provides signal limiting, auto power on and off and an out-of-phase output for bridging a stereo amplifier for even more power. There are two front panel controls (cutoff frequency and level) and two LEDs (“Standby” and “On”. The cutoff frequency control is used to match the subwoofer controller to the actu- al speaker enclosure. Some speakers have a narrow range of opera­ tion, while others can handle a much wider bass range. In effect, the frequency control acts like a tunable crossover. The range of cutoff frequencies is demonstrated in the graph of Fig.4. The level control is used to set the output audio level, from the subwoofer speaker, to match the rest of the system. The compressor-limiter is basically an automatic volume control that only works when the input level is too high. This means that in the normal signal range, the signal goes straight through. Above a certain input level though, the gain is progressively reduced to prevent the sub­woofer amplifier from being driven into over­load. A built-in power-up circuit is incorporated to sense the audio input. When an input signal is present, the controller switches on a relay, which can be used to turn on the subwoofer amplifier. The relay switches off approximately 13 minutes after the signal is removed. The reason for the long delay is so that quiet patches in the program do not cause the unit to prematurely switch off. Circuit operation The circuit diagram of Fig.1 has four distinct sections comprising: (1) VOX and timer; (2) input conditioning and filter; (3) compressor and December 1995  39 40  Silicon Chip D1 D2 Fig.2: the component overlay diagram for the PC board. Leave sufficient lead length to enable to two LEDs to protrude through the front panel. output amplifier; and (4) the power supply. Input signals from the left and right channel source (usually the preamp output signals of a stereo amplifier) are mixed by resistors R2 and R29 and then fed via R4 to op amp IC2a which is wired as a Schmitt trigger. Its gain is very high so that virtually any input signal will drive it into clipping. Its output is fed to diodes D1 & D2 to develop a DC voltage across C8, a 10µF electrolytic capacitor. Op amp IC2b amplifies and inverts the voltage from C8 and its output pulls pin 7 of IC3 low, via diodes D9 & D8. IC3, a 555 timer, normally has its output (pin 3) high, so that transistor Q2 can drive the relay. As noted above, Fig.1 (left): the circuit has four distinct sections. IC2 & IC3 function as the VOX and timer, IC4 is the adjustable low pass filter, and IC1 is the signal limiter. Op amps IC7a and 7b provide out-of-phase outputs to drive a stereo amplifier in bridge mode. the relay is used operate the external subwoofer amplifier. To this end, the relay switches 240VAC to the 3-pin panel mount socket on the rear panel. LED1 indicates that the subwoofer amplifier is on. Transistors Q4, Q5 & Q6 are also driven by pin 3 of IC3 and are used to clamp the signal outputs to ground, when the unit is switched on and off. If the audio signal stops, at the end of a CD for example, the voltage across C8 drops to zero and so pin 7 of IC3 is no longer pulled low. C1, at pin 6, can now charge up and when it reaches about +8V, the output at pin 3 goes low, turning off Q2 and the relay. As noted above, this takes about 13 minutes. The mixed input signal from R2 and R29 is also fed into a 4-pole low pass filter comprising IC4, VR1 and associated compon­ents. This circuit has a low frequency cutoff at around 15Hz but has an option for a lower frequency cutoff by inserting jumper JP2 which then changes the input coupling capacitor to C28, a 10µF electrolytic. In normal practice though, there is little point in doing this as it will only add unwanted signals such as recorded rumble. IC4’s output is fed to VR2, the 10kΩ level control and then to IC1, an NE571 variable gain amplifier. IC5 and associated components are used to derive the gain control voltage for IC1. IC5 is wired as a window comparator, where the output goes low (pins 1 & 7) when the input (fed via C35) is above or below the thresholds set by resistors connected to pins 2 & 5. When a signal exceeds the threshold levels, the comparator switches low, turning on transistor Q3 and charging C19 rapidly via R26. The voltage across C19 is the gain control for IC1 and this causes IC1’s gain to fall so that high level signals are compressed but not clipped. IC1’s output is fed to IC7, a TL072 dual op amp. IC7a provides an inphase output to be fed to the external subwoofer amplifier, while IC7b provides an out-of-phase output if you want to drive a stereo amplifier in December 1995  41 bridged mode. In bridged mode, the two channels of the external stereo amplifier drive a single speaker system, connected between the two positive terminals of the amplifier’s left and right outputs. In this case, the amplifier’s full power will be delivered to the speaker but if it is an 8Ω speaker, you must make sure that the amplifier is rated to drive 4Ω loads because that it is the load that each channel will effectively “see”. Power for the controller comes from a 12.6V transformer which feeds a bridge rectifier (D4-D7) and a 1000µF capacitor. The filtered DC is fed to IC6, a 7812 12V regulator which powers the entire circuit. Note that there is no fuse in the power supply as the transformer is internally fused. Construction All the components with the exception of the power trans­ former are mounted on the PC board which measures 160 x 90mm (code K5562. PCB). The parts layout diagram for the board is shown in Fig.2. First check the board for any defects and fix them before proceeding. Then mount all the resistors and diodes. Follow with ICs Inside the subwoofer controller virtually all the circuitry is mounted on the PC board. The AC socket is used to power the external subwoofer amplifier. and electrolytic capacitors. Make sure that the diodes and ICs are the right way around. Now the rest of the components can be inserted and soldered. Having completed the PC board, let’s look at the base plate and mains wiring. All the details of the off-board wiring and the hardware details are shown on the diagram of Fig.3. You will need to attach two tapped metal spacers to the baseplate. These support the rear of the PC board when PARTS LIST 1 PC board code, (K5562.PCB) 2 jumper shunts and pin headers 1 12V relay (S-4170) 1 screened & punched front panel 1 screened & punched rear panel 1 3-core mains cord & moulded 3-pin plug 1 plastic instrument case, 203 x 156 x 69mm 1 cordgrip grommet 2 knobs (1 spline, 1 grub-screw type) 1 steel base plate 1 flush mount GPO socket 1 M-2852 12.6V transformer 2 binding posts (1 red, 1 black) 4 RCA sockets (chassis mount type) 2 solder lugs 1 3-way insulated terminal block 1 50kΩ dual gang 16mm pot (VR1) 1 10kΩ (log) 16mm pot (VR2) 1 50kΩ horizontal trimpot (VR3) 42  Silicon Chip Semiconductors 1 NE571 compander (IC1) 1 LM358 low power op amp (IC2) 1 555 timer (IC3) 1 TL071 op amp (IC4) 1 LM393 dual comparator (IC5) 1 7812 +12V regulator (IC6) 1 TL072 dual op amp (IC7) 1 BD139 NPN transistor (Q2) 2 BC558 (Q3,Q4) PNP transistors 2 BC548 NPN transistors (Q5, Q6) 4 IN914, IN4148 diodes (D1,D2,D8,D9) 5 IN4002 diodes (D3,D4,D5,D7) 2 5mm red LEDs (LED1, LED2) Capacitors 1 1000µF 16VW electrolytic 1 470µF 16VW LL electrolytic 2 100µF 16VW electrolytic 14 10µF 35VW electrolytic 5 1µF 63VW electrolytic 3 0.1µF metallised polyester (greencap) 2 .082µF greencap 1 .039µF greencap 1 .022µF greencap 1 .01µF greencap 2 .001µF greencap 1 180pF disc ceramic Resistors (0.25W, 5%) 1 3.3MΩ 1 12kΩ 1% 1 2.2MΩ 9 10kΩ 1 1.5MΩ 3 10kΩ 1% 2 1MΩ 1 8.2kΩ 1% 1 220kΩ 1 4.7kΩ 7 100kΩ 5 1kΩ 3 47kΩ 2 680Ω 2 39kΩ 1 560Ω 1 33kΩ 3 100Ω 1 22kΩ 4 0Ω links Miscellaneous Solder, PC pins, spacers, nuts, machine screws, washers, mains rated cable (three colours – brown, blue & green/yellow), shielded cable. Fig.3: note that the cases of the two pots must be earthed with a length of tinned copper wire which connects to the solder lug at one side of the PC board. Do not forget the 0.1µF capaci­tor between the main Earth solder lug and the output shield connection on the PC board. it is mounted. Feed the power cord through the relevant hole on the back panel (later it will be anchored with a cordgrip grommet). Connect the Active and Neutral wires to the insulated terminal block and connect the Earth wire to a solder lug which will be anchored by one of the transformer mounting screws. The base plate is secured to the case with four self-tapping screws. Place a solder lug underneath the lefthand front self-tapping screw. This will be used to earth the cases of the pots, with a length of tinned copper wire. For the front panel you will need to use two pot washers on the dual pot, VR1. Secure the front panel using the nuts for both pots and attach the knobs. The two LEDs should be poked through their respective holes in the front panel. This done, attach the PC December 1995  43 board to the spacers towards the rear of the case. The rear panel requires the flush mount GPO to be attached first, after which the RCA connectors and binding post terminals can be mounted. Slide the finished rear panel into the vertical slot at the rear of the case and then insert and secure the cordgrip grommet for the power cord. Wire up the rear panel according to the wiring diagram shown in Fig.3, making the connections as short and as neat as possible. Keep the shielded leads away from the transformer to avoid hum pickup. Check all wiring carefully before proceeding to test the unit. Testing To test the subwoofer controller you will require a multi­meter, an audio program source such as a CD player, an amplifier and a speaker. Connect the power and note that the power LED comes on. If not, check that the LED is the right way around. Then connect your multimeter between the anode of LED2 and the regulator heatsink tab and switch on again. There should be 12V present. Now connect an audio source to the input. Set jumper JP1 to the appropriate position (remove for line level input). The relay should click in after a short delay and LED1 should come on. If not check pin 1 of IC2. It should be between +4V and +8V. Pin 7 of IC2 should be close to 0V, with the input source on. It should go high (+11V) when the input signal is removed, after a delay of a few seconds. The timer will then start to time out and the relay should drop out after approximately 13 minutes. If these tests all check out, connect an amplifier and speaker. Feed in a music source, from a CD player, or the tape monitor output from your main stereo amplifier and have a listen. It will sound very muffled and boomy. Why? Because you are mainly listening to bass signals below 300Hz. If this checks out, the only adjust- AUDIO PRECISION FREQRESP AMPL(dBr) vs FREQ(Hz) 10.000 0.0 -10.00 -20.00 -30.00 -40.00 -50.00 -60.00 -70.00 10 100 1k 10k Fig.4: this is the range of cutoff frequencies provided by the subwoofer controller. Note that the actual gain of the circuit is set by the output level control VR2. Performance of Prototype Signal to noise ratio: 67dB unweighted with respect to 173mV in and 1V out. Total harmonic distortion: 0.05% at 40Hz and 1V out; 0.25% at 250mV out Input impedance: 10kΩ Output impedance:1kΩ Filter slope:12dB/octave Crossover frequency range: see Fig.4 ment to be made is to VR3. This is to set the maximum level to the sub­woofer amplifier. It should be set just below the clipping level of the amplifier. If you have a signal generator, then connect this to the input and set it to around 100Hz. Rotate VR2 fully clockwise. Connect your digital multimeter (set to a low AC voltage range) across one of the RCA output sockets (either one) of the controller and adjust VR3 Where to buy a kit of parts The Subwoofer Controller was designed by Altronics and they own the copyright. The kit is available in two forms. The short form kit, comprising the PC board and all the on-board components, is $49.00 (K-5562). The full kit, including the case with screen printed and punched front and rear panels, is $99.00 (K-5563). These kits are available from Altronics in Perth (phone 1 800 999 007) or from any of their interstate resellers. 44  Silicon Chip 18 SEP 95 15:02:59 to the rated sensitivity of the amplifier, typically 1V RMS. Note: do not do this adjustment with the external sub­woofer amplifier connected as it will drive it to full power or beyond. If you do not have a signal generator, you can do the ad­justment with your subwoofer amplifier connected but you will need to limit the power delivered to the subwoofer itself. To do this, connect a 100W 240VAC lamp in series with your speaker. The cold resistance of the lamp will be around 50Ω or thereabouts and this will safely limit the power although it will still be more than adequately loud while you are driving the subwoofer amplifi­er to full power. Now connect a CD player and select a disc with plenty of bass present. Adjust VR3 until clipping is heard from the speaker. This will sound like a buzzing or high distor­tion of the bass signal. Back off VR3 slightly until the clipping is no longer present. The best signal source for the controller is a line level output, derived from just after the volume control in your stereo amplifier. Note that “Tape Out” and similar outputs are unsuit­ able, as they are not volume dependent; ie, the signal from these points does not vary with the volume control. Alternatively, take the signal from one of the speaker outputs on SC your stereo ampli­fier.