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HASIB'S ARTICLES AND AUDIO PROJECT REVIEWS
120-WATT (4 OHM) PLATE SUBWOOFER POWER AMPLIFIER Introduction This project was intended for amplifying subwoofer. I’m too lazy to build a case just to put one channel of power amplifier. After searching the new ideas from commercial active subwoofers – I used their approach that mounts the power amplifier including the controller at rear panel. The idea is good because no need more outside world connections - just put in the signal/AC main and you’re ready for bass business. This built in power amplifier/EAS subwoofer controller now feeding my 12” sealed subwoofer system. Subwoofer Controller I use a circuit diagram that based from ESP page (EAS subwoofer controller - Project 48). I already built it for my stereo subwoofer evolution and it works great. The original circuit is mono (stereo L and R summed together). From the original circuit I have done some modifications (for this project). Here there are:
I have several reasons by doing these. I have measured my main speaker F6 – 38-Hz. So I must change R5/R8 and VR1 to higher value. The new frequency response (using 22-kohm resistors and 50-kohm potentiometer) is 22-Hz to 72-Hz. Lower the 4.7-uF? Oh, that’s because I don’t have 4.7-uF handy J. Power Amplifier The power amplifier also based from ESP Project 3A (60-watt power amplifier). My subwoofer driver has a load impedance of 4-ohm so I can safely said that the amplifier output can put out close enough to 120-watt. Read the description on this project for full detail from ESP page linked below. I laid down all the components on a piece of strip board for EAS controller, power amplifier and power supply section excluding bridge diode and power transformer. I build the circuits as compact as possible to save space as minimum as I can manage. The circuits will well behaved if you separates the circuits nicely (high/low impedance block etc) – mine appears no problem. I have lower quiescent current to 50-mA (voltage measured between output transistors collectors is 47-mV) because the heatsinks I used were too small – only 370-cm square @ 2.6 degree Celsius/watt. I don’t recommend anyone to do this; this will shorten the component life especially power transistors. Listening to moderate level the heatsinks goes warm but still not too hot. As an experiment I have higher the quiescent current to recommended value 100-mA, as I have expected I can’t even touch the heatsinks even for 5 seconds – very very hot. Power supply I use simple power supply for powering the power amplifier/subwoofer controller. The transformer is 200-VA, a 35A bridge diode and two 4700-uF 50V electrolytic capacitors. I use heavy gauge wires for connections and keep them short as possible. Note: I know that two 4700-uF electrolytic capacitors are too small for this project but I can’t make it larger because of limited space. Supply for EAS controller needs a regulated +/- 12-volt and again it simple. I just use 680-ohm 5-watt dropping resistors, 12-volt 1-watt zener diode, 470-uF 35V electrolytic capacitors and two 100-nF Mylar capacitors. I didn’t use voltage regulators (say 7812/7912) because it tends to be pricey and need a lot of mounting space. Also two TL072 op-amps didn’t need high current to operate. Module Arrangements/Constructions I laid out all circuits as compact as possible. Heatsink(s) must be installed outside because it(s) will run cooler. I mount the circuit board behind the heatsinks, this is just the location that make sense because I can make the wiring for power transistors shorter and not eaten too much space. At front panel of plate amp, there are only a control for gain, frequency cut-off, phase switch, AC power cord, panel mounting fuse, power switch, power indicator, input (stereo) and the heatsinks. Module panel was made from 3-mm and 11-mm plywood laid together (nailed and glued). I cannot find an aluminum sheet that thick enough to make it as module panel. If I use thin aluminum sheet the force that bouncing inside the subwoofer box will make the module damaged (remember the subwoofer box is sealed). Also take care to mount the EAS controller/power amp module to plate amp surface as strong as possible. Before this I just use double tape and some hot glue, after I open back (to take this picture) the circuit PCB was fell off (caused by bouncing air) – luckily it doesn’t caused short circuit. Lastly I use a lot of hot glue and screws. Because the box is sealed I must take caution to keep the module airtight – air must not leak from the plate amp panel i.e. heatsinks, gain/frequency control, power switch etc. I use lots of hot melt glue for sealing process. The panel painted with flat black spray paint. Behind the plate amp module (between plate module and subwoofer back panel) I put double-sided foam tape to keep the joint airtight. Lastly I use ten of 1 ½” wood nails for securing the plate module to subwoofer back panel. Markings were done using Microsoft Paint (invert color mode). Well, that’s all. Simple isn’t it? It is! Cost for building this project is around RM150.00. Again it is simple and cheap to build. The sound – well it sound like a subwoofer (a real one). After considering the total cost for this project (around RM300) including the subwoofer driver/box I can safely said I have just made the commercial subwoofer goes dead. What you can expect from RM300 commercial subwoofer – 8” driver, variable low-pass “subwoofer controller”, “100-watt” power amplifier that uses STK chip, puny power supply and so on. If you have tools handy and a willing to do this project – just go on, you will be VERY happy (just like me). From the photo I supplied you can see the completed module – inside and outside, mounted and not mounted. Maybe I have give you some ideas from this project, to modify this original plan is up to you. Enjoy. Completed Project Pictures
Fig1 - 140Watt Power Amplifier (Rear Views)
Fig2 - 140Watt Power Amplifier (Internal Views)
Fig3 - Complete Views 140W Mounted On Subwoofer Unit Link to ESP Page 1. Project 48 – Active subwoofer and controller 2. Project 3A – 60-watt power amplifier Muhammad Hasib bin Md. Nor 2002 Any Comment And Question Email to : hasib_tangkak@yahoo.com
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