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OFF ~ ON POWER VOL UME Build this mini amplifier for personal stereos Have you ever wanted to use your personal stereo player with speakers instead of those tiny and annoying headphones? This little project allows you to do just that. It uses only one low-cost IC and a few external components. By DARREN YATES Personal stereos have revolutionised the way we listen to our favourite music and radio stations. Not only can we now listen to Beethoven on the train going to work but also while jogging, studying for exams and during those tedious hours at work (oops .. not that it happens here, Boss!) - in fact, just about anywhere we want. If you stop and think about it, you can now buy a sensitive FM stereo receiver with a stereo cassette deck that fits easily into the palm of your hand - and many now come with a 48 SILICON CHIP 3-way graphic equalizer and Dolby noise reduction. Some even feature auto-reverse for the cassette deck as well. About the only thing the personal stereo lacks is a couple of pint-sized speakers that plug in when you've had enough of those tiny earphones poking holes through your noggin. These days, there are a few amplified speakers available but most of these come with an equally amplified price-tag to boot! Of course, many solutions to the problem have been found, including mounting a couple of LM386 audio amplifier !Cs on a board to drive a couple of small loudspeakers. The only problem with this approach is that you have to use two !Cs and these inevitably take up a fair amount of board space. "Well, what about stereo amplifier chips?" I hear some of you ask. There are a few of these around, some of which have already been used in previous projects. In the Portable AM Stereo Radio project published in September 1989, we used a TA7376P chip, which was a single in-line (SIL) package, but it was physically a bit on the fragile side if you weren't careful. The next chip we used was National Semiconductor's LM831 . This time the package was a 16-pin DIL, but it didn't have the same output power as the LM386. Its main prob- OFF 47 16VW L1 + 0 ~: o v I gvoc PLUGPACK TDA2822M 470 16VW l • v ~ R I G HOUTPUT T 10n 0.1 ! L2 INPUT S TEREO, L3 vtErr,1E ~ - - - - - f VR1b 10kLOG -:- 470 alf 10n 0.1 L1, L2 AND L3 • 6T, 0.68mm ENCW, AIR CORED WOUND ON A 5mm DIA. MANDREL i ~ V OUTPUT LEFT I PARTS LIST 1 PC board, code SC01110921, 60x 37mm 1 TDA2822M 1W + 1W stereo amplifier IC 1 plastic utility zippy case, 131 x 68 x 45mm 1 front panel label, 124 x 62mm 1 9V 300mA DC plugpack 1 miniature toggle switch 3 mono 3.5mm sockets 1 3.5mm stereo socket 2 3.5mm stereo plugs 4 rubber feet 2 10Q 0.25W 1% resistors 1 miniature 10kQ logarithmic dual potentiometer 1 knob to suit potentiometer Capacitors MINI STEREO AMPLIFIER Fig.1: the circuit is based on the TDA2822M stereo amplifier chip from SGS. Inductors Ll, L2 & L3 ensure that the RF signal picked up by the input leads is not shunted away by the amplifier inputs, since this signal is used by the tuner. 2 470µF 16VW electrolytics 2 100µF 16VW electrolytics 1 47µF 16VW electrolytics 2 0.1µF 63VW MKT polyester lem is that NS has now discontinued it - strike out one IC. Then we came across a tiny 8-pin DIL device, the same size as the old LM386, but with two amplifiers packed inside, capable of delivering 1W into 8 ohms at 9V. Delving into the specs a bit more revealed that it would also work down to 1.8V - a definite · plus over most other ICs - and that it has only 0.2% distortion, which is pretty good for such a tiny package. The device in question is the TDA2822M. Miscellaneous Circuit diagram Let's take a look at the circuit diagram in Fig.1. As you can see, there's very little to it. The input signal comes direct from your stereo player into the two volume pots VRla and VRlb. Notice that it also comes via three aircore inductors. The reason for these is that if your personal stereo has an AM/FM radio built in, it uses the headphone leads as the antenna. The inductors prevent the RF signal from being shunted away by the amplifier inputs. A dual-ganged potentiometer provides the volume control for both channels and we have omitted a balance control. This saves a little money on a control which is very rarely used on any stereo amplifier. The stereo signal passes into ICla for the right channel and IClb for the left. The gain of both amplifiers is set internally to 100 (40dB), while the low frequency roll-off is set by the lO0µF capacitors at pins 5 & 8. The output is coupled to the speakers by two 470µF capacitors (pins 1 & 3) and each amplifier has a Zobel network consisting of a 0. lµF capacitor and a lOQ resistor, which prevents any high frequency instability from occurring. The supply voltage can be anywhere from 1.8V to 12V, with obvious improvements in sound quality and output power available at the higher voltages. At 3V, the IC will put out Hookup wire, screws, nuts, solder. more than 100 milliwatts per channel into 4Q, which is quite adequate for personal listening. And that's about it as far as the circuit workings are concerned. As we said before, there's very little to it. Construction All of the components for the Mini 9VDC PLUGPACK RIGHT OUTPUT Fig.2: mount the parts on the PC board & install the wiring as shown in this diagram. Inductors Ll, L2 & L3 are made by winding six turns of0.8mm enamelled copper wire on a 5mm former (eg, the shaft of a drill bit). OCT0BER1992 49 Once the coils are in, you can solder in the resistors and the capacitors. Again, follow the wiring diagram carefully to make sure that each part is correctly located and take care with the polarity of the electrolytic capacitors. The last item to be mounted on the board is the TDA2822 IC. Note that pin 1 of this device is adjacent to a notch or dot in one end of the plastic body. Once the board is complete, check it thoroughly for any errors such as solder splashes across tracks or between IC pins. When you're happy with it, you can begin work on the case. Installing the board The PC board is secured to the bottom of the case, using machine screws & nuts, with extra nuts used as spacers. Use rainbow cable or light duty hook-up wire for the wiring connections & take care to ensure that the supply is wired with the correct polarity. Amplifier (except for the volume control, on/off switch and sockets) fit onto a small PC board measuring 60 x 37mm (code SC01110921). Before you begin construction, check the PC board carefully for any shorts or breaks in the copper tracks. If you find any, use a small artwork knife or some wire links as appropriate to fix the problem. Next, you can wind the three coils. This is really quite easy. All you need to do is get a 5mm (or 7/32-inch) drill ~ fl!#- bit and wind on six turns of 0.8mm enamelled copper wire. Make sure that you wind the turns close together and reasonably tightly, but don't make the turns too tight otherwise you won't be able to get them off the drill bit. When you've wound all three coils, scrape the enamel off the ends and solder them to the board. Check the overlay wiring diagram (Fig.2) to make sure you insert them into the right holes. MINI STEREO AMPLIFIER Before iI.1stalling the board into the zippy case, you will have to drill holes for the volume control, the power switch and the four 3.5mm sockets. We suggest that you drill some small guide holes first and then use a tapered reamer to enlarge the holes to suit. When you go to fit the 3.5mm sockets, you will probably find a small problem. The threaded bush of each socket isn't quite long enough to protrude sufficiently through the plastic case and hence you won't be able to secure the nut. The solution is to use an 8mm drill bit and slightly countersink each hole by hand. This will provide more access to the thread and allow you to get the nuts on. Once all the hardware has been mounted, the wiring can be completed using light-duty hook-up wire or rainbow cable - see Fig.2 . Finally, the board can be secured to the bottom of IO SCOl110921 - ~~ ! OFF 0 ON Q MIN POWER MAX VOLUME Fig.3: the front panel & PC board artworks. Check the board for etching defects before installing any of the parts. 50 SILICON CHIP 0 I the case using 3mm machine screws and nuts, with extra nuts used as spacers. Connecting plugs The next step is to wire up the connecting cables. Let's start with the signal cable that connects your personal stereo player to the amplifier. This needs to be a 1-metre length of figure-8 shielded cable fitted at either end with a 3.5mm stereo plug. You can either purchase two stereo plugs and wire them together or you can save time by buying the complete lead; eg, Arista Cat. No. AL161 for $3.95. Next, you can work on the speaker cables. Alternatively, you could buy a pair of small speakers which already have the cables connected. If you're "rolling your own", make sure that you connect the positive and negative speaker terminals to the same terminals on each of the two 3.5mm mono plugs. You should use some light duty (figure-8) speaker cable for this. Here's what the PC board looks like close up. Scrape away the enamel from the leads of the inductors before soldering them to the board & make sure that all polarised parts are correctly oriented. amplifier's current drain by connecting your multimeter (set to the ZOOmA range) in series with the 9VDC supply and one of the supply rails. With no input signal, you should find that the current drain is about lOmA or so. Ifit is any more than this, switch off and check that you have the circuit wired up correctly. When Testing The test procedure is quite simple. First, it's a good idea to check the the speakers are connected, the "no signal" current should still be the same, at about lOmA. Next up, plug in your personal stereo receiver and set the volume control on it to about halfway. Now increase the volume on the stereo amplifier and you should be able to hear the fruits of your labour. SC Electronics Workbench® EmonaLAB4 Four instruments in one package! The electronics lab in a computer! • aocr, .... 66600666 The LAB4 has four full function test instruments: ■ DC POWER SUPPLY Triple output; 5V, 15V, 0-50V ■ with neatly laid out and labelled front panel controls. Also, the LAB4 is compact and only takes up a small space, compared to four bench-top instruments. DIGITAL MULTIMETER Full function, auto-ranging ■ FUNCTION GENERATOR 0.02Hz-2MHz, with SWEEP ■ FREQUENCY COUNTER 1Hz-100MHz input range INDEPENQENTINSTRUMENTS Each instrument in the LAB4 is completely independent and can be switched ON or OFF as required, The LAB4 has only one 240V mains input, saving on multiple power points. Each instrument is clearly defined A simple, intuitive and very powerful teaching tool, Electronics Workbench lets students and hobbyists design and test both analog and digital electronic circuits, without the delays and expense of a laboratory. FEATURES EMONA INSTRUMENTS NSW ......................... (02) 519 3933 VIC ............................ (03) 889 0427 OLD .......................... (07) 397 7427 DISTRIBUTORS Perth Adelaide Hobart ............... (09) 244 2777 ........... (08) 362 7548 (002) 34 2233 • Quick and simple circuit entry • Digital and Analog Modules included, complete with all components • Simulated instruments: dual trace scope, spectrum analyser, function generator, multimeter, digital word generator and logic analyser • Complete control over all component values and parameters • Print: circuit schematics, parts list, instrument readings, macros • Logic conversion - truth table to Boolean formula to logic gates • Customisable hypertext help system 0 M. . I THREE VERSIONS • Professional Version: EGA/VGA colour display; unlimited components • Personal Plus Version: Monochrome display; unlimited components • Personal Version: Monochrome display; limited to 20 components per .. EMONA INSTRUMENTS NSW ......................... (02) 519 3933 VIC ............................ (03) 889 0427 OLD .......................... (07) 397 7 427 DISTRIBUTORS Perth .............................. (09) 244 2777 Adelaide .... (08) 362 7548 Hobart (002) 34 2233 OCTOHEH 1992 51