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A compact audio powerhouse This rugged, single channel amplifier module comes complete with power supply and a fan-cooled heatsink. It is based on the once-popular “Pro Series One” originally featured in “Electronics Australia” magazine and now re-designed by Altronics. 200 Watt Mosfet by LEO SIMPSON W e have had quite a few en- power output of 140W into 8Ω and TO-3 metal cases. These are no longer quiries from readers who 200W into 4Ω. Frequency response available and their plastic equivalents want to build a Mosfet am- is within 1dB from 20Hz to 80kHz are quite difficult to obtain as well. (Fig.1). Total harmonic distortion is Altronics looked at this situation plifier with a rating of about 200W. We had not designed such a module rated at less than 0.1% up to full power and have used essentially the same (Fig.2) and signal-to-noise ratio with circuit designed around some equivand as has been mentioned previalent plastic Mosfets made by Exicon ously in SILICON CHIP magazine, our respect to 200W is better than 100dB of the UK. This preference has been to has necessitated design high performance Performance of Prototype a re-design of the amplifier circuits around PC board so that all bipolar transistors rather Output Power (RMS):.... 140W into 8 ohms; 200W into 4 ohms the Mosfets and the than Mosfets. Frequency Response:.. 20Hz – 80kHz at -1dB points (see Fig.1) driver stage tranHowever, many people sistors all line up prefer Mosfets because Input Sensitivity:........... 830mV for 200W into 4 ohms along one edge, alof their legendary rugHarmonic Distortion: .. <0.1% (20Hz – 20kHz) (see Fig.2) lowing them to be gedness. Altronics had a mounted vertically Mosfet amplifier module Signal-to-Noise Ratio:.. >102dB unweighted; 105dB A-weighted on the heat-sink. which produced 200W             with respect to 200W into 4 ohms Apart from usinto a 4Ω load and so we Stability:........................ Unconditional ing plastic power decided to take a look it. transistors which It turned out to be based greatly simplify on the “Pro Series One” as mentioned unweighted. mounting compared to metal TO-3 above, although this version by AlOriginally, the “Pro Series One” power transistors, Altronics have emtronics has been derated and adapted was based on Hitachi Mosfets with ployed spring clips to mount adjacent to different Mosfets. It has a rated 32  Silicon Chip www.siliconchip.com.au Amplifier Module Ideal amplifier for:  ✪ hifi   ✪ subwoofer  ✪ public address  ✪ guitar transistor pairs, to make things simpler again. The spring clips apply just the right amount of tension to the transistors and there is no danger of damaging a transistor due to over-tightening a mounting screw. The heatsink is a black-anodised aluminium extrusion with fins on one side. It measures 300mm long and is fitted with a cover which allows it to be cooled by an 80mm 24V DC fan. The fan runs continuously and this means that the heatsink is always cool (or at least, at little above ambient temperature). A really attractive feature of the module is that it comes with its own power supply, consisting of a 300VA toroidal power transformer and accompanying power supply board. The bridge rectifier is mounted on the same vertical heatsink as the amplifier and so it also gets the benefit of continuous fan cooling. The whole assembly is mounted on www.siliconchip.com.au a sheet of black enamelled steel measuring 300 x 214mm. This could be built into a larger chassis for a PA system, stereo system, active sub-woofer or whatever. Easy connections Both the amplifier and power supply boards are designed for easy connection to supply wires, input and output lines and so on, by virtue of the insulated terminal blocks lined up along one edge. The particular advantage of this feature is that it makes it very easy to connect and disconnect the boards, without any need to resort to the soldering iron. Circuit details The circuit of the amplifier appears to be loosely based on an original Hitachi design produced about 20 years ago but it still rates as a very good design today. Fig.3 shows the circuit. Q3 & Q4 are the input differential pair with the input signal applied to the base of Q3 via a 0.47µF capacitor and 1kΩ resistor. Negative feedback is applied to the base of Q4. Q3 and Q4 amplify the “difference” between the input and negative feedback signals and the output signals appear at their collectors. Q1 & Q2 form a constant current “tail” circuit for the differential pair, ensuring that it has high gain and good common mode rejection ratio. Following the input differential pair of Q3 & Q4 is the voltage gain stage involving differential pair Q5 & Q6 and their balanced current mirror load of Q7 & Q8. The bases of Q5 and Q6 are driven from the collectors of Q4 & Q3 respectively and their collector loads are provided by Q7 & Q8, the current mirror. Current mirror loads for differential gain stages are widely used in op amps as they give very good linearity (ie, distortion free). In fact, most of the August 2001  33 AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz) 10.000 06 MAY 100 05:15:08 AUDIO PRECISION SCTHD-W THD+N(%) vs measured LEVEL(W) 10 06 MAY 100 04:52:33 8.0000 6.0000 1 4.0000 2.0000 0.1 0.0 -2.000 0.010 -4.000 -6.000 -8.000 0.001 -10.00 .0005 10 100 1k 10k 100k 200k Fig.1: as measured in the SILICON CHIP laboratory, frequency response is a very wide 20Hz-80kHz (-1dB). distortion in this amplifier will be generated in the Mosfet output stage although most of this is then corrected by the negative feedback applied back to the base of Q4. The output of Q6 drives the gates of Mosfets Q9, Q10, Q11 & Q12 which 0.5 1 10 100 300 Fig.2: maximum power output before it “hits the wall” is about 220W into 4Ω at <0.1% distortion. operate as complementary source followers, ie, with a voltage gain of a little less than unity. It may appear that Q6 drives Q11 & Q12 while Q8 drives Q9 & Q10 but Q6 is the driver while Q8 is merely part of the current mirror load for the second differential amplifier stage. The gates of Q9 & Q10 are “separated” from the gates of Q11 & Q12 by the 200Ω trimpot RV1. The trimpot actually provides a small amount of forward bias to the gates of the Mosfets so that they are always conducting Viewed from a steeper angle than the previous photo, this gives a good idea of the layout of all the major components in the amplifier. Note the insulated cover over the fuse in the foreground: this is essential for your safety. 34  Silicon Chip www.siliconchip.com.au Fig.3: the circuit is relatively conventional, using four power Mosfets in the output stages. Q13 and 14 form an overload indicator, detecting the large spikes generated at the onset of clipping. to some extent. This is done so that the effects of crossover distortion are minimised. Readers may wonder why trimpot RV1 does not have an associated NPN small signal transistor across it to function as a “Vbe multiplier”. This is required in Class A, AB and Class B amplifiers using bipolar output transistors to ensure that the quiescent current is thermally stable. Without such a transistor to monitor and continually adjust the bias setting, the quiescent current just www.siliconchip.com.au continues to rise until eventually, the output transistors get so hot that they are destroyed. This is called “thermal runaway”. So why don’t Mosfet amplifiers need this same transistor? It is not true to say that Mosfet circuits cannot experience thermal runaway in particular circumstances but generally the power Mosfets used in high-quality audio power amplifiers have a negative temperature coefficient for currents above a particular level, typically 100mA. Above that current, if the Mosfet gets hotter, it tends to throttle back its operating current and thus reduce its dissipation and so there is no tendency to thermal runaway. We’ll discuss the quiescent current setting later in this article in the section on testing and setup. The drive signal to the gates of the four Mosfets is limited to about 12.6V peak-to-peak by the diode network consisting of D1, D2, ZD1 & ZD2. This limiting is included because Mosfets can be destroyed if they have excessive August 2001  35 Fig.4: there are two parts to the power supply; a centre-tapped bridge to give the main plus and minus rails, along with a conventional bridge fed by a 6.8µF capacitor, stabilised with a zener diode for the 24V fan supply. Note that the voltages are nominal, being “off load” and the type of figures you would expect to measure with a digital multimeter. gate drive. In normal signal conditions this does not occur but if the output of the amplifier is shorted or fed into a very low impedance, the negative feedback action causes the amplifier to compensate by increasing the gate drive. Also part of the circuit of each Mosfet is a 220Ω resistor in series with the gate. This is referred to as a “stopper” resistor because it is there to prevent (ie, to stop) spurious or parasitic oscillation at very high frequencies. Mosfets can oscillate at much higher frequencies than bipolar transistors and it is not unheard of for a badly designed Mosfet amplifier to oscillate at 100MHz or more (ie, in the middle of the broadcast FM band). So those stoppers are vital for stable amplifier operation. By the way, amplifiers with bipolar transistors often have stopper resistors as well but the potential frequency of oscillation is much lower than for power Mosfets. Another interesting feature of the Mosfet output stages in this amplifier is the presence of 47pF capacitors between Drain and Gate of Q9 & Q10. These are incorporated to equalise the gate capacitance of the N-channel Mos36  Silicon Chip fets (ie, Q9 & Q10) to the higher gate capacitance of the P-channel Mosfets (Q11 & Q12). Each Mosfet has a 0.22Ω 5W source resistor. This is included to improve current sharing in each pair of Mosfets and it also contributes to overall thermal stability. Right at the output of the amplifier, at the junction of the four 0.22Ω source resistors, is a Zobel network consisting of a .022µF capacitor and 6.8Ω resistor. This is necessary in virtually all solid-state amplifiers to ensure that a predictable load impedance (ie, around 6.8Ω) is presented at high frequencies where the inductance of typical speakers means their impedance is becoming very high. The overall gain of the amplifier is set by the negative feedback network consisting of C4, C6, R8 & R12. R12 and R8 set the overall gain to 34 while C4 sets the low frequency rolloff to 3.3Hz. However, the main determinant of the amplifier’s low frequency response is the 0.47µF input capacitor C1 and the 33kΩ input resistor which together set the -3dB point at 10Hz. The high-frequency response of the amplifier is mainly determined by the 1kΩ input stopper resistor R2, in conjunction with the .001µF input shunt capacitor C2. They set the high frequency -3dB point to about 160kHz. Overload indicator Finally, an interesting feature of the amplifier is the overload indicator involving transistors Q13 & Q14. It depends on the fact that, at the onset of clipping, large spike signals appear at the collector of Q5. These are fed via the voltage divider comprising R25 & R26 to the base of Q13. Each time a positive spike is fed to Q13, it charges capacitor C20 in its collector circuit. As the capacitor subsequently discharges, it turns on Q14 and lights LED1, the overload indicator. Thus Q14 and C20 function as a pulse extender so that each momentary signal overload is made visible on the overload indicator. Power supply The power supply (Fig.4) is fairly conventional, employing a 300VA toroidal transformer with 45V secondaries feeding a bridge rectifier, BR1. Each diode in the bridge rectifier is bypassed with a 0.22µF capacitor to provide suppression of rectifier hash. The bridge rectifier feeds four 4700µF 80VW capacitors to provide balanced supply rails of ±67V (nominal). On www.siliconchip.com.au O P T O PA C K 1 0 4 D E V I C E S : various colours & types. Top brands. Siemens etc. just $10 VISIBLE LEDs...5mm...14X Yellow clear, 6X Red (clear) 24deg, 2X Yellow (clear) 24deg, 16X Red (clear) 24deg,38X Green (clear) 24deg.VISIBLE LEDs... 3mm...14X Red diffused 70deg. 4X 3mm or rect. Yel. diffused 70deg SPECIAL...1X 5mm IR,3X 3mm Clear Phototransistor, 3X 5mm Phototransistor, 1X IR RX module. 2X DIL rect. black PIN Photodiode. PELTIER EFFECT DEVICES. 4A T 65deg. Qmax 42W $24 6A T 65deg. Qmax 60W $26 8A T 65deg. Qmax 75W $28 Comes with info to build cooler / heater All 40 X 40mm. PELTIER CONTROLLER KIT this kit is a switch mode design and correctly controls the temperature of peltiers to 10A using a very efficient design. Inc PCB, all on-board components . (k140) $19 (NEW) MULTI FUNCTION BATTERY CHARGER / DISCHARGER: New in original box with instructions. This unit was designed to charge NI-CD & NIMH mobile phone batteries of 4.8V, 6.0V and 7.2V. Operates from 12-24V DC input. Features include processor control & multi stage charge indicator. By changing the value of one resistor it can charge higher voltages, although a higher voltage plugpack is required for 9.4V or higher. Includes cigarette lighter lead, 12V / 1A DC plugpack & instructions for modifications for higher voltages. The unit has battery charging terminals but the user will have to make their own adaptor to interface to a battery. The plugpack supplied alone is worth around $30 retail. Weight is 0.9kg. $29... 15V DC / 1A Plugpack for charging batteries 9.4V or higher: (ZA0055) $6 If you ask when ordering you will receive a free 6-pack of batteries. CFL INVERTER KIT our very popular inverter. Very Efficient Driver kit can drive a number of CFL’s from 12vdc$25. QUALITY AUSTRALIAN MADE FEATURE PACKED MINI ALARM SYSTEM CONTROL Features inc. boot release, central locking output, imobiliser output, indicator flash relay. With 2 key-fob transmitter keys. $99 12V AUTOMOTIVE RELAY: Has 30A SPDT Contacts with 73ohm relay coil. These are the standard size and normally retail for around $7 each: (RL3) $3 each SUBSCRIBE TO NEW KITS FROM “OATLEY’S” We are constantly developing many electronic projects, but there is only a limited amount of these that the electronics magazine can publish. If you wish to receive a regular Email and be informed about these projects just send a blank Email with the following text in the subject heading: newkits-subscribe<at>oatleyelectronics.com Where possible our Emails will include descriptions, PCB overlays, parts lists and pictures. We will also offer you regular kit specials and where necessary, additional notes and or errata. In the future you will be able to access this same information at www.newkits.com but for the moment the ONLY WAY you can do this is by subscribing to the above Email address. As an example if you do it now you would be Emailed the following two projects within the next few weeks. MULTI PURPOSE INVERTER This modified square wave inverter can be used to convert 12-24V DC to 120V AC or 240V, or any other voltage. Power and voltage O/P’s depend on transformer. O/P freq. is adjustable between 50 and 60Hz and a beat indicator cct. is included (LED) so you can easily adjust the freq.. to be the same as the mains freq..With one pair of MOSFETS and no additional heat sinks 100W power O/P is possible, 200W with two pairs of MOSFETS and no H/S’s, 400W+ with two pairs of MOSFETS and additional H/S’s, etc…PCB plus all on-board components kit (No transformer):$18...Two additional MOSFETS: $6...US Plugpacks with a 30VA transformer: $2.50Ea. We will include notes on how these can be rewound for 120V O/P (1 needed) or 240V o/p (2 needed) FINALLY IT'S HERE!!! THE RIGHT WAY TO DRIVE STEPPER MOTORS. Now stepper motors can give high torque at high revs with our new 2 part kit driver system K142C Constant Current Source and K142B New Stepper Motor Driver. As a stepper motor's speed increases the current drawn and the power output slowly drop until it reaches a certain speed (varies greatly with motor type) then suddenly drops to almost nothing. Some drives like "Chopper drives" try to overcome this with a linear response to a non-linear problem. Our new K142C Constant current source drive senses the drop in current and increases the voltage to the motor and thus the current as speed increases. This gave similar torque at around 290 RPM as at 1 or 2 RPM (this is as high as we tested with a 200 step motor) Because of the wide voltage output range of the constant current source we had to re-design our stepper motor driver to cope. K142C: features easy construction, kit inc. PCB, heat-sink with fan and all onboard components. K142B: features inc. 4 or 6 wire motor drive, Opto Isolation to protect your computer, MOSFET placement for ease of heat-sink installation (if required) (heat-sink not supplied). Kit inc. PCB and all onboard components inc. high power MOSFET's. Both kits inc. full instructions, component ID and orientation printed on PCB for easy assembly. All Circuit boards (PCBs) are solder masked for easy soldering. GENUINE MAGLITE TORCHES So new it’s hard to tell that they are used, (during the Olympics). The same type as used by police, security guards etc. Complete in original box with booklet, rechargeable batteries, charger and charger clip /wall bracket etc. $150 SOLAR PANELS: Quality SIEMENS brand Polycrystalline cells. Open circuit voltage 5.7V, Short circuit current 0.22A, Peak power 1W <at> 100mW per square cm. 4 panels req. to charge 12V batteries. 160 x 55 x 5mm. Terminated with a 25cm long figure eight cable. $10 ea. or 4 for $36. SONY UNIVERSAL CAMCORDER BATTERY + CHARGER: Brand new in original packing Less than 1yr. old. 7.2V 1500mAh lithium-ion As commonly used with SONY digital cameras, camcorders, SONY and some other brand products . US made OPREX brand. Charger has an unusual plug that is easy to adapt. Requires power plug-pack (not supplied) 9V 1A (2A peak for 5 minutes)...$39. SERIAL SERVO CONTROLLER KIT: This kit is ideal for robotics kits etc, it controls up to 5 servos via the serial port of your computer. A lot of shareware and support for this kit on the Internet. Features inc. small kit size & hi servo resolution. Kit inc. software, PCB & all ELECTRONIC HOT WATER BOTTLE This kit would be ideal for sports injuries, pets, home-brewing etc Features include onboard com-ponents.:$24 insulated heating wire & thermostatic control. Some suitable power supplies may be GEARED AC MOTORS available, check when ordering. Kit inc. PCB, all onboard components & heating wire. Brand new small mains operated BOOK SHELF LIGHT SHOW K170 This 4 channel light controller is ideal for processional musicians or DJs. It is sound geared motors, triggered with adjustable gain or it will change through lots of different patterns at very strong, random by its self when its quiet. It is designed with 4 high powered MOSFETs to made for generate minimal heat while switching high loads and easily switches 4 12/50W rotating microwave halogen down lights. Kit inc. PCB, all onboard components inc. 4 MOSFETS. turntables, 240V/ Some suitable transformers may be available, check when ordering. 50Hz/3W/5RPM., SOOPER SNOOPER / STETHOSCOPE $4Ea. or or 4 for $12. This amazing parabolic microphone can listen in on all sorts of things from a distance, like bird calls and wildlife sounds, etc. Or by attaching the microphone to a metal rod NEW 500W Tungsten Halogen Lamps or screwdriver handle it can be used to listen to white ants chewing on your house! It is (All are new but packing may be shop also ideal for detecting engine knocks and worn bearings etc. We even heard water soiled) Ideal replacement or spare bulbs rushing through a radiator hose! Kit inc. PCB, all onboard components, stethoscope for yard and security lights. $2ea pickup, electret microphone and 300mm parabolic dish. SOLAR FURNACE /PARABOLIC REFLECTOR This is the same 300mm dish as used in our Sooper Snooper project. It is mill finished ie. unprotected aluminum and is reflective enough to ignite paper allmost instantly, With the use of some automotive cutting compound / polish it could easily be made W e h a v e m o r e u s e d t e s t equipment. we need to clear some highly reflective:$25 ea. VIDEO SYNC. STABILISERS Various forms of copy protection are used on video tapes & DVDs, that may cause playback problems like the jitters. This device removes the copy protection. thus cleaning the ONE / TWO CHANNEL UHF REMOTE picture. It has CONTROL On freq. of 304MHz, been transmitter is suggested to us that assembled, receiver is a these units could be used to copy commkit, inc. 2 12V/ ercial videos & DVDs but we do not 12A relays, 1Tx + condone any breach of copyright. This 1Rx kit:$45, additional Tx: $15 item comes as a ready built PCB with a I CHANNEL Kit just $25 new recycled metal case to suit. Just...$29 NEW 80mm 12V FANS WE HAVE TOO MANY ITEMS TO Ideal replacement for ADVERTISE HERETHE ONLY WAY TO computer power supply fans. SEE IT ALL IS TO CHECK OUT OUR 12V <at> 0.15A..$4 or 4 for $12 WEB STIE oatleyelectronics.com NEW SHIPMENT to make way for the next lot. But you may have already missed it. The only way to make sure you don’t is to subscribe to our bargain corner and receive advanced notice of what’s comming... Just send us a blank E-Mail to.... bargaincorner-subscribe MONOCHROME CCD VIDEO CAMERA <at> o a t l e y e l e c t r o n i c s . c o m VIDEO CAMERAS The output of these cameras below is std video & can be plugged into the "VIDEO IN" socket of any Australian std VCR, video monitor or TV, or via an RF Modulator to an Ant. Input. The B/W cameras are Infra Red responsive & can be used in total darkness with IR Illumination. B&W Camera built on a PCB with auto iris. (0.1 lux). Can be focused sharply down to a few mm(useful for people with visual impairment). Spec.: Power req.: 10V to 12V <at> approx. 50mA.CCD: 1/3", 30grams: $89, with 92° lens: www.oatleyelectronics.com Orders: Ph ( 02 ) 9584 3563, Fax 9584 3561, sales<at>oatleyelectronics.com, PO Box 89AOatley NSW 2223 www.siliconchip.com.au ugust 2001  37 major cards with ph. & fax orders, Post & Pack typically $7 Prices subject to change without notice ACN 068 740 081 ABN18068 740 081 SC_AUG_01 Fig.5: the component side of the PC board. The eight large empty holes are for the 5W resistors – see below. Fig.6: the four 5W resistors mount on the underside (copper side) of the PC board and should be 3-4mm away from the surface to avoid heat damage to the tracks or board. the amplifier board, further filtering is provided for the early stages via diodes D3 & D4 and associated 100µF capacitors. Another bridge rectifier, BR2, is fed from the two 45V secondaries (90V total) via a 6.8µF capacitor to supply a zener stabilised 24V for the fan. The high impedance of the 6.8µF capacitor throttles back the voltage to the bridge rectifier so that little power is wasted in zener diode ZD1. Assembly details Three PC boards need to be assembled: one for the amplifier, one for the power supply and one for the bridge rectifier. The amplifier PC 38  Silicon Chip board measures 159 x 65mm while the power supply board measures 122 x 65mm. We suggest you start on the amplifier board first, placing all the small components, then the insulated terminal blocks, followed by the power transistors and Mosfets along one edge. Use the component overlay diagram of Fig.5 as a guide to the assembly. When soldering the transistors along the edge, you will need to make sure that each Mosfet is pushed right down onto the board while the four other transistors are mounted with a lead length of about 11mm. This is not really critical though because spring clips are used to mount these transistor packages to the heatsink. The four 5W source resistors are mounted underneath the PC board, as shown in Fig.6. They should be mounted so that the resistor bodies are clear of the copper tracks by about 3 or 4mm. The rectifier is attached to the heatsink with a long screw and nut and a small PC board is fitted over the four rectifier lugs and soldered to them, as shown in the diagram of Fig.7. Before you do that though, make sure you mount and solder the four 0.22µF 250VW capacitors onto the rectifier board. The power supply board is also quite straightforward – see Fig.8. Mount and solder the insulated terwww.siliconchip.com.au Fig.7: the bridge rectifier and its associated capacitors are mounted on this small PC board which mates with the board at right. minal blocks first, followed by the small components, the 6.8µF capacitor and finally, the four 4700µF capacitors. Make sure that all electrolytic capacitors, diodes and zener diodes are mounted the right way around, as shown on the component overlay diagrams of Figs.5 & 8. Note that the 2A fuse is a slow-blow type. When all the boards are finished, check your work carefully and make Fig.8: the main power supply board holds the rest of the power supply components. sure that you have not missed any solder connections or have made any solder splashes to short between tracks. The next step is to mount the power transistors and Mosfets to the heatsink. Line up the board so that its lower edge is about 4mm above the lower edge of the heatsink channel. Each transistor is mounted using an appropriately sized mica heatsink washer and this is coated on both sides with heatsink compound. Then it is a matter of attaching the heatsink clips to secure each pair of transistors in place. The heatsink is then attached to the chassis plate with two self-tapping screws and the forward edge of the amplifier board is secured with metal spacers and screws. Fit the tunnel plate and the 24V fan to the heatsink at the same time. A heatsink-side view of the amplifier showing the fan and the way the heatsink is assembled and all mounted on the “chassis”. www.siliconchip.com.au August 2001  39 Fig.9: follow this inter-board wiring diagram when you’re assembling the amplifier and you shouldn’t have any problems. The chassis is designed to be mounted “as is” into other equipment, such as a PA or guitar amplifier. 40  Silicon Chip www.siliconchip.com.au The power supply board is also mounted on the chassis plate with four metal spacers and screws. Next the power transformer can be mounted using a large bolt, nut, flange washer and rubber washer. All the transformer leads, with the exception of those to the main bridge rectifier (BR1) are connected to the insulated terminal block along one end of the power supply board. All the inter-board wiring is shown in Fig.9. Solder the two 45V windings to the AC inputs on the bridge rectifier on the heatsink, together with two white wires which become the AC input to the bridge rectifier (BR2) on the power supply board. The white wires are connected to the relevant 45V terminals on the power board. Don’t forget to scrape the varnish (enamel) off the ends of all the transformer wires, to make sure you make good connections before soldering or terminating them. Make all the input connections to the power board but don’t connect any of its DC outputs to the amplifier. Do connect the 24V output to the fan. The green/yellow wire of the 3-core mains flex is terminated to the solder lug on the chassis adjacent to the transformer. Also connected to this lug is a 0.1µF capacitor which connects to the 0V line and centre-tap of the transformer. This is done instead of directly earthing the amplifier as it can help to avoid hum and buzz due to earth loops. The Active and Neutral wires in the mains cord connect to the relevant insulated terminals on the power supply board. Note that the slow-blow mains fuse should be fitted with a plastic shroud to stop any possibility of accidental contact. Initial power up Before connecting the amplifier supply rails, it is a good idea to check out the power supply operation. So carefully check out all your power supply connections and then apply power. The fan should immediately run and you should be able to measure around ±70V at the DC outputs of the board. Turn off the power and discharge the main filter capacitors with a 5W resistor of 100Ω or higher connected across the DC outputs. Now connect the DC supply rails Winning Gold . . . . . .After the Games EX OLYMPIC GENUINE MAG LITE TORCHES Made in USA, complete with 240V battery charger kit and in car battery charger kit. As used by Police, Navy & RTA from the power board to the amplifier, each via a 10Ω 5W resistor. Rotate RV1 fully anticlockwise – this is to set minimum quiescent current. Do not connect any signal source or a speaker load. Then apply power. If the fuses go up in smoke or get red hot, turn off the power and recheck your work. If all is well, check the ±67V rails and the DC voltage at the speaker output; it should be within ±30mV of 0V. To set the quiescent current, monitor the voltage across one of the 10Ω 5W resistors and adjust RV1 for a reading of 0.8V. Leave the amplifier running for a period of ten minutes and if necessary, readjust RV1 for the correct reading. Then turn off the power, remove the 10Ω resistors and directly connect the supply rails from the power supply board to the amplifier. The signal leads and loudspeaker can be connected to the relevant terminals on the connector strip and then you are ready to roll. Where to buy the kit This 200W amplifier is exclusive to Altronics (Cat K-5172) and is priced at SC $349. Phone 1 800 999 007. HURRY! This is your LAST CHANCE to grab some of the equipment left over from the Sydney 2000 Olympic Games at never-to-be-repeated prices! CALL NOW! PH: (02) 9879 6782 FAX: (02) 9879 6993 180 ea $ EX OLYMPIC SECURITY DURACELL 9 VOLT BATTERIES Brand new (expiry dates 2004) Boxed lots of 48 1 00 $ 79 ea $ 80 ea EX OLYMPIC TOA MEGAPHONES Complete with shoulder harness and alkaline batteries www.siliconchip.com.au ALL PRICE INCLUDS E GST ® DON’T MISS OUT! ® Registered Trade Mark Australian Video Systems August 2001  41