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Rugged Mosfet Audio Amplifier Module By LEO SIMPSON Want a big powerful amplifier module based on Mosfets? This one uses eight plastic Mosfets to deliver just over 200 watts into an 8W load and a whisker over 350 watts into a 4W load – just the ticket for heavy duty amplification. For many audio enthusiasts, Mosfets rule supreme and Hi­tachi Mosfets are the best there are. But in the last few years, Exicon, a manufacturer from England, has appeared on the scene with a range of plastic power Mosfets. This new design features these plastic devices which are rated at 20 amps, 200V and 125W. Eight of these devices – ie, four Exicon ECX10P20 p-channel and four ECX10N20 n-channel – are used in this amplifier module. As the graphs of Fig.1 & Fig.2 demonstrate, the amplifier module will deliver just over 200 watts into an 8Ω load or just over 350 watts into a 4Ω load, at the onset of clipping. The onset of clipping is where the harmonic distortion graph suddenly becomes almost vertical. While we’re talking about performance graphs, we might as well refer 30  Silicon Chip to a few more. Fig.3 shows the frequency response which is 0.7dB down at 10Hz and 20kHz. While it is just off the graph, the -3dB point is at 54kHz. Fig.4 shows the harmonic distortion versus frequency for the power amplifier module when delivering 250 watts into a 4Ω load. Fig.5 shows harmonic distortion versus frequency at 150 watts into an 8Ω load. As these graphs show, the performance is quite respectable. The amplifier module is also very quiet, which is as it should be for any modern design. We measured a signalto-noise ratio of 117dB unweighted (22Hz to 22kHz) and 123dB A-weight­ ed with respect to full power into an 8Ω load. The PC board is designed so that the eight Mosfet power devices are mounted onto a heatsink angle bracket which then mounts on a large finned heatsink as part of the amplifier chas­ sis. Our photos show only the heatsink bracket. The amplifier must not be operated without a larger heatsink as it will rapidly overheat. Circuit description Fig.6 shows the circuit diagram. This amplifier is unlike most direct-coupled circuits in that it has three differential stages to give it high open-loop gain before negative feedback is applied. Two BC546 NPN transistors, Q4 & Q5, form the differential input stage and their operating current is set by the constant current source, Q7. The signals at the collectors of Q4 and Q5 are then fed into the voltage gain stage which comprises Q1, Q2, Q3, Q6, Q8, Q9 and associated components. This can best be described as a “double differential pair with TO N I W 200 S; 8-OHM TO IN 350W MS 4-OH current mirror load”. This stage works as follows. PNP transistors Q2 and Q3 form the first dif­ferential pair with R8 as the common emitter resistor. The output of Q2, Q3 provide differential drive to NPN transistors Q6 & Q8. The collector load for these two transistors is provided by the current mirror transistors Q1 & Q9. The current mirror ensures equal current sharing in the associated differential pair and thereby provides high gain and good linearity. Finally, we come to the power output stage which is the business end of the amplifier; it employs the eight Mosfets mentioned earlier. These are connected as complementary source-followers which means that they behave in a similar way to emit­ter followers – their voltage gain is a little less than unity but they have oodles of current gain. In effect, the Mosfets act as a buffer stage for the amplifier, transforming the voltage drive from the earlier stages to a low impedance output which can deliver a great deal of power – 350 watts in fact! The signal at the collectors of Q8 and Q9 (ignore VR1 for the moment) is applied to the gates of the paralleled Mosfets, via the 390Ω resistors. As the signal rises towards the positive rail, the top Mosfets (10N20’s) start to conduct, allowing current to flow to the load. Conversely when the signal goes towards the negative rail, the bottom Mosfets (10P20’s) conduct, pulling current out of the load. Performance Output power ......................... 200 watts into 8Ω; 350 watts into 4Ω Frequency response .............. -0.7dB down at 10Hz and 20kHz (see Fig.3) Input sensitivity ...................... 1.7V RMS (for 200 watts into 8Ω) Harmonic distortion ............... less than .01% (see Figs.1 & 2) Signal to noise ratio ���������� 117dB unweighted (22Hz to 22kHz); 123dB A-weighted with respect to full power into 8Ω Stability .................................. unconditional August 1996  31 LEVEL(W) AUDIO PRECISION SCTHD-W THD+N(%) vs measured 10 29 MAY 96 14:55:34 1 AUDIO PRECISION SCTHD-W THD+N(%) vs measured 10 LEVEL(W) 29 MAY 96 14:58:49 1 0.1 0.1 0.010 0.010 0.001 0.001 .0005 .0005 0.5 1 10 100 300 0.5 1 10 100 500 Fig.1: total harmonic distortion versus power into an 8Ω load. Power at the onset of clipping is 212W. Fig.2: total harmonic distortion versus power into a 4Ω load. Power at the onset of clipping is 353W. AUDIO PRECISION SCFREQRE AMPL(dBr) vs FREQ(Hz) 5.0000 AUDIO PRECISION SCTHD-HZ THD+N(%) vs FREQ(Hz) 5 28 MAY 96 11:10:30 29 MAY 96 15:11:42 4.0000 1 3.0000 2.0000 1.0000 0.1 0.0 -1.000 0.010 -2.000 -3.000 -4.000 0.001 -5.000 .0005 10 100 1k 10k 50k Fig.3: frequency response of the amplifier. While it is just off the graph, the upper -3dB point is at 54kHz. AUDIO PRECISION SCTHD-HZ THD+N(%) vs FREQ(Hz) 5 29 MAY 96 15:02:38 1 0.1 0.010 0.001 .0005 20 100 1k 10k 20k Fig.5: total harmonic distortion versus frequency at 150W into an 8Ω load. The 390Ω gate resistors are there to act as “stoppers” for the Mosfets. They act in conjunction with the high gate 32  Silicon Chip 20 100 1k 10k 20k Fig.4: total harmonic distortion versus frequency for the amplifier module when delivering 250W into a 4Ω load. ca­pacitance of the Mosfets to reduce their gain at very high fre­quencies. This prevents the tendency of Mosfets to “parasitic oscillation” which is typically manifested as bursts of high frequency oscillation (typically at 10MHz or higher) superimposed on the audio signal. Sometimes parasitic oscillation in Mosfets can be at such a high frequency that it will not be seen on typical 20MHz oscillo­ scopes; 100MHz or higher bandwidth scopes are necessary to show it. However, even though it may be invisible on a typical oscil­loscope, it is most important to stop it happening because para­doxically, even though it is at such a high frequency, it will cause the harmonic distortion to be much higher than it otherwise would be and the amplifier will sound unpleasant as a result. Anyhow, that’s why the stoppers are included. Capacitors C16-C19 are included to match the input ca­pacitance of the n-channel devices to that of the p-channel types. This improves the gain linearity at high audio frequen­cies. The 0.22Ω 5W resistors in series with the source of each Mosfet are there to provide a degree of local negative feed- back and to help improve the current sharing between devices. Trimpot VR1 is connected between the collectors of Q9 and Q8 and is there is provide a voltage offset between the gates of the n-channel devices at the top and the p-channel devices below. This voltage offset becomes a forward bias which turns on the Mosfets slightly in the absence of any audio signal. This quies­cent (ie, no signal) bias is necessary to operate the Mosfets in the more linear region of their transfer curve and thus reduces crossover distortion. Zener diodes ZD1 & ZD2 and diodes D3 & D4 protect the gates of the Mosfets from overdrive. The zeners and diodes clamp the drive voltage between gate and source of each Mosfet to a maximum of about +12.7V. Since the Mosfets act as source-followers you might wonder how the gate voltage could go this high. Normally, the peak current (at full power into a 4Ω load) would be no more than about 3-4A. Since the transconduct­ ance of these Mosfets is about 1 Siemen or 1V/A, then the gate-source voltage can be expected to rise to no more than about 4V or so under normal drive. So how could the gate voltage ever rise much above this figure? The answer is that the gate drive becomes excessive when the load of the amplifier is short-circuited and it is being driven hard. Under these conditions, the gate voltage to the Mosfets could easily rise above 20 volts. However, the zener diodes do not provide short-circuit protection to the amplifier. That is provided solely by the fuses. The Mosfets are rugged enough to withstand short circuits until the fuses blow. Negative feedback is applied from the output of the ampli­fier, via R21, a 22kΩ resistor, to the base of Q5, part of the first differential pair. The AC gain is set by the ratio of the 22kΩ and 1kΩ resistors at the base of Q5 and this gives a value of 23. The resulting input sensitivity of the amplifier is 1.7V RMS for 200 watts into 8Ω and 1.6V RMS for 350 watts into 4Ω. The low frequency response of the amplifier is set by two time-constants. The first is made up of the 1µF input capacitor C1 and the 47kΩ input bias resistor R3, giving a -3dB point of 3.3Hz. The second time-constant is provided by the 1kΩ feedback resistor Fig.6: this power amplifier is unlike most direct-coupled circuits in that it has three differential stages to give it high open-loop gain before negative feedback is applied. August 1996  33 hot. Choke L1 is wound with 20.5 turns of 0.8mm enamelled copper wire onto a 14mm plastic former. Once it is wound, scrape the enamel off the wire ends and then tin them with solder before installing the choke on the board. When installing the fuse clips, take note of their little lugs which should be on the outside ends of the fuse. Heatsink bracket Fig.7: this diagram shows a suggested power supply for the amplifier. The power transformer is rated at 500VA. R19 and the 100µF capacitor C8, giving a -3dB point of 1.6Hz. Combined, they result in a response which is only -0.7dB at 10Hz. At the high frequency end, the main determinant of the response is the double time-constant provided by the input net­work consisting of R4, R5, C2 & C3 which produce a rolloff above 80kHz. Other factors affecting the high frequency response are the 10pF capacitor shunting the feedback resistor R21 and the output coupling network consisting of R30, R31, L1 & C10. The latter network is included to ensure stability of the amplifier under reactive load conditions. Power supply And now a few words about the power supply. Ideally, you need a supply which can deliver over 300 watts if you are using an 8Ω load and almost 600 watts if you are using a 4Ω load. A good compromise is to use a 500VA transformer and six 10,000µF capacitors, as shown in Fig.7. Note that the DC supply rails are ±70V, a total of 140V between rails. This is a potentially lethal voltage so be very careful when making measurements around the circuit! Construction As supplied in the kit from Altron­ ics, the PC board has a solder mask and screen printed component overlay, to make assem­bly straightforward. The component overlay diagram is shown in Fig.8. Start construction by fitting the PC pins and the resis­tors, then install the diodes, capacitors and small signal tran­sistors. Watch the orientation of the electrolytic capacitors, diodes and transistors. Don’t confuse the 1N914s and 12V zener diodes. Mount the 5W resistors about 3mm off the PC board, just in case they get The next task is to assemble the MJE340 and MJE350 transis­tors onto the heatsink bracket. A total of six transistors need to be mounted. If you hold the bracket so that it’s facing you, three MJE350s (Q2, Q3, Q1) are mounted on the left, then the two MJE340s (Q6, Q8) and then another MJE350 (Q9). It is most import­ant not to mix them up. We should also make a note about the brand of MJE340s and 350s. As we have stated in the past, Motorola devices are the best. Other brands will work but they are nowhere near as good, giving rise to less power output and higher distortion. Fig.9 shows the details of how each MJE340 and MJE350 is mounted to the heatsink bracket. You can use mica washers and heatsink compound for each transistor or use silicone impregnated thermal washers. Do not overtighten the mounting screws. When all six TO-220 transistors are mounted on the bracket, it can be installed on the PC board and the transistor leads soldered. An easier method is used to secure the power Mosfets to their heatsink RESISTOR COLOUR CODES ❏ No. ❏  1 ❏  2 ❏  4 ❏  1 ❏  1 ❏  2 ❏  2 ❏  8 ❏  2 ❏  1 ❏  4 ❏  1 ❏  1 ❏  1 34  Silicon Chip Value 470kΩ 47kΩ 22kΩ 4.7kΩ 3.3kΩ 1kΩ 470Ω 390Ω 270Ω 150Ω 100Ω 10Ω 4.7Ω 1Ω 4-Band Code (5%) yellow violet yellow gold yellow violet orange gold red red orange gold yellow violet red gold orange orange red gold brown black red gold yellow violet brown gold orange white brown gold red violet brown gold brown green brown gold brown black brown gold brown black black gold yellow violet gold gold brown black gold gold 5-Band Code (1%) yellow violet black orange brown yellow violet black red brown red red black red brown yellow violet black brown brown orange orange black brown brown brown black black brown brown yellow violet black black brown orange white black black brown red violet black black brown brown green black black brown brown black black black brown brown black black gold brown yellow violet black silver brown brown black black silver brown PARTS LIST 1 PC board, code PEDK5180, 205 x 97mm 4 3AG fuse clips 2 5A 3AG fuses 1 large heatsink bracket 1 large single sided heatsink 1 small heatsink bracket 8 TO-3P mica insulating washers 6 TO-220 mica insulating washers 4 transistor mounting clips 7 PC pins 1 plastic bobbin 1 1.2m length of 0.8mm enamelled copper wire 1 200Ω horizontal trimpot (VR1) Semiconductors 4 ECX10N20 n-channel Mosfets (Q12,Q13,Q14,Q15) 4 ECX10P20 p-channel Mosfets (Q10,Q11,Q16,Q17) 4 MJE350 PNP driver transistors (Q1-Q3,Q9) 2 MJE340 NPN driver transistors (Q6,Q8) 3 BC546 NPN transistors (Q4,Q5,Q7) 4 1N914, 1N4148 signal diodes (D1-D4) 2 12V 400mV zener diodes (ZD1,ZD2) Capacitors 2 100µF 160VW electrolytic 1 100µF 25VW electrolytic 1 1µF 63VW electrolytic 1 0.22µF metallised polyester 2 .047µF monolithic 1 .001µF greencap 1 470pF disc ceramic 1 330pF disc ceramic 1 220pF disc ceramic 4 22pF disc ceramic 1 10pF disc ceramic Fig.8: the parts overlay for the PC board. Note that the 5W resistors should be spaced 3mm off the board. Take care to ensure that all polarised parts are correctly oriented. bracket. Spring clips are used to clamp adjacent transistors. The screw which retains the spring clip also secures the heatsink bracket to the PC board. A cross-section diagram of the mounting is shown in Fig.10. All eight Mosfets are soldered to the PC board first, making sure that there is about 8mm of lead length above the board. This allows them to be bent over without placing too much strain on the leads. When the eight Mosfets are soldered in place, the heatsink bracket and spring clips can be assembled together. Do not forget to use a mica washer and heatsink com­pound for each device. Place a spring clip over two Mosfets Resistors (0.25W, 5%) 1 470kΩ 8 390Ω 2 47kΩ 2 270Ω 4 22kΩ 1 150Ω 1 4.7kΩ 4 100Ω 1 3.3kΩ 1 10Ω 2 1kΩ 1 4.7Ω 1W 2 470Ω 1 1Ω 1W 8 0.22Ω 5W wirewound 4 zero-ohm links 2 100Ω 5W (for biasing setup) Miscellaneous Screws, nuts, washers, solder, heatsink compound. August 1996  35 and then, using a 4mm screw from under the board, secure it to the heatsink bracket. The screw for each clip should be fully tightened; the beauty of these spring clips is that you cannot apply too much force to the Mosfets. Make sure that all devices are insulated from the heatsink bracket. Check that all six TO-220 devices are insulated from their heat­sink bracket as well. Now check over all your assembly work, making sure that the component installed in each position agrees with that on Fig.8. Setting up and testing You will need a power supply (see Fig.7), a multimeter and a small screwdriver to set up the module. Remove the two fuses and solder a 100Ω 5W wirewound resis­tor across each fuseholder. Rotate trimpot RV1 fully anticlock­wise. This setting will Fig.9: here’s how the six TO-220 transistors are mounted on the heatsink bracket. You can use mica washers and heatsink compound for each transistor or silicone impregnated thermal washers. 36  Silicon Chip result in the minimum quiescent current through the output stage. Connect the ±70V supply rails and ground to the board. Don’t connect a signal or a load at this stage. Set your multimeter to DC volts and connect it across one of the 100Ω resistors on the fuse clips. Now switch on. No smoke? Good! If all is not well, switch off immediately! Assuming no smoke, measure the voltage across the 100Ω fuse clip Fig.10: this diagram shows the mounting details for the power Mosfets. Spring clips are used to clamp adjacent transistors. Kit Availability resistor. It should be quite low, about 1V or so. Now rotate trimpot RV1 anticlockwise until the meter reads about 7V. This means that the output state quiescent current is 70 milliamps. Now measure the voltage across the other 100Ω fuse clip resis­tor; it should be about the same. Next, measure the voltage across the speaker outputs. The voltage can positive or negative but should be less than 50mV. Let the amplifier run in this condition for 10 minutes or so, to let the bias stabilise. Re-measure the voltage across the 100Ω resistors and adjust trimpot RV1 if necessary. The next job is to fit the amplifier with a suitable heatsink and mount it inside a case with a cooling fan and power supply. You can then connect a loudspeaker and signal source and listen to your heart’s content. Troubleshooting If the 100Ω resistors smoked when power was applied, then check the following: (1). Bias pot turned wrong way (should be anticlockwise). (2). Power Mosfets transposed (N types with P types). (3). Power supply wrongly connected. (4). Short on underside of PC board. (5). Output device(s) shorted to heat­ sink(s). (6). Shorted capacitor on power supply (check greencaps and electro­ lytics). If the current is unstable (ie, jumps all over the place), or the sound us crackly or hissy, then the amplifier is possibly unstable. Check the following: (1). Wrong values of resistors in the signal section (check them all). (2). Ceramic capacitors are incorrect value. (3). Earth or ground connection missing. SC (4). Mosfet shorted to heatsink. SILICON CHIP SOFTWARE Now available: the complete index to all SILICON CHIP articles since the first issue in November 1987. The Floppy Index comes with a handy file viewer that lets you look at the index line by line or page by page for quick browsing, or you can use the search function. All commands are listed on the screen, so you’ll always know what to do next. Notes & Errata also now available: this file lets you quickly check out the Notes & Errata (if any) for all articles published in SILICON CHIP. Not an index but a complete copy of all Notes & Errata text (diagrams not included). The file viewer is included in the price, so that you can quickly locate the item of interest. The Floppy Index and Notes & Errata files are supplied in ASCII format on a 3.5-inch or 5.25-inch floppy disc to suit PC-compatible computers. Note: the File Viewer requires MSDOS 3.3 or above. ORDER FORM PRICE ❏ Floppy Index (incl. file viewer): $A7 ❏ Notes & Errata (incl. file viewer): $A7 ❏ Alphanumeric LCD Demo Board Software (May 1993): $A7 ❏ Stepper Motor Controller Software (January 1994): $A7 ❏ Gamesbvm.bas /obj /exe (Nicad Battery Monitor, June 1994): $A7 ❏ Diskinfo.exe (Identifies IDE Hard Disc Parameters, August 1995): $A7 ❏ Computer Controlled Power Supply Software (Jan/Feb. 1997): $A7 ❏ Spacewri.exe & Spacewri.bas (for Spacewriter, May 1997): $A7 ❏ I/O Card (July 1997) + Stepper Motor Software (1997 series): $A7 POSTAGE & PACKING: Aust. & NZ add $A3 per order; elsewhere $A5 Disc size required:    ❏ 3.5-inch disc   ❏ 5.25-inch disc TOTAL $A Enclosed is my cheque/money order for $­A__________ or please debit my ❏ Bankcard   ❏ Visa Card   ❏ MasterCard Card No. Signature­­­­­­­­­­­­_______________________________ Card expiry date______/______ Name ___________________________________________________________ PLEASE PRINT Street ___________________________________________________________ Suburb/town ________________________________ Postcode______________ Send your order to: SILICON CHIP, PO Box 139, Collaroy, NSW 2097; or fax your order to (02) 9979 6503; or ring (02) 9979 5644 and quote your credit card number (Bankcard, Visa Card or MasterCard). ✂ ✂ The design copyright for this project is owned by Altron­ics in Perth. They can supply a complete kit for the amplifier module, heatsinks, chassis and power supply components. The amplifier module is priced at $189. (Cat K-5180). August 1996  37