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Ultra-LD Mk.3 200W Amplifier Module; Pt.2 Second article has the assembly details By NICHOLAS VINEN The Ultra-LD Mk.3 amplifier module introduced last month is by far our lowest distortion Class-AB amplifier design. This month, we present the construction details and give some additional information on its performance. B UILDING THIS new high-performance amplifier module is really quite straightforward although there’s a fair amount of work involved. When building a high-power amplifier like this, it’s important to take your time and double-check each stage of the assembly as you proceed. The double-sided PCB shown in the photos is critical to the performance of this module. It not only simplifies the supply wiring but has also been carefully designed to largely cancel the magnetic fields produced by the asymmetric currents drawn by each 62  Silicon Chip half of the class-B output stage. In addition, the double-sided board eliminates the need for wire links, the exception being a couple of 0Ω resistors. We’ll describe how to assemble the module shortly but first let’s take a look at a few more aspects of the design. Increased fuse rating The previous Ultra-LD Mk.2 had two on-board 5A fuses which we changed in the Mk.3 to 6.5A (7.5A is also OK). That’s because the amplifier is capable of delivering around 300W of music power into 4Ω. Under this condition, each fuse carries around 4.4A RMS (including the quiescent current). It could be even higher with a highly reactive speaker load. While this is unlikely to blow a 5A fuse, it could eventually lead to fuse failure due to thermal stress. We have not had any reports of blown fuses in the Mk.2 version but we thought it best to make the change anyway. Inductor value The Ultra-LD Mk.3 amplifier uses a higher value inductor (10µH) than siliconchip.com.au ing direction are very important. The reason is that the current flowing through this coil creates a magnetic field that partially cancels the magnetic field generated by the speaker current loop on the PCB. With a positive output voltage, the speaker current flows in a clockwise direction around the loop which includes the 10µH inductor and the speaker. However the current in the coil itself flows in an anti-clockwise direction and so the magnetic fields partially cancel. This reduces the magnetic coupling back to the input stage. As described later, the inductor itself is actually wound in a clockwise direction. It must also be installed exactly as shown on the PCB layout diagram (Fig.10), otherwise the distortion will be higher than it otherwise would be. If you have already built an UltraLD Mk.2 amplifier, check its inductor orientation. If it’s the wrong way around, its 20kHz distortion figure will be around 0.012% rather than the specified 0.006%. Input filtering previously for better distortion cancellation. This results in slightly worse high-frequency response for 4Ω loads (the difference with an 8Ω load is insignificant), the response being -1dB at 20kHz compared to -0.7dB. We feel that -1dB at 20kHz is acceptable. If you really want the extra 0.3dB though, you can have it in exchange for slightly higher distortion (peaking at 0.0048% at 20kHz rather than 0.0038%). All you have to do is wind the inductor with five fewer turns of wire. Either way, its orientation and windsiliconchip.com.au As stated last month, the 4.7nF capacitor at the amplifier’s input (increased from 820pF in the Mk.2) provides better RF signal attenuation. This prevents radio signals picked up by the input leads from being rectified and amplified by input transistor Q1. The value chosen assumes a low source impedance (ie, 220Ω or less) which suits most modern program sources (eg, CD or DVD players). If RF pick-up is still a problem (unlikely), the input filter can be improved by using a 4.7nF ceramic capacitor. This will have less inductance than an MKT type and so its impedance is lower for RF signals. A ferrite bead can also be slipped over the 100Ω resistor lead between the input connector and this capacitor, further increasing RF attenuation. Note that a 4.7nF input capacitor will cause an audible reduction at high frequencies if the signal source impedance is too high, so a lower value (eg, 1nF) should be used in that case. Resistor ratings The 6.8Ω 1W snubber resistor in the output filter is adequately rated for music or speech at maximum power. However, if the amplifier is made to deliver high-frequency signals at high WARNING! High DC voltages (ie, ±57V) are present on this amplifier module when power is applied. In particular, note that there is 114V DC between the two supply rails. Do not touch the supply wiring (including the fuseholders) when the amplifier is operating, otherwise you could get a lethal shock. power (eg, during testing), this resistor can overheat and burn out. To avoid this, you can use a 6.8Ω 5W wirewound resistor instead. The final board design has extra pads for fitting a larger resistor, which can sit on top of the adjacent 2.2kΩ resistor. This should withstand a continuous 20kHz output at 135W into 8Ω or 100W into 4Ω. Frequency response Last month, we published the frequency response plots for 4Ω and 8Ω loads, showing the low and highfrequency roll-off. However, we didn’t show the no-load frequency response. Because of the output filter, the noload response is actually up by +0.3dB at 20kHz. For more details on this condition, see Neville Thiele’s paper “Load Circuit Stabilising Network for Audio Amplifiers” (Proceedings of the IREE 299, September 1975). No-load operation of the amplifier will only occur when it is driving headphones (ie, via the on-board 390Ω 1W resistor), so it is of little consequence. Alternative transistors The amplifier’s second (transimpedance) stage (also called the voltage amplification stage or VAS) incorporates two medium-power transistors with flag heatsinks, Q7 (BF470) and Q9 (BF469). These were selected for good linearity. Each dissipates around 15mA x 57V = 855mW continuously, so small signal transistors are not suitable. Unfortunately, BF469s and BF470s are becoming harder to acquire because they were designed for cathode ray tube (CRT) driver circuits. With the advent of LCD and plasma TVs, the demand has dropped dramatically and so they can be hard to get. As a result, we have made provision for alternative parts from Toshiba: August 2011  63 MJE15030 BD139 MJE15031 Q16 NJL1302D Q15 NJL1302D Q11 470nF 120 330 100 Q10 Q14 220 NJL3281D 100 NJL3281D Q13 Q12 F2 6.5A 11170110 3.K M REIFILP MA DL-ARTLU VR1 100nF 1000 F 63V 100nF 100nF + 0.1  5W 1000 F 63V 0.1  5W 0.1  5W F1 6.5A 0.1  5W + Q1,Q2: 2SA970 Q5,Q6: BC556 Q1 Q2 100 47 F NP 100 510 12k 1M 4.7nF R01 + 1000 F 10 CON1 0 BC639 D1 4148 2.2k 180pF 10  1W 100nF 2 x BC546 D2 +57V 0V 390  1W S Q3 Q4 R02 4148 L1 10 H 180pF Q8 0 1102 © 22k –57V 470 F 63V CON2 220nF 400V 100 Q9 Q7 6.8  1W 2.2k 12k F 68 Q6 BF469, 2SC4793* 2.2k Q5 6.2k 2.2k 6.8k 1W 100 47 F 68 BF470, 2SA1837* 47 F 35V 68 6.2k 100nF Speaker CON3 Phones GND SIGNAL INPUT * SEE TEXT & FIGS.11 & 12 FOR MOUNTING DETAILS Fig.10: follow this parts layout diagram to build the Ultra-LD Mk.3 Amplifier module. Note that you should install a tinned copper wire feed-through in the middle of each group of five vias. Note also that this overlay shows the arrangement for BF470 & BF469 transistors for Q7 & Q9 while the photo at right the alternative arrangement when using 2SA1837 & 2SC4793 transistors. 2SA1837 (PNP, replaces Q7) and 2SC4793 (NPN, replaces Q9). These are designed for audio amplifier use and are still in production. They are currently available from DigiKey. Note, however, that their pinouts are reversed compared to the BF469/ BF470 so we added an extra set of pads on the opposite side of each small heatsink. The slightly different assembly method is explained later in this article (see also Figs.11 & 12). In fact, using these alternative transistors gives a slight reduction in high-frequency distortion compared to the BF469/BF470 combination, ie, the THD + N at 20kHz is reduced from .0038% to .0031% at 100W into 8Ω CRANKING THE TRANSISTOR LEADS The leads of the TO-92 transistors should be cranked to fit their mounting holes in PC board using a pair of needle-nose pliers. Here’s how it’s done. 64  Silicon Chip (the graphs published last month were generated using BF469/BF470). PCB assembly Fig.10 shows the parts layout on the PCB. Before starting the assembly though, it’s a good idea to carefully inspect the board. This will not only familiarise you with its layout but will also reveal any defects (however unlikely). Next, to prevent the high-current vias on the board from fusing if there is a fault, solder wire feed-throughs to the six vias with pads. You can use tinned copper wire or component lead off-cuts. Solder these feed-throughs on both sides and then trim off the excess. The next step is to install all the small (0.25W or 0.5W) resistors. Check each value using a digital multimeter set to Ohms mode before soldering it in place (the colour bands can sometimes be difficult to read). That done, siliconchip.com.au This fully-assembled module uses 2SA1837 & 2SC4793 transistors for Q7 & Q9 (note how they are mounted) plus the optional vertical connectors for CON1 & CON2. Make sure the inductor (L1) goes in with the correct orientation. install the two small 1N4148 diodes with their striped ends to the left, as shown on the overlay diagram. Follow with the four 1W resistors (or three, if you are upgrading the 6.8Ω resistor), again being careful to check the values. The two 180pF polypropylene capacitors can then go in, along with the 4.7nF and 100nF MKT capacitors. Fit the small signal transistors in the TO-92 packages next. There are four different types so read the markings carefully and be sure to install the correct type at each location. Note that their leads should be cranked using needle-nose pliers so that they fit properly – see the adjacent panel. siliconchip.com.au You can now solder in the four 0.1Ω 5W resistors. These should be mounted about 2mm proud of the PCB so that air can circulate beneath them for cooling. A cardboard spacer slid under the resistor bodies before soldering their leads can be used to ensure consistent spacings. The four M205 fuse clips are next. Press them down fully onto the board before soldering and ensure that the retention clips are on the outside (if in doubt, test-fit a fuse but remove it before soldering so that it won’t be heat-damaged). The best approach is to fit a dummy fuse to hold the fuse-clips in position, then tack solder them on the top of the PCB first. They can then be soldered to the pads on the underside of the PCB. The 10µH inductor is next on the list. It must be orientated as shown, with its start (S) lead (ie, the one nearest the centre of the bobbin) going through the pad on the PCB nearest the 10Ω 1W resistor. Now fit the connectors, ie, the RCA socket and the two Molex plugs. These should all should be installed flat on the board. Check that each is perpendicular to the adjacent PCB edge before soldering it in place. Note that either vertical or horizontal connectors can be used for the August 2011  65 Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o o o o No.   1   1   1   1   2   4   1   1   1   1   1   6   3   1   1   4   2 6021 TYPE TO-220 HEATSINK BF469/470 TRANSISTORS Value 1MΩ 22kΩ 12kΩ 6.8kΩ 6.2kΩ 2.2kΩ 390Ω 330Ω 270Ω 220Ω 120Ω 100Ω 68Ω 10Ω 6.8Ω 0.1Ω 0Ω SILICONE INSULATING WASHER M3 x 10mm SCREW M3 NUT FLAT WASHER FLAT WASHER PC BOARD Fig.11: follow this diagram if you are using BF469 & BF470 transistors. They go on the inside of the heatsink and require an insulating washer. 6021 TYPE TO-220 HEATSINK M3 NUT FLAT WASHER 2SA1837 (Q7) OR 2SC4793 (Q9) TRANSISTORS M3 x 10mm SCREW FLAT WASHER PC BOARD Fig.12: the alternative 2SA1837 (Q7 and 2SC4793 (Q9) transistors are attached to the outside of the heat­ sinks & do not require insulating washers. RCA socket (CON1) and power input connector (CON2). The vertical RCA socket (if used) goes in a different position than the horizontal one, ie, 66  Silicon Chip 4-Band Code (1%) brown black green brown red red orange brown brown red orange brown blue grey red brown blue red red brown red red red brown orange white brown brown orange orange brown brown red violet brown brown red red brown brown brown red brown brown brown black brown brown blue grey black brown brown black black brown blue grey gold brown brown black silver brown single black stripe it uses the pads to the left (see photo). Follow with the 220nF and 470nF metal film capacitors, then fit trimpot VR1 with its adjustment screw towards the bottom as shown. The 47µF and 470µF electrolytic capacitors can then go in. The 47µF NP (non-polarised) electrolytic can go in either way around but the others must all be orientated correctly. Don’t install the 1000µF capacitors yet. These parts are left out until after the assembly is fitted to the heatsink, otherwise you won’t be able to tighten the mounting screws for Q10 & Q11. Flag heatsinks Before fitting Q7 and Q9, you must first attach their flag heatsinks. If you are using BF470 (Q7) and BF469 (Q9) transistors, these should be attached to their heatsinks as shown in Fig.11. Alternatively, if using 2SA1837 (Q7) & 2SC4793 (Q9) transistors, fit the heatsinks as shown in Fig.12. Note that you must use insulating washers with the BF469/BF470 transistors. The 2SA1837 & 2SC4793 types have insulated tabs, so no washers are required. Do the mounting screws up fingertight initially, then push the transistor leads through the PCB so that the heatsinks line up with the indicated positions. If you are using BF469 & BF470 transistors, push each one down so that its heatsink sits flush against the PCB. That done, solder the leads and 5-Band Code (1%) brown black black yellow brown red red black red brown brown red black red brown blue grey black brown brown blue red black brown brown red red black brown brown orange white black black brown orange orange black black brown red violet black black brown red red black black brown brown red black black brown brown black black black brown blue grey black gold brown brown black black gold brown blue grey black silver brown black brown black silver brown single black stripe Table 2: Capacitor Codes Value 470nF 220nF 100nF 4.7nF 180pF µF Value IEC Code EIA Code 0.47µF 470n 474 0.22µF 220n 224 0.1µF 100n 104 .0047µF   4n7 472 NA 180p 181 tighten the mounting screws. It’s best to lightly solder one lead of each transistor first, then make any necessary adjustments before soldering the other two leads. Make sure that the insulating washers are properly aligned before tightening the mounting screws and be careful not to get the two transistors mixed up. The 2SA1837 & 2SC4793 transistors have their base and emitter leads transposed compared to the BF470/ BF469 types and so are mounted on the back of each heatsink (see Fig.12). Once they are attached, its just a matter of fitting them to the outside sets of pads so that the heatsinks are in the correct locations. Push the transistors down as far as they will go (in this case, the heatsinks will sit about 2mm proud of the PCB) before soldering their leads. As before, take care not to get the two transistors mixed up. Note that the parts layout diagram (and the photo in Pt.1 last month) indicate the arrangement for BF470/BF469 siliconchip.com.au Making A Winding Jig For The 10μH Inductor ➊ ➋ START ➌ F ➍ S Wind wire on bobbin clockwise The winding jig consists of an M5 x 70mm bolt, two M5 nuts, an M5 flat washer, a piece of scrap PC board material (40 x 50mm approx.) and a scrap piece of timber (140 x 45 x 20mm approx.) for the handle. In use, the flat washer goes against the head of the bolt, after which a collar is fitted over the bolt to take the bobbin. This collar should have a width that’s slightly less than the transistors. The photos published this month show 2SA1837/2SC4793 transistors in position. Winding the inductor The inductor (L1) is wound using a 2-metre length of 1mm-diameter enamelled copper wire on a plastic bobbin. Use a winding jig, as shown in the above panel; without it, it’s a much more difficult procedure and you risk damaging the relatively fragile bobbin. siliconchip.com.au These photos show how the winding jig is used to make the 10mH inductor. First, the bobbin is slipped over the collar on the bolt (1), then an end cheek is attached and the wire threaded through the exit slot (2). The handle is then attached and the coil tightly wound onto the bobbin using 30.5 turns of 1mmdiameter enamelled copper wire (3). The finished coil (4) is secured using a couple of layers of insulation tape and a band of heatshrink tubing. width (height) of the bobbin and can be wound on using insulation tape. Wind on sufficient tape so that the bobbin fits snugly over this collar without being too tight. Next, drill a 5mm hole through the centre of the scrap PC board material, followed by a 1.5mm exit hole about 8mm away that will align with one of the slots in the bobbin. The bobbin can be slipped over the collar, after Attach the bobbin to the jig as shown, then wind on 30.5 turns of 1mm diameter wire in the direction indicated (ie, clockwise), leaving about 20mm-long leads at each end. When finished, secure the winding with a narrow strip of insulation tape, then slip a 10mm length of 20mmdiameter heatshrink tubing over the bobbin and heat it gently (be careful not to melt the bobbin). That done, use a small, sharp hobby knife to scrape which the scrap PC board “end cheek” is slipped over the bolt (ie, the bobbin is sandwiched into position between the washer and the scrap PC board). Align the bobbin so that one of its slots lines up with the exit hole in the end cheek, then install the first nut and secure it tightly. The handle can then be fitted by drilling a 5mm hole through one end, then slipping it over the bolt and installing the second nut. away the enamel from the protruding lengths of wire (around the whole circumference) and then tin them. The inductor can then be installed on the PCB, orientated as shown. Preparing the heatsink The next step is to drill and tap the heatsink – see Fig.13 and the accompanying panel. This is the most painstaking part of the assembly process and it’s worth taking your time with it. August 2011  67 Drilling & Tapping The Aluminium Heatsink CL (SCALE 50%) 50.75 50.75 30.5 A 30.5 A A A A 75 A 42 A 30 25 10.25 10.25 200 100 HOLES A: DRILL 3mm DIAMETER OR DRILL 2.5mm DIAMETER & TAP FOR M3 SCREW. DEBURR ALL HOLES. Fig.13: this half-size diagram shows the heatsink drilling details. The holes can either be drilled and tapped (using an M3 tap) or can be drilled to 3mm and the transistors mounted using machine screws, nuts & washers. Fig.13 above shows the heatsink drilling details. If tapping the holes, they should be drilled to 2.5mm diameter right through the heatsink plate and then tapped to 3mm. Alternatively, the holes can be drilled through using a 3mm drill and the transistors mounted using screws, nuts and washers. It’s somewhat more work to tap the holes but it makes mounting the transistors quite a bit easier (no nuts required) and gives a neater appearance. Before drilling the heatsink, you will have to carefully mark out the hole locations using a very sharp pencil. That done, use a small hand-drill fitted with a 1mm bit to start the location of each hole. This is important as it will allow you to accurately position the holes (the locations are critical) before stepping up to larger drills in a drill press. Be sure to use a drill press to drill the holes (there’s no way you’ll get the holes perfectly perpendicular to the mounting face without one). Use a small pilot drill to begin with (eg, 1.5mm), then carefully step up the drill size to either 2.5mm or 3mm. The holes have to go between the fins so it’s vital to accurately position them. In addition, you can drill (and tap) three holes in the base of the heatsink so that it can later be bolted to a chassis. Be sure to use a suitable lubricant when drilling the holes. Kerosene is the recommended lubricant for aluminium but we found that light machine oil (eg, Singer or 3-in-1) also works well for jobs like this. Don’t try drilling the holes in one go. When drilling aluminium, it’s important to regularly remove the bit from the hole and clear away the metal swarf. If you don’t do this, the aluminium swarf has a nasty habit of jamming the drill bit and breaking it. Re-lubricate the hole and the bit with oil each time before you resume drilling. Tapping To tap the holes, you will need an M3 intermediate (or starting) tap (not a finishing tap). The trick here is to take it nice and slowly. Keep the lubricant up and regularly wind the tap out to clear the metal swarf from the hole. Relubricate the tap each time before resuming. Do not at any stage apply undue force to the tap. It’s all too easy to break a tap in half if you are heavy-handed and if the break occurs at or below the heatsink’s face, you can scratch both the tap and the heatsink (and about $25). Similarly, if you encounter any resistance when undoing the tap from the heatsink, gently rotate it back and forth and let it cut its way back out. In short, don’t force it or it will break. Having completed the tapping, deburr all holes using an oversize drill to remove any metal swarf from the mounting surface. The mounting surface must be perfectly smooth to prevent punch-through of the transistor insulating washers. Finally, the heatsink should be thoroughly scrubbed cleaned using water and detergent and allowed to dry. 68  Silicon Chip Ideally, the transistor mounting holes should be tapped with an M3 thread. If you don’t want to (or can’t) tap the holes, you can simply drill 3mm holes instead. In this case, accuracy is of the utmost importance as it’s difficult to fit the screws if the holes aren’t correctly positioned between the fins. You also need to drill and/or tap the heatsink to mount it in the chassis. You can either drill and tap three holes in the bottom of the heatsink (see photo) or you can attach right-angle brackets to the sides or face. Once all the holes are drilled, deburr them using an oversized drill bit and clean off any aluminium particles or swarf. Check that the area around the holes is perfectly smooth or else the insulating washers could be damaged. Final assembly Now it’s time to mate the PCB with the main heatsink but first re-check the face of the heatsink. All holes must be deburred and it must be perfectly clean and free of any grit or metal swarf. Start the heatsink assembly by mounting transistors Q10, Q11 & Q16. A silicone rubber washer goes between each of these transistors and the heatsink. Q10 and Q11 also require an insulating bush under each screw head. Fig.14 (A & B) shows the mounting arrangements. Because these three transistors are so close, you may need to trim the sides of Q16’s washer so they fit sideby-side. Be careful not to get Q10 & Q11 mixed up as their type numbers are similar. If the holes are tapped, these transistors can be secured using M3 x 10mm machine screws. Alternatively, if you have drilled non-tapped holes, you will need to use M3 x 15mm machine screws, with the screws coming through from the heatsink side (ie, the screw heads go between the heatsink fins). Make sure the three transistors and their insulators are properly vertical, then do the screws all the way up but don’t tighten them yet; ie, you should still just be able to rotate the transistors in each direction. The next step is to fit an M3 x 10mm tapped spacer to each corner mounting hole on the PCB. Secure these using M3 x 6mm machine screws. Once they’re on, sit the board down on the spacers and lower the heatsink so that siliconchip.com.au MAIN PLATE OF HEATSINK MAIN PLATE OF HEATSINK MAIN PLATE OF HEATSINK SILICONE INSULATING WASHER SILICONE INSULATING WASHER M3 FLAT WASHER INSULATING BUSH M3 x 10mm SCREW M3 FLAT WASHER M3 x 15mm SCREW M3 x 10mm SCREW M3 TAPPED HOLE M3 TAPPED HOLE A AMPLIFIER PC BOARD M3 TAPPED HOLE NJL3281D OR NJL1302D TRANSISTOR (TO-264) BD139 TRANSISTOR (TO-225) MJE15030 OR MJE15031 TRANSISTOR (TO-220) AMPLIFIER PC BOARD (HEATSINK FINS) B AMPLIFIER PC BOARD C Fig.14: this diagram shows the mounting details for the TO-220 driver transistors (A), the BD139 VBE multiplier (B) and the four output transistors (C). After mounting these transistors, use your multimeter (switched to a low ohms range) to confirm that they are properly isolated from the heatsink – see text. the transistor leads pass through the appropriate holes. The four output transistors (Q12Q15) can now be fitted. Two different types are used so be careful not to mix them up (check the layout diagram). As shown in Fig.14(C), these devices must also be insulated from the heatsink using silicone insulating washers. Start by fitting Q12. The procedure here is to first push its leads into the PC mounting holes, then lean the device back and partially feed through an M3 x 15mm mounting screw with a flat washer (or M3 x 20mm for untapped holes). That done, hang the insulating washer off the end of the screw and then loosely screw the assembly to the heatsink. The remaining three devices are then installed in exactly the same way but take care to fit the correct transistor type at each location. Once they’re in, push the board down so that all four spacers (and the heatsink) are in contact with the benchtop. This automatically adjusts the transistor lead lengths and ensures that the bottom of the board sits exactly 10mm above the bottom edge of the heatsink. Now adjust the PCB assembly horizontally so that each side is 32.5mm in from its adjacent heatsink end. Once you are sure it is properly positioned, tighten all the transistor screws just enough so that they are held in place siliconchip.com.au while keeping the insulating washers correctly aligned. The next step is to lightly solder the outside leads of Q12 & Q15 to their pads on the top of the board. The assembly is then turned upside down so that the heatsink transistor leads can be soldered. Before soldering the leads though, it’s important to prop the front edge of the board up so that the PCB is at rightangles to the heatsink. If you don’t do this, it will sag under its own weight and will remain in this condition after the leads have been soldered. A couple of cardboard cylinders cut to 63mm can be used as supports (eg, one at each corner adjacent to CON1 & CON3). With these in place, check that the board is correctly centred on the heatsink, then solder all 29 leads. Make sure the joints are good since some can carry many amps at full power. Once the soldering is completed, trim the leads using a steel rule as a straight edge to ensure consistent lead lengths. That done, turn the board right way up again and tighten the transistor mounting screws to ensure good thermal coupling between the devices and the heatsink. Three M3 (or M4) holes can be drilled and tapped in the base of the heatsink so that it can later be attached to a chassis. These should be about 10mm deep. August 2011  69 Music Power, Dynamic Headroom & Slew Rate W HILE WE PUBLISHED comprehensive performance data and graphs on the Ultra-LD Mk.3 in last month’s article, we did not include figures for music power, dynamic headroom or slew rate limit. These are less indicative of raw performance than the figures already published but readers have asked for them so here they are. The power output specifications presented last month are for continuous sinewave output. But this is quite a rigorous test which is far removed from normal signals involving music, speech and sound effects which usually have a peak power to average power ratio of at least 10dB. For classical music, the ratio is usually at least 20dB. So for a more realistic indication of how much power is available with music signals, we can apply a sinewave burst signal with a specific dynamic range and measure the peak power delivered before clipping or serious distortion. With the right signal characteristics, this gives us a “music power” rating. The ratio of music power to maximum continuous power is the “dynamic headroom”. The standard (IHF-A-202/EIA RS-490) specifies the test signal as a 1kHz sinewave which alternates between full scale (0dB) for 20ms and -20dB (ie, 1/10th the voltage) for 480ms. Fig.15 shows a digital scope grab of this test being run into an 8Ω load (the green input trace is hidden under the yellow output trace as they track very closely). The reason that the music power rating is higher than the continuous power rating has to do with supply regulation, which is mainly determined by the power transformer and bridge rectifier diodes. By “regulation” we mean how much the supply voltage changes depending on the current drawn from it. Because the music power tests involve a lower average current, the transformer isn’t loaded as heavily and so the supply voltages don’t sag as much. The dynamic headroom for this amplifier is a healthy 1dB for 8Ω loads and 1.3dB for 4Ω loads. You can verify the 8Ω music power rating with Fig.15 by converting the peak voltage to RMS and using the formula P = V2 ÷ R. Slew rate We are sometimes asked what the slew rate is for our amplifier designs. Slew rate is often touted in some hifi magazines as being important for “fast” audio amplifiers. It is really a lot of rot as slew rate is primarily a relevant figure for video amplifiers but we’ve measured it anyway and we will describe its significance. It’s measured by applying a square-wave signal with very fast rise and fall times; much faster than occur in any normal audio signals. Fig.16 shows the unloaded square-wave response of the Ultra-LD Mk.3 module. The input signal (after the RF filter) is in cyan and the output in yellow. For this test, the square-wave generator must have a low output impedance (<100Ω) otherwise the RF filter limits the slew rate more than the amplifier. As typical for an audio amplifier, the transient response shows some overshoot (about 20%). Since audio signals do not have such fast transitions (ie, no frequencies above about 20kHz), it isn’t a problem. To accurately measure the slew rate, we put the amplifier on-load and zoomed in on the rising and falling edges. For an 8Ω load, the voltage ramps 12V over a 500ns interval, so the slew rate is 24V/μs (with a load, it’s the same for both directions). With a 4Ω load, it drops slightly, to 20V/μs. The maximum slope of a sinewave is computed (in V/s) as 2π x frequency x peak voltage. So to obtain a full-scale (55V peak) 20kHz sinewave output we need a slew rate of 2π x 20,000 x 55 ÷ 1,000,000 = 6.9V/μs. So the Music Power ............................................... 170W (8Ω), 270W (4Ω) slew rate of this amplifier (24V/μs) Dynamic Headroom.........................................1dB (8Ω), 1.3dB (4Ω) is far more than Slew Rate .......+35,-60V/μs (no load), ±24V/μs (8Ω), ±20V/μs (4Ω) adequate. Don’t over-tighten the mounting screws though. Remember that the heatsink is made from aluminium, so you could strip the threads if you are too ham-fisted. Checking device isolation You must now check that the transistors are all electrically isolated from the heatsink. That’s done by switching your multimeter to a high ohms range and checking for shorts between the heatsink mounting surface and the collectors of the heatsink transistors (note: the collector of each device is connected to its metal face or tab). For transistors Q10-Q15, it’s simply a matter of checking between each of the fuse-clips closest to the heatsink and the heatsink itself (ie, on each side of the amplifier). That’s because the device collectors in each half of the output stage are connected together and run to their respective fuses. Transistor Q16 (the VBE multiplier) is different. In this case, you have to check for shorts between its centre (collector) lead and the heatsink. In either case, you should get an open-circuit reading. If you do find a short, undo each transistor mounting screw in turn until the short disappears. It’s then simply a matter of locating the cause of the problem and remounting the offending transistor. Additional Specifications 70  Silicon Chip This power supply board can run two Ultra-LD Mk.3 amplifier modules and will be described in Pt.3 next month. siliconchip.com.au Fig.15: this shows the pulsed sinewave that’s used to test the amplifier’s music power (-20dB for all but 20ms every 500ms). The yellow trace is the output while and green trace (underneath it) is the input signal. The level is set as high as possible without the output clipping during the high amplitude burst. The music power can then be calculated based on the peak output voltage, in this case 170W (8Ω). Be sure to replace the insulating washer if it has been damaged in any way (eg, punched through). Completing the PCB assembly The PCB assembly can now be completed by installing the two 1000µF 63V capacitors. You must also remove the two support spacers from the edge of the board adjacent to the heatsink. In fact, it’s quite important that the rear edge of the board be supported only by the heatsink transistor leads. This avoids the risk of eventually cracking the PCB tracks and pads around the heatsink transistors due to thermal expansion and contraction of their leads as they heat up and cool down. siliconchip.com.au Fig.16: the slew rate test waveform. The cyan square wave is the input signal (somewhat distorted by circuit capacit­ ances and parasitic inductance). The yellow waveform is the amplifier’s output. As can be seen, its edges are less vertical than the input signal and by measuring their slope we can calculate the amplifier’s maximum slew rate. 20% overshoot is typical for an audio-optimised amplifier. In short, the rear spacers are installed only while you fit the heatsink transistors and must then be removed. They play no part in securing the module. Instead, this edge of the module is secured by bolting the heatsink itself to the chassis. As previously stated, this can be done by tapping M3 (or M4) holes into the main plate on the underside of the heatsink or by using right-angle brackets. The front of the board is secured using the two M3 x 10mm spacers fitted earlier. That completes the assembly of the power amplifier module. The next step is to build the power supply module and we’ll describe how that’s done next month. We’ll also describe how to power up and test the amplifier and give some basic details on housing it SC in a metal case. Stability At Very High Power L AST MONTH, we explained how the 180pF compensation capacitor values are selected to not only prevent the amplifier from oscillating under normal conditions but also during recovery after being driven into clipping. While this is true for a continuous sinewave, we discovered that the Ultra-LD modules can sometimes “misbehave” in response to very large but brief signal excursions with a 4Ω load (this can be observed with peak power levels in the range of 300W). At this power level, occasionally we can observe an output deviation as the signal swings away from the negative rail. This is nothing like the high-frequency oscillation that can occur if the compensation capacitors are undersized; it certainly won’t blow the fuses and we don’t think it’s likely to cause transistor or speaker damage. But it does result in a temporary, massive increase in distortion. Mind you, driving any amplifier beyond clipping also causes a massive increase in distortion! The reason for this behaviour seems to be that the power supply is sagging badly under such a load and this causes a small amount of clipping during the latter portion of the signal peak. This is based on the fact that for a 20-cycle sinewave burst, typically only one of the cycles will be distorted. So if you plan to drive the Ultra-LD Mk.3 amplifier right at its power limit into a 4Ω load, it may be worthwhile increasing the compensation capacitors from 180pF to 220pF (Rockby stock code 31943). This will slightly worsen the distortion performance overall but should allow 4Ω peak power in excess of 300W without this problem. We did not observe this phenomenon when driving an 8Ω load. August 2011  71