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Upgrading an Ultra-LD Mk.2 amplifier to Mk.3 performance By NICHOLAS VINEN This teensy PCB lets you add the vital modifications to an UltraLD Mk.2 amplifier to bring it up to Mk.3 performance. We are doing this so that all those people who built the Ultra-LD Mk.2 from the August-September 2008 issues will not be too annoyed with us for superseding it with the Mk.3 version. After all, we want to keep our readers happy and content! 84  Silicon Chip siliconchip.com.au Table 1 C A 180pF 180pF 330 C B Q16 BD139 E VR1 1k G F 220 470nF 2.2k Wire/Pad A B C D E F/G Length 70mm 80mm 60mm 50mm 85mm 75mm 120 B D E 330 470nF D 220 2.2k E 100V F 100V G 180pF B C E B C 180pF BD139 VR1 1k 11190210 120 Q16 Fig.1: the upgrade board circuit. Q16 is the new VBE multiplier transistor, with the voltage across it adjusted by VR1. Next, the 220Ω resistor and parallel 470nF capacitor are connected between the driver transistor emitters for faster output transistor switch-off. And finally, the two 180pF capacitors and 2.2kΩ resistor form a double-pole filter across the transimpedance stage transistor on the main board, providing increased open loop gain in the audio band while retaining stability. A incorporated on the upgrade board while the remainder involve component replacements on the main PCB. Fig.2: the parts layout on the upgrade PCB. Transistor Q16 is mounted on the component side, as far above the PCB as possible. Its leads are then bent around the board’s edge so that it “hangs” upside-down from it, ultimately supporting the PCB on the heatsink – see photo. T HIS TINY upgrade board carries the new components, ie, the VBE multiplier transistor (Q16), the new driver transistor emitter arrangement and the 2-pole compensation filter. A few other changes are made by replacing components directly on the board. The upgrade board is mounted on the heatsink via the VBE multiplier transistor and wired to pads on the main amplifier board via flying leads. In summary, the changes to upgrade an Ultra-LD Mk.2 to Mk.3 standard are: (1) Two of the ThermalTrak diodes in the bias voltage generator are replaced with an adjustable VBE multiplier, allowing quiescent current adjustment and providing better thermal stability. The constant current source resistor is also changed to 68Ω for correct biasing. (2) The two driver emitter resistors are replaced by a single resistor, bypassed by a 470nF capacitor. This speeds up output transistor turn-off and so resiliconchip.com.au duces high-frequency distortion. (3) The 100pF Miller capacitor, connected between the collector of Q9 and the base of Q8, is replaced with two 180pF capacitors and a 2.2kΩ resistor. This replaces the single-pole compensation scheme with a 2-pole filter for more open loop gain at audio frequencies. (4) The feedback capacitor goes from 220µF to 1000µF, which reduces distortion and flattens the response at very low audio frequencies. It also slightly improves the signal-to-noise ratio. (5) The 820pF input filter capacitor is increased to 4.7nF, for more effective RF filtering. (6) The output filter inductor and capacitor values are increased, improving magnetic field cancellation and thus lowering high-frequency distortion further. Of these changes, the first three are Construction The first step to upgrade the amplifier module is to assemble the upgrade board. This is built on a PCB coded 01209111 and measuring 20.5 x 36.5mm. Begin by fitting the four resistors. Check each one with a DMM set to Ohms mode before installing it. Follow with the two 180pF polypropylene capacitors, then fit the 470nF MKT capacitor and trimpot VR1. The latter should go in with its screw terminal to the right side of the board – see Fig.2. That leaves Q16, the BD139 transistor. It should be soldered to the top of the board, with its metal tab facing away from the nearest edge and with its leads just protruding through the bottom of the board by a millimetre or two. Solder one pin, then ensure it is in straight before soldering the other two. With all three leads soldered, bend it around the edge of the PCB until its leads form a “J” shape, as shown in the photo. Now solder lengths of wire to the pads marked “A” through “G”. The length required for each wire is shown in Table 1. Modifying the module If the module has already been installed in a chassis, remove it. Undo the six screws holding the transistors to the heatsink and separate the PCB. The silicone rubber washers can sometimes “stick” so you may need to gently pry the transistors off the heatsink. Remove any silicone rubber washers that are stuck on the heatsink. Now mark out and drill the extra mounting hole shown in Fig.4 (for the VBE multiplier transistor). It goes in the same place whether you are tapping the heatsink or drilling right through; in either case it is between the fins. September 2011  85 Q12 Q13 NJL3281D MJE15030 MJE15031 NJL3281D Q14 Q15 NJL1302D NJL1302D A Q11 100 1000F 63V 100nF 100nF (2) REMOVE RESISTOR 100 Q1 Q2 (7) REPLACE CAPACITOR 510 12k 1000F 1M 47F NP 4.7nF CON1 SIG COM 10 0 D1 4148 4148 D2 (6) REPLACE ELECTROLYTIC As we stated in the Ultra-LD Mk.3 construction article last month, you must use a lubricant such as Kerosene or 3-in-1 oil when drilling or tapping aluminium. Regularly clear the swarf from the hole, especially during the tapping process. It’s easiest to start with a small pilot drill and slowly enlarge the hole to size. You can either drill the hole to 2.5mm and tap it for an M3 thread, or just drill a 3mm hole right through and use a longer machine screw and a nut between the heatsink fins. When finished, de-burr it using a large drill bit and check that the surface of the heatsink is perfectly smooth. Then wash the oil residue off G 390 1W H E Q8 BC639 470F 63V 0 Q3 Q4 CON2 L1 6.8 1W 2.2k 10 1W (4) REMOVE CAPACITOR 2 x 2SA970 Q9 2 x BC546 68 2.2k 12k 68 100 F 6.2k Q5 Q6 (5) REPLACE RESISTOR 100 BF469 Q7 6.8k 1W 100 68 BF470 2.2k 47F REPLACE (8) R EPLACE L1 Q5,Q6: BC556 47F 35V 100nF 220nF 220nF 250V 250V 6.2k 100nF B (3) REMOVE RESISTOR 18080110 FUSE 2 (5A) reifilpmA 2.KM DL-artlU 100nF (1) CUT TRACK (TOP LAYER) 0.1 5W 1000F 63V D 0.1 5W 100 0.1 5W 0.1 5W FUSE 1 (5A) C 100 100 Q10 Fig.3: the locations where the original Ultra-LD Mk.2 board is modified to upgrade it to the Mk.3 standard. There are nine numbered component changes; some are removed entirely and others are replaced with components having different values. The vias/pads labelled A-G indicate where the corresponding wires from the upgrade board are soldered. Pad E is the only place where a wire is soldered to a pad which still holds a component lead. 22k +55V 0V 55V CON3 SPEAKER + SPEAKER PHONES OUT (9) REPLACE CAPACITOR with some water and detergent and leave the heatsink to dry. Use the same procedure to remove the remaining lead. For components where you can’t get at the leads, such as the inductor and the capacitors, the easiest method is to “rock” the component out. Heat one of its leads and gently pull it up on that side. The component will bend over slightly as the lead is withdrawn by a millimetre or two. Then heat and pull up the other side, bending the component over in the other direction. Repeat a few times and it will lift free of the board. Once the component has been removed, the holes must be cleared of solder before it can be replaced. Use a solder sucker; heat the pad for a few Removing parts While the heatsink is drying you can make the necessary changes on the main amplifier PCB. This involves removing and in some cases replacing components. This can be a little tricky on a double-sided PCB with plated through holes. To remove a resistor, cut one of its leads close to its body, then grasp the resistor with pliers and gently pull on it while heating the pad on the underside of the board. It should come out easily after a few seconds; if not, let the board cool down and try again. Table 1: Resistor Colour Codes o o o o o o No.   1   1   1   1   1 86  Silicon Chip Value 2.2kΩ 330Ω 220Ω 120Ω 68Ω 4-Band Code (1%) red red red brown orange orange brown brown red red brown brown brown red brown brown blue grey black brown 5-Band Code (1%) red red black brown brown orange orange black black brown red red black black brown brown red black black brown blue grey black gold brown siliconchip.com.au CL ADDITIONAL HOLE LOCATED HERE* 75 40 100 200 * EITHER DRILL HOLE 2.5mm DIAMETER & TAP FOR M3 SCREW OR DRILL 3mm DIAMETER (SCALE 1/2) Fig.4: this half-size drilling template shows where the additional hole is drilled. It is in the same place regardless of whether you are going to tap it or not. If you are, drill it to 2.5mm and then use an M3 tap (it’s easiest to drill right through the heatsink, then you don’t need to use a finishing tap). Otherwise drill it to 3mm and use a longer (15mm) machine screw and nut to secure the VBE multiplier transistor. seconds before using it to ensure all the solder has melted. Solder suckers work best when the tip is right up against the hole to maximise suction. If removing the solder from the upper pad doesn’t clear the hole, apply the same technique to the opposite pad. If the hole is still blocked (even partially so), add some fresh solder to the pad (this also adds flux and helps the solder flow) and then try again. Making the changes Here are the changes you need to make. Be careful not to touch any plastic components (eg, capacitors) with the soldering iron while doing so. They are shown on the overlay diagram of Fig.3: STEP 1: cut the top layer track con- Table 2: Capacitor Capacitor Codes Value 470nF 220nF 4.7nF 180pF µF Value IEC Code EIA Code 0.47µF 470n 474 0.22µF 220n 224 .0047µF   4n7 472   NA 180p 181 STEP 3: remove the 100Ω resistor connected to pin 3 of Q11. STEP 4: remove the 100pF 100V capacitor near Q9. STEP 5: if the current resistor value is not 68Ω, replace it with a 68Ω resistor. STEP 6: remove the 220µF 16V capacitor and replace it with a 1000µF 16V capacitor (you may need to bend the leads of the adjacent resistor a little for it to fit). STEP 7: remove the 820pF capacitor and replace it with a 4.7nF MKT capacitor (do not make this change if you will be driving the amplifier from a high source impedance, ie, more than 220Ω). STEP 8: remove the inductor and wind a new one with 30.5 turns (rather than 25.5 turns) of 1mm-diameter enamelled copper wire. We published detailed instructions on how to do this in last month’s Ultra-LD Mk.3 construction article. It must be wound and installed with the correct orientation, as shown on the overlay. You can re-use the inductor bobbin if desired. STEP 9: remove the 150nF 400V capacitor and replace it with a 220nF metal film capacitor (250/400V DC or 250V AC). Re-attaching the board necting DQ14 and DQ15 using a sharp hobby knife. It’s best to cut the track in two locations and lift out or obliterate the section between the cuts. Check that the vias at either end are not connected using a DMM on continuity mode. STEP 2: remove the 100Ω resistor dir­ ectly below Q13. Presensitized PCB & associated products When all those changes are complete, re-attach the board to the heatsink. This is best done by re-inserting the machine screws through the tabs, hanging the insulating washers off them and then screwing each transistor to the heatsink with a couple of turns before tightening them all up. Make sure that the silicone insulat- IN STOCK NOW! •Single Sided Presensitized PCBs •Double Sided Presensitized PCBs •Fibreglass & Phenolic •UV Light Boxes •DP50 Developer •PCB Etch Tanks, Heaters & Aerator Pumps •Thermometers •Ammonium Persulphate Etchant •PCB Drill Bits (HSS & Tungsten) For full range, pricing and to buy now online, visit 36 Years Quality Service siliconchip.com.au www.wiltronics.com.au Ph: (03) 5334 2513 Email: sales<at>wiltronics.com.au September 2011  87 Parts List 1 PCB, code 01209111, 20.5 x 36.5mm 1 1kΩ 25-turn vertical trimpot (VR1) 1 10µH air-cored inductor (or 1 pot core bobbin and 2m of 1mm diameter enamelled copper wire plus a short length of 20mm diameter heatshrink tubing) 2 6.3A, 6.5A or 7.5A M205 fastblow fuses 1 50cm length wire-wraping wire (Kynar) or light duty hookup wire 1 TO-126 or TO-220 insulating washer 1 M3 x 15mm machine screw, M3 flat washer and nut 1 M3 x 10mm machine screw Semiconductors 1 BD139 1A NPN transistor (Q16) Capacitors 1 1000µF 16V electrolytic 1 470nF MKT 1 220nF 250/400V DC or 250V AC MKT/MKP 1 4.7nF MKT or ceramic 2 180pF 100V Polypropylene (Rockby stock code 36350) Resistors 1 2.2kΩ 1 330Ω 1 220Ω 1 120Ω 1 68Ω ing washers are all aligned properly before finishing the job. The upgrade board can then be fitted. Start by soldering its wire “D” to to the pad on the main PCB adjacent to Q11, labelled “D” on the overlay diagram (Fig.3). The upgrade board is then secured to the heatsink, with the component side up and Q16 on the bottom, using an M3 x 10mm machine screw, flat washer and silicone rubber insulating washer (TO-126 or TO-220 size). If your hole isn’t tapped, use a 15mm machine screw and feed a nut between the fins using small pliers. With the upgrade board firmly attached to the heatsink, re-check the isolation of all transistors. To do this, remove the fuses and set your DMM to Ohms mode. Measure between the 88  Silicon Chip face of the heatsink and the nearest fuse clips on either side. If the reading is less than several megohms then one or more of the insulators has been punctured or is improperly installed and must be fixed. You also need to check that Q16 is properly isolated. To do this, check the resistance between the heatsink and the right-most lead of the the 330Ω resistor (the one closest to Q16). This too should read very high. You can then set about soldering the six remaining wires from the upgrade board to the appropriate pads on the amplifier PCB. These are labelled with letters A-G on the overlay diagram, corresponding to the pads labelled A-G on the upgrade board. Wires A and B are soldered to vias, which have enough exposed copper for solder adhesion (they are essentially small pads). Wires C, F and G are soldered to now-empty pads. Wire E is soldered to the bottommost lead of the 10Ω 1W resistor. Strip that wire back a bit further than the others and wrap it around the resistor lead before soldering it in place. Alternative upgrade method It is possible, though not recommended, to install the upgrade board without the need to detach or drill the heatsink. In this case, Q16 is mounted on the tab of Q10 or Q11, using the same screw (an insulating washer is still required). The disadvantage of this approach is that the extra thermal resistance between Q16 and the heatsink means that the thermal tracking is inferior and so the quiescent current is not as well controlled. If you do decide to take this approach, fit a rectangle of insulating material to the bottom of the upgrade PCB (eg, presspahn or plastic cut from a takeaway container or “blister pack”). This can be held on using cable ties. Otherwise, the bottom of the PCB could short against the top of the 1000µF 63V capacitors. Testing and adjustment With all the connections made, the amplifier can be tested and the quiescent current set, as follows: STEP 1: wind trimpot VR1 (on the upgrade board) fully anti-clockwise. Since this is a 25-turn trimpot it can take many turns before it is at its endstop. The screw normally continues to turn once the end has been reached but you should hear a soft click with every revolution. STEP 2: remove the fuses (if installed) and replace them with 68Ω 5W safety resistors. These can be soldered to blown fuses as shown in the Ultra-LD Mk.3 construction article elsewhere in this issue. These then plug straight into the fuse-holders. STEP 3: wire up the supply leads to the module; be very careful to get these right or damage is likely once power is applied. These voltages are high enough to be lethal (especially the 110V or so between the positive and negative rails) so you must be careful to avoid touching the wiring and fuseholders while power is applied to the board. In fact the safest thing to do is to avoid touching the board at all. STEP 4: connect a DMM set in volts mode across one of the safety resistors. If possible, use alligator clip leads. STEP 5: apply power and watch the voltage reading. Cut power immediately if you get a reading of more than 10V across the safety resistor. In fact it should be below 1V (but may “dance around” a bit). If the power is left on and there is a fault, the safety resistors can quickly burn out. STEP 6: slowly wind VR1 clockwise. The voltage across the safety resistor should stabilise and start to rise. Adjust it so that the reading is about 9.5V. STEP 7: measure the voltage across the speaker output terminals. This should be no more than ±50mV (it’s typically around +20mV). Assuming that’s all OK, you can cut the power, wait for the filter capacitors to discharge fully and reinstall the fuses (using the specified 6.5A types, or the closest you can get). The module can then be reinstalled in the chassis. A quick test with a signal source and some speakers should confirm that the upgraded module is working correctly. Once the amplifier is restored to full working condition, it’s a good idea to run it with the lid on for an hour or so with normal program material and then check the voltage across one of the 0.1Ω emitter resistors on each amplifier module. If this is more than about 15mV, wind the associated adjustment trimpot back a little. Although the upgraded module will have improved thermal tracking, its quiescent current can still increase a bit as the module warms up so it’s a good idea to re-adjust the setting once it has been operating for a while. SC siliconchip.com.au