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Ultra-LD Mk.3 200W Amplifier Module; Pt.3 Power supply & quiescent current adjustment By NICHOLAS VINEN This month, we describe the power supply for the new high-performance Ultra-LD Mk.3 amplifier. We also describe how to test and adjust the completed amplifier module and give some details on building it into a metal case. T HE POWER SUPPLY circuit is virtually identical to the UltraLD Mk.2 power supply described in September 2008. The changes are in the PCB, which is longer and narrower. This allows it to fit between two amplifier modules mounted on either side of a rack-mount case. In this configuration, the two DC output connectors line up with the power supply sockets on each amplifier, simplifying the wiring. The AC 72  Silicon Chip input terminals face towards the back of the case, where the transformer is normally situated. Note that if a single power supply is shared between two amplifier modules, the continuous output power is reduced when both channels are driven. However, the music power will not drop so much. Power supply circuit Fig.17 shows the circuit details of the power supply. It’s based on a toroidal mains transformer (T1) with two 40V windings and two 15V windings. The two 40V windings are connected together to give 80VAC centretapped and this arrangement drives bridge rectifier BR1. This in turn feeds six 4700µF 63V electrolytic capacitors (ie, 14,100µF on each side) to provide balanced ±57V DC rails to power the amplifier. siliconchip.com.au ~ T1 CON1 TERM1 BR1 35A/600V + ~ 4700 F 63V 4700 F 63V 4700 F 63V 0V F1 5A A TERM2 – 0V 3.3k 5W –57V A 40V 0V 4700 F 63V TERM3 4700 F 63V 4700 F 63V 15V N  LED1 K 40V POWER S1 +57V A CON2  LED2 +57V 3.3k 5W 0V K 0V –57V CON4 15V CON5 30V AC 0V E T1: 240V TO 2x 40V/300VA, 2x 15V/7.5VA CON6 D1–D4: 1N4004 K 0V A A K K A +20V K REG1 7815 IN +15V GND 2200 F 25V A CON3 OUT 100 F 16V 0V 2200 F 25V LEDS 1N4004 A IN –15V OUT REG2 7915 K A K 100 F 16V GND 7815 7915 GND SC 2011 ULTRA-LD AMPLIFIER MK3 POWER SUPPLY GND IN GND IN OUT IN OUT Fig.17: the power supply is based on a toroidal transformer (T1) with two 40V windings and two 15V windings. The two 40V windings drive bridge rectifier BR1 and six 4700mF filter capacitors to produce the ±57V rails. Two LEDs are connected in series with 3.3kΩ 5W current-limiting resistors across these ±57V supply rails. These serve two purposes: (1) they provide a handy indication that power is present on the supply rails (or when it is not present) and (2) they discharge the filter capacitors when the power is switched off (see warning panel). The two 15V windings are also connected together to provide 30VAC centre-tapped. These drive bridge rectifier D1-D4 and two 2200µF filter capacitors to derive unregulated rails of about ±20V. These rails are then fed to 3-terminal regulators REG1 & REG2 to derive regulated ±15V supply rails to power a preamplifier module. The +20V rail is also made available as an output, along with a 30VAC output. The +20V rail is used to power the “Universal Speaker Protector siliconchip.com.au & Muting Module” (described next month), while the 30VAC output is connected to the “AC Sense” input of this module. This latter input is used to quickly disconnect the speaker when the power goes off, to avoid switch-off thumps. Power supply assembly Start by checking the PCB (code 01109111) for defects such as hairline cracks or under-etched areas and repair if necessary. Fig.18 shows the parts layout on the PCB. Begin by fitting the two wire links using 0.71mm or 1mm-diameter tinned copper wire (1mm diameter is better but you may need to enlarge the holes slightly). Follow with the four 1N4004 diodes (D1-D4), orientating them as shown. Install the two 3-terminal regulators next. You will need to bend their leads down by 90° so that they fit the PCB pads with the tab mounting hole lined up correctly. Attach each regulator to the board using an M3 x 6mm machine screws, shakeproof washer and nut, taking care not to get the two different types mixed up. Solder the leads after the screws have been tightened. The two LEDs can now be installed. These sit flush against the PCB with the flat side of the lenses orientated as shown on the overlay. Follow these with the two 3.3kΩ 5W resistors. These should be stood off the board by about 2mm, to allow the air to circulate beneath them for cooling (use a card­ board spacer during soldering). The two 5-way screw-terminal connectors are made by dovetailing 2-way and 3-way blocks together. Be sure to fit these assemblies with the wire entry September 2011  73 CA V 5 1 TCT C 15V CAV 0 3 ~ 5 1 30VAC 15V 1 tuptu O OUTPUT 1 3.3k 5W A LED2 – 4700 F 63V 4700 F 63V + 4700 F 63V + A NI- + TERM3 –IN 4004 4004 CON5 K A K A 4004 4004 K K 2200 F 2200 F 25V 25V REG2 7915 REG1 7815 D3–D6 TC TERM2 4700 F 63V 4700 F 63V + 4700 F 63V + + CT NI+ TERM1 +IN LED1 + CON2 OUTPUT 2 2 tuptu O–57V 0V +57V - 3.3k 5W + 220 F 16V 11190110 uS r e woP reifilpmA 2.k M DL-artlU 01109111 CON4 CON1 Ultra-LD Mk.3 Power Supply +57V + 0V 0 –57V - 220 F 16V CON3 CON6 +20V –15V V 5 1- 00 +15V V 5 1 + 00 V 02+ Fig.18: install the parts on the power supply board as shown here, taking care to ensure that all the electrolytic capacitors are mounted with the correct polarity. Be sure also to use the correct regulator at each location. The two LEDs indicate when power is applied and remain lit until the 4700mF capacitors discharge after switch-off. holes facing towards the adjacent edge of the PCB. The two 3-way terminal blocks for the ±57V outputs can then go in. Alternatively, instead of fitting these blocks, you can solder the DC supply leads directly to the PCB pads if it will be mounted right next to the amplifier modules. The three Quick-Connect (spade) terminals are next on the list. If you are using PCB-mount connectors, simply push the pins through and solder them in place. It will take a while to heat the connectors so that the solder will “take”. However, be careful not to overdo it, as the solder could “wick” through the hole and onto the spade section. If you are using 45° chassis spade lugs instead, screw them down tightly using M4 machine screws, nuts and washers – see Fig.19. If you can’t get single-ended chassis lugs, cut one side off double-sided lugs. Finally, fit the electrolytic capacitors, starting with the two 220µF units and finishing with the six large 4700µF units. Be sure to orientate them correctly and make sure that they all sit flush with the PCB. Cabling The new plug-in connectors on the power amplifier modules make installing and removing them much 74  Silicon Chip easier than before. However, they also mean that you have to crimp the leads to the plugs before the wiring can be completed. You can buy a special tool to crimp the pins but it’s expensive. The solution is to crimp the pins with small pliers and then carefully solder the exposed wire to them before pushing the pins into the plastic shell. The pins are designed for wires with up to 1.29mm conductor diameter (18AWG) and 3.1mm outer diameter. This is equivalent to heavy-duty hookup wire and is quite adequate for the power supply wiring. For the speaker wiring, it is perhaps a little thin but it will be sufficient provided the wires to the speaker protection module are kept fairly short. To wire the plugs, first cut the wires to length and strip about 5mm from the ends. Rest the wire in the channel within the pin, then use small pliers to fold the small metal tabs around the exposed conductor. The larger tabs can then be folded around the insulated portion of the wire and compressed to hold the wire in place. You can then apply a small amount of solder to each crimped joint, heating it for a few seconds until the solder wicks into it. Try to avoid getting any solder on the outside of the pin or it may not fit into the plastic shell. When all the pins are crimped and QUICK CONNECT PC BOARD M4 STAR WASHER M4 FLAT WASHER M4 x 10mm SCREW & NUT Fig.19: here’s how the single-ended male Quick Connects are secured to the power supply PC board. soldered to the wires, push the pins into the rear of the plastic connector shells so that they click into place. Chassis assembly We plan to provide detailed instructions for building a complete Ultra-LD Mk.3 Stereo Amplifier in a future issue. In the meantime, here are the basic details for those who wish to go it alone or build a basic mono power amplifier. The chassis layout is important to achieve the stated performance, so be sure to follow these instructions. In addition, safety is of the utmost importance, especially for mains wiring and chassis earthing. Basically, the amplifier module(s) and the power supply (along with the transformer) must be housed in an earthed metal case. This must be large enough to provide sufficient room between the transformer and the amplifier modules to avoid hum cousiliconchip.com.au The power supply board has ±57V output connectors on either side (CON1 & CON2) so that it can be easily connected to two separate power amplifiers. Note that this particular module has the alternative Quick Connect terminals from Altronics (ie, they are soldered to the PCB). The preamplifier supply section with the two 3-terminal regulators can be separated from the high-voltage supply section if necessary. Power Supply Parts List 1 PCB, code 01109111, 141 x 80mm 4 3-way PCB-mount terminal blocks, 5.08mm pitch (Altronics P2035A or equivalent) (CON1-4) 2 2-way PCB-mount terminal blocks, 5.08mm pitch (Altronics P2034A) (CON5-6) 3 PCB-mount or chassis-mount spade connectors (Altronics H2094) 3 M4 x 10mm screws, nuts, flat washers and shakeproof washers (if using chassismount spade connectors) 4 M3 x 9mm tapped Nylon spacers 6 M3 x 6mm machine screws 2 M3 shakeproof washers and nuts 150mm 0.7mm diameter tinned copper wire Semiconductors 1 7815 1A 15V positive linear regulator (REG1) 1 7915 1A 15V negative linear regulator (REG2) 4 1N4004 1A diodes (D1-D4) pling. It’s also critical to use shielded cable for all the audio signal wiring, ie, between the input connectors and amplifier module(s). siliconchip.com.au 1 5mm green LED (LED1) 1 5mm yellow LED (LED2) Capacitors 6 4700µF 63V electrolytic 2 2200µF 25V electrolytic 2 220µF 16V electrolytic Resistors 2 3.3kΩ 5W Parts For Complete Stereo Power Amplifier 2 Ultra-LD Mk.3 amplifier modules 1 Ultra-LD Mk.3 power supply module 1 speaker protection module (to be described next month) 1 vented metal case, 2U/3U rack-mount or similar size (eg, Altronics H5047) 1 chassis-mount IEC mains input socket with fuseholder (use Altronics P8324 for recommended case) 1 M205 5A fuse 1 mains-rated power switch (eg, Altronics S4243A) You will need a 2U or 3U extra-deep rack-mount metal case (or a similar enclosure) to fit a complete stereo amplifier. It will need to be quite 1 300VA transformer with two 40VAC 300VA windings and two 15VAC 7.5VA windings 1 35A 400V chassis-mount bridge rectifier 1 white insulated chassis-mount RCA socket 1 red insulated chassis-mount RCA socket 2 red and 2 black chassis-mount speaker terminals (or two double speaker terminals) 1 10kΩ dual-gang log potentiometer with suitable knob (optional, for volume control) M3 and M4 screws, washers & nuts for mounting bridge rectifier, PCBs and heatsinks Mains flex (approximately 2m) Mains-rated heavy duty wire (approximately 2m) Shielded wire for input signals (approximately 2m) Speaker cable (about 0.5m) Heatshrink tubing Fully-insulated 6.3mm spade crimp connectors (about 20) strong to support the weight of the heatsinks and the transformer. Good ventilation is also important and ideally there should be vents immediately September 2011  75 EARTH LUGS SECURED TO CHASSIS 11170110 3.K M REIFILP MA DL-ARTLU HEATSINK + + MALE IEC CONNECTOR WITH INTEGRAL FUSE T1 230V PRIMARY LEADS 1102 © 0V 4148 15 4148 0V 1 0V 5V +57V 0 –57V CON1 CON4 CON5 CON3 CON6 NI- + + + TC + 2 x 10k LOG POT (OPTIONAL) + + CT NI + TERM1 +IN CON2 –57V 0 +57V 2 tuptu O OUTPUT 2 ±57V - POWER SUPPLY BOARD (RIGHT CHANNEL INPUT WIRING NOT SHOWN) + +20V –15V V 5 1- 00 +15V V 5 1 + 00 V 02+ INSULATE ALL MAINS CONNECTIONS WITH HEATSHRINK SLEEVING Fig.20: here’s how to wire the completed power amplifier and power supply boards into a metal case. Make sure that all exposed terminals on the IEC connector and mains switch are fully insulated – see text. surrounding the heatsinks. The power transformer and IEC connector should be mounted towards the back (either in the lefthand or righthand rear corner), while the amplifier modules can be positioned on either side of the case, near the front. The power supply board can then fit between the amplifier modules, with its ±57V outputs lined up with the supply connector(s) on the module(s). It’s also vital to include a loudspeaker protector module (not shown in Fig.20) – see panel overleaf. The speaker protection module can be mounted towards the centre-rear of the chassis, while the RCA input connectors can be mounted in the opposite corner to the mains input. The volume control is optional but most constructors will want one, unless they are using an external pream- BR1 ~ + CA V 5 1 TCT C 15V CAV 0 3 ~ 5 1 30VAC 15V 1 tuptu O 1 OUTPUT ±57V TERM2 76  Silicon Chip – ~ TERM3 –IN (RIGHT INPUT) V TO SPEAKER TERMINALS VIA SPEAKER PROTECTOR – + 00 – RCA PLUG LEFT INPUT 4 40 –57V AMPLIFIER BOARD DIRECT WIRING IF POT IS NOT USED V 0V 11190110 uS r e woP reifilpmA 2.k M DL-artlU 01109111 +57V 0V 0V Ultra-LD Mk.3 Power Supply + plifier. No input switching is shown on Fig.20 but this will be a feature (with remote control) of the full stereo amplifier to be featured later. Checking the wiring Fig.20 shows the wiring connections. Make sure that the chassis is securely earthed via the mains and be sure to insulate all exposed mains terminals with heatshrink sleeving. Fig.21 shows how the earth lugs are secured to the chassis using an M4 x 10mm screw, a lock-washer and two nuts. Make sure that the earth leads are securely crimped or soldered to these lugs before bolting them to the chassis. Once you’ve done this, use a multimeter to confirm the earth connection. You can do that by checking for continuity between the earth terminal of the IEC socket and the chassis. S1 (TOP REAR) INSULATED CRIMP EYLETS LOCKING NUT M4 x 10mm SCREW, NUTS AND STAR LOCKWASHER BASE PLATE OF CASE NB: CLEAN PAINT AWAY FROM MOUNTING HOLE Fig.21: the chassis earth point is installed as shown here. Two nuts are used to permanently lock the assembly in place. Make sure that it forms a good electrical contact with the chassis. Testing the power supply Once the assembly is complete, check your wiring very carefully. In particular, make sure that BR1’s positive and negative terminals connect to the correct terminals on the power supply board. It’s now time to check that the power supply is functioning correctly but first siliconchip.com.au Adjusting The Quiescent Current Through The Power Amplifiers The quiescent current flowing in the output stage of each power amplifier is initially adjusted by installing 68W 5W resistors in place of the fuses. The voltage across one resistor is then monitored and trimpot VR1 adjusted for a reading of 9.5V – equivalent to a quiescent current of 70mA. The easiest way to connect the resistors is to “blow” the a warning: the metal strap that runs from the Active terminal to one end of the fuse has 230V AC on it. You should insulate this terminal using neutralcure silicone sealant or you can cover the IEC socket with a rubber boot, eg, Jaycar Cat. PM-4016. To check the power supply, first make sure that the supply wiring is disconnected from the amplifier. That done, apply power and check the various DC outputs. You should be able to measure close to ±57V on CON1 & CON2, +20V on CON6, ±15V on CON3 and 30VAC on CON5. If you don’t get the correct voltages, switch off immediately and check for wiring and component errors. Testing the power amplifier Each power amplifier must be tested with 68Ω 5W “safety” resistor in series with its fuse clips. These are necessary to limit the current through the output stage to about 840mA if there is a fault in the module that turns the output transistors on hard. This protects the output transistors from damage but note that the 68Ω resistors will quickly burn out under such circumstances (since they would be dissipating over 40W). As well as protecting the output stage, the 68Ω resistors allow you to initially set the quiescent current. That’s done by monitoring the voltage across one resistor and adjusting trimpot VR1 to give a reading of 9.5V (equivalent to a quiescent current of about 70mA). siliconchip.com.au fuse wires in a couple of spare M205 fuses, then drill holes in the end caps and solder the resistors in place as shown. The original fuses can then be removed and the “modified” fuses clipped into place – see photo. The easiest way to connect the 68Ω safety resistors is to solder them across two blown M205 fuses. If you don’t have any blown fuses, you can purposefully blow some by connecting them directly across a power supply. It’s then just a matter of drilling holes in the end caps, bending the resistor leads to go through them and soldering them in place. These two modified “fuses” are then clipped into the fuse clips on either side of the module – see photo. Each amplifier module is now ready for testing. STEP 1: check that the safety resistors are installed and that their leads can’t short to any adjacent parts (note: do NOT connect the loudspeaker to the amplifier during this procedure). DANGER: HIGH VOLTAGE High DC and high AC voltages are present in this circuit. The power supply uses a total of 80V AC and the amplifier power supply rails are a total of 114V DC. Do not touch any part of the amplifier circuitry when power is applied otherwise you could get a severe electric shock. The two LEDs on the power supply board indicate when power is present. If they are alight, the power supply and amplifier boards are potentially dangerous. STEP 2: connect a DMM set to volts across one of the safety resistors (alligator clip leads are extremely handy in this situation). STEP 3: turn trimpot VR1 fully anticlockwise. This can take as many as 25 turns but it will continue to turn even so. Many (but not all) multi-turn trimpots click when they are at the end-stop. If in doubt, check the resistance across it – it should be about 1kΩ. STEP 4: check that the power supply is off and that the filter capacitors are discharged (LEDs off!), then connect the ±57V supply to the module. Check that the supply polarity is correct, otherwise the amplifier will be damaged when power is applied. STEP 5: apply power and check the voltage across the 68Ω resistor. It should be less than 1V (it may jump around a bit). If the reading is over 10V, switch off immediately and check for faults. STEP 6: using an insulated adjustment tool or a small flat-bladed screwdriver, slowly adjust the trimpot clockwise. Be careful not to short any adjacent components. STEP 7: after a few turns, the resistor voltage should stabilise and start to rise. Continue until it reads around 9.5V. It may drift a little but should be quite steady. STEP 8: check the voltage across the amplifier’s output terminals (ie, across the speaker terminals). It should be less than ±50mV. STEP 9: switch off, wait for the capacitors to fully discharge (LEDS off) and September 2011  77 WHAT’S COMING: in future issues, we intend to describe a low-noise stereo preamplifier board (above) with remote volume control plus a 3-input selector board (right) which is controlled by the same remote. The pre­amp is a slightly modified version of the unit described in August 2007 for the 20W Class-A Stereo Amplifier while the input board is completely new. The latter uses relay switching and features internal RCA output sockets which connect to matching input sockets on the preamp. on. If the reading is more than 15mV, readjust VR1 anti-clockwise to bring it back below this figure. The stability is such that it should stay below this figure but it’s a good idea to check. That completes the quiescent current adjustments. Note, however, that if you wish to repeat the above procedure (ie, with the 68Ω resistors in place), you will first have to reset VR1 to minimum (ie, fully anti-clockwise). If you don’t do this, the amplifier may latch up when power is reapplied and burn out the safety resistors. Troubleshooting The power supply can sit in the chassis with its ±57V output connectors aligned with the supply connectors on two power amplifier modules. replace the safety resistors with 6.5A fuses. STEP 10: connect a DMM set to volts across one of the 0.1Ω 5W emitter resistors. STEP 11: reapply power and check that 78  Silicon Chip the DMM reads close to 7mV. If necessary readjust trimpot VR1 to bring the voltage close to this figure. It’s a good idea to recheck this resistor voltage after the amplifier has been idling for an hour or so with the lid If there’s a fault in the module, a likely symptom is either excessive voltage across the safety resistors or the amplifier output voltage is pegged near one of the ±57V supply rails. If this happens, switch off and wait for the power supply capacitors to discharge. That done, check that all the transistors are properly isolated from the heatsink. If this checks out, apply power to the amplifier without the fuses or safety resistors in place – ie, so that the output stage (Q10-Q15) is left unpowered. Now check the voltage between the bases of transistors Q10 & Q11. This should be close to 2.2V. If this voltage is too high and you can’t reduce it with the trimpot, there could be a fault in the VBE multiplier (Q16 and associated components) or an open circuit between it and the siliconchip.com.au You MUST Use A Loudspeaker Protector diode leads of Q12-Q15. This could be due to an open-circuit track on the PCB or more likely, missed solder connections on the output transistor leads. If the voltage between the bases of Q10 & Q11 is correct (ie, 2.2V), check the other voltages indicated on the circuit diagram. Note that the supply rails can vary by a few volts depending on your exact mains voltage, so some of the voltages can vary somewhat. In addition, check the base-emitter voltage of every transistor in the amplifier. In each case, you should get a reading of 0.6-0.7V if the transistor is working correctly. If not, then either the transistor is faulty or the wrong type has been used in that location. Making repairs If you need to remove a faulty com                      A S STATED in the main body of the article, it’s essential to use a loudspeaker protector with the Ultra-LD Mk.3 amplifier module (and with any other high-power audio amplifier module for that matter). That’s because if a fault occurs in the amplifier (eg, if one of the transistors fails), this could apply one of the full 57V supply rails to the loudspeaker’s voice coil. As a result, the voice coil would quickly become red hot and burn out, irreparably damaging the speaker. This may also cause a fire! This slightly-modified version of the “Universal Speaker Protector & Muting ponent from a double-sided PCB, the best approach is to first cut the body of faulty component away from its leads. It’s then just a matter of grabbing them one at a time with pliers, heating the  Module” described in the July 2007 issue of SILICON CHIP will prevent this from happening. This device not only quickly disconnects the loudspeaker(s) in the event of a DC output fault but also provides muting at switch-on and switch-off to prevent audible thumps. It also includes an input for an optional temperature sensor to disconnect the loudspeaker(s) if the output stage heatsink rises above a preset temperature. The full details of this modified loudspeaker protector will be published next month. solder joint and pulling gently until the lead comes out. Once the leads have been removed, use a solder sucker or vacuum desoldering tool to clear the holes. SC              siliconchip.com.au September 2011  79