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Universal Loudspeaker Protector By Nicholas Vinen This extensively revised Speaker Protection module protects your expensive loudspeakers from catastrophic faults in your amplifier. As well, it mutes switch-on and switch-off thumps, disconnects the speakers if you plug in your headphones and has heatsink temperature sensing in order to control a cooling fan in the amplifier. It also has on-board LEDs to indicate various fault conditions. A LL HIGH-POWER amplifiers should have an in-built Loudspeaker Protector. It will disconnect the speakers if the amplifier develops a serious fault which could result in a high DC voltage being applied to the speaker(s). Without the Loudspeaker Protector, the resulting high current would damage the speaker and in the worst case, set the speaker on fire! You can imagine the scenario: you have the volume pumped up to enjoy your favourite music and suddenly one speaker emits a loud PFFTT and then nothing. Maybe the other channel stays just as loud. But before you realise you have a catastrophic fault, the damage is done; expensive damage. Mind you, it could be a lot worse if you’re having a party and you’re not even in the room when the fault occurs – the speaker could catch fire! This can happen within seconds! In fact, in this article we have reproduced a series of Features & Specifications •  Suits Ultra-LD Mk.4 110W/200W modules and other amplifiers with similar ratings •  Fast speaker disconnection with a sudden onset DC fault (typically <50ms) •  Compact PCB •  Operating voltage range: 15-55VAC or 22-50V DC (minimum 18VAC/24V DC •  •  •  •  •  •  •  if fan output used) Operating current: ~50mA (not including fan current) Switch-on and switch-off thump muting Temperature monitoring with overheat indicator and speaker disconnection Headphone speaker disconnection option using commonly available 3PST sockets Temperature sensing with thermostats or NTC thermistors Fan switch-on at a preset heatsink temperature Six on-board LEDs and two optional off-board status LEDs 62  Silicon Chip photos which were taken for the article on our previous Loudspeaker Protector, featured in the October 2011 issue. Some amplifiers have PTC thermistors in series with the outputs to prevent speaker damage and fire. These work because the high current which flows during a fault causes them to heat up and thus increase in resistance, limiting the power delivered. However, because they are non-linear devices, PTC thermistors can seriously affect distortion performance during normal operation of the amplifier. Relays are much better in this regard but even they can have a secondary effect on distortion performance. Which is why we have been careful to verify that the relay used in this project does not have any significant effect. Other roles played by the Loudspeaker Protector include muting any switch-on and switch-off thumps, disconnection of the speakers if the amplifier modules overheat (eg, due to being driven too hard), disconnecting the speakers when headphones are plugged in and running a small fan before the amplifier’s heatsinks get too hot. As noted above, we have included on-board indicator LEDs so you siliconchip.com.au Don’t let this happen to you! Without a Loudspeaker Protector, a serious fault in a high-power amplifier could cause enough current to flow through a speaker’s voice coil to set the speaker on fire. These three photos, taken at 3-second intervals, illustrate just how quickly a fire can take hold once the cone ignites. don’t have to guess what’s going on. These show the presence of DC and AC (mains) power, relay operation, left and right channel fault status and over-temperature fault status. This makes testing the module and verifying its normal operation much easier. The new design will run over a wide range of voltages without modification and has simplified supply wiring, partly because it has an on-board bridge rectifier; in many cases, one pair of wires from one of the transformer secondaries is all that’s required to power it. All the extra features have been incorporated on a smaller PCB because we have employed a fair proportion of surface-mount devices, although these are relatively easy to install and solder. Circuit description The Loudspeaker Protector module monitors seven inputs to determine when to connect or disconnect the speakers or turn on the fan or various LEDs. These seven inputs are: AC power, left channel DC offset, right channel DC offset, left and right channel temperature-sensing thermistors (if fitted), headphone jack socket switch (if fitted) and heatsink thermostat(s) (if fitted). siliconchip.com.au The full circuit is shown in Fig.1. The inputs mentioned above connect via CON1, CON3, CON4, CON5, CON9, CON13 & CON17. Let’s go through these in sequence. One of the amplifier’s power supply transformer AC secondaries is connected to pin 3 of CON1. This voltage is applied to the emitter of NPN transistor Q3 and the base of NPN transistor Q4 via a 10kΩ resistor, with a 100kΩ pull-down to improve noise immunity. If the voltage at pin 3 of CON1 is above about 0.7V, Q4’s base-emitter junction is forward-biased and so its collector sinks current, turning on AC sense indicator LED2. Similarly, if the voltage at this pin is below about -0.7V, Q3’s base-emitter junction is forward biased (it’s operating as a commonbase amplifier) and it pulls current from the base of PNP transistor Q12, switching the latter transistor on and thus the result is the same – LED2 turns on. So LED2 is on while ever the absolute voltage at pin 3 of CON1 is more than 0.7V, which is true most of the time when mains power is applied to the transformer. As soon as the mains supply is switched off, the voltage at pin 3 drops to zero and LED2 turns off within milliseconds. When Q4 or Q12 are on, they not only power LED2 but they also sink current via D1 and the series 100Ω resistor. This discharges the connected 470nF capacitor and thus NPN transistor Q9 is off. It in turn allows the 47µF capacitor connected to its collector via a 100Ω resistor to begin charging. After a few seconds, this capacitor has charged to 14V and Q10’s baseemitter junction becomes forward biased, because its emitter is limited to 13.5V, due to the voltage across zener diode ZD2 and regular diode D3. Q10 therefore switches on, sinking current from PNP transistor Q15’s base which in turn powers the relay coil. With the relay on, LED3 also illuminates (as well as any external LED connected to CON8). While the relay switch-on is delayed by several seconds while the 47µF capacitor charges, it switches off almost immediately when power is removed, since Q9 discharges it via a 100Ω resistor, a much lower value than the 100kΩ charging resistor. Thus, the speakers are disconnected before the collapsing power supply can cause the amplifier modules to deliver a transient and cause a thump, regardless of how long the main power supply reservoir capacitor charge lasts. Amplifier fault detection Relay switch-off must be similarly fast should either power amplifier module (left or right channel) develop a fault which results in its output being pegged to a high positive or negative voltage – for example, a shorted output transistor. This is a bit tricky since normal output signals can contain significant excursions, eg, loud low-frequency bass. The solution is to filter out the AC component of the signal from each channel with a simple RC low-pass filter comprising 22kΩ and 10kΩ reNovember 2015  63 sistors and a 47µF capacitor for each channel. The filtered, attenuated signal is fed to transistors Q5, Q6 & Q13 (left channel) or Q7, Q8 & Q14 (right channel) which are arranged in the same manner as Q3, Q4 & Q12 as described above and thus are also triggered by voltages exceeding about ±0.7V. If either fault detector channel is triggered, LED4 or LED5 illuminates and the 47µF time delay capacitor is quickly discharged via D2 and its 10Ω series resistor, disconnecting the speakers from the amplifier. We tested this filter arrangement using LTSPICE simulations to check both that normal audio waveforms will not cause false triggering and that a serious amplifier fault will result in a quick switch-off. For example, a 15Hz sinewave delivering 200W into an 8-ohm speaker will not trigger this circuit, even with other higher-frequency, high-power signals mixed into it. This equates to a voltage swing higher than an Ultra-LD Mk.4 200W amplifier module is able to deliver. However, if the amplifier offset goes from 0V to either +40V or -40V and stays there indefinitely, the fault detection circuit triggers in approximately 20ms. The relay itself takes a little time to switch off too but even taking this into account, the speakers should be disconnected in less than 50ms under these conditions. By comparison, our October 2011 design takes nearly 50ms to even detect the fault, ie, more than twice as long, and the even earlier July 2007 design takes over a quarter of a second (250ms)! Should the fault detection be triggered by, say, an extreme signal overload which is then removed, the speakers will be re-connected within a few seconds after amplifier operation returns to normal. In addition, when the speakers are disconnected from the amplifier, the terminals are effectively shorted out, to blow out any arc which may occur between the relay contacts due to the high DC current being interrupted. You might notice that there are empty pads on the PCB near the low-pass filter components. These were included for the connection of 100nF 100V capacitors across the 22kΩ resistors. Simulation shows that this speeds up fault detection by around 10%. However, they could potentially couple 64  Silicon Chip ground ripple voltage into the speaker outputs and thus affect distortion performance so we decided to omit them from the final design. Detecting other faults As with the earlier designs, amplifier overheating can be detected by a normally-open thermostat bolted to each heatsink and wired across CON3. Multiple thermostats can be connected in parallel to monitor multiple heatsinks. When any one closes, the 47µF capacitor is discharged via D4 and thus the speakers are disconnected. When it cools down and opens, the speakers are re-connected after the normal delay. However, this latest module also has provision to sense heatsink temperature using one or two 10kΩ NTC (negative temperature coefficient) thermistors. These are cheaper than thermostats and smaller, requiring only a single M3 screw for mounting. They are also more accurate (typically within 1°C or so) and since they can sense a range of temperatures, they can be used to control a cooling fan which comes on at a lower temperature, to cool the heatsink and possibly avoid ever disconnecting the speakers, even though you may have the volume “pumped up”. Also, since the thermistor temperature thresholds are set using a simple resistive divider, you can easily change them to suit your needs. With thermostats, you are limited in choice of temperature thresholds and hysteresis. The thermistor(s) connect via CON4 and/or CON5. They form a voltage divider across the 24V supply, in combination with a 10kΩ resistor to ground. At 25°C, this results in ~12V at the junction, increasing as the thermistor heats up. Comparators IC1a-IC1d monitor these voltages. These are part of an LM339 quad low-power comparator. IC1a compares the voltage from the thermistor at CON4 to a reference voltage formed by a 15kΩ/5.6kΩ/100kΩ divider across the 24V supply. This voltage will vary with the supply rail but so will the voltage from the thermistor, ie, it is a ratiometric comparison. Pin 5 of IC1a is approximately 4.1V below the 24V rail. The specified thermistor has a beta of 3970. Using the calculator at www.daycounter.com/ Calculators/Steinhart-Hart-Thermistor-Calculator.phtml, we determine its Fig.1: the full circuit for the revised Speaker Protector. Transistors Q3, Q4 & Q12 monitor the presence of AC power while Q5/Q6/Q13 and Q7/ Q8/Q14 monitor the left and right amplifier channel DC offsets in a similar manner. IC1 monitors the NTC thermistor resistance at CON4 and CON5 and either switches on a fan at CON6 or switches off the main speaker relay RLY1 if the temperature gets too high. Onboard rectifier BR1 and the 220μF filter capacitor provide a DC supply for the circuit while Q1 and ZD1 regulate the voltage to an average of around 24V DC. Similarly, Q2 and ZD3 provide a current-limited 12V supply for the optional cooling fan. resistance is 2070Ω at 65°C. We will therefore have a voltage that’s 4.12V below the 24V rail at pin 4 of IC1a. So at this temperature, the output of IC1a will go low, sinking current through PNP transistor Q16. Q16 operates as an emitter-follower, powering the optional 12V fan at CON6. Comparator IC1b works exactly the same way for the other thermistor and since the open-collector outputs are joined together, the fan will turn on when either heatsink exceeds 65°C. If either reaches 75°C, the associated thermistor resistance drops to 1477Ω. Thus, pin 8 of IC1c or pin 10 of IC1d will be just 3V below the 24V rail. Both non-inverting inputs (pins 9 & 11) are 2.98V below this rail, so above 75°C, the output of IC1c or IC1d will go low. Over-temperature indicator LED6 will then light and the 47µF time delay capacitor will be discharged via D4, switching off RLY1. A 10MΩ resistor from the common IC1c/IC1d output (pins 13 & 14) to their common non-inverting input pins (9 & 11) provides a little hysteresis, so that the relay does not switch on and off rapidly. Once the relay is off, the heatsink temperature must drop by several degrees before it will switch back on. Similarly, the 10MΩ hysteresis resistor for IC1a/IC1b ensures that once the fan has switched on, the heatsink temperature must drop by a degree or two before it will switch back off. The two 100pF capacitors filter out any RF that may be picked up by the thermistor leads. If either or both thermistors are not connected, the associated pin will be pulled down to ground by the 10kΩ resistor and this siliconchip.com.au siliconchip.com.au November 2015  65 SC 20 1 5 RIGHT THERM 10k 10k 100k 5.6k 4 5 6 7 8 B 14 13 50V 470nF C 12 IC1a 10M IC1b 2 1 IC1: LM339 IC1c 10M IC1d 3 B E Q12 BC856 2 B 10k A K B 100k TEMP B Q16 BC856 ~12.4V C Q2 BDP953 E 10k K 10k K A C E λ K2 K1 A 100k D1 BAW56 1 3 3 1.8k 1 3 2 D7 BAV99 D8 BAV99 B +24V 2 2 1 A K1 E C LED1 E C A CATHODE DOT A K D4 BAW56 B Q9 BC846 LEDS ZD3 13V A K K1 100Ω 1 E 2 3 B B K1 C E C E D3 BAW56 A 10k B 10k B K2 BAW 5 6 C LED5 Q14 BC856 K LEFT λ FAULT A C LED4 RIGHT FAULT ZD2 13V K2 K1 100k Q13 BC856 K λ A 10k Q10 BC846 D2 BAW56 E C Q6 BC846 K1 BAV99 Q8 BC846 C E C A +24V 16V B 33k 47 µF K2 10Ω 100k +24V 22k K2 POWER 10k K λ A D5 BAW56 CON6 1 + FAN 2 – K2 B 50V 470nF Q11 BC846 BAV99 AC SENSE LED2 2 D6 TO HEADPHONE 100k SWITCH CON17 1k 100Ω 35V 220 µF LED6 E Q1 BDP953 C λ OVER A THERMOSTAT (N/O) CON3 1 CON7 OVER TEMP LED K +24V C Q4 BC846 E ZD1 27V 22Ω 1 UNIVERSAL LOUDSPEAKER PROTECTOR 1 2 100pF 9 11 CON4 100pF 470Ω * NOT NORMALLY FITTED – SEE TEXT ~ Q3 BC846 E C LK1* 10 2 – 4.1V 3V 15k 100k 10k ~ + 1 LEFT THERM CON5 3 2 1 CON1 AC/ DC GND AC SENSE BR1 W04M OR MBS4 A B E 6.3V TAB (C ) 47 µF 6.3V K λ A RLY1b 10k B E C BC846, BC856 10k 22k 10k RLY1a 22k 10k LED3 RELAY ON Q15 BC856 C 47 µF BDP953 +24V B C E Q7 BC846 B E +24V Q5 BC846 A K RLY1 24VDC B E K +~~– W04 CON16 CON15 CON14 CON13 CON12 CON11 CON10 LSPKR OUT+ LSPKR OUT– LSPKR IN– LSPKR IN+ RSPKR OUT+ RSPKR OUT– RSPKR IN– RSPKR IN+ ON LED CON9 2 CON8 1 sistor Q11 is pulled high via a 100kΩ resistor. Thus Q11 turns on, discharging the 47µF capacitor and switching RLY1 off. When the plug is removed, it switches on again after the usual delay. If this feature is not needed, CON17 is shorted out (eg, with a jumper shunt) to disable Q11. Power supply Fig.2: use this layout diagram to build the speaker protector. Fit the SMDs first, followed by the remaining components in order of ascending height. BR1 and the 220μF electrolytic capacitor can be either SMD or through-hole parts. It’s recommended to fit a heatsink where shown if the fan output is being used. Take care with LED orientation; if in doubt, use a DMM to check which end is the cathode. LK1 is normally not fitted (see text).   Attaching Non-Solder Spade Lug Connectors Fig.3 (right) shows how double-ended non-solder spade lugs are mount­ed. Each lug is secured using an M4 x 10mm screw, a flat washer (which goes against the PCB pad), an M4 star lockwasher and an M4 nut. The trick to installing them is to first do the nut up finger-tight, then rotate the spade lug assembly so that it is at a right-angle to the PCB. A screwdriver is then used to hold the M4 screw and the spade lug stationary while the nut is tightened from below using an M4 socket and ratchet. will disable temperature monitoring for that channel. Headphone switching Generally we want to disconnect the speakers when a headphone jack is inserted. The signal for the headphones 66  Silicon Chip Fig.3: attach the six double-ended spade lugs to the PCB as shown in this diagram. Do the nut up nice and tight to ensure a good connection and to ensure that the assembly does not rotate. is taken from before the speaker protector relay so that output is not affected. Most jack sockets have a switched ground terminal for the sleeve which opens when a jack is inserted. This is wired to pin 1 of CON17 so that when a plug is inserted, the base of NPN tran- Our previous loudspeaker protector designs required changing a power resistor (or linking it out), depending on the supply voltage used, so that the relay’s coil was supplied with the correct voltage. We’ve now eliminated that requirement by using a transistor and zener diode to provide a semiregulated supply for the relay coil. The specified relay will operate just fine with a little ripple voltage across its coil, as long as it stays within the range of about 18-27V. The easiest way to power this unit is from the transformer winding(s) which are used to power the amplifier modules. One end goes to pin 3 of CON1, for both AC sensing and connection to bridge rectifier BR1, while pin 2 of CON1 goes to ground/earth, as shown in Fig.5. This results in half-wave rectification, giving a pulsating DC voltage at the positive terminal of BR1. This charges the 220µF capacitor via NPN transistor Q1. The 27V zener diode at its base is biased from the output of BR1 and prevents the 220µF capacitor charge from exceeding 27V, as ZD1 shunts Q1’s base drive above this voltage. While the diodes in BR1 are forward-biased, the voltage in this capacitor is maintained via Q1. The rest of the time, this capacitor supplies the load current and recharges on the next mains cycle. The voltage drop across Q1 results in heat dissipation of less than 1W during normal operation or around 2W with a fan attached and running (drawing around 100mA). The PCB acts as a heatsink to safely dissipate this heat. Q1 is rated for 5W dissipation as long as the PCB temperature is kept below 100°C. With the copper plane provided, junction-to-ambient thermal resistance is about 40°C/W so, in theory at least, no heatsink is required. In practice, Q1 gets very hot to the touch if the board is driving a fan so we strongly recommend you glue a small heatsink on top of Q1 if connecting a fan. There are more details on this below. siliconchip.com.au tor replaced with a 0Ω resistor (or a wire link). Changing thresholds This view shows the completed prototype Loudspeaker Protector. Note that you can either use PCB-mounting spade lugs as shown here or use double-ended chassis-mounted types (see panel). Dual series diode D6, in combination with the 22Ω base resistor, limits Q1’s base current to 0.6V ÷ 22Ω = 27mA which in combination with its beta of around 100 provides a peak collector current of around 3A. This is close to its continuous rating of 3A and well below its 10ms peak rating of 5A. If the unit is run from a centretapped transformer secondary with an AC voltage less than 35V-0-35V, both ends of the winding should be connected to CON1 (pins 1 & 3), giving full-wave rectification and 100Hz recharge of the 220µF capacitor. Otherwise the ripple may be so great that RLY1 can’t remain latched over the whole mains cycle. With higher supply voltages this is not only unnecessary but will likely increase dissipation in Q1. The key point is that the voltage across the 220µF capacitor should not drop below about 18V as RLY1’s “must operate” voltage is 16.8V. Powering a fan Most small DC fans run from 12V so we’ve provided a 12V current-limited supply. This can be bypassed if a 24V type is used. Use a 24V fan if you can get a suitable type, since the fan will receive more power and less will be dissipated in the speaker protector power supply. Either way, the fan negative terminal is pulled to ground to turn the fan on or left floating to turn it off. The ~12V rail is derived by NPN transistor Q2 from the 24V rail in a siliconchip.com.au similar manner to the way that the 24V rail itself is derived by Q1. The 1.8kΩ base resistor, in combination with dual series diode D8 limits its base current to 0.6V ÷ 1.8kΩ = 0.33mA which, with a beta of 350, gives a maximum collector current of just over 100mA. This protects Q1 and Q2 in case the fan terminals are shorted out. It also protects both transistors against excessive dissipation should the fan try to draw more than 100mA (as many 12V fans would). Basically, if that happens, the supply voltage will drop and it will simply run slower. Diodes D5, and D7 absorb any inductive spikes which may be generated by the fan motor, especially when it is switched off, and prevent Q16’s base-emitter or emitter-collector junctions from becoming reverse-biased. If a fan is to be used, dissipation in Q1 will be lower if the module is driven from a centre-tapped low-voltage transformer winding of at least 18V0-18V. With lower voltages, depending on the fan current draw, supply ripple may be too high for the unit to operate properly. In this case, you have to run the unit from the higher voltage windings on the transformer, as shown in Fig.5. If driving a fan and running the unit off a much higher voltage transformer secondary, it’s a good idea to fit a small heatsink to Q1, as explained earlier and described below. If using a 24V fan, ZD3 should be omitted and the nearby 22kΩ resis- If using NTC thermistors, the temperature thresholds can be changed by varying the value of two resistors. As explained above, the fan will normally switch on at 65°C (TFAN) and the speakers are disconnected at 75°C (TOVERHEAT). Let’s say you want TFAN = 60°C and TOVERHEAT = 70°C instead. First, calculate the expected NTC thermistor resistance at these temperatures using the website listed previously: www.daycounter.com/Calculators/ Steinhart-Hart-Thermistor-Calculator.phtml Plug in beta = 3970, R1 = 10,000Ω, T1 = 25°C and T2 = 60°C to get RFAN = 2468.7Ω. For T2 = 70°C we get ROVERHEAT = 1744.4Ω. Since the reference voltage divider string has a 100kΩ resistor at the bottom, compared to the 10kΩ divider resistor for the NTC thermistors, the total value of our two new resistors (to replace the 5.6kΩ & 15kΩ resistors) will need to be RFAN x 10, ie, 24,687 ohms (RTOTAL). We can now calculate the new value for the 15kΩ resistor as (100,000Ω + RTOTAL) x ROVERHEAT ÷ (ROVERHEAT + 10,000Ω) = 18,519Ω. 18kΩ is close enough. The 5.6kΩ resistor is then replaced with a value of 24,687Ω – 18,000Ω = 6687Ω, which is close to 6.8kΩ. Substitute different thermistor resistance values into these formulae to calculate the required components for other temperatures. Construction The Speaker Protector module is built on a double-sided PCB coded 01110151 and measuring 88 x 79mm. Most of the components are SMDs. The exceptions are the connectors, RLY1 and optionally, bridge rectifier BR1 and the 220µF electrolytic capacitor. Fig.2 shows the assembly details. Start with the sole IC, comparator IC1. Locate its pin 1; this will normally be indicated by a dot, divot or failing that, a bevelled edge. Rotate the IC so that pin 1 is at upper left as shown. Tack-solder one pin, then check that all the other pins are properly centred over their pads. You can either solder them individually or you can add flux paste down both sides of the IC and then use a wave-soldering technique. November 2015  67 Parts List: Loudspeaker Protector 1 double-sided PCB, code 01110151, 88 x 79mm 8 6.3mm PCB-mounting spade terminals, 5mm pitch (CON9CON16) (Jaycar PT4914, Altronics H2094) OR 6 double-ended chassis-mounting spade terminals with M4 x 10mm machine screws, shakeproof washers, flat washers and nuts 1 3-way terminal block, 5.08mm pitch (CON1) 1 2-way terminal block, 5.08mm pitch (CON3) 6 2-way polarised headers, 2.54mm pitch (CON4-CON8, CON17) 1 24V DC coil, 10A DPDT cradle relay with integral LED (RLY1) (Altronics S-4313) 4 M3 tapped spacers 4 M3 x 6mm pan-head machine screws 1 shorting block 2 10kΩ lug mounting NTC thermistors (Altronics R4112) (optional) 1 12V or 24V DC fan (optional) 1 9V battery (for testing) 1 or 2 small finned heatsinks with adhesive pads (optional, see text) Once all the pins have been soldered, check for bridges between them and use flux paste and solder wick to remove any bridges you find. The next job is to mount the 27 SOT23 package transistors and diodes. These are quite small but the pins are spaced fairly far apart. The orientation of each is obvious due to the fact that they have one pin on one side and two on the other but do be careful not to get the six different types of components mixed up and make sure they are not fitted upside-down, ie, their leads should be in contact with the PCB pads. The basic technique is the same as for the IC – tack solder one pin, check the positioning, solder the other two pins and then refresh the initial solder joint with a little flux paste or added solder. The two larger BDP953 transistors can be soldered in place now. These are in SOT-223 packages which do an excellent job of transferring heat to the PCB due to their large, thin tabs on which the silicon die is mounted. 68  Silicon Chip Various lengths of hook-up wire and header plugs as required Semiconductors 1 LM339 comparator (SOIC-14) (IC1) 2 BDP953 100V 3A high-gain NPN transistors (SOT-223) (Q1,Q2) 9 BC846 NPN transistors (SOT-23) (Q3-Q11) 5 BC856 PNP transistors (SOT-23) (Q12-Q16) 1 27V 0.25W zener diode (SOT-23) (ZD1) 2 13V 0.25W zener diodes (SOT-23) (ZD2,ZD3) 1 400V 0.5A SMD bridge rectifier, MBS4 type (BR1) OR 1 W04 1A bridge rectifier (BR1) 5 BAW56 dual common anode diodes (SOT-23) (D1-D5) 3 BAV99 dual series diodes (SOT23) (D6-D8) 1 high-brightness blue LED, 3216/1206 size (LED1) 1 high-brightness yellow LED, 3216/1206 size (LED2) 1 high-brightness green LED, 3216/1206 size (LED3) 2 high-brightness red LEDs, 3216/1206 size (LED4,LED5) The easiest technique (excluding hot-air or IR reflow) is to put a little flux paste on the large pad, then tack the part down at one of the smaller end pins. You can then solder the large tab; it will take a little while before the part and PCB heat up sufficiently to form a proper solder joint but the flux paste should help the solder flow under the tab. You then immediately solder the centre small pin (which is connected electrically to the tab) and finally the two outer pins. Fitting the LEDs There are five different colour LEDs and the first step before soldering each one is to verify its polarity. Use a DMM set on diode test mode and carefully probe both ends of the LED package. When it lights up, the red probe is on the anode and the black probe on the cathode. The polarity is marked on the PCB and shown in Fig.2 so solder it to the appropriate pad with the correct orientation. Often, the cathode is marked with a green dot – but not always so be careful! 1 high-brightness amber LED, 3216/1206 size (LED6) 1 or 2 chassis-mounting LEDs for status indication (optional, see text) Capacitors (3216/1206 unless stated) 1 220µF 35V SMD or throughhole electrolytic, up to 8mm diameter (eg, Nichicon UCW1V221MNL1GS; Digi-Key 493-9430-1-ND) 2 47µF 6.3V X5R 1 47µF 16V X5R (3224/1210 or 3216/1206 size) 2 470nF 50V X7R 2 100pF 50V C0G Resistors (all 3216/1206, 0.25W 1%) 2 10MΩ 1 15kΩ 7 100kΩ 14 10kΩ 1 33kΩ 1 5.6kΩ 3 22kΩ 1 1.8kΩ 1 1.8kΩ 0.25W through-hole (for testing) 1 1kΩ 0.25W through-hole (for testing) 1 1kΩ 2 100Ω 1 470Ω 0.5W 1 22Ω 2 10Ω (one optional for LK1) Since LED1 is blue and has a forward voltage of more than 3V, depending on your DMM it might not light up either way around. In this case you’ll either have to trust the cathode marking or use a 9V battery with a series current-limiting resistor and a couple of short lengths of wire to probe it. Solder them using the usual method of tacking down one end, soldering the other and then refreshing the first. Try to solder them with the base flat on the PCB. The resistors and ceramic capacitors can now be mounted using the same basic technique. The resistors will be printed with a 3-digit or 4-digit code indicating their value (eg, 10kΩ = 103 or 1002) while the capacitors will not have any markings and you will have to check the packaging. Make sure you don’t get the 47µF 16V capacitor mixed up with the two 47µF 6.3V capacitors; the former is likely to be bulkier. Similarly, the 470nF capacitors will be thinner than the 47µF capacitors and the 100pF thinner again. Fit the electrolytic capacitor next, siliconchip.com.au FROM RIGHT CHANNEL AMPLIFIER MODULE 2 3 0V PRIMARY LEADS + ~ + ~ 473 101 TO RIGHT CHAN. NTC THERMISTOR RSPKOUT+ CON2 T1 + + HP – INSULATE WITH SILICONE UNIVERSAL SPEAKER PROTECTOR MK3 – SPK + RSPKIN+ LSPKIN/OUT– 0V +57V 391 LSPKIN+ SILICON CHIP LSPKOUT+ CON3 –57V RSPKIN/OUT– NTC THERMISTOR (LEFT CHAN.) k.4 fier 0V 01107151 RevB LEFT CHANNEL AMPLIFIER BOARD 15 V 0V 1 5V 0V 40 V 0V 40 V – THESE FRONT PANEL LEDS ARE OPTIONAL OVERHEAT CA V 5 1 TCT C 15V CAV 0 3 ~ 5 1 30VAC 15V 1 tuptu O 1 OUTPUT ±57V CON4 CON5 CON3 CON6 11190110 uS r e woP reifilpmA 2.k M DL-artlU 0110 9 111 NI- + + TERM3 –IN TC TERM2 + + + CT NI + TERM1 +IN CON2 –57 V 0 +5 7 V 2 tuptu O - OUTPUT 2 ±57V POWER SUPPLY BOARD ~ + ~ SPKRS ON CON1 + BR1 + +20V –15V V 5 1- 00 +15V V 5 1 + 00 V 02+ NOTE: 0V OUTPUT MUST GO TO EARTH – VIA PREAMP OR DIRECTLY (IF NO PREAMP)! TO PREAMPLIFIER Ultra-LD Mk.3 Power Supply + –– 00 +57V 0 –5 7 V FROM RIGHT CHANNEL HEADPHONE OUTPUT MALE IEC CONNECTOR WITH INTEGRAL FUSE RIGHT CHANNEL OUTPUTS 01110151 473 LEFT CHANNEL OUTPUTS EARTH LUGS SECURED TO CHASSIS INSULATE ALL MAINS CONNECTIONS WITH HEATSHRINK SLEEVING S1 (TOP REAR) Fig.4: here’s how to connect the speaker protector module when the transformer has a low-voltage set of secondaries. In this case, they’re being fed to a power supply board to provide regulated ±15V rails for a preamplifier. Note that in this case, the speaker protector will only work if the transformer centre tap is connected to Earth via the regulator board and preamp. Note also the single wire from the headphone socket to the module; this is done to avoid an Earth loop (the ground connection is provided by the power supply). orientated as shown in Fig.2. It will normally have a bevelled base indicating the positive end and a black stripe on the negative end of the can. Solder its two metal pins similarly to the ceramic capacitors. You can use a regular through-hole electrolytic if you prefer, as mounting holes are provided. In this case, the longer lead goes though the pad towards the bottom of the board (ie, positive). Similarly, the bridge rectifier can be an SMD or W02/W04 through-hole type. The SMD type will have a notch at the right-hand end (between the two AC terminals) while the through-hole type has a “+” printed above one lead (longer than the others) which goes in the pad indicated on the PCB. There’s no need to fit both bridges. Solder the polarised pin headers in siliconchip.com.au place next, orientated as shown, followed by the two terminal blocks, with their wire entry holes towards the top of the board. Note that they are not dovetailed, ie, they’re fitted separately. The spade lugs are now soldered in place in the positions shown in Fig.2. Two are required for the negative polarity speaker terminals but only one for the positive polarity terminals. Try to solder them in at right angles to the PCB but don’t hold them while soldering as they get extremely hot! Alternatively, you can attach chassis-mount spade lugs to the PCB using M4 x 10mm machine screws, nuts and washers, as shown in Fig.3. Use shakeproof and flat washers to ensure good electrical connections between the spade lugs and the PCB pads and to ensure that they don’t come loose. All that’s left is the relay. Ensure this is pushed all the way down onto the PCB before soldering its pins. It will only go in one way. Changing component values Remember that you may want to change a few of the components to suit your application. These include: the 15kΩ and 5.6kΩ resistors if you are changing the temperature thresholds and ZD3 and its associated 22kΩ resistor if using a 24V fan. Note that LK1 is normally NOT fitted. Fit LK1 if and only if both the following are true: the transformer winding powering the module is not connected to earth or anything else (including any other taps on the same winding) and the total winding voltage connected to the unit is no more than November 2015  69 EARTH LUGS SECURED TO CHASSIS FROM RIGHT CHANNEL AMPLIFIER MODULE LEFT CHANNEL OUTPUTS 473 T1 2 3 0V PRIMARY LEADS + ~ + ~ 101 CON2 TO RIGHT CHAN. NTC THERMISTOR INSULATE WITH SILICONE + + HP – RSPKOUT+ – SPK + UNIVERSAL SPEAKER PROTECTOR MK3 +57V RSPKIN+ LSPKIN/OUT– SILICON CHIP 0V 391 LSPKIN+ LSPKOUT+ CON3 –57V RSPKIN/OUT– (LEFT CHAN.) 473 RIGHT CHANNEL OUTPUTS 01110151 NTC THERMISTOR k.4 fier 01107151 RevB 0V LEFT CHANNEL AMPLIFIER BOARD 40 V 0V 40 V – OPTIONAL LEDS ON FRONT PANEL ~ OVER- SPKRS HEAT ON + MALE IEC CONNECTOR WITH INTEGRAL FUSE –– 00 +57V 0 –5 7 V BR1 ~ + 1 tuptu O 1 OUTPUT ±57V CON1 FROM RIGHT CHANNEL HEADPHONE OUTPUT NI- + + + TERM3 –IN TC TERM2 + + + CT NI + TERM1 +IN CON2 –57 V 0 +5 7 V 2 tuptu O - OUTPUT 2 ±57V POWER SUPPLY BOARD INSULATE ALL MAINS CONNECTIONS WITH HEATSHRINK SLEEVING + S1 (TOP REAR) Fig.5: here’s how to wire up the speaker protector module so that it runs off the same transformer secondary as the power amplifier module(s). This will be necessary if the transformer only has the one set of secondaries (eg, in an amplifier with no preamp) or if you want the extra current to run a fan. Only one pair of supply wires is required unless the transformer secondary voltage is below 35V-0-35V. Note how the supply wiring is routed – this is to minimise any hum radiation due to the pulsating current draw of the unit. 24V. You can use a 10Ω SMD resistor for LK1 if required. Basically, if unsure whether you need LK1, leave it out! Fitting a heatsink While it certainly won’t hurt to attach a small heatsink to Q1, as mentioned earlier, it isn’t strictly necessary unless you are running a fan. Q2 typically dissipates less than 1.5W but it also has a smaller copper pad on the PCB and will get pretty hot when the fan is running, so you may want to put a small heatsink on it too. Just about any small finned heatsink will work. We used a small anodised aluminium heatsink on our prototype. This is designed to be attached to the top of a 14-pin DIP IC (or similar). We got it from Rockby (Stock No. 26001) and attached it using a piece cut from an adhesive silicone insulating washer. 70  Silicon Chip The Jaycar HH8580 pin grid array heatsink should also be suitable and comes with an adhesive pad preattached. Simply peel off the backing tape and press it down hard on top of Q1 and it should stay there. There’s less room around Q2 so you’d have to offset it a bit but the same comment applies. Testing Thanks to the on-board indicator LEDs, testing the module is pretty simple. First, place a shorting block on CON17. Then hook up a source of medium-voltage AC or DC power to pins 1 and 2 of CON1 (middle and right-hand terminals, looking at the board as shown in Fig.2). If using DC, ensure the negative terminal is connected to pin 2, otherwise the supply will be shorted out by BR1. Ideally, use 18-25VAC or ~30V DC. A series resistor of say 100Ω 5W can be connected to protect the supply and PCB in case of a construction error. Apply power and check that LED1 (Power) lights. None of the other LEDs should light yet. If possible, measure the supply current (eg, by measuring the voltage across the safety resistor). You should get a reading of around 10mA. If the LEDs do not light up as expected, or the current drain is excessive, switch off and check for soldering or component placement mistakes. Assuming it’s all OK, measure the voltage across the electrolytic capacitor by touching the DMM’s probes to the pads on the top of the PCB. If the incoming supply voltage is high enough for the unit to attain regulation, you should get a reading close to 27V. Now temporarily connect a convenient resistor (eg, 1kΩ 0.25W) between siliconchip.com.au   Here’s another view of the fully-assembled prototype PCB. Be sure to install all the SMDs before installing the larger through-hole parts (see text). pins 1 and 3 of CON1, eg, by touching it across the two screw heads. You should see LED2 (AC presence, yellow) light up when it is connected and LED2 should switch off immediately upon removal. Hold it in place for a few seconds and RLY1 should click on. At the same time, LED3 (Relay On, green) should light up. Remove the resistor and the relay should immediately turn off, along with LED3. Power down and move the supply lead from pin 1 of CON1 (righthand end) to pin 3 (lefthand end). Power back on and wait for the relay to switch on. Then connect a spare 9V battery between supply ground (ie, pin 2 of CON1) and the LSPKIN+ terminal. LED4 (Left Channel Fault, red) should immediately light up and the relay should click off, along with LED3. Reverse the polarity of the battery and check that the same thing happens. Now perform the same tests but this time with the RSPKIN+ terminal. LED5 (Right Channel Fault, red) should light and the relay should again click off for both polarities. If using a fan, connect it up now, then clip a 1.8kΩ resistor across one of the thermistor terminals. The fan should switch on but RLY1 should remain engaged and LED3 should not go out. Test the other thermistor terminal; it should behave the same way. Now do the same test on both siliconchip.com.au terminals with a 1kΩ resistor. In both cases, RLY1 and LED3 should switch off and LED6 (Over Temperature, red) should light. You can also check that shorting out the terminals of CON3 has the same effect, ie, RLY1 and LED3 switch off and LED6 lights. Installation The unit mounts in the amplifier chassis on four tapped spacers. Mark out the holes using the PCB as a template and drill them to 3mm. The basic wiring arrangement is shown in Figs.4 & 5. Fig.4 shows the arrangement when the transformer has suitable low-voltage secondaries, while Fig.5 shows the wiring when powering the unit from the same transformer secondaries as the amplifier module(s). Note that high AC & DC voltages are present in the power supply – see the warnings on pages 39 & 44 of the October issue. Use heavy-duty figure-8 cable for the loudspeaker connections. It doesn’t matter if you swap the left and right channels around if it simplifies the wiring but either way, ensure that the polarity (±) is correct. You can check by using a DMM to test for continuity between the LSPKIN– and RSPKIN– terminals and chassis earth (once the power amplifier supply wiring is complete). Temperature Sensors Temperature sensors are variously called “thermostat switches”, “thermal cutouts” and “thermal circuit breakers” and are available in a range of trip temperatures from 50-100°C. Both NO and NC (normally-closed) temperature sensors are available but the temperature sensor used with this unit must be a normally open (NO) type. A thermal cutout rated at 70°C (eg, Jaycar ST3833) is suitable for the Ultra-LD Mk.4 amplifier (and for earlier versions). Alternatively, use the Altronics S5591 which is rated at 60°C. Note the way we’ve shown the power supply wiring in Fig.5. This minimises the AC magnetic field around the wires. Also note that the AC supply connection must go to pin 3 of CON1 as shown. For a centre-tapped connection (as in Fig.4), use three lengths of hook-up wire twisted together and/ or encapsulated in heatshrink tubing. The power supply current is normally 50-150mA so medium/light-duty wire is OK. If using thermostat(s), wire these in parallel to CON3. Their polarity does not matter. If using NTC thermistors, connect these to CON4 & CON5 as shown in Figs.4 & 5. The thermistors are also non-polarised. For speaker disconnection when headphones are inserted, run a single wire from the switched terminal on the jack socket to the relevant terminal of the headphone switch connector (CON17) on the PCB as shown. The ground connection is made through the power supply wiring. If not using this feature, place a shorting block across this header instead. Indicator LEDs The two front-panel indicator LEDs are optional and you can fit none, one or both. No series resistor is needed as 10kΩ resistors are already on the PCB. These will supply around 2mA which is sufficient for high-brightness LEDs but you can reduce the value if necessary, to drive less efficient LEDs. SC November 2015  71