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Loudspeaker protector and fan controller for the Ultra-LD amplifier This simple project will save your valuable loudspeakers if a fault occurs in the output stages of the Ultra-LD Stereo Amplifier. As a bonus, it includes fan control so that the fan only runs when necessary. By PETER SMITH & LEO SIMPSON This loudspeaker and fan controller has been specifically designed to suit the Ultra-LD 100W per channel amplifier de­ scribed in the March and May 2000 issues. Not only does it pro­vide muting at switch-on and switch-off to prevent any thumps 54  Silicon Chip from the loudspeakers, it also protects the loudspeakers against catastrophic failure in the amplifier. In addition, it provides temperature control for the fan-cooled heatsink, switching the fan on if the heatsink temperature rises above 60°C. However, while the circuit has been specifically designed to suit the above amplifier, it can be used to mute and protect the loudspeakers in other amplifiers and also provide fan switch­ing if that is required. This is not the first loudspeaker protector we have pub­lished as we featured similar designs in April & October 1997. However, this latest design provides two methods of temperature sensing for the fan control as well as a temperature cutout for the speakers, if the heatsink rises above 80°C. Why you need protection By the far the biggest reason for incorporating speaker protection into Fig.1: each channel of the amplifier is connected to one of the moving contacts of the double-pole relay and is monitored for DC faults by a triplet of transistors, Q1, Q2 & Q3 for the left channel and Q4, Q5 & Q6 for the right channel. Two methods of temperature sensing for the fan control are shown. The section in the bottom lefthand corner of the circuit shows the optional thermistor temperature sensing, using an LM393 comparator. any amplifier is for insurance – to save money in the case of a serious amplifier fault. For example, in the Ultra-LD amplifier, the main supply rails are ±55V DC. If one of the output transistors fails it means that more than 50V DC will be applied to the speaker’s voice coil. For a nominal 8Ω speaker the voice coil will have a DC resistance of around 6Ω and so the total power dissipation will be around 400W until the supply fuse blows. But maybe the fuse won’t blow. Either way, the speaker is likely to be history. On the one hand, the huge DC power applied is likely to push the voice coil right out of the gap, damaging the voice coil and suspension in the process. But a worse scenario is if the on-board supply fuse doesn’t immediately blow – a strong possibility since a current of around 8.5A may not blow a 5A fuse straight away. If the fuse doesn’t blow straight away, there is a strong possibility that the voice coil will immediately become red-hot and set fire to the speaker cone material. Now we are really in trouble because the acetate filling material in the enclosure and the grille fabric can also catch fire and then generate huge quantities of acrid black smoke. Don’t think this can’t happen. It has happened before and will happen again to some unsuspecting owner of high-power audio equipment. Stereo systems do fail and they can cause house fires. That is why they should not be left on for long periods of time, especially if no-one is present to turn them off if a fault does occur. OK, we have established the risk associated with any audio power amplifier of more than about 40W per channel. The way to avoid the problem is to build a loudspeaker protector like the one featured here. Apart from the fire insurance angle, the circuit will mute any thumps and pops which occur when you turn your amplifier on and off and it does the August 2000  55 The 80°C thermal switch is attached to the side of heatsink using self-tapping screws. A second 60°C thermal switch (for the fan) can be mounted next to it if you elect not to use a thermistor temperature sensor. fan control which we’ll come to later. The whole circuit fits onto a PC board measuring 124 x 60.5mm and includes a DPDT relay with 10A contacts, plus a 3-terminal regulator on a finned U-shape heatsink. Fig.1 shows the complete circuit. On the lefthand side of the circuit you will notice that each channel of the amplifier to be protected is connected to one of the moving contacts of the double-pole relay and then out to the speaker terminals. Each channel of the amplifier is also monitored for DC faults by a triplet of transistors, Q1, Q2 & Q3 for the left channel and Q4, Q5 & Q6 for the right channel. For the sake of simplicity, we’ll just talk about the left channel since the identical process occurs for the right channel. Let’s see how the triplet of transistors operate together. The active signal from the amplifier’s left channel is fed via a low-pass filter consisting of the 22kΩ resistors and two 47µF BP (bipolar or non-polarised– NP) electrolytic capacitors. The filter network removes any audio frequencies and just leaves DC to be monitored by the three transistors. This is because we don’t want audio signals to trip the protection circuit in any way. The line from the low pass filter is connected to the emitter of Q1 and the base of Q3. Q1 monitors for negative DC signals while Q3 monitors for positive DC sign­als. If a DC signal of more than +0.6V is present, Q3 will turn on. Similarly, if a signal of more than -0.6V (ie, neg56  Silicon Chip ative voltage) is present, the emitter of Q1 will be pulled low, and so Q1 will turn on and it will turn on Q2. Both Q2 & Q3 have a common 56kΩ collector load resistor and this normally feeds base current to Q7. Q7 feeds base current to Q8 and so both these transistors and the relay are on. But if Q1 or Q3 are turned on by an amplifier fault condition, the base current for Q7 is shunted away and so Q7, Q8 and the relay are turned off, disconnecting the speakers. As noted above, Q4, Q5 and Q6 do exactly the same monitoring for the right channel of the amplifier and they switch Q7, Q8 and the relay in exactly the same way. Heavy duty relay The relay selected for the job has contacts rated at 10A and there are several reasons for this. First, and most important, we want the contact resistance in the relay to be as low as possible so that it has negligible effect on the amplifier performance, in respect of distortion, damping factor and so on. Second, the relay contacts have to pass and break the heavy DC currents which would otherwise flow through the loudspeaker if a fault ever occurs in the amplifier. However, we don’t merely use the relay to disconnect the amplifier’s output from the loudspeakers. If we simply did this, there is a fair chance that the contacts would just arc across and the heavy DC current might continue to flow through the loudspeaker. That might seem unlikely but when you have a heavy DC cur­rent and a high DC voltage pushing it along, it can be quite hard to break the circuit. That is why the moving contacts of the relay are shorted to the loudspeaker ground lines via the “un­used” contacts. By shorting the moving contacts of the relay to the loudspeaker ground lines, the arc current is diverted to chassis and the fuses will blow if the arc still persists. Muting delay So far we have described the protection function of the circuit. Now we’ll look at the muting function, to prevent thumps at switch-on. This is achieved with resistors R1, R3 and the 220µF capacitor C1. When power is first applied, C1 is discharged and so no base current can flow to Q7 via 56kΩ resistor R1. C1 then charges via the 220kΩ resistor R3 and after three seconds or thereabouts, enough voltage is present to allow base current to pass via R1 to Q7. It then turns on Q8 and the relay to connect the loudspeakers. If power is removed from the protection circuit, the relay opens within less than half a second and this prevents any turn-off thump being heard. Fan control Fig.2: the speaker protection board is powered from the 35V secondary wind-ings on the Ultra-LD amplifier’s power transformer. We have provided two methods of temperature sensing for the fan control and both are shown on the circuit. The section in the bottom lefthand corner of the circuit shows the optional thermis­ tor temperature sensing, using an LM393 The leads of the thermistor are insulated with heatshrink tubing. It is then slid into a channel in the TO-220 heatsink clip, which holds it firmly in place. The thermistor/heatsink clip assembly is clipped onto one the fins of the large tunnel heatsink, as shown here. Be careful not to damage the thermistor body during this procedure. comparator. But first we’ll talk about the simple version of the circuit which involves a 60°C thermal cutout TH1 and transistor Q9. The thermal cutout is mounted on the tunnel heatsink, preferably somewhere near the centre. The thermal cutout has a set of normally closed contacts but when the temperature rises above 60°C, they open and this allows the associated 2kΩ resistor to turn on transistor Q9 and thereby run the fan. It is fed via a 33Ω 5W resistor so it does not run at full speed but still pumps a fair amount of air through the tunnel heatsink. When the heatsink temperature cools down to around 40°C, the thermal cutout will close again and the fan will be switched off. Note that 40 degrees C is relatively cool so the fan will probably run for a long time and on a hot day would continue to run until the amplifier was switched off. While the thermal cutout has the virtue of simplicity, its relatively wide hysteresis (ie, difference between switch-on and switch-off temperatures) means that once the fan comes on, it may not turn off until the amplifier is switched off. Thermistor circuit As an alternative to the thermal switch, we have provided the optional thermistor circuit mentioned above. This uses a negative temperature coefficient (NTC) bead thermistor in a comparator circuit based on an LM393, IC1. Pin 2 is connected to the thermistor (RT1) while pin 3 is connected to trimpot VR1. Naturally, the thermistor is mounted on the tunnel heatsink. At room temperatures, trimpot VR1 will be set so that the voltage at pin 3 is below that at pin 2 and so the output at pin 1 will be low. This means that transistor Q9 will be off and the fan is not running. When the heatsink temperature rises, the resistance of the thermistor goes low and at some point pin 2 will be pulled below pin 3 of IC1. This will cause pin 1 to go high (or actually, the open-collector transistor inside IC1 to turn off) and allow Q9 to turn on and run the fan. The 1MΩ positive feedback resistor between pins 1 & 3 of IC1 ensures a degree of hysteresis so that the fan does not cycle on and off repeatedly. We suggest that VR1 be set to turn on the fan for heatsink temperatures of around 55-60°C. We’ll discuss that setting later on in the article. Finally, there is another thermal cutout in the circuit and that is in series with the base of Q8, the transistor controlling the relay. This second thermal cutout is a failsafe device so that if the amplifier is overheating due to a serious overload or a failure of the fan circuit, the relay will be turned off to disconnect the loudspeakers. Power supply Deriving a low-voltage supply from that of the Ultra-LD Amplifier presents a problem because of the August 2000  57 The loudspeaker protection module was mounted inside the disk drive cage of the Ultra-LD Amplifier, adjacent to the power amplifier module. Note that the heatsink gets quite hot, so make sure it goes towards the top. relatively high AC voltage from the transformer secondary and the need to provide a total current of around 200mA at 12V to power the fan and relay. Our solution is to connect diodes D1 & D2 to the 35V secondary windings (as shown in Fig.2) and then pass the full-wave recti­fied DC from the 470µF capacitor via a 33Ω 5W resistor to the input of an LM317HVT high voltage 3-terminal regulator, REG1. This provides a regulated 11.7V to power the speaker protection circuit. PC board assembly All the parts are mounted on a PC board measuring 124 x 60.5mm and coded 01108001. The wiring diagram is shown in Fig.3 and it shows both temperature measurement options; ie, thermal cutout TH1 and the optional thermistor, RT1. If you are going to use thermal cutout TH1, you can leave out IC1, VR1, RT1 and the associated resistors apart from the 2kΩ resistor which supplies base current to Q9. Mount the PC pins first and make sure they are a tight fit in their holes before they are soldered. Then fit the links (these must be done before the two wirewound resistors are in­ stalled). Most of the resistors and diodes are mounted vertically (end-on) to save space. Mount them as shown in the diagram of Fig.3. In each case, do not mount the end-on diodes and resistors so that they are right down on the board; you should have a lead length of about 2-3mm above the board to make sure the component is not overheated while being soldered. The two 33Ω 5W wirewound resistors should be mount­ed so that they are about 2-3mm above the board, to allow cooling. The four 47µF electrolytic capacitors can go in either way around since they are non-polarised (BP or NP). The other elec­trolytics are polarised and must be inserted the correct way around. Next, insert the IC and the transistors and make sure you put the correct one in each spot. The relay is intended for mounting in a socket but we have not used a socket in this case, because it takes up more space on the board and it will be an extra source of contact resistance which we particularly want to avoid. Therefore the relay is mounted by soldering short lengths of stout (say 1mm) tinned copper wire to each relay pin. These wire leads are then pushed through the relay mounting holes in the PC board and soldered. Alternatively, if you are supplied with a PC board which has slotted holes for the relay, you can solder it in directly. The 3-terminal regulator is mounted on a U-shaped heat­sink using a standard insulating kit (see Fig.3) This assembly is then attached to the PC board with two M3 screws, nuts and washers, and the regulator leads soldered. Installation When the PC board is complete, check your work carefully. If you Resistor Colour Codes             No. 1 1 1 2 4 3 2 1 1 2 2 58  Silicon Chip Value 1MΩ 220kΩ 68kΩ 56kΩ 22kΩ 10kΩ 2kΩ 1.5kΩ 240Ω 22kΩ 33Ω 5W 4-Band Code (1%) brown black green brown red red yellow brown blue grey orange brown green blue orange brown red red orange brown brown black orange brown red black red brown brown green red brown red yellow brown brown red red orange brown not applicable 5-Band Code (1%) brown black black yellow brown red red black orange brown blue grey black red brown green blue black red brown red red black red brown brown black black red brown red black black brown brown brown green black brown brown red yellow black black brown red red black red brown not applicable 56k 1.5k 56k 10k HEATSINK TH2 80C VR1 50k 10k 1M 220k Q5 68k Q9 2k 10k 220F 220F 10F 0.1F Q7 IC1 A LM393 D3 K 1 Q2 Q4 470F Q1 Q6 33 5W A 33 5W K NOTE: CAPACITORS MARKED "BP" ARE BIPOLAR 47F BP 47F BP 22k 22k 22k 22k Q8 D2 LED1 FRONT PANEL POWER LED REG1 LM317HVT 240 K A 2k D1 Q3 K Fig.3: both temperature sensing options are shown on the wiring diagram. If you are using thermistor TH1, you can omit IC1, VR1, RT1 and the associated resistors apart from the 2kΩ resistor which supplies base current to Q9. TO LEFT SPEAKER 47F BP RLY1 22k 1W _ 22k 1W + FROM LEFT AMP FROM RIGHT AMP TO RIGHT _ SPEAKER + 47F BP + _ + _ A Parts List M3 x 10mm SCREW HEATSINK INSULATING PAD PLAIN WASHER INSULATING BUSH REG1 MOUNTING DETAILS TO 12V DC FAN RT1 TH1 60C LM317 0V FROM T1 SECONDARY 35V 35V M3 NUT HEATSINK _ + HEATSINK have a DC power supply capable of around 15-20V then you can do some initial checks which we describe in the setup procedure below. Your DC supply can be connected to the input of the 3-terminal regulator. Failing that, your next step is to install the board in the amplifier case. You can see from the photos how we mounted the prototype. Quite a lot of wiring is involved in the installation. You will need to run three wires from the 35V transformer secondary windings, two wires to the 12V fan and another two pairs of wires to the thermal cutouts (TH1 & 1 PC board, code 01108001, 60.5 x 124mm 1 10A 240VAC DPDT power relay (Jaycar SY-4065) 1 Universal “U” heatsink (Jaycar HH-8511) 1 TO-220 clip-on heatsink to mount thermistor (Jaycar HH-8504) 1 TO-220 insulating bush and washer 16 PC stakes 9 M3 x 6mm screws 2 M3 x 10mm screws 3 M3 nuts 7 M3 flat washers 4 M3 x 10mm tapped spacers 1 50kΩ horizontal trimpot (VR1) 1 Thermal circuit breaker, 80°C, normally closed (TH2) (Altronics S-5610) 1 Thermal circuit breaker, 60°C, normally closed (TH1) (Jaycar ST-3821, Altronics S-5600) 1 NTC thermistor, 100kΩ <at> 25°C (RT1) (DSE R 1797) Wire and cable 200mm length of 0.8mm tinned copper wire; hook-up wire; heavy-duty speaker wire Semiconductors 3 1N4004 1A 400V diodes (D1-D3) 1 LM317HVT high voltage adjustable regulator (REG1) 1 LM393 dual comparator (IC1) 5 BC547 NPN transistors (Q1, Q3, Q4, Q6, Q7) 2 BC557 PNP transistors (Q2,Q5) 1 BC327 PNP transistor (Q8) 1 BC337 NPN transistor (Q9) Resistors (0.25W, 1%) 1 1MΩ 3 10kΩ 1 220kΩ 2 2kΩ 1 68kΩ 1 1.5kΩ 2 56kΩ 1 240Ω 4 22kΩ 2 33Ω 5W 2 22kΩ 1W Capacitors 1 470µF 63VW PC electrolytic 2 220µF 16VW PC electrolytic 4 47µF 50VW non-polarised PC electrolytic 1 10µF 63VW PC electrolytic 1 0.1µF 63VW MKT polyester Miscellaneous Heatsink compound. August 2000  59 +11.7V is present at pin 8 of IC1, at the emitter of Q8 and the collector of Q9. Initially the fan should be off. The relay should operate about three seconds after turn-on. When the relay is closed, LED1 should be alight. To check that the fault protection works, connect a 1.5V battery to the left and right channel inputs on the protection PC board. In each case the relay should open immediately, indicating that the protection circuit is working correctly. Fig.4: this is the actual size artwork for the PC board. Check your board carefully before installing any of the parts. TH2). By the way, the wires to TH2 also connect the front panel LED, as shown on Fig.3. You also need to run the heavy cabling from the amplifier outputs and to the loudspeaker terminals. Since so much extra speaker wiring is required you must use heavy cabling such as 2 x 79/02mm speaker cable (or heavier) to avoid power losses and any reduction in amplifier damping factor. However, do not make the speaker cable connections to the PC board until initial checks are done. We’ll come to these in a moment. If you are using the bi-metallic thermal cutouts you will need to mount them somewhere near the centre of the tunnel heat­sink. Our photo shows just one thermal cutout (TH2). The thermistor option is actually easier because you can just use a TO-220 clip-on heatsink (Jaycar HH8504) to mount the thermistor. We have a series of photos showing how the leads of the thermistor are individually sleeved and then a TO-220 clip is used to secure the thermistor to one of the fins of the tunnel heatsink. Setting up With all of the wiring complete, apart from the speaker cabling, it is time to power up the protection board. First check that the output of the 3-terminal regulator is around +11.7V. You can also check that Temperature setting Ideally, VR1 should be set so that the fan cuts in at around 55-60°C. To do this setting you need a thermom­ eter which will read to 100°C. Our suggestion is to boil some water in a jug and then add it slowly to a small container of water while stirring it with the thermometer. As it comes up to 60°C, you can adjust VR1 to turn the fan on. Oh, you will of course have to immerse the thermistor in the water container for this adjustment. The thermistor and its leads should be sealed into a small plastic bag or plastic shrink-wrap. Once you are satisfied with the adjustment of VR1, you can clip the thermistor back onto the heatsink fin, connect the loudspeaker cables and SC the system is ready to roll. Thumps In The Ultra-LD Amplifier As published in the March & May 2000 issues, the Ultra-LD 100W amplifier does produce a thump several seconds after switch-off although it does not sound particularly loud. However during our testing of this loudspeaker protection circuit, we noticed that if the power was turned on and then off fairly quickly, there was quite a sharp turn-on thump as the relay reconnected the loudspeakers. This was puzzling as normally there is very little turn-on thump from this amplifier. We then connected up our digital scope to monitor the output of one amplifier channel, both before and after the relay. Setting the timebase to 0.5 sec/div, we were able to easily 60  Silicon Chip observe what was going on. When the amplifier was turned off, it did produce a turn-off thump which was muted by the delay cir­cuit. However, the turn-off thump was really quite severe, amounting to a 20V spike which then decayed to zero over a period of 20 seconds or more. So if the power was reapplied shortly after turning off, the mute delay capacitor had not had enough time to discharge and it connected the amplifier before it had time to stabilise again, producing a sharp thump. Having seen just how severe the turn-off thump was, we realised it was due to the regulated -55V rail collapsing prema­ turely. This was due to the fact that the current drain from the -55V rail is higher than from the +55V rail. The solution was to increase the 100µF 63V capacitor connected to the -55V rail on each amplifier board, to 220µF. This change means that the input differential stage (Q1, Q2) maintains control over the amplifier DC conditions for much longer so that the main ±52.5V rails are almost completely discharged before the amplifier ceases to work. The result – no turn-off thump. So regardless of whether you build this protection circuit of not, we recommend that owners of the Ultra-LD amplifiers increase the 100µF capacitor for the -55V rail on each board to 220µF.