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Keeping a high-power amplifier cool is vital to its longevity. Designing the chassis properly is important for achieving the best possible cooling performance. It’s even possible to improve the cooling of existing amplifiers if necessary. This photo shows the Silicon Chip 500W Power Amplifier from AprilJune 2022. Part 1 by Julian Edgar Cooling Audio Amplifiers L ow-power amplifiers are easy to cool; a reasonably modest heatsink is sufficient for cooling to occur through natural convection in the air. That’s satisfactory in many domestic situations. But if it’s a powerful amplifier that you push really hard, or it’s mounted in a hot location, things aren’t so easy! I recently ran into major problems with amplifier cooling. First, the two amplifiers were working at higher power levels than I’d ever previously used them. Second, rather than being located inside a cool house, they were stacked in a much hotter roof space. The outcome was fried amplifiers... So it’s important to design an amplifier for proper cooling – and if it’s already built, you might need to make some adjustments to fix a less-thanideal design. This series will cover both aspects. Requirements Amplifiers generate heat in three key siliconchip.com.au areas. The most important heat generators are the output devices, whether they are transistors or ICs. Perhaps 3/4 of the heat generated by a typical amplifier is created by these components. However, significant heat is also generated by the power supply, mainly in the bridge rectifier, the transformer and assorted other devices like voltage regulators (if present). Cooling an amplifier falls into two categories: specific cooling, typically by thermally connecting certain high-temperature components to a large heatsink, and general cooling, typically by allowing ventilation or forced air through the enclosure. Where possible, these two requirements should be kept separate. For example, if the main heatsink is buried deep within the case (which is not at all uncommon), the heatsink will warm nearby components. Conversely, if the heatsink is mounted on the outside of the case, this heat can move straight to the wider Australia's electronics magazine environment, so it won’t impact interior case temperatures so much. Another option is to mount the output devices on a tunnel heatsink with a fan sucking air in through a vent on one side of the case and blowing the warm air out a vent on the other side. Unless that warm air is being sucked back in somewhere else, it will have minimal effect on other components in the amplifier. Heatsinks Heatsinks work in two quite different ways. As it names suggests, a heatsink absorbs heat. As it does, its temperature rises. Say we are using a huge 1kg block of aluminium as a heatsink. The specific heat value of aluminium is 0.9J/°C/g, so to raise the temperature of our block of aluminium by 1°C requires 900J (0.9J × 1°C × 1000g). That’s equivalent to 900W of power for one second, 450W for two seconds or 225W for four seconds. So after 60 August 2025  23 seconds at 225W, the heatsink temperature will have risen by 15°C. If the ambient temperature is 25°C, our 1kg heatsink will already be at 40°C after just a minute! If we ran our very powerful amplifier (that we are assuming dissipates 225W) in only 10-minute bursts, we’d be fine. But running it for an hour, the transistors will get hot enough to burn out. So our heatsink will be quite inadequate. You can see that the name ‘heatsink’ is a bit of a misnomer; what we call heatsinks primarily work as heat exchangers. Heat exchangers are devices that transfer heat, often to the air (or sometimes to water, or even oil). Heat exchangers While we have referred to amplifiers throughout this article, any piece of equipment that needs to dissipate a lot of heat will benefit from these techniques. This includes inverters, speed controllers and electronic loads. Heat exchangers shed their heat in three different ways. The first is conduction. If you run an amplifier at full power, switch it off, then pick it up and moved it, you might find that the shelf it was sitting on is warm. The amplifier has heated the shelf primarily through conduction – although that’s more likely if the prototype amplifier is yet to gain feet, and there was a big contact area between the amplifier and the shelf. Conduction is important to amplifier cooling in two ways. First, the heat source (output transistor, output IC, bridge rectifier etc) needs to conduct heat to the heat exchanger (heatsink). You could have the best heatsink in the world, but if the device can’t transfer heat into it fast enough, the device could still fail. The heat transfer depends on numerous factors such as the device’s packaging, which will act as an insulator to some extent, but must be present to transfer the heat onto a large, flat surface suitable for clamping to the heatsink. It also depends on how flat the surfaces are and how firmly they are pressed together. Because perfectly flat surfaces are unlikely, thermal paste is usually applied between them, to help fill in the gaps. But it isn’t a perfect heat conductor either. Thermal paste should not be used to bridge large gaps – the mounting surfaces of both the electronic device and heatsink need to be as flat as possible. Ensure the compound is still runny; if it has started going hard, discard it. Second, in many amplifier designs, the case itself can act as a heat Australia's electronics magazine siliconchip.com.au A car amplifier I built with the cover removed (shown at the bottom). The smallest possible enclosure dimensions were required, preventing the use of conventional finned heatsinks. The front, rear and bottom aluminium panels of the case all act as the heatsink. They are bolted together with generous flanges coated with heatsink grease. The car amplifier fan is controlled by an off-the-shelf module (lower right) that can be easily set to different temperatures using DIP switches. The bottom sheet metal panel of this car sound amplifier was replaced with clear acrylic. A fan has been added under the sheet (a thin fan was required) and it draws air out of the case. Air is admitted to the case through the chamfered holes shown inset, positioned above added finned heatsinks. Cooling other equipment 24 Silicon Chip An amplifier I built that uses thermostatically controlled fan cooling. The temperature controller and display are on the front panel. A fan in the centre of the top panel draws air out of the amplifier, aiding natural convectional flow. There is a similarly sized vent on the bottom panel. At the rear of the amplifier, the main heatsink is positioned horizontally, with a fan blowing air along the fins. The fans switch on at 40°C. Despite working hard during some hot days, in 10 years, the 250W amplifier’s heatsink has never exceeded 45°C. exchanger. That’s especially so if the enclosure is made from aluminium, which is a decent conductor of heat (good electrical conductors are also usually good heat conductors). To do this effectively, the various enclosure panels need to be in intimate contact so the heat is readily conducted to all parts of the enclosure. When a heat exchanger conducts its heat to the adjacent air, it takes very little time for that thin layer of air against the heat exchanger fins to warm up. Once the temperature difference between the air and the heat exchanger drops to nothing, the heat transfer stops. The trick is to move that air away, replacing it with cooler air. This can occur due to natural convection; the warmed air is less dense and so it rises, being replaced with cooler air that is drawn in from below. Convectional flow is largely vertical, so for a heatsink to work effectively by convection, it requires vertical fins along which the air can slide, and no obstructions above or below those fins. The amount of heat that will be exchanged with the air in a given period is heavily dependent on the exposed surface area of the heat exchanger – more is better. Increased surface area is provided by using fins and having a textured (rough) surface to each fin. Fins in most large amplifier heat exchangers are often relatively thick and few. Having numerous very thin An amplifier during construction. The two finned heatsinks have been mounted face-to-face to form a tunnel. One fan is used at each end of the tunnel – one blowing & one sucking. An efficient fan-forced heatsink design – note the fins on the fins, giving a massive surface area. This main heatsink is external to the case, preventing heat being shed from this heatsink and warming internal components. siliconchip.com.au Australia's electronics magazine August 2025  25 This 250W amplifier was originally cooled just by convection. However, this proved insufficient, so two fans were added (see below). Note how the fan shrouds (upturned baking dishes) cover the top of the heatsink fins, drawing air past them. With a setup like this, nothing can be placed on top of the amplifier! Note the heavy gauge aluminium angle used to thermally link the output devices to the exterior finned heatsinks, and how the rear and bottom panels are aluminium and are thermally connected to also act as heatsinks. Heavy aluminium angle is also used to cool the two bridge rectifiers. fins is more effective, but thinner fins are more easily damaged. A good example of this is an air conditioner, which will usually have lots of very thin fins, but if you bump it, they will be squashed. Convectional airflow can also be used to cool the interior of the amplifier – the ‘general’ cooling we mentioned earlier. To achieve this, we need to take a similar approach to heat exchanger cooling – placing vents on the top and bottom of the amplifier enclosure and then ensuring there are no restrictions to that gentle natural air movement. Vents in amplifier enclosure side panels do very little unless there is forced airflow (ie, fans). One major downside to vertical convectional flow is that it is easily impeded by stacking equipment on top of each other, using mounting feet that are too short, and decorations (like flower pots) that may be placed on the top of exposed amplifiers to make them look better. We’ve also seen cats lying on top of amplifiers to keep warm – it may be great for the cat, but not the amplifier! The final heat exchange mechanism is radiation; however, this is the least important. Black heatsinks will radiate heat more effectively than silver or light-coloured heatsinks, but the 26 Silicon Chip difference is relatively small unless the heatsinks are getting very hot. Black anodised heatsinks are around 6-8% more effective than silver ones under normal circumstances. So it’s clear that while amplifier heatsinks are heatsinks, more importantly, they are heat exchangers with the air. Conduction and convection are critically important in cooling heat exchangers. Convectional flow requires careful design and construction, especially in giving free vertical movement to cooling air. Heat exchangers should have the maximum possible exposed surface area. Fans As we suggested above, convectional flow can be thought of as being quite fragile – easy to disrupt and requiring specific heat exchanger fin orientation. Rather than relying on convection, we can use a fan or fans – either to aid the natural convectional flow, or to replace it. Let’s look first at aiding convectional flow. Say we have a commercial amplifier that is running very warm. Its heatsink is located in the middle of the enclosure, with its fins orientated vertically. There are grilles in the top and bottom enclosure covers, and convectional flow is supposed to provide the cooling. To increase this convectional flow, Australia's electronics magazine we can add a fan to either the top or bottom of the case. If it’s on the top, it should draw air out of the enclosure and blow it up. If it’s on the bottom, it should draw cool air from below the amp and blow it into the enclosure. Either way, because it is aiding natural convectional flow, the result will be much more effective than, say, attaching a fan to the side of the heatsink itself. In some cases, the new top or bottom fan can be fitted within the enclosure – even a quite small fan will, in my experience, massively improve flow over purely convectional air movement. If the amplifier is too tight inside to do this, and the amplifier is not normally able to be seen, cutting a hole in its lid and adding an external fan sitting on top will work well. Rather than aiding convectional flow, you can instead decide to organise the heat exchanger purely to suit the fan. For example, the heat exchanger fins can be horizontal. The key criterion is that the air movement provided by the fan must pass along as much of the exposed area of the heatsink fins as possible. For example, two long finned heatsinks can be mounted facing one another, forming a heatsink tunnel. A fan at one end blows into the tunnel, while one at the other end extracts siliconchip.com.au heat from the tunnel (one fan may be enough to do both jobs). The electronic devices bolt to the outside of the heatsinks. For its size, this approach is one of the most efficient ways of cooling an amplifier. This is the approach used in our Variable Speed Drive Mk2 (November & December 2024; siliconchip.au/ Series/430) and it proved very effective. Fans should always move air along heat exchanger fins – we want air to slide along the fins, pick up heat, then depart. We don’t want air to just be turbulently whizzing around! It’s also important to consider what happens to the warm air after it has picked up the heat from the fins. We don’t want it to end up pushed against a solid panel where it will splash back and heat up other components. We also don’t want it to circulate around back to the input side of the fan, or the air will just end up getting hotter and hotter. Ideally, it should go straight out of the case once it’s warm. Conventional PC-type axial fans are the most common and cheapest fans available, and they are also easily salvaged at no cost from many discarded consumer items. There’s also the significant advantage (for use in amplifiers) that many silent or almost-silent types are available that still move a reasonable amount of air. However, squirrel cage (cross-flow) fans can move a huge amount of air, can be very quiet (or very loud, depending on their design) and their long, thin shape lends itself to low-­ profile amplifiers. In the past, this type of fan has been quite expensive, but they’re now cheaply available from Chinese suppliers, including low-­ voltage designs. However, if you decide to use one of these fans, be prepared do so some sheet metal work – they typically don’t just bolt into place, but instead need some baffles made. The flow of air through an enclosure needs to adequately cool the various hot components. This will not occur if the airflow can take a ‘short-cut’ route, for example, passing straight from an inlet grill to the adjacent outlet fan. However, it can be difficult to picture where the airflow will go just by looking at the amplifier. Two airflow visualisation techniques can be used, though. The first is to stick short (eg, 10mm) siliconchip.com.au An older hifi amplifier heatsink, pictured with normal and thermal cameras. The thin fins give an excellent surface area, while the thick metal base conducts heat along the heatsink from the widely separated output devices. The temperature is only about 10°C over ambient, even after testing at high loads with the top cover in place. Sometimes individual components can run very hot. Typically, they have been fitted with small heatsinks, but they seem quite ineffective. This component is running at nearly 49°C with a 20°C ambient temperature. Australia's electronics magazine August 2025  27 Sizing inlets and outlets Any fan that draws air out of an amplifier must have an equivalent inlet vent area. For example, if a 90mm diameter fan is fitted (a cross-sectional area of about 6000mm2), the inlet vent area must also be about 6000mm2. This inlet can comprise a single 90mm diameter opening, or multiple openings that add up to the same cross-sectional area. However, note that as the diameter of the inlets decreases, their restriction to airflow increases – so if the inlet area comprises mesh with small openings, the total of the openings will need to be greater. There is no immediate disadvantage in oversizing the ventilation inlet area, although having too many vents may make it difficult to control the airflow patterns. If the inlet vent area is too small compared to the outlet fan area, the result will be a reduction in air pressure inside the case. This can make the fan(s) less effective, increase noise and dust collection and sometimes result in uneven cooling. In general, it’s preferable to have neutral or a slightly positive pressure inside the case. pieces of cotton thread inside the amplifier and then temporarily replace the lid with a sheet of clear glass or plastic (don’t leave the lid off – the airflow direction will be quite different with the lid removed). With the fan switched on, the direction that the cotton pieces point will show the directions of airflow. Ensure that the power supply capacitors have fully discharged before opening the amplifier. The same applies after you have finished your flow testing and need to remove the threads. The other approach, which works very well, is to again temporarily replace the top cover with a clear sheet, but this time use a source of smoke, like an incense stick, to make the airflow visible. Light the incense stick, allow it to flame for a few moments, then blow it out. A thin stream of smoke will be released from the end of the stick. Allow the smoke to be drawn in by the fan and watch where the airflow goes by looking at the smoke pattern. If the amplifier has multiple inlet openings, place the incense stick in front of each in turn. It’s almost certain that the internal airflow will show unexpected patterns. We will use this technique next month when modifying an amplifier’s cooling. If the cooling airflow is bypassing key components, the easiest solution is to place one of more baffles or guides to redirect the airflow. Cardboard can be temporarily used during flow testing. Then, when effective baffle designs have been developed, it can be replaced with aluminium sheet or, if there is insufficient clearance to live areas, with Presspahn, acrylic or a similar insulating material. Fan control Because of the noise, people often object to the use of fans in hifi amplifiers. After all, who wants a quiet passage ruined by the whirr of a fan? Two approaches can be used to overcome this objection. The first is to use a thermal switch to switch on the fan only when the heat exchanger temperature is too high. A normally open mechanical temperature switch, closing at say 40°C, is the simplest way of achieving this. However, such switches are not as widely available as they once were, and so it may be easier to use an electronic temperature switch. These prebuilt boards are available with relays, remote sensors and adjustable temperature setpoints. They are very cheap, and some have panel temperature displays – which can be reassuring to watch when your fan-cooled amplifier is belting out the tunes! One disadvantage of this approach is that, unless your fan(s) are totally silent at full speed, you may notice them switching on and off. Also, given that the ambient temperature may vary, and amplifiers dissipate power even when idle, it’s almost certain that the fans will be on (and running at full pelt) some of the time when the amplifier is in use. Another approach, which works very effectively, is to have the fan(s) operate at a slow speed whenever the amplifier is switched on. Experiment with suitable series resistor values until you find one that slows the fan to the point of inaudibility, but still allows the fan to flow a reasonable amount of air. You can then use the temperature switch to short out the resistor, changing the fan to full speed when an elevated temperature occurs. Because the heat exchanger is always fan cooled, When selecting amplifier and power supply modules, look carefully at the heatsinking. This bridge rectifier heatsink has vertical fins (good), but the bottom of the heatsink is completely blocked to convectional airflow (bad). While designed to be mounted horizontally, mounting this amplifier module with the heatsinks fins vertical and the board slightly raised to give bottom clearance will dramatically improve cooling. The two bridge rectifiers on the right need to be raised on extension wires to give clearance for fitting heatsinks, with their fins aligned with those on the main heatsink. 28 Silicon Chip Australia's electronics magazine siliconchip.com.au albeit at a low speed, it will take a lot longer for the heat exchanger to reach the ‘fan full speed’ temperature. Note that some fans use bearings that require a certain minimum speed before the bearing operates properly. If the bearing squeaks or makes any other noise at low speed, increase the minimum fan speed. Another great option is to use our Fan Controller & Loudspeaker Protector (February 2022; siliconchip. au/Article/15195), which controls the speed of up to three PWM-­ capable fans. You can set it so that the fans are off at low temperatures, switch on at low speed as the temperature rises, then increase in speed until the temperature stabilises. This gives you the best of all worlds: complete silence (passive cooling) when possible, effectively silent fan-forced cooling under most conditions, and highly effective cooling when the ambient temperature is high and/or the amplifier is producing a lot of heat. While it’s a little on the expensive side, Jaycar’s YX2584 is a good example of a fan that runs basically silently at full speed. It’s a 120mm, 12V DC type with maglev bearings (that run virtually forever; the rated life is 100,000 hours) and it flows 1795L/min with a noise level of 25dBA. Even in a quiet environment, you’d be unlikely to notice that noise. You could also consider a fan from a manufacturer like Noctua or BeQuiet!, both known for fans with a good balance between airflow and noise. That’s all we have space for this month. Next month, we’ll show you how to test an amplifier at high loads SC and improve its fan cooling. Measuring heatsink temperature under full load using an infrared thermometer. A good amplifier cooling system should keep the heatsink temperature less than 25°C above ambient – in this warm room, this reading is just on that limit. Both Thermalright and Noctua make excellent fans. Although Noctua’s are very reliable, they are much more expensive compared to other manufacturers. Squirrel cage fans, sometimes call cross-flow fans, work well for amplifier cooling, especially where the enclosure is not very tall. Air can be drawn-in through one or more vents, then discharged through a rear slot against which the fan is positioned. These fans can be quiet and flow a lot of air. Both mains-powered and low-voltage DC designs are available. Modules like the one shown to the right can easily have the overly small heatsink unbolted and a very much larger heatsink substituted. I use four of these modules in an amplifier with a fan-cooled heatsink about ten times as big as the one provided! siliconchip.com.au Australia's electronics magazine August 2025  29