Please 'Boom' Responsibly As most of you have noticed, the noise ordinances have become much tougher lately. Most of this is due to idiots, yes IDIOTS, who drive through residential areas with their windows down while their system is playing at full power. To make things worse, the music they listen to has all sorts of foul language that's not suitable for small children, (who may be playing outside). There are even a few people, who are even beyond idiot status, that play their systems at full power through residential areas after 10:00 PM (when many people go to bed). I don't believe that this type of behavior is good for the industry. If the fines get too stiff, people will stop buying large systems. If this happens, more people will get out of car audio (who wants a mediocre system). People get interested in things because they're exciting. A deck and four 6.5" speakers are not going to interest many of the younger car audio enthusiasts. If car audio enthusiasts keep annoying more and more people, the fines will keep getting tougher. All of this will only reduce interest in the equipment that fuels the industry. If you want to listen to your system at full volume, get out on the highway where there's little chance of bothering anyone. When you get to a red light, turn it down. If the only thing attractive about you is your 'system', you have some work to do. Bottom line... Think about what you're doing. Think about other people. It's not the end of the world if you have to turn the volume down for a little while.

Heat Sink Basics:
The reason an amplifier has a heavy aluminum outer case (heat sink) is to help dissipate heat. The heat sink is needed because no electronic component is 100% efficient. This is especially true in most audio circuits because they require a voltage drop across the output transistors in order to reproduce the audio signal. As you already know, if there is a voltage drop across a device (wire, transistor... anything) and current flowing through that device, power will be dissipated (in the form of heat) in the device. All semiconductors (transistors, rectifiers, regulators...) which dissipate significant amounts of power are mounted tightly to the heat sink (to provide a thermal connection between the two). You should also know that electronic components become more susceptable to failure as their temperature increases. This is the reason that amplifiers (or any device which supplies or converts power) must have a heat sink. If there were no heat sink, the temperature of the electronic components would quickly reach unsafe levels and they would die a horrible painful death. For example... If an amplifier is taken out of it's heat sink and played at normal volume levels into an otherwise safe speaker load, the amplifier would fail in just a few minutes.
Surface Area:
Most heat sinks have fins and/or ridges. The fins are used to increase the surface area. The extra surface area allows more metal to come into contact with the air which allows the heat to be dissipated more quickly.
NOTE:
Do not paint a heat sink. Most heat sinks are anodized aluminum. Painting a heat sink (especially if it's a thick coat of paint) is like putting a blanket on the amplifier. If you absolutely must paint the heat sink, use the lightest, thinnest coat of paint possible.

Thermal Conductors and Electrical Insulators

Thermal Grease:
No matter how good production methods are, the metal surfaces of the heat sink and the metal mating surface of the semiconductor will not be perfectly smooth (as shown below). When mating semiconductors to a heat sink, you'll generally use a 'heat sink compound' (the white grease that you often see when you open an amplifier or other electronic device with high power dissipation) between the metal surfaces to help fill in the microscopic irregularities. Without the grease (compound), the effective surface contact/area would be greatly diminished and result in higher semiconductor operating temperatures and therefore higher failure rates of the semiconductors. This metal to metal contact (no insulator between the sink and the semiconductor) is the most efficient way to transfer the heat from the semiconductor to the sink (except for soldering or welding to 2 pieces together).

Mica Insulators:
Mica is a hard clear crystalline material that can be split into thin sheets. It is a very good electrical insulator and a good thermal conductor. It's used to insulate semiconductors from a heat sink. If the heat sink is electrically conductive, you must have some sort of insulator between the semiconductor and the sink to prevent a short circuit. The following 2 images show a semiconductor and its respective mica insulator.

Kapton©/Polyamide:
Polyamide film is used by some manufacturers for electrical insulators. It is an amber colored film that can handle temperatures as high as 800°F. The following picture shows what polyamide film looks like.

Sil-Pads:
Sil-pads come in a few different varieties. The most common is a silicone rubber material reinforced with fiberglass. A different version has 2 thin silicone rubber layers sandwiching a layer of polyamide film. The following is an example of a glass reinforced sil-pad. Many manufacturers use sil pads because they don't require grease to perform to spec (which makes assembly quicker and easier). All others listed on this page require thermal grease to properly conduct heat.

Metallic Insulators:
Some manufacturers use a metallic insulator like anodized aluminum. The anodizing process creates a thin aluminum oxide layer which is an electrical insulator. This is a very effective thermal conductor. The following is an example of a metallic insulator.

Thermal Impedance/Resistance:
Some electrical insulators are more efficient than others in transferring heat from a semiconductor to the heat sink. The thermal transfer efficiency is expressed by the formula Rth=°C-in²/W and says that... for every square inch of area, the temperature will rise a given number of degrees celcius for every watt of dissipation. If the specification says 0.5°C/W and you have 10 watts of dissipation, the temperature of the semiconductor is is going to be 5°C higher than the heat sink. From best to worst (of the types listed) I prefer... aluminum oxide, mica, kapton and lastly sil-pads. Remember that the formula expresses the RATE that the insulator can conduct the heat. All of them can transfer the heat but some can do it more quickly.

 

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