402 Chapter 13
13.5.1 Cooling
All power amplifi ers will have a heat sink that needs cooling, usually by free convection,
and the mechanical design is often arranged around this requirement. There are three
main approaches to the problem.
a. The heat sink is entirely internal and relies on convected air entering the bottom
of the enclosure and leaving near the top (passive cooling).
13.5.1.1 Advantages
The heat sink may be connected to any voltage, which may eliminate the need for thermal
washers between power device and sink. However, some sort of conformal material is still
needed between transistor and heat sink. A thermal washer is much easier to handle than
the traditional white oxide-fi lled silicone compound, so you will be using them anyway.
There are no safety issues as to heat sink temperatures.
13.5.1.2 Disadvantages
Because of the limited fi n area possible inside a normal-sized box and the relatively
restricted convection path, this system is not suitable for large dissipations.
b. The heat sink is partly internal and partly external, as it forms one or more sides
of the enclosure. Advantages and disadvantages are much as just described; if
any part of the heat sink can be touched, then the restrictions on temperature and
voltage apply. Greater heat dissipation is possible.
c. The heat sink is primarily internal, but is fan cooled (active cooling). Fans always
create some noise, which increases with the amount of air they are asked to move.
Fan noise is most unwelcome in a domestic hi-fi environment, but is of little
importance in PA applications. This allows maximal heat dissipation, but requires
an inlet fi lter to prevent the build-up of dust and fl uff internally. Persuading
people to clean such fi lters regularly is near impossible.
E f fi cient passive heat removal requires extensive heat sinking with a free convective air
fl ow, and this indicates putting the sinks on the side of the amplifi er; the front will carry
at least the mains switch and power indicator light, while the back carries the in/out and
mains connectors so that only the sides are completely free.
The internal space in the enclosure will require some ventilation to prevent heat build-up;
slots or small holes are desirable to keep foreign bodies out. Avoid openings on the top