Noise and Grounding 403surface as these will allow the entry of spilled liquids and increase dust entry. BS415 is a
good starting point for this sort of safety consideration, and this specifi es that slots should
be no more than 3 mm wide.
Reservoir electrolytics, unlike most capacitors, suffer signifi cant internal heating due
to ripple current. Because the electrolytic capacitor life is very sensitive to temperature,
mount them in the coolest position available and, if possible, leave room for air to
circulate between them to minimize the temperature rise.
13.5.2 Convection Cooling
It is important to realize that the buoyancy forces that drive natural convection are very
small and that even small obstructions to fl ow can seriously reduce the rate of fl ow, and
hence the cooling. If ventilation is by slots in the top and bottom of an amplifi er case,
then the air must be drawn under the unit and then execute a sharp right-angle turn to go
up through the bottom slots. This change of direction is a major impediment to air fl ow,
and if you are planning to lose a lot of heat then it feeds into the design of something so
humble as the feet the unit stands on; the higher the better for air fl ow. In one instance
the amplifi er feet were made 13 mm taller and all the internal amplifi er temperatures
dropped by 5°C. Standing such a unit on a thick-pile carpet can be a really bad idea,
but someone is bound to do it (and then drop their coat on top of it); hence the need for
overtemperature cutouts if amplifi ers are to be fully protected.
13.5.3 Mains Transformers
A toroidal transformer is useful because of its low external fi eld. It must be mounted so
that it can be rotated to minimize the effect of what stray fi elds it does emit. Most suitable
toroids have single-strand secondary leadouts, which are too stiff to allow rotation; these
can be cut short and connected to suitably large fl exible wire such as 32/02, with carefully
sleeved and insulated joints. One prototype amplifi er I have built had a sizeable toroid
mounted immediately adjacent to the TO3 end of the amplifi er PCB; however, complete
cancellation of magnetic hum (hum and ripple output level below 90 dBu) was possible
on rotation of the transformer.
A more diffi cult problem is magnetic radiation caused by reservoir charging pulses (as
opposed to the ordinary magnetization of the core, which would be essentially the same if
the load current was sinusoidal), which can be picked up by either the output connections