Icom IC-746 Faq page 24

http://www.arrl.org/members-only/prodrev/pdf/pr0009.pdf
Transportability
The larger the capacity, the larger and heavier the supply. If size and weight are critical for
you, consider a "switching" supply as they are usually less than half the size of a "linear"
supply.
Electrical Noise
All supplies generate some electrical noise. Typically the noise from linear supplies will be
in the form of AC hum while from switching supplies the noise will be RF hash.
Communications grade power supplies of either type are designed to minimize the noise, and
generally cause no problem. (i.e. this is not a good place to cut corners)
"Computer" power supplies that are pressed into radio service are particularly bad when it
comes to electrical noise.
Usually, the harder you push a supply (the heavier the load), the worse the noise will
become.
Mechanical Noise
Many supplies have cooling fans. Sometimes, the fans are loud enough to be a noise
problem themselves. Fan noise is variable from supply to supply (even within the same
manufacturer and model). Likewise, hearing tolerance varies from operator to operator.
Thus, it's hard to tell in advance if a specific supply will have a problem. Time may yet tell.
We do know that fans are driven by load. The closer you operate to a supply's capacity, the
more the fan runs.
Also, some of the transformers used in linear supplies will generate an audible hum. The
transformer itself vibrates with the AC supply. Mass seems to help here. Usually (but not
always) the heavier the transformer is, the less the hum.
Its pretty easy to put a noisy power supply under a table, behind a cabinet, or elsewhere to cut
down on the noise level. Just be careful to allow proper airflow for cooling.
Features
Most of us consider current limiting, over voltage protection, and high temperature shutdown
to be mandatory in a new supply.
Fuses or breakers are required for UL certification. Buy spare fuses right away; they never
blow at a convenient time.
Meters are non-essential; they are luxury items. They make the supply bigger and more
expensive. However, they can be very handy and even fun. Lets face it; most hams enjoy
gadgets.
Adjustable output voltage is a useful feature, more useful than you might realize. Most new
supplies have it (usually internally).
IC-746 FAQ
Page 24 of 113
2003-02-08