[Richard Crowley]

"Mike Rivers" wrote...
genericaudioperson wrote:
I think there is a rule that if you have an internal power supply, you
have to pay $20 or $30k to have United Laboratories certify the device
for safety. But if you use an external supply, you can simply use an
off-the-shelf box and do not require the certification.

It's sort of like that. The device with the AC power plug on it has to be
safety certified. In Europe, it's CE, and that's even more stringent than
UL. So if they buy an already-certified external power supply that has a
sufficiently low voltage output, then this saves the manufacturer money.
Another advantage of an external power supply for analog audio products is
that it gets the transformers away from the innards where it could induce
hum. Still, some decide to use an internal power supply because they think
it makes a better product.

It's true that you're likely to find an external power supply in a low
priced product, but the $300 Behringer mixer I recently reviewed had an
internal power supply. It's a choice the manufacturer has to make. Part of
that choice is ergonomics, part is perception, and part is cost.

Not to mention that very high-end mixers (and other high end equipment)
often use external power supplies for other reasons (such as space
utilization, heat/noise management, etc. etc.)

Another factor is globalization. If you want to make your product
attractive to anyone on the planet, an easy way to accommodate the
various mains power standards (and the various certifications as
you mentioned) is to use a pre-certified external PS which either
is made specifically for 100/120/240V and 50Hz/60Hz, or one
which automatically accommodates all the variations (typically the
more contemporary switchmode power supplies, etc.)

Here is one manufacturer's explanation for your amusement...
http://www.fmraudio.com/FAQ.htm#question1