On 4/22/10 1:28 PM, steve roche wrote:
Hello-
I've got a Pismo which puts an awful 60hz hum when I plug it into the AC
power supply.
(it is most noticeable if I am using iTunes and hooking it up to an
amplifier at a party.)
It's almost as loud as the music itself.
I am staying in Mexico, and there is basically no such thing as a
grounded outlet here,
so what I do is unplug it. and run on battery for 45 minutes (no more hum)
I checked out some other forums; perhaps I have a 'ground-loop' problem;
I haven't run
my own ground wire outside yet (i think it needs to be driven 3 feet
into the ground? Or
hooked to a water pipe, etc)
but I have tried plugging in the sound system and the laptop into TWO
separate circuits (I used the house next door). Still, a ridiculous hum.
I'v got 2 white iBooks which do not have this problem.
So I'm guessing that once again my sound card is bad, on the Pismo?
I've been through two so far. Sometimes vocals on a song sound distant.
I've got a new (generic) charger, but the original Apple hummed louder.
One person suggested running a car power supply unit (110v to 12v DC),
then plugging in an inverter into that, which brings the power back up
110v, and plugging the laptop into that.
He uses that when onstage and he needs the laptop to work without a hum.
He says it's insane but works perfectly. Is there a filter made to
refine the AC or DC power,
or a way to create a 'clean' power supply?
Another suggestion was to REMOVE the grounding pin... I though the lack
of ground was my problem, originally.
Perhaps the PISMO is my problem, instead?
The power adapter, does it have a 2 prong AC plug or a 3 prong. If it's
two prong then it doesn't have a ground and therefore it's not a ground
loop problem.
A simple old trick from the dark days of electronics is to reverse a two
prong plug. Try it on both the computer and the amplifier, one at a time.
One option is to try using a USB sound device instead of the built-in
sound.
The idea of going to 12V and then back up to 110 may eliminate the 60 Hz
hum but, depending on the equipment, it can introduce some very hefty
noise. Computers are usually immune to this noise but things like amps
often are not.
--
Clark Martin
Redwood City, CA, USA
Macintosh / Internet Consulting
"I'm a designated driver on the Information Super Highway"
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