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- In reply to: Jim Green: "RE: Noise from "Power Source"????"
You only wish that the power supplied everywhere were simply a smooth sine
wave with a root-mean-square (RMS) amplitude of 120V and a frequency of
60Hz. In fact, it can be far from that, at least at times.
The power company contract with you (the implicit one, anyway) only says
they will give you a certain amount of power at *nominally* 120VAC and
60Hz. Of these two specifications, they care a whole lot more about the
average frequency being right than anything else. (That's so your kitchen
clock will keep good time.) But even the frequency can wander a fair amount
during the day. The voltage is subject to great variation (called surges
and brownouts), and in addition there often are spikes on the line.
A laptop power supply is sort-of like a small UPS. And it can do some good
filtering to get rid of much of this noise. But a very good surge protector
in front of that can help as well. (And help protect the laptop from being
destroyed by a really big spike--which, given the relative prices of a
surge protector and a laptop computer, is a very greatly cost-effective
reason to use a surge protector.) Another UPS might serve as an additional
stage of filtering and if one stage reduces line noise by a factor of 100,
then two might reduce it by a factor of 10,000--which may be what you need.
I have no direct experience on VR on a laptop in the presence of line
noise, but these general considerations certainly could explain what the
Dell representative said and some of the experiences others have reported
here.
John
At 03:16 PM 7/10/98 -0400, you wrote:
>This response from Dell sounds like bull s__t to me. The wall current is
>120V 60Hz, everywhere. You plug in an AC adapter that changes it to 12VDC
>or 5VDC, with some ripple of course. If this is charging a battery in your
>laptop, the battery acts like the largest capacitor in the universe, and
>should filter out all the ripple. If it doesn't, and some gets into your
>sound card, then Dell has a very poor power supply design. Even so, all
>that should get to the sound card is 60Hz humm. Does the noise sound like
>hummmmmmmmm?
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John M. Goodman, Ph.D., author of "Peter Norton's Inside the PC,"
Seventh Edition (Sams 1997, ISBN 0-672-31041-4)
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