Re: gEDA-user: buzzing board

[Larry Doolittle <ldoolitt@xxxxxxxxxxxxxxx>]

DJ -

On Thu, Sep 13, 2007 at 12:34:38AM -0400, DJ Delorie wrote:
> It's really hard to tell which part is doing it, just from listening.
> The sound may be a combination of a high pitch whine and a 100hz buzz
> (the oled is 100hz).
> 
> I *think* the sound may be coming from the switching power supply (it
> has three power inductors in it), but neither the +15v power input nor
> the 3.3v rail have any noise on them (outside of the 150KHz from the
> switcher itself).  Bypassing two of the inductors (the filter ones)
> does nothing.  Pressing on any component does nothing.
> 
> 3. If you know what's causing it, how do I fix it?

A lot of switching power supplies have a low-current mode
where they don't switch at the full rate.  Those modes
tend to be noisier, both electrically and acoustically,
than the normal fixed-frequency mode.  If that's the
culprit, most switching controller chips have a control
pin to disable it.  They don't make it the default, because
it decreases efficiency.

    - Larry


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