Bluetooth headsets to the rescue

Ed Kapitein ed at
Mon Mar 23 15:04:09 CET 2009

On Mon, 2009-03-23 at 16:51 +0300, Paul Fertser wrote:
> "arne anka" <openmoko at> writes:
> >> And don't forget that the proposed rework doesn't fix wired headset
> >> mic, it will still produce a lot of buzz.
> >
> > how's that?
> > i lived under the impression that the buzz is basically one buzz and the  
> > hw fix is intended to end all buzz.
> >
> > any sources proving your statement available?
> Schematics. Or ask Joerg.
> Basically the idea is that the buzz gets to the MICBIAS line via the
> 4th pin of the headphone receptable (that is mic input). The longer
> antenna you attach to it, the more RF you get.
> Then it's detected somewhere inside the Wolfson and comes back via the
> same MICBIAS line this time as a very strong audio frequency signal.
> With internal mic you can use a large cap to filter it on one side of
> the mic and get the signal from the other side because it's connected
> in a differential way. With the headset mic it's not possible because
> it has only one line.
> The rework for that would take lifting the can, desoldering one small
> R, soldering two of them in parallel to the same place (already tricky
> enough!) and adding a big cap to gnd (and free space inside the can is
> very limited). So, not practically possible.
> Read the schematics.
> OTOH you can use headphones and _internal_ mic for GSM after
> buzz-fixing it.
> NB: Factory-produced A8 will have ferrite beads in place (so RF will
> not get anywhere it can be detected in the first place), and that
> should probably solve all known buzz problems.

Just a wild guess, but is it possible to replace the headphone
receptable with a shielded one?
Would perhaps be more of a DIY job then replacing SMD resistors and add
SMD caps.

Kind regards,

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