GSM-noise "buzz" issue

Alastair Johnson alastair at
Thu Oct 30 15:12:43 CET 2008

Joerg Reisenweber wrote:
> Am Mi  29. Oktober 2008 schrieb Ole Kliemann:
>> Hi Joerg and others,
>> I haven't followed the whole thread as I only understand 1%. So just one
>> quick and dirty question:
>> I have the two years EU warranty and am going to send my FR to my
>> distributor anyway because GPS is not working with the internal antenna.
>> (Independant of SD card.)
>> So if I send it in, is there any fix that could be applied for the
>> buzz? Or is it worth to wait, say another week, because there will be
>> something then?
> Seems you always could "upgrade" to the 'botch' fix found in A7 (It *should* 
> work, as our TPE engineers found it worth to implement in A7 ;-):
> replace R4403: 0R -> 2k2
> add C4306: NC -> 100uF (probably you won't find a 100uF to fit in anywhere, so 
> a 47uF might kinda suffice. C4306 pads aren't existent in A5/A6, but you can 
> use pads of NC R4304 which isn't mentioned in public schematics)
> A _yet_unapproved_(!) variant is to use a LM336-2.5 in place of the huge 
> capacitor, and to 
> change R4305: 2k2-> 860R (to make LM336 operate on >400uA, when setting 
> MICBIAS to VDD*0.9)
> You also have to
> replace R4403: 0R -> 2k2
> In theory this rework should yield even better stabilization of MICBIAS 
> voltage, the path where ripple caused by GSM RF is introduced to audio path.
> See all but first 6 pics at
> (first 6 pics are a nasty approach to cut pin4 using some dremel-like mill. 
> Those are not related to LM336 fix)

All of this is on the Mic2 side of the micbias supply. Is Mic1 also buzz 
free after this mod, or does it only help for the handset mic?

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