my buzz fix method
joerg at openmoko.org
Tue Apr 7 04:47:43 CEST 2009
Am Di 7. April 2009 schrieb Rask Ingemann Lambertsen:
> On Mon, Apr 06, 2009 at 12:59:21PM +0100, Joerg Reisenweber wrote:
> > Am So 5. April 2009 schrieb Rask Ingemann Lambertsen:
> > > Joerg, could you clarify the significance of R4303 a bit? For
> > > how would I tell if it has 2.2 kOhms or 0 Ohms without measuring it (on
> > > device without the buzz fix as well as with the buzz fix)?
> > Probing is the *only* way you can tell afaik.
> > We are basically grounding upper end of mic with a huge capacitor (AC), so
> > need some R >>0Ohms on the lower end of mic to get some signal at all.
> I see. So
> 1) on an unfixed device, we'll get away with 0R R4303 with no perceptible
> loss of mic volume?
> 2) preferably, we'd put the 100 uF cap on MICBIAS, i.e. before the 2k2
> resistors R4305 and R4408 and parallel to 4U7 C3015, so that we don't fix
> the voltage of one of the mic terminals?
Of course this has been tested, but source impedance of the MICBIAS voltage
regulator which is also creating the ripple (totem pole output it seems) is
so low we needed a 860uF and even then there was buzz left. To form a decent
lowpass filter for ripple we need to have some R between C and MICBIAS
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