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1. Re: R-44 issue-geek reply

Subject: 1. Re: R-44 issue-geek reply
From: "Raimund" animalsounds
Date: Sun Jun 24, 2012 11:40 pm ((PDT))
Hi again,

Because I had some doubts on my quick-and-dirty test setup (there is no shi=
elding around the dummy microphone circuit, which means that it could theor=
etically have picked up "only" electromagnetic interferences from the recor=
der), I just did another test with that unbalanced (and unshielded) microph=
one dummy, but with the phantom power switched off:

It finally turned out that it is most likely the noisy phantom power supply=
 voltage that is causing the noise.


--- In  "Jez" <> wrote:
> still, for me is the fact that none of this should be needed at all. Rola=
nd need to sort this.
> --- In  "sainganrob" <unbalancedmike@> w=
> >
> >
> >
> > --- In  "Raimund" <raimundspecht@> wro=
> > >
> > > Hi Jez,
> > >
> > > I just did a quick test with a Roland R-26 in order to investigate th=
e quality of its phantom power supply circuit:
> > >
> > > I made a simple dummy load consisting of a 150 ohm resistor connected=
 between the XLR pins 2 and 3. In order to create a load on the phantom pow=
er supply, I also added two 2.2 kohm resistors, one connected between pin 2=
 and pin 1 (ground) and the other from pin 3 and pin 1. This creates a curr=
ent flow of about 10 mA (48V / (6.8 + 2.2 kohm) * 2).
> > >
> >
> > >
> > > Finally, I removed one of the 2k2 load resistors, which simulates an =
unbalanced microphone circuit:
> >
> > >
> > > The last recording reveals that the phantom power supply of the recor=
der contains indeed some digital noise that becomes audible, once a poorly =
designed (unbalanced) microphone is attached to the recorder (a balanced ci=
rcuit would eliminate the phantom power noise).
> > >
> > > So, I believe that you could fix the problem with your DPA microphone=
s by using a better phantom power adapter that draws the same current from =
pin 2 and 3.
> > >
> > > Regards,
> > > Raimund
> > >
> >
> > Hi Raimund, this is indeed a very telling test, but the the imbalance i=
s not as much as you perhaps think. i.e. by leaving the 150R across the pin=
s, from the 48V you have 6K8 to the 2k2 direct and 6K95 to the 2K2 via the =
Message: 150R. 
Subject: i.e. a 2.2 percent imbalance.
> >
> > And its supposed to be a pure DC, not contaminated with system noise!
> >
> > Anyway, the point I take issue with is the "poorly designed (unbalanced=
) microphone". The only poor design appears to be the R44 phantom supply, e=
ither a poor return path, poor decoupling or both.
> >
> > In principle there is absolutely no reason why one couldn't short eithe=
r of the two signal pins to ground, save that the common mode rejection wou=
ld of course negated.
> >
> > Rob
> >

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