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5. Re: R-44 issue-geek reply

Subject: 5. Re: R-44 issue-geek reply
From: "Jez" tempjez
Date: Sun Jun 24, 2012 1:05 pm ((PDT))
still, for me is the fact that none of this should be needed at all. Roland=
 need to sort this.

--- In  "sainganrob" <> =
> --- In  "Raimund" <raimundspecht@> wrote=
> >
> > Hi Jez,
> >
> > I just did a quick test with a Roland R-26 in order to investigate the =
quality of its phantom power supply circuit:
> >
> > I made a simple dummy load consisting of a 150 ohm resistor connected b=
etween the XLR pins 2 and 3. In order to create a load on the phantom power=
 supply, I also added two 2.2 kohm resistors, one connected between pin 2 a=
nd pin 1 (ground) and the other from pin 3 and pin 1. This creates a curren=
t flow of about 10 mA (48V / (6.8 + 2.2 kohm) * 2).
> >
> >
> > Finally, I removed one of the 2k2 load resistors, which simulates an un=
balanced microphone circuit:
> >
> > The last recording reveals that the phantom power supply of the recorde=
r contains indeed some digital noise that becomes audible, once a poorly de=
signed (unbalanced) microphone is attached to the recorder (a balanced circ=
uit would eliminate the phantom power noise).
> >
> > So, I believe that you could fix the problem with your DPA microphones =
by using a better phantom power adapter that draws the same current from pi=
n 2 and 3.
> >
> > Regards,
> > Raimund
> >
> Hi Raimund, this is indeed a very telling test, but the the imbalance is =
not as much as you perhaps think. i.e. by leaving the 150R across the pins,=
 from the 48V you have 6K8 to the 2k2 direct and 6K95 to the 2K2 via the 15=
Message: 0R. 
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, eit=
her a poor return path, poor decoupling or both.
> In principle there is absolutely no reason why one couldn't short either =
of the two signal pins to ground, save that the common mode rejection would=
 of course negated.
> Rob

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