--- In "Raimund" <> wrot=
e:
>
> Hi Jez,
>
> I just did a quick test with a Roland R-26 in order to investigate the qu=
ality of its phantom power supply circuit:
>
> I made a simple dummy load consisting of a 150 ohm resistor connected bet=
ween the XLR pins 2 and 3. In order to create a load on the phantom power s=
upply, 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 current =
flow of about 10 mA (48V / (6.8 + 2.2 kohm) * 2).
>
>
> Finally, I removed one of the 2k2 load resistors, which simulates an unba=
lanced microphone circuit:
>
> The last recording reveals that the phantom power supply of the recorder =
contains indeed some digital noise that becomes audible, once a poorly desi=
gned (unbalanced) microphone is attached to the recorder (a balanced circui=
t 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 pin =
2 and 3.
>
> Regards,
> Raimund
>
Hi Raimund, this is indeed a very telling test, but the the imbalance is no=
t as much as you perhaps think. i.e. by leaving the 150R across the pins, f=
rom the 48V you have 6K8 to the 2k2 direct and 6K95 to the 2K2 via the 150R=
. 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) mi=
crophone". The only poor design appears to be the R44 phantom supply, eithe=
r 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 o=
f course negated.
Rob
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