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7. Re: R-44 issue-geek reply

Subject: 7. Re: R-44 issue-geek reply
From: "sainganrob" sainganrob
Date: Sun Jun 24, 2012 1:20 pm ((PDT))

--- In  "Raimund" <> wrot=

> > And its supposed to be a pure DC, not contaminated with system noise!
> Sure, but it is often a challenge to generate such a perfectly pure DC vo=
ltage out of a low battery voltage.

Not really the concern of the microphone manufacturer although the mic inte=
rnals will almost certainly include RC filtering of the supply.

> > 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.
> I beg to differ here! The phantom power specification demands perfectly b=
alanced currents on both rails. For that reason, the two internal 6.8 k res=
istors must be matched to within at least 0.4% (see
/wiki/Phantom_power). The microphone itself should therefore also be perfec=
tly balanced. Otherwise, the matched 6.8 k resistors within the recorder wo=
uld be meaningless.

The 0.4% is purely to maintain a good common mode rejection. As an aside, u=
nless the input capacitors, amplifier resistors and gain elements are also =
tightly matched then the 6K8 match really is meaningless.

> > 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.
> Hmmm, I'm not sure if that would be a good idea, given that it is quite d=
ifficult to design a perfectly ripple-free phantom power supply.

Again, not the concern of the microphone manufacturer.

Anyway, it was a good test, with real figures. Something tangible that the =
Roland chaps can hopefully reproduce.

cheers Rob

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