I think I have answered my questions to my own satisfaction.
John
--- In "John Crockett" wrote:
>
> Hi David,
>
> I had sort of figured that out sitting here thinking about it. So EIN is =
not the way to talk about it. But I do note that when I engage the -20 dB s=
ensitivity switch on my H2, the H2 noise level drops by 20 dB. If I didn't =
have an external preamp, the signal would also drop by 20 dB. but with an e=
xternal preamp providing the gain, now the recorder noise has dropped but t=
he signal level is still high.
>
> What I am still unsure about is exactly how to quantify this. How to calc=
ulate the amount of noise at the end of the chain when you have microphone =
output noise, preamp 1 EIN and preamp 2 EIN.
>
> I mean, if one just uses the EIN for each preamp in the calculation, ther=
e appears to be no advantage to using an external preamp with lower noise a=
nd feeding it into a recorder preamp with higher noise. Yet I can see by pr=
actical application, that if I feed a quieter preamp into my Zoom H2, and I=
engage the -20 dB pad on the H2, I get a much better result than if I feed=
the same mic directly into the H2 without the -20 dB pad.
>
> It appears to me that the numbers give an accurate prediction of the resu=
lt if I pretend that the EIN of the H2 is now -120 dB instead of being -100=
dB.
>
> But you seem to be saying that is wrong, so what is the correct way to ca=
lculate the total noise at the end of this chain?
>
> Am I making any sense?
>
> John
>
>
>
> --- In "Avocet" wrote:
> >
> > > For example, if your H4n has an EIN of -107 (at full gain), and
> > > adding your preamp to the chain allows you to reduce the gain on the
> > > H4n by 35 dB, does that mean the EIN of the H4n has now been reduced
> > > to -142 dB? I'm really not sure I am understanding this correctly!
> >
> > John,
> >
> > No. The Equivalent Input Noise takes any gain setting into account.
> > Another way to define EIN is the input level which would increase the
> > audible noise by 3 dB (twice the energy level)
> >
> > The measured EIN should stay constant whatever the gain settings, but
> > if not, the lowest EIN should be that measured at high gain. If EIN
> > goes up with gain, there is extra noise in the recorder. This sounds
> > odd but you subtract the gain, so the EIN should stay the same.
> >
> > Another way of looking at this is putting a 10dB attenuator on the
> > input. The noise level will be the same, but relative to the overall
> > gain the EIN will be 10dB higher.
> >
> > The EIN of a line input could be quite high, but that would normally
> > be swamped by the higher input level going into the line input. What
> > the Telinga preamp does with PIP inputs is to swamp any recorder EIN
> > which can be quite high with "affordable" recorders.
> >
> > What makes EIN difficult to pin down is different noise frequency
> > profiles, for instance adding LF noise to HF noise. That is why
> > weighting is used to make measurementrs more standard.
> >
> > Hope this helps.
> >
> > David
> >
> > David Brinicombe
> > North Devon, UK
> > Cogito cogito ergo cogito sum - Ambrose Bierce
> >
>
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