[LAU] [Music] Sans titre - Without title

[Fons Adriaensen]

On Fri, Jul 15, 2016 at 09:12:03PM -0400, jonetsu at teksavvy.com wrote:

> Regarding the previous discussion on preamp, is it possible to
> summarize that in this case, with the 1010LT card, the (ART MPAII)
> preamp would raise, bring focus to the sounds captured by the mic over
> the noise level ?  I had to fade down at the end the piece mentioned in
> that discussion because the noise level became so apparent as the
> acoustic guitar sounds faded that it was an incompatible artistic
> statement.  If a preamp actually raises the sound of the instrument
> well above the noise level, then I'm sold.  The MPA II claims 75dB.

I don't find that number in the specs. But they do mention an
EIN (Equivalent Input Noise) of -129 dBu(A), which is not bad
at all (4 dB more than the theoretical minimum).

Then if you use a dynamic mic, the result will depend on its
sensitivity and impedance. Let's take the SM58 as an example: 

  Impedance = 300 ohm.
  Sensitivity = 1.85 mV / Pa = -52.5 dBu at 94 dB SPL

The impedance is twice the value used to measure EIN (150 ohm),
so the effective EIN of the preamp when used with this mic 
could be a few dB higher. But let's ignore this.

Then the equivalent acoustic noise level is 

  -129 - -52.5 + 94 dB(A) = 17.5 dB(A).

So if you are recording a sound with an SPL of say 70 dB,
the S/N ratio of this signal will be

  70 - 17.5 dB(A) = 52.5 dB(A).

To this you need to add the effect of any EQ (which 
could go either way).

Now is this OK or not ? It depends on loud this signal
is reproduced. At the original level, the noise would be
17.5 dB(A). This you need to compare with the ambient
noise level. 

You could gain a few dB by using a preamp with a lower
EIN, but very few will be better.

Or you could use a condenser mic with a lower self-noise.
For example a Neumann KM184 has 13 dB(A), a TLM103 goes
down to 7 dB(A), and you could gain 10 dB. With condenser
mics, the self-noise of the mic will dominate the noise,
unless the preamp is really bad.

If you really have excessive noise, there could be some
other reason. How did you combine the MPA and the 1010LT ?
You should use a line input on the latter (using the +4 dBu
and not the -10 dBV setting if you have the choice), and
have all the gain in the MPA. 

P.S. if you want to do this sort of calculations, have
a look at the slides of my Audio Measurements workshop:
<http://kokkinizita.linuxaudio.org/papers/amworkshop1.pdf>

Ciao,

-- 
FA

A world of exhaustive, reliable metadata would be an utopia.
It's also a pipe-dream, founded on self-delusion, nerd hubris
and hysterically inflated market opportunities. (Cory Doctorow)