Re: very high gain preamp+mic combination

"Don Pearce" <spam@xxxxxxxx> wrote in message
news:49adc4c0.1298029125@xxxxxxxxxxxx
On Thu, 19 Feb 2009 20:36:22 GMT, "Paul Stamler" <pstamler@xxxxxxxxx>
wrote:

<Don Pearce> wrote in message news:49a509c1.1250158437@xxxxxxxxxxxx
<Don Pearce> wrote in message news:49a16e75.1210401515@xxxxxxxxxxxx
On Wed, 18 Feb 2009 15:12:30 -0500, "Soundhaspriority"
<nowhere@xxxxxxxxxxx> wrote:


Do the sums.A preamp with zero excess noise and a 150 ohm mic gives an
input noise of -139.37dBV in a 20kHz bandwidth.

How did you come up with that figure? I get -133.04dBV (-130.82dBu) of
Johnson noise for 150 ohms in a 20kHz bandwidth.

Oops - missed a 4 in my calc. Sorry about that.

OK, this is where we stand. Using the NT1-A, a perfect preamp imposes
a noise penalty of 0.9dB. The Gordon Audio preamp identified by Hank
adds 3dB of noise (the same for a cheap Behringer, incidentally) and
an RNP adds 6.5dB. So the choice of preamp is critical, and designing
for ultimate low noise will be worthwhile.

I'm still not following you. The equivalent noise figure of the NT1-A
presumably includes the thermal noise of its output impedance, no? In
which
case, a perfect preamp would add no noise at all.


A perfect preamp has noise - the voltage is given by the equation
sqrt(4kTBR) (k is Boltzmann's Constant, T is the temperature, B is the
bandwidth and R is the total resistance at the input).

No, it doesn't. A perfect preamp is one that adds no noise when its input is
shorted (in other words, the source impedance is zero). Under those
circumstances, the preamp (a mythical beast) will generate an output noise
of zero volts.

Take away the short and put a resistor in its place. The resistor will
generate Johnson noise based on the formula you gave. The resistor -- not
the preamp -- is generating the noise, just by virtue of being a resistor
that's not at absolute zero.

Now replace the resistor with a condenser microphone. It generates noise, a
lot more noise than a resistor with the same value as its output impedance.
We know how much noise it generates because the company specifies how much,
and (assuming they're telling the truth) they specify how much by measuring
it at the output.

And that measure will include the output resistance of the microphone as one
of the sources of the microphone's noise, along with
electronically-generated noise from the microphone's head-amp.

If you use the microphone's noise figure, and then add to it the noise of
the equivalent impedance, in effect you're counting the noise of the output
impedance twice, which is wrong.

As for the Gordon or Behringer...as I recall, you gave the noise figure of
the NT1-A as -121dBV (correct me if my memory's wrong). If the
Gordon/Behringer has an equivalent input noise of -127dBV then the total
noise will be -120.02dBV, almost exactly a 1dB penalty. If the preamp's
noise is less than that, the total noise will be slightly less as well.

Yup, it is possible to get about an extra 5dB of noise performance out
of the preamp by careful design.

Not my point. My point is that adding two noise sources, with one 6dB lower
than the other, will add only 1dB to the noise level of the noisier source,
assuming they're uncorrelated.

Do the arithmetic. Say one noise source is 1V and the other is 6dB lower, or
0.5V. The total noise will be sqrt(1^2 + 0.5^), or sqrt(1.25), or about
1.118. That's about 0.969dB worse than the noisier signal, 1V.

Peace,
Paul


.

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