Looks to me like your -134 dBV is the noise contributed by the power supply
ripple only. Given your assumption, that it is running at 50 KHz, then the
noise contribution is zero if we use the standard 20 - 20,000 Hz passband
for calculating audio noise.
-134 dBV is about the 150 ohm resistor shot noise figure. The preamp noise
must be higher than that as there must be many sources of resistance beside=
s
the overall impedance, each noise source geometrically adding to the rest.
For practical preamps (those which use impedances above 10 ohms and can
actually be built) the lower 20 - 20,000 Hz noise limit seems to be about
-125 dBu (-127 dBV). These are quite small in comparison to the noise from
all known mics at atmospheric pressure. The preamp noise could be lessened
by going to a lower system impedance, but that introduces (dramatically)
line loss and the effect of static charges and radio-induced voltages.
Bruce Wilson
http://science.uvsc.edu/wilson
-----Original Message-----
From:
On Behalf Of jpbeale
Sent: Friday, September 15, 2006 12:44 PM
To:
Subject: [Nature Recordists] Rolls PB224 noise floor (estimate)
--- In Rob Danielson <> wrote:
> Eric[...] He measured the noise floor with two MK 2/CMC6's [11
dB(A), > 15 mV/Pa] and found it to be -120 dBV.
For what it's worth, I estimate the Rolls contribution to be -134 dBV at th=
e
switching frequency (well beyond audible range), assuming two 150 ohm mics
drawing 10 mA each. Lower impedance mics and lower current draw would mean
less noise.
I don't have a Rolls and I haven't measured one. This is based on a
first-principles analysis of the schematic online at
http://www.rolls.com/data/pb224man.pdf as outlined below.
---------------------------------------
Unknown: Q1 switching frequency Sf.
Noise is inversely proportional to Sf.
Sf assumed to be 50 kHz. Would have less noise if it is higher.
Unknown: mic supply current Is.
Noise is directly proportional to Is, assumed to be 20 mA (worst case for
phantom power: 10 mA x 2)
Unknown: mic impedance Z.
Noise is inversely proportional to mic impedance (for Z<<6.8K) Z assumed to
be 150 ohms at frequency of interest.
---------------------------------------
peak ripple Voltage at D2-C11 node:
I =3D C dV/dt, so (I/C)*dt =3D dV
(20 mA / 47 uF) * (20 us) =3D 8.5 mV (at 50 kHz)
R7 & C6 form a one-pole lowpass filter at frequency 1/2*pi*R*C =3D 1/(2 * 3=
.14
* 100 ohms * 10 uF) =3D 159 Hz
A single-pole filter is good for 20 dB per decade, so a 50 kHz signal is
attenuated by 50 dB at the R7-C6 node
The phantom power supply has Rs =3D 6.8K in series with the mic.
if Z is 150 ohms, the voltage divider attenuates by 150/(150+6800) which is
1/46.33, or a 33 dB attenuation.
So our 8.5 mV ripple is attenuated by (50db + 33dB) leaving us 0.58 uVpp. I=
f
we ignore the higher ultrasonic and RF harmonics and call our 50 kHz
sawtooth ripple a simple sinewave, that would be 0.20 uVrms, which is -134
dBV. That seems pretty quiet to me.
This is a simple analysis but probably pessimistic. Most mics don't draw th=
e
full 10 mA, and the noise you actually care about is only the fraction that
gets by your anti-aliasing filter and is aliased down into the audio band.
I only considered the main switching frequency and not any subharmonics or
other noise. I see the Rolls design has a feedback mechanism through Q2 to
regulate the power by pushing the U1F input out of the active switching
range when the zener current nears 0.7 mA.
This might be effectively a PWM control depending how the 4069 input stage
works in the linear region, but I don't know what impact if any that has in
the audio range. I'd guess negligible. Zeners themselves generate broadban=
d
noise but that would be much less than the ripple voltage.
-----------
John Beale
www.bealecorner.com
"Microphones are not ears,
Loudspeakers are not birds,
A listening room is not nature."
Klas Strandberg
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