Heres what John Siau had to say. Note his comments are for the CMC6, the CMR have a very slightly higher noise spec, so it seems with the mod hes talking about, the noise level of the AD would be comparable to that of the mic, and more importantly inaudible in all real world field recording environments
mk4+cmc6, sensitivity = 13 mV/Pa, max SPL = 132 dB, EIN = 24 dB CCIR/ 15 dB A weighted
mk4+cmr, sensitivity = 8 mV/Pa, max SPL = 130 dB, EIN = 29 dB CCIR/ 18 dB A weighted
John's comments:
Jamie,
Your analysis is basically on track but has some errors that are
throwing the final numbers off a bit.
I am including my microphone data spread sheet which already has the
MK4V+CMC6 combination. This shows a self noise of 14 dB SPL and given
the sensitivity of the microphone this means that the noise at the
microphone output is -115.5 dBu. The AD2402 has a signal to noise ratio
of 117 dB and a maximum sensitivity of 0 dBFS at +14 dBu. This means
that the equivalent input noise of the AD2402 is +14 dBu-117dB = 103
dBu. The self noise of your microphone (measured at its output) is
actually 12.3 dB lower than the equivalent input noise of the AD2402.
In a perfectly quiet room the AD2402 would add about 12 dB of noise to
your recording. Obviously the noise level in a live venue exceeds 14 dB
SPL even when nobody is in the room. If the noise in the room measured
14+12.3=26.3 dB SPL the AD2402 would add 3 dB of noise to your
recording. At a room noise level of 32.3 dB SPL the AD2402 will only
add 1 dB of noise. Your system is working because you are recording in
a noisy environment. In a quiet studio you would benefit from a
microphone preamplifier.
Here are some easy rules-of-thumb for adding noise sources:
1) Two equal amplitude noise sources added will degrade the SNR by 3 dB.
2) If a new noise source is added that is 6 dB lower than the existing
noise, the resulting noise will increase by 1 dB.
3) If the noise sources differ by more than 6 dB, the quieter source can
be ignored.
4) Use RMS calculations for summing noise sources when you need exact
numbers.
The above assume random noise and no correlation between the two noise
sources. This is usually the case with microphones and electronics, but
be careful about room noise because it is often not random when the room
is empty. On the other hand, the random rustle of a crowd can approach
white noise under certain conditions.
Another interesting and useful rule of thumb:
The human ear can detect a tone that is 30 dB lower than the ambient
noise level. I frequently demonstrate this is the lab. What this means
is that a TPDF dithered 16-bit signal with an SNR of 93 dB can audibly
reproduce a signal that is recorded at -123 dB FS.
FFT analyzers can easily resolve signals that are 50 dB or more below
the noise floor. Our ears have about the same resolving ability as a 4k
to 8k point FFT - this is amazing. In the lab we have the luxury of
using 64k FFT analyzers so that we can analyze distortion signals that
are too small to be heard. Every time the number of points is doubled,
the resolution of an FFT analysis increases by 3 dB (sound familiar? -
see rule of thumb #1 above).
Here is a modification that will boost the input sensitivity of the
AD2402 by 8.2 dB (making the self noise of the AD2402 only 4.1 dB higher
than the microphone self noise - please note that the signal to noise
ratio of the AD2402 will still be 117 dB). With this modification the
AD2402 will only add about 1 dB of noise to your microphones in the
quietest studio.
Modification to increase sensitivity of AD2402 by 8.2 dB:
Each of the input attenuators has a 5.62K Ohm and a 649 Ohm resistor.
There are 4 of each. If you swap the position of the 649 and 5.62k
resistors, the +14 dBu switch position will become a +5.7 dBu switch
position (meaning that 0dBFS = 5.7 dBu). All other switch positions
will remain unchanged.<snip>
