I've figured out the Marantz 670 numbers:
The measured EIN (level controls at full) = -65.2 dB relative to
full scale.
When we subtract the gain of the mic preamp (48 dB), we get an
apparent EIN of -113 dB relative to full scale.
Since the full scale reading corrsponds to -8 dBu (300 mV), the EIN
is -121 dBu. The trouble with this reading is that you can never use
the top 8 dB of the scale, since the recorder is full scale at -8
dBu.
So here's my proposal for measuring EIN:
1. Record the noise of a 150 ohm resistor (placed between pins 2 and
3 of the XLR, or between tip and sleeve of a miniplug) at full level.
2. Use Audition/Analyze/Statistics to get the average rms level of
the noise (I haven't decided if the square-wave rms or the sine-wave
rms value is better; the square-wave value if 3 dB lower than the
sine-wave value, and my guess is that thermal electron noise is a
square-wave function). If you don't have Audition then do step 3 and
we'll finish for you.
3. Upload the file to some convenient server where others can listen
to the file, and analyze it on their own. This is also good at
identifying preamps that are "grainy" or "smooth" with low signal
levels.
4. Subtract the preamp gain from the noise level to get the Apparent
EIN. This may be the most comparable number we can find.
5. If you know the full-scale level of your recorder, report that
also.
I think the things we need in the database are:
A. The actual average noise level of the 150 ohm resistor relative
to full scale.
B. The gain of the preamp in dB.
C. The calculated Apparent EIN in dB full scale for your recorder (A-
EIN?).
D. If known, the full-scale level, in dBu or volts [dBu=20log
(volts/0.775)].
E. If know, the maximum input before clipping the preamp in dBu or
volts (with pads in and with pads out, if you have an attenuator
before the preamp stage).
Note that B-D are best gotten off the spec sheet, so if you have a
recorder and don't know dB calculations, just record the resistor
and upload it--we can do the rest for you.
This is just a proposal, some of you may know of bettrer ways to
collect and present this sort of info, and I'd be very interested in
what you think the advantages and disadvantages of the other methods
might be.
Bruce
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