Core Sound Mic2496 is sold as a quiet preamp and A/D converter. However, specifications on the website do not include any figure representing its equivalent input noise. Len Moskowitz doesn't like the EIN specification very much, as he has explained for example on rec.audio.pro here and here, and also to me in a private communication. I won't enter into this argument here, but I believe that a published EIN figure can't do any harm. Since I have just purchased a Mic2496 from Core Sound I have done some measurements and will now share my results.
As you will see, at 44 and 48 kHz sampling frequency and at minimum gain I have found an EIN of -113 dBV A-wtd i.e. 2.1 µV A-wtd. At higher sampling frequencies it seems to get up to 3.5 dB noisier in the left channel, 9 dB in the right channel. Tuning up the gain, the SNR seems to get remarkably better, and at maximum gain I found an EIN of -118 dBV A-wtd i.e. 1.3 µV A-wtd at any sampling frequency.
Since I own no fancy instrumentation, all that I did is to hit record with no microphones attached, process the digital track with some mathematics, and use the provided values of full-scale sine wave (575 mV RMS) and gain range (40 dB between maximum and minimum). I hope there aren't flaws in this procedure. However, it seems to be confirmed by the fact that the results I've found agree with Mike Rivers' review on Pro Audio Review, where an EIN of -73dBFS at maximum gain is stated. Any comments are welcome: please write me an email at firstname.lastname@example.org or answer to my post. Thank you.
All following results are in dB relative to a FS (full-scale) sine wave.
Sampl./Gain RMS L RMS R 2444/min -105.89 -106.00 2444/max -70.62 -69.85 2448/min -105.74 -105.67 2448/max -70.30 -69.61 2488/min -100.08 -94.77 2488/max -67.24 -66.57 2496/min -99.71 -94.46 2496/max -66.95 -66.23
Note that the RMS levels calculated by spectrum.pl are in excellent agreement with those found by CoolEdit at point 4.Sampl./Gain RMS L RMS R | dBA L dBA R 2444/min -106.01 -106.09 | -108.69 -108.77 2444/max -70.63 -69.86 | -73.26 -72.44 2448/min -105.85 -105.80 | -108.84 -108.85 2448/max -70.32 -69.63 | -73.28 -72.54 2488/min -100.11 -94.78 | -105.27 -99.87 2488/max -67.26 -66.58 | -72.99 -72.18 2496/min -99.74 -94.47 | -105.27 -99.91 2496/max -66.96 -66.24 | -73.12 -72.21
To convert these figures to mV or dBV you can use the value specified as the FS sine wave at minimum gain in the specs of Mic2496 on the website: 575 mV RMS (that is -4.8 dB re. 1V).
For example, taking the value of -108.7 dBFS A-wtd as a typical level at minimum gain, then the EIN can be evaluated of -4.8 − 108.7 = -113.5 dBV A-wtd, that is 2.1 µV A-wtd. At the higher sampling rates the noise level seems to raise considerably: about 3.5 dB in the left channel and 9 dB in the right channel.
However, it also appears that the noise floor doesn't scale with the gain: tuning the Mic2496's gain from min to max, the signal gets 40 dB (per specs, also roughly verified on my own device within about 1 dB), while the noise raises only 35 dB or less, so the SNR is better with higher gains. At maximum gain, the noise floor is about -73 dBFS A-wtd for all sampling frequencies.
In the light of that, and assuming the FS sine wave at maximum gain is 40 dB lower than that at minimum gain, the EIN value at maximum gain is -4.8 − 40 − 73 = -118 dBV A-wtd (with some rounding), i.e. 1.3 µV A-wtd.
For a PDF version of the plots click here.