I'd think you're within 2 dB of the actual value at this point, probably better. Keep in mind that the

*absolute physical limit*, the 150 ohm resistor's voltage noise itself, is -130.8 dBu (20 kHz flat) or -130.4 dBu (22 kHz flat) at 300 K. The best audio interfaces according to

@Julian Krause's measurements make it just below -131 dBu(A), so somewhere around 129 dBu flat. At the very least you're no worse off than a Focusrite Scarlett 3rd gen.

I suggest taking some more data points for better accuracy.

Using your preferred method, please record the noise level (

**at full preamp gain**) for the following source resistance values R:

- 0 ohms (a dead short)
- 150 ohms
- ...and a few values in between that you might have floating around (maybe 22 - 47 - 100 ohms or 15 - 33 - 68 ohms).

Then try to match the following function to the values obtained in a spreadsheet:

N[dB] = 10 log(1 + R/R_eq) + C

Vary C and R_eq.

C is just some constant we're not overly interested in, it should roughly be N[dB] for the shorted input so start with that. You could also just look at N[dB](R) - N[dB](short) and ignore C altogether, with a slight loss of accuracy.

R_eq is the preamp's

*equivalent input noise resistance*, the resistor value which would result in the same amount of voltage noise if in series with an ideal noiseless input. This is what we want. Once you have a good approximation for its value, adding that to 150 ohms and calculating unweighted noise voltage is trivial, the old √(4kTRB), you can use my RMS summing calculator for that. (Adjust T according to actual ambient temperature, which I suspect may be closer to 300 K where you are.)

This model assumes that input current noise is negligible, which at 150 ohms and below it should generally be. The good thing about it is that it eliminates a lot of variables, leaving only resistor noise and input noise.

I can try to explain the math later if so desired, just wanted to get this one out quickly.

You could also just post a list of the corresponding resistor and noise floor values you get and I'd have a go in LibreOffice Calc. I can't say my last attempt at regressions was overly successful though...

As a further simplification, you can even calculate R_eq directly from the "150 ohm" and "short" data points (though I would assume that accuracy improves with more of them).

N[dB](150Ω) - N[dB](0Ω) = 10 log(1 + 150Ω/R_eq) - 0

10^((N[dB](150Ω) - N[dB](0Ω))/10) = (R_eq + 150Ω)/R_eq

Let's call the left-hand side "dB2linpwr(ΔN)". Then

(dB2linpwr(ΔN) - 1) R_eq = 150Ω

or

R_eq = 150Ω / (dB2linpwr(ΔN) - 1)

Let's say we're seeing a 6 dB difference in noise level. Then

R_eq = 150Ω / (10^0.6 - 1) ~= 50.3Ω.

Then the thermal noise of 150Ω + 50.3Ω over a 20 kHz bandwidth (unweighted) at 300 K would come out as -129.6 dBu - there's your EIN figure. Still unweighted, but hey. Consult REW for the delta between 20 kHz flat and A-weighted in the 150 ohm measurement (e.g. 1.7 dB as shown above), then you should get a pretty good estimate for A-weighted.