But, as best as I can tell with my test gear, it measured .7nV/rtHz which isn't too shabby really.
As only Cohen & the Earthworks have proven sub nV/rtHz performance for a properly protected P48V preamp, your friend joins a very short list. The best of the rest, Millenia & Sam at 1nV/rtHz, are also a very short list.
I don't know what he plans to do with this little circuit but, if I can, I'll try and post a schem.
So yes please, pretty please!
I have to say, I'd use THAT1512 (optimised for slightly less gain than 1510*) if I was building a preamp tomorrow. The only reason for the complexity of Cohen would be an original STC4038 ribbon with nominal 30R output. But even that has a workround eg PRR'shttp://www.prodigy-pro.com/diy/index.php?topic=5743.80
or my own efforts on MicBuilders.
Regarding Rbe: Linsley-hood demonstrated that some mighty fine noise specs can be had by using just a single pair of medium power input devices rather than the small signal TO-92 type. An Rbe of 4 is not uncommon for some of these medium beefy globs of silicon.
Baxandall & I both looked at many medium power devices in da old days. The problem with these is usually poor hfe which means more current noise. But much nastier is they often had huge 1/f noise so you had to do a lot of selection.
When I found Hitachi 2sa1084/2sc2546, measured rbb 5R, I stopped looking. Later the Rohm 2sb737/2sd786, rbb 2R, that JR likes came out but I've never used them.
There's also Tosh 2sc3329 rbb 2R, which has escaped from the Unobtainium Store
These three types are the lowest noise devices I've come across. IIRC, better than LM394 etc but those have the advantage of 2 matched devices in one package.
My best effort is 0.28nV/rtHz with a huge total of 2 devices and 3mA but that's for Moving Coil cartridges and no way could that be a P48V mike preamp.
I don't think JLH understood noise. Some of his "low-noise" designs were pretty dire.
At the end of the day, the active electronics inside the mic will probably dominate the path performance. I know this is an area of Ricardo's interest and expertise, not mine.
Actually once you get below 20dBA spl, the acoustic resistance noise starts to be important and you'll want to listen to the noise too.
Hence the above about STC 4038 nominal 30R. The Coles version is nominal 300R and -65dBV/Pa. THAT1510 can ju..ust do 1dB NF at 300R but not 30R. I think THAT1510 or even 1512 is beyond reproach for any condensor even old ones with big step downs. I may be wrong PRR
* Having 2 different versions is an example of JR's stuff on feedback resistors. But Cohen also doesn't have evil current sources at sensitive points to muck up low gain noise.