JohnRoberts said:
I was not aware that Cohen was credited with this topology until several years ago. I had seen it around well before he published his AES paper.
The difference between Cohen and other stuff using that topology is 0.7nV/rtHz.
IMHO, it is perhaps the only sensible topology for a practical mike preamp with <1nV/rtHz. If we define 'Cohen' as something using that topology with <1nV/rtHz, that rules out practically all the DIY copies. Once you add P48V, protection & RFI stuff, you are struggling to achieve even 1nV/rtHz .. which highlights Millenia'a achievement with this topology.
Circuit & topology are only one thing you have to get right for <1nV/rtHz. The layout, decoupling, earthing & construction have equal, if not greater importance. That's why my recommendation for the 7th Circle stuff. Cohen 1984 is their C84 .. geddit?
I have a small preference for Eden over THAT in certain applications (which is reversed for other apps) but we really need to wait for Rochey's own box & PSU for its full potential.
I'm only talking about
noise though I have my own strong opinions about clarity & definition bla bla for various circuits. Noise is something anyone with a bit of nounce can reliably test for himself without an AP. My MicBuilders stuff on modifying DMP3 has some details.[.]
To put all this stuff into perspective ... the normal 150-200R mikes can achieve their full noise potential with the 1.6nV/rtHz of a good THAT or IN163 (Eden) implementation.
Supa LN transistors like 2sb737 et al in Cohen will show little practical noise improvement over THAT for most mikes. But if you have a low output ribbon like 4038 .. especially the 30-50R versions, you will be looking for every dB of better noise performance. Even then, you are talking about at best, 1.5dB between good implementations of both.
Removing P48V and other stuff is likely to be more rewarding for a dedicated ribbon preamp.
This Millenium, I'm involved with experimental mike stuff that has some benefits from the 0.6nV/rtHz of the Earthworks. I'd dearly love to know how Earthworks do it with P48, full protection & RFI stuff. Their stuff is all encapsulated.
They've only recently started to claim their stuff is all discrete ???
[.] If you read this carefully, you'll see some possible reasons for audible differences between preamps .. audible as in reliably detected in Double Blind Listening Tests.