If you look at the NF curves for BC109 and 549 you will see the lowest NF of 1.75dB can be obtained with a collector current between 0.1 and 1mA and a source impedance of around 800 ohm which I think I said is the typical impedance a 200 ohm mic presents to the circuit.

I think we're talking at crossed purposes here
So we can bias for lowest NF at a source of 800 ohms then.  You said that 4K8 was where the minima of the BC109 was. 

It is difficult to see how simply altering the collector current could make much difference in NF. (pdf of NF curves attached)

By bumping it up to where I said it was better at that source impedance -  at 1mA or a bit more, as it is in the Jensen 990 which was specifically designed for a similar transformer turns ratio. 


 

Which low rbb high Hfe BJT do you have in mind?

Ah well.  Those devices are now moot for the most part since they were EOL about 20 years ago.   Rohm 2SB7... something, something was one.  Great for mic amps and phono pre-amps and used by many folks.
Now, people seem to use the 2N4401  which is really a switching transistor.  Again paralleling 2 devices will get the rbb contribution down though and a theoretical 3dB improvement which, in practicality usually ends up about a 2dB improvement.
Only probably worth it anyway for in the 31102 with its own line amp buffer.   

Yeah but that is just tinkering not proper design.

For sure.  Which is why I added that bit in, to dissuade anyone thinking of just throwing in another transistor and expecting major improvements without doing a proper redesign. 

Edit:  a redesign also looking at the impedance of feedback network too of course.



 

Almost sounds like dimmers being used nearby.....but maybe that would've shown itself before?/

I salvaged two of these tiny boards from Aliexpress mini u87. So this is the idea, based on Josephson e22s, basically to have least possible acoustical interference around the capsule. We'll see how it turns out in the end with the sleeve.

The good thing is the capsule housing is grounded so there is no emi even when used like this with no sleeve.

Ian, I love the English people, really! But I wanted to point out that since 1818 no one wanted to take a step towards a much simpler and more efficient system. I wonder what would have happened if they asked the British to consider the possibility of moving to the “right” side of the roads. 

Well, if all this has been done to keep measuring in inches.

LOL. Yes, I'm pretty sure that something like the above will also feature together with other explanations in future history books.