abbey road d enfer said:I understand all that, but your remark, IIUC, was addressed only to the posted schemo.
Regarding the Helios and its 1:10 xfmr, the preamps in my mixers used a similar one (Melodium, if you remember them) with a BC109C operating at 50uA.
I measured -128dBu input noise with a 200r dummy.
About the Neve, the operating point must be chosen for the highest gain, since S/N ratio can only improve, unless someone does crazy things with attenuators.
I have designed a preamp where the input device (BC560) runs at 200uA and normally sees a 2k secondary. I dedicate it to drums and such brutal sources. It is not the quietest in the world but nobody complained so far.
The input pad is based on an alternate primary (20k nominal), so the transistor base sees about 20 ohms+ some DCR, so talk about mismatch! Anyway, the resulting S/N ratio is perfectly manageable and inobtrusive.
And yes, the output stage is class A.
I knew you knew that, sorry
Well, I may be mistaken about the Helios then, but I seem to remember an input current higher than your 50uA so, mea culpa.
On Neve, I remember that noise is optimized not for the highest gain which sees an impedance of 600 - 800 ohms (transformer secondary attenuator full up) but somewhere between that and the 1K4 when it's halfway down.
Maybe I'm just full of crap and remembering wrong!
I don't remember Melodium no, I'd be interested in any info if you know of any online somewhere.