The suggestion to switch off the phantom to V/2 came from THAT themselves. But I do have to prevent anyone might plug a ribbon into it. So what should I do?
Don't worry that I got your point on the protection diodes, and I will not use the zeners. Will also add the 1N4148s as you suggest.
The reason for going for film types, is because when I made my tests, the HFQs recommended by AD made the amp noisy. There were some Siemens 'lytic caps that stopped the noise, but the Siemens film types were much better sounding.
Let's clear up one thing: I am first of all what they call a user. My experience is as location sound recordist, coming from the Nagra 4.2 times. So please do respect my gray hairs.
My electronics knowledge is limited and hopefully you will acknowledge that as a not too serious disadvantage.
So instead of using instruments to measure what I build, I rely mostly on my ear. Of course I get help from my friends to measure things up, but listening comes first. On my listenings film caps did sound cleaner in all aspects, or at least much better than all 'lytics types I tried. I must confess I didn't try the "series inverted caps with resistors in the middle" arrangement, mostly because of space reasons. I did consider going to 2.2uF film types, but they were a bit larger, more expensive and not that easy to find. Things might be different now, also for 'lytic types. The one you call "permanent HPF" at the input I wish it to be at 40-50Hz, so I think I'd the choice of going for 2.2uF at the input or 2K2 for R1-R2, right. Not sure which might be better.
Safety wise, if switching my phantom off to chassis ground will prevent from harming ribbon mics, and that is fine with film input caps, then that's the way I want to go.
The supply cap is not shown, but it should be at least 120uF, but going higher or choosing specific types is something I am very much open to suggestions on. Same thing for going with a star grounding scheme, if it proves useful.
About the output, I will probably need balanced and unbalanced options, due to the units it will be connected to. Some will be unbalanced, some balanced. I'm already making up my mind on the output arrangement I will use, as it will be a thick cable carrying several things.
The output will have another stage, because I wish to put a level setting pot in between. Also a HPF passive filter is essential, so it should to before the output chip.
On my previous preamp I used an OP275 with 10x gain, which allowed me a fake balanced option and XLR outputs, which could also be used unbalanced. This time the main output will go through the thick cable mentioned above.
Besides all that there will be a headphone amp, a 1KHz generator and VU-meter driver. Undecided yet is whether I will also add a mid-stage limiter.