I am a newbie in terms of mic building and modding, I have studied electronics in university, but have done only digital design for about 15 years now.
I am trying to design (for fun) a different preamp to fit into a t.bone EM700. I like the response of the capsule, but would like a bit more level out of the mic (about 10 dB more than the original preamp offers).
I have considered just plugging a FetHed Phantom in between, but based on some images I found and based on the schematic of the regular (which I have confirmed by tracing one I have), I believe that it will forward the equivalent of ~15V phantom to the mic, which is not what it is designed for. So here I am, dusting off some old skills and learning new ones.
The capsule is FET-less, the original preamp has a FET and a 1G resistor in the input stage. I like the Schoeps input stage, pictured below, including the ability to adjust Vgs (I have done some simulations with different FET models, and having a pot there is very useful).
The effect of the adjustment is that the DC value of the gate voltage is now set by the split of the 1M pot, which may set Vg at let's say around 1 volt.
This effectively attempts to polarize a pre-polarized electret capsule by 1 volt or so. Will this impact the response of the capsule?
Of course I could mitigate this by adding a series capacitor (probably a 1 nF polypropylene cap between the capsule and the gate/ 1G resistor), but do I really have to?
I am grateful for your replies. While I know my way around transistors and electronics in general, how a pre-polarized (electret) capsule will behave under these conditions is a mystery to me.