Gentelmen
this DIY preamp sounds like an interesting undertaking.
I think you'll manage to interface the transformer and the INA103 just fine.
Looking at the 018 schematic, my suggestion is to lift the grounded leg of the transformer and connect the "center" of the transformer to ground (ground the middle of the transformer "output-short" by using two resistors (50% of single res value) and two caps (200% of single cap value) for RN, CN and RL mounted symmetrically around ground.
Then there is no DC flowing in the secondary winding either.
Then the INA (which has bjt inputs) sees the same source impedance
and potential on both inputs. And voila - the resulting offset might just be
of academic interest. (And you can rip out the entire branch with R3, R5 C3 R6 etc).
Now you have a nice, truly balanced input signals for the INA.
I don't think the PSpice will yield that much. The transformer is a known (spec sheet wise -not wrt to spice modeling). The INA is well described.
The true unknowns are really the stray elements C and L, ground planes or lack of such, bypass caps .. None of which are easily captured for modeling. And the pulse/freq responce can be manipulated by the transformer's RN CN & RL load.
But do a very carefull _noise_ analysis. The INA want's the lowest source
impedance. Can you provide that?
I am not sure why you need all that gain. And I not sure why you don't just reproduce
the original circuit using the AD797.
But heck, I am all for experimenting! And I will suggest you build the circuit first with cheapo xformers from Radio Shack just to get going.
JK