[sdiy] OPA604 anyone?

[JH.]

> > I have 100k feedback resistor (output to -input), with _no_way_ to
> > add a capacitor across this, and a FET (with its capacitance when turned
> > off)
> > from -input to GND.
>
> I think that the 100k could well be the largest noise contributer here.

No. The impedance is only 100k when the FET is off, i.e. gain is zero.
With any gain to speak of, the impedance is defined by the on-resistance of
the FET.

JH.