I have a number of the Stanford Research Systems SR560 voltage pre-amplifiers in my labs. I also see them in use in my scientific labs around the world.
A common issue with them is the front-end gets noisy, going from an input referred noise of ~5 nV/sqrt(Hz) to ~50. In this case, what we usually do is scour the internet for some obsolete NPD 5565 (dual JFET) to swap in and this fixes the problem.
It seems to me like the LSK389 is a better part: its supposed have a voltage noise 5x smaller. Does anyone have any experience with this or possibly something similar when doing SR560 repair?