Question about Mic booster schematic

[Rodgre]

I have been searching for a schematic for something like the Cloudlifter, which is basically a 20db preamp for microphones with low output levels (ribbons, Shure SM7s, etc). I have a Cloudlifter already but was curious if I could build a few more channels for the studio.

I found this:




The device itself is a simple box with jacks on either end. No switches that I can see. The Cloudlifter has no switches either. So on the schematic, I'm not following what S1 and S2 are. Can anyone shed light on this?

Thank you!

Roger