Posting for attachment. Note to you young-ins, this only works for galvos with Torsion Bars, Ie GSI open loop scanners such as G330 or G124.
This derives it's Back EMF feedback from the difference voltage between the purely resistive network and the inductive load from the Galvo. Aka a Bridge Circuit.
The value of "C3" is critical in the design of the bridge. Terminals CDJ go to the external DC offset potentiometer. A value of 1OK Ohms is just fine for that part.
Steve