I have a continuing interest in recovering the schematic of a 2 stage DC magnetic amplifier that was used in my graduate school. I was in the physics lab at the University of New Orleans working with superconductors. The magnetic amplifier was used to provide feedback from a rotating gaussmeter coil to the big magnet current source in our lab. It had a gain of about 1000. I have procured toroids and tested their characteristics. I also made a one stage magnetic amplifier with a saturable reactor. I am looking for help with the two stage circuit because my designs are very crude and use more parts than the schematic which I saw. Is there a similar thread still open? The circuit which I saw and used was powered by audio frequency AC. It was a DC amplifier connected in a standard operational amplifier configuration. It was not a chopper type amplifier and I do not remember how the output ac was converted to DC, possibly without using diodes.