From Graham Patten -
Hello Walt
The preamp chip is a Burr-Brown INA217AIP low noise amp with THD+N of 0.004% @ 1Khz.
I cannot really speak to the decision to control the gain with a dip switch. I can only imagine that some of the major recording studios at the time of design were requesting specific levels instead of continuously variable gain. Since it would have been cheaper to design it as continuously variable they most likely had reason to believe the dip switch was a customer requirement.
Best regards,
Jerry
Jerry Allgood
Senior Service Technician
The Isis Group, Inc.
119 E. McKnight Way
Grass Valley, Ca 95949