The Twin Servo mic preamp design was published as Jensen application note #83. The design employs Jensen’s transformers as well as the 990C discrete opamp designed by Deane Jensen. (This design is commercially available from the John Hardy Company.)
- Pre-built 990 opamps are available from John Hardy and Steven Hogan
- The Jensen App. Note AS083 can be viewed at Jensen’s website (PDF).
- DIYer Mnats has graciously shared his PCB layout for a DIY 990 opamp.
In app note #83, Jensen warns that “This is not a project for electronic beginners.” Indeed the 990 opamp is relatively difficult to DIY and is prone to oscillation if layout and grounding conditions aren’t just right. As an alternative, Jensen’s app note #82 details a similar circuit using inexpensive IC opamps, which can be viewed here: AS082 app. note.