Deane Jensen developed the 990's circuit design in the 1970s. The 990 was revolutionary in that it provided and ensured quiet and stability, something that had yet to be achieved up until that point. Deane Jensen pursued and was awarded a US patent for aspects of the original design. It is interesting to note the 990 was never produced by Jensen Transformers. The John Hardy company made them from 1980-1981. As changes & improvements were made to the design, John Hardy would get Deane Jensen's approval before he changed the way he made the opamp modules for the mic preamps he developed for The John Hardy Company.
After Deane Jensen died in 1989, John Hardy would consult with Deane's long-time colleague and friend, Bill Whitlock regarding any proposed changes before they were implemented. Following that, Steve Hogan, who was working at Jensen Transformers at the time, made some modifications. Later, Hogan would produce them under his company The Sound Steward.
We lost Deane Jensen in 1989 though his design and vision lives on in the 990. In fact, in 2014 when Bill Whitlock sold Jensen Transformers to Canadian company Radial Engineering, they were already in the throws of remaking a Deane Jensen 990 design inspired preamp, though in a 500 series format. This product is the now Jensen Twin-Servo 500. The Radial 500 series Twin-Servo incorporates two 990 opamps cascaded in series, two servos and two Jensen transformers, thus culminating in the Twin-Servo name. The Radial design team faithfully followed Deane's original recipe and this has culminated in strong sales numbers for the product. It's apropos that Deane Jensen's design be honored by the NAMM Hall of Fame in the same year his design was greatly responsible for Radial's single largest 500 series order in the company's history with 640 Jensen Twin-Servos specified and installed for the project.
See the presentation here.
Tens of thousands of Jensen transformers are in use today throughout the world, and maintain a flawless reputation for superior quality and reliability. Jensen Transformers is now a subsidiary of Radial Engineering.