The Jensen JE-990 Operational Amplifier is a discrete operational amplifier originally designed by Deane Jensen. It was described in a paper published by the Audio Engineering Society in 1980. The paper is available at a cost from the AES or for free from Jensen Transformers.
It is something of a legendary opamp; not long ago several manufacturers used the "990" name for their own, completely unrelated products. But in the end, there is really only one '990 (well, there are at least four versions that I know of) designed by Deane Jensen and, in later versions, by Jensen in conjunction with Steve Hogan.
The JE-990 is manufactured by The John Hardy Company for a very reasonable price and also in an unpotted, repairable form from Steve Hogan at The Sound Steward. No one in their right mind would bother attempting one from scratch, although many have - with mixed results. But quoting from the AES paper, "The circuit is "public domain" and can be used for any purpose without license or permission." So there isn't any law against making your own. In fact, Jensen seems to actually encourage it in his report.
The heart, and apparently the inspiration for the '990 is the LM394 SuperMatch pair transistor. Oddly enough, about the first active device in this design is an integrated circuit.
For some unknown reason printed circuit board layouts based on the circuit are amazingly rare. There are a couple that use substitute devices but were seemingly designed without the AES paper in hand. Deane Jensen is specific in his recommendations for a layout of the JE-990 in section 7.1 of the AES paper and gives many hints throughout the text as to how the opamp should be constructed. It also seems a shame to cut the heart (LM394) out of the circuit when it is still available and actively manufactured.
None of the existing DIY layouts I'm aware of take into account the sheer size of the recommended inductor. Fortunately, I was able to locate a core with the same physical size (but not the same properties) of the original core for the layout. But the original core was apparently not manufactured anymore by Fair-Rite (the original supplier).
After much agonizing over the inductor and few exchanges with the more inductor-savvy Lab members, the most important characteristics were identified. This opened up the door to several off-the-shelf replacements.
In a final twist, however, I finally heard back from Jensen Transformers having emailed them very early on in the piece. The inductor core is still available directly from Jensen. You can see it at the very bottom of their price list. The part number is FB-6.
It seemed sensible that there should be a JE-990 printed circuit board layout for the masochistic DIYer. Such a PCB should conform to the "2520" footprint (which, according to Walt Jung, seems to have originated with George Philbrick's 1962 design, the PP65). This would allow the opamp to be plugged into existing systems for evaluation and comparison purposes.
Here is the layout (links on left sidebar). I don't claim that it is anything but my interpretation of Deane Jensen's description of the JE-990. Certainly nothing has been added and hopefully nothing has been subtracted from Jensen's design. Your comments and suggestions are welcome. Please post here: http://www.groupdiy.com/index.php?topic=14422.0
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