Some general comments about the 990, Deane Jensen, Steve Hogan and myself. The original JE-990 design was done by Deane Jensen and was finished in 1979. The engineering paper that Deane wrote about the JE-990 in 1979 was turned into an AES Preprint that appeared in the AES Journal in 1980. You can go to www.aes.org
and pay for this preprint and download it, or I can send a copy to you if you provide a postal mailing address. In the latter half of 1979 I developed packaging for the 990 that was compatible with the API-2520 and offered it as a product. Various mic preamp cards followed over the next few years, with the M-1 Mic Preamp coming out in 1987.
The 990A and the 990C were developed in the mid/later 1980s, certainly in time to be used in the Jensen Twin Servo 990 Mic Preamp that I began manufacturing for Deane toward the end of 1988 (or so). This version of the Jensen Twin Servo Mic Preamp was a combined development of Deane, Steve Hogan, Bill Whitlock and myself, using the M-1 package as the foundation. It replaced the Twin Servo that was built for Deane from 1986 to 1988 by the Boulder Company.
Steve Hogan was instrumental in the development of the 990A and 990C versions. He has been in touch with Beno May about Beno's modifications, but I have no knowledge about the details of those modifications.
The 990 (or any op-amp) does not have "30dB of gain", or even "28dB of gain". It has whatever amount of gain you design it to have. There are limits to how much gain an op-amp can provide, and they are determined by a variety of factors, including open loop gain, the desired bandwidth, maximum allowable distortion, etc.
I never had a specific discussion with Deane about the reason for the two stage design of the Jensen Twin Servo. But I am almost certain that he looked at the combination of the JE-16-A (as it was known at the time) input transformer and the JE-990, and thought to himself something like this: "Wow, the JE-16-A is a perfect match for the noise characteristics of the JE-990, and it's my best input transformer because it has the lowest impedance ratio (150:600 ohms) of all my input transformers, but the low ratio only provides 5.6dB of voltage gain compared to 20dB of gain that the JE-115K-E provides (150:15k-ohms). If I need 60dB of gain, a high-ratio transformer would provide 20dB, the op-amp following it would provide 40dB. But the JE-16-A only provides 5.6dB, so the 990 will have to provide 54.4dB to get me up to a total of 60dB. That is a lot of gain for one op-amp (relatively speaking). I think I'll add a 2nd 990 in series with the first one so that each 990 will provide 27.2dB of gain for a total of 54.4dB of gain from the TWO op-amps." So each 990 in the Jensen Twin Servo mic preamp is adjustable from about 6dB to 27.2dB, for an overall gain range of about 18-60dB (including the transformer voltage gain).
In the M-1 mic preamp, the 990 is adjustable from 5.6dB to 54.4dB of gain, for an overall range of about 12-60dB including the transformer voltage gain.
I have modified quite a few M-1 and M-2 mic preamps, and a few Jensen Twin Servo mic preamps, for greater than 60dB maximum gain (the standard "maximum" gain). If you look at the schematic for the M-1 mic preamp on page 7 of my M-1 data package: http://www.johnhardyco.com/pdf/M1_M2_M1p_20031025.pdf
You will see that R7
determines the maximum gain of the 990. It is listed as 20 ohms, but it has actually been 19.1 ohms for many years. One approach to increasing the maximum gain is simply to add another 19.1 ohm resistor on the bottom of the p.c. board in parallel with the original 19.1 ohm resistor. This cuts the resistance in half, providing 6dB additional gain. It is easy to add the resistor, and easy to remove it if desired. I have put two of those resistors on a few preamps to increase the gain by 10dB.
So, how much gain is too much gain? Hard to say. YMMV. But the Twin Servo design might have a performance edge when venturing beyond 60dB because the gain is being split between two op-amps, each one working at a much lower and more relaxed gain. On the other hand, it seems that customers that have M-1 preamps with the high gain modification are happy with the results.
Hopefully this clears some things up. Thank you.
The John Hardy Co. www.johnhardyco.com