If you are interested in the discrete 990 op-amp, John Hardy has been making them for his mic pre amps for almost 20 years. It is based on the Deane Jensen design with some very functional modifications. What is unique about his version is the entire unit is encapsulated to improve temperature tracking. However, I don't believe the design is cascode or A but rather differential.
He sells them for about 50 dollars in small quantities.
The Boulder 993 Gain Stage
The sonic signature of any audio component depends largely on what circuit is used for achieving gain and buffering outputs. From the very beginning, rather than seeking any particular characteristic sound, the goal of maintaining sonic neutrality has been held high by Boulder.
The 990 is a discretely built gain stage, developed in the Hollywood audio community in 1978 under the coordination of the late Deane Jensen. It became widely recognized as the best universal gain stage ever, and was quickly retrofitted into many existing recording consoles at great expense.
Designed around a revolutionary matched dual transistor fabrication process, the 990 set a new standard for low noise and distortion.
Because the original 990 could not be DC coupled, Boulder added bias injection when it started using the 990 design and called it the BA990DC.
DC coupling eliminates the need for interstage capacitors. Otherwise, these form multiple high-pass filters and become a source of bass clarity problems.
Widely used for two decades, the 990 has indeed earned its reputation for critical music recording and reproduction. With the coming of the digital age, additional improvements make it the best choice for another decade or two.
In the new version, called the 993, Boulder doubled the slew rate, thus yielding the lowest distortion numbers in the industry. A special cascoded front-end circuit increases input impedance, eliminating this potential source of distortion.
All this is done without losing the phase margin and stability that the 990 is famous for. (Stability is absolutely essential for sonic clarity.)
The Boulder 993 is housed in a new metal case that provides extra shielding. DC performance is further improved by a special thermally conductive encapsulating compound.
The 993¡¯s capabilities include high slew rate, wide bandwidth, low distortion, high output current, and low output impedance. This combination cannot be matched by any integrated circuit.
The 993 continues to be the basis for new Boulder Series 2000 products into the next millennium.