All-discrete Jensen 990 op amp based signal path, high-nickel transformer balanced output
You can read about this op amp for days. From Deane Jensen's original design documents and John Hardy's updates to countless articles and posts discussing what makes this amplifier so special.
The Jensen design is a complex device. It contains not only discrete transistors, but also inductors and capacitors. This makes the 990 differ from even a good audio IC op amp in two ways. First, since it is a discrete, transistor-based op amp, there will be slight non-linearities prior to full clipping. That plus some phase distortion due to the complex reactance will lend tracks recorded through this configuration with a bit more weight and depth.