Digital Mastering System
Active Member
Please study Sam Groner's tome:
There you can see several discrete opamps vs. many classic IC opamps. The discrete opamps, including the 990 are not great. Dean Jensen's 990 actually uses IC technology by using a dual transistor LM394 as the input long tailed pair - that's one reason it works as well as it does. Groner's study is now a little dated as some of the latest SOA opamps are even better that any he measured. The main takeaway I took from Groner's work is that no matter what opamp you use, the connection with significant common mode signal is always worse that the connection with none. Inverting is always lower distortion, but higher noise, than non-inverting. I therefore always use the inverting infinite-gain-multiple-feedback topology over the Sallen-Key configuration in line level filters (as in a crossover).