These are popular microphones among modders, so I decided to add them to the list as well. The 603S is probably popular because of the Neumann KM84-style capsule, as explained here. At the moment, I have only acquired the MXL 991, which probably has the same capsule as the 603S. There are many modifications published on the web, of which the capsule modification and the replacement of some ceramic caps with better types are probably the most popular ones. These mods have been described here in detail. I would not be eager to do such capsule mods, afraid that I would destroy the membrane. I think I would rather buy capsules from 3U Audio or Micparts, which get good reviews.
The MXL 603S and 991 electronic circuits are quite similar, but not identical. I’ve seen MXL 603S schematics and PCBA pictures that have a trim potmeter to adjust the JFET bias, which at least my MXL 991 does not have. Instead, the 991 has two unnamed resistors as a fixed voltage divider, which biases the JFET. And perhaps MXL changed to SMD versions in the meantime? I don’t know, but that’s a general trend.
In the following sections, I will discuss the schematic and PCBA, present some measurements, and discuss modifications.