Shure’s SM81 was designed to be a durable, road-ready condenser microphone with studio-quality sound. While the unmodded microphone is excellent enough to have become a modern classic, its sound can be improved at the expense of a little durability. The following mod is reported improve the accuracy of the SM81’s high-frequency response while making it more susceptible to damage from plosives, kick drums, and other blasts of air. Unlike the comparatively easy Shure SM57 mods, however, this mod requires working with some small, sensitive components and can damage your mic.
Full instructions for the mod are included in Scott Dorsey’s article for Recording Magazine.