Q: What does Focusrite’s “ISA” stand for, and what’s special about it?
A: ISA is an abbreviation for “Input Signal Amplifier” — Focusrite’s name for a high-end family of preamps, equalizers, originally commissioned from Rupert Neve by George Martin for Montserrat Air Studios in 1985. All of the ISA products feature a Lundahl LL1538 transformer specified by Rupert Neve for its particular sound — in particular the non-linearities and 2nd-order harmonic distortion, as well as the compression characteristics.
Neve also designed the circuit to have an extremely wide and flat frequency response. Nearly two dozen consoles were built by Focusrite featuring the ISA technology, around half of those are still in use today. Beginning in the 1990s, Focusrite began offering the ISA preamps and EQs as outboard gear, which continues to be available today. The ISA family also continues to expand with new form factors and new products in the family.