Focusrite ISA One Classic Mic PreampDescription At the heart of the ISA range is the microphone preamplifier with its Lundahl input transformer essentially the same circuit, based on the original ISA110, that has been used for 30 years in successive Focusrite designs. The input transformer imparts a special character to the sound that can't be truly emulated in any other way: inseparable from the microphone itself, the transformer input stage is ideal for use with classic microphones.