The ISA microphone pre-amplifier was first introduced back in 1985. This transformer-based pre-amplifier formed the first half of the ISA110 mic-pre and EQ module on Focusrite's critically acclaimed Forte console. This console, and the later Studio console were hailed by many as the finest sounding consoles ever built. The ISA microphone pre-amplifier went on to form the cornerstone of all ISA and RED Focusrite products. The topology has never changed, except for the addition of a variable impedance circuit, allowing ISA users to either perfectly match the pre-amp with any microphone, thus maximizing level, or to use different settings creatively to shape the sound of the microphone being used. The original ISA110 setting was supplemented by three further impedance settings. The Focusrite pre amp is renowned for its transparency, but there's also that subtle warmth that transformer core saturation contributes. This seeming contradiction of warmth plus beautifully open, unrestricted high frequencies, is a classic hallmark of the mid 80's Focusrite designs.
With processors whose bandwidth is strictly limited to the audible (20Hz to 20kHz) bandwidth alone, significant roll-off can still occur, affecting the higher audible frequencies of the signal. Therefore, in order to capture every subtle nuance, exactly as it occurs whilst avoiding any roll-off, the pre-amp must operate over a far greater bandwidth. The extremely broad frequency response of Focusrite pre-amps - close to 200kHz at the top end - guarantees accurate phase response across the much narrower range of human hearing, the roll-off point being moved entirely outside of the audible hearing range. All critical inputs are transformer-coupled, providing a dual gain structure - an expensive option, but the optimum route to high quality signal handling. Focusrite use Lundahl LL1538 transformers. Also originally employed was a 'Zobel network' to counteract the reactance of the primary coil of the transformer. A Zobel network is a damping circuit that is applied across the secondary of the mic input transformer. This damping network tailors the naturally occurring high frequency resonant point of the input transformer. The Zobel network featured in ISA and RED pre amps was set by ear, has never changed, and as such is a key contributor to the Focusrite sound. Finally, all the components featured in the ISA pre-amp were always chosen with a specific frequency response in mind. Rather than accepting the effect of the components chosen, the designer worked diligently with the values of every component in a relentless effort to achieve the desired response.