The ISA Pre

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.

Studio Magazine review quotes
August 2003
"Conclusion: The ISA 428 effortlessly claims its place in the high-end class of multichannel pre-amp solutions, and this is all the more impressive given the attractive price. In particular, the quality of the workmanship and components are wonderfully high. Four sonically impressive and noiseless mic. pre-amps including Lundahl-transformers and a well-equipped environment, combine together with the excellent converter option to provide an especially attractive package that is ready to deal with the challenges of the DAW era."

Mix (US) Magazine review quotes
November 2003
Kevin Becka
"The ISA 428 is not simply four preamps in one box, but four preamps that sound so good. The box is pro inside and out with Lundahl 1538 input transformers, as originally spec'd by early Focusrite designs. The fit and finish are what you'd expect from Focusrite: every knob, button, connector is top-notch. And it's a tweaker's delight, with added touches such as variable input impedances, dual metering, HP filters and optional digital back end (with added inputs)."

Key Features

• Four ISA-series transformer-based pre-amps
  The benefits of Focusrite’s classic pre-amp topology include superb common-mode rejection, an excellent overload margin, an extremely low noise floor and super-wide bandwidth.
• Dedicated analogue insert points
  Switchable insert points are also featured on every channel, allowing routing (between pre-amp and output) to additional external processing.
• Accurate ‘moving coil’ peak meters
  Clear metering designed to catch even the fastest and most elusive of transients.
• Four front panel instrument inputs
  Instant access on the front fascia and no separate DI box required.
• Eight line inputs
  Four additional A-D converter inputs combine with the four pre-amp inputs to provide eight line inputs, routable discretely to the eight-channel A-D converter option.
• Eight Channel optional 192kHz A/D conversion card
  With the additional four ADC inputs, two ISA428’s can be used with a single ADC, providing eight channels of pre-amplification, A-D conversion and VU metering.
• Custom Global A-D converter ‘soft clip’ limiter
  Focusrite’s custom optical design protects the A-D converter by preventing ‘digital overload’. It requires that an A-D converter be installed.
• Switchable Impedance
  Ideal for mic matching or extended microphone creativity. Settings include the ‘Vintage ISA110’ setting, providing the original Focusrite signature sound.
• Variable Focusrite high pass filter
  Provided on every channel, this is the same filter as featured on the original ISA110 and the Focusrite forte console.