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.

Reverb Magazine review quotes
January / February 2008
vol 02 iss 07
"The ISA828 provides a massive degree of flexibility with the sources you can record and recordings you can use it on. It’s sound is subtle yet distinct, with just the amount of presence so as not to overpower other elements in your mix. If you’re finding that your recordings are somewhat lack lustre and missing that professional edge, probably due to the use of a cheap interface, then the ISA 828 is the solution for you."

Future Music Magazine review quotes
February 2008
iss 197
"As a testament to the quality of the pres design, noise is super-low even at high gain settings and it’s difficult to get any audible distortion. It’s one of the quietest pres I’ve used in this respect and there’s no way you’d ever be short of headroom.

Where it excels is on bass/acoustic guitar and on punchy sounds where the pre really retains and emphasises the attack transients.

I can see the ISA 828 getting used in many different applications including as a standalone converter, (using the ADC card) as a front end for your DAW and generally if you want eight excellent workhorse pres in a compact unit you need to consider this box seriously"

Sound On Sound Magazine review quotes
October 2007
Paul White
"Even discounting the optional digital capabilities included with the review model, the ISA828 provides the most cost-effective ‘per channel’ means of buying this particular design of preamplifier."

With its Rupert Neve heritage, the ISA preamp — in its various guises — is a popular piece of kit, and is used in many top studios around the world, so it doesn’t really need any help from me to establish its credentials. What the ISA828 offers is a very cost-effective way of adding eight channels of ISA preamp to any studio system, with the highly-specified digital output card available as an option, which means you don’t have to pay for it if you don’t need it. Having to buy cables to mate with the D-Subs is a minor inconvenience, but it is more than made up for by the space-saving format and does benefit Pro Tools HD users. Because the price is so attractive, those project-studio owners prepared to spend a little more on quality equipment can now afford to add truly professional, transformer-input preamps to their own systems."

Key Features

• Eight ISA series transformer-based pre amps
  In a single robust 2U chassis, offering the classic Focusrite pre at its lowest cost-per-channel to date.
• New Optional Eight Channel 192kHz ADC
  Embodies cutting-edge conversion technology, incorporating Focusrite analogue circuitry to deliver the best A/D performance on the market, bar none
• Convenient 25 pin D-Type connectors
  The ISA828 integrates quickly and seamlessly with ProTools HD, as well as other popular hard disk recorders and mixing desks.
• Switchable Impedance
  Selectable per channel; choose one of four carefully selected input impedances, including the original ISA110 setting, to suit any microphone.
• Direct Instrument inputs
  Each instrument input features high and low impedance options to provide a comprehensive and accessible DI solution.
• Inserting available on every channel
  Allows external devices to be inserted between the pre amp and the ADC, using the 25-pin D-type cable.
• High pass filter, Phase reverse and Phantom power
  Each has a dedicated switch, independent on each channel.
• Six LED input meter
  Complete with precise meter trim - Designed to utilise the same reference points as Digidesign’s HD™