Four classic ISA preamps with DI inputs and a premium eight-channel A-D converter option.

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The ISA428 MkII provides four of Focusrite’s prestigious transformer-based microphone preamps. It features the same classic circuitry and renowned audio quality found in the original at a new level of affordability. With selectable input impedance, direct instrument inputs and an optional eight channels of high-quality analogue to digital conversion, the ISA428 MkII creates the perfect front end for the discerning recording professional.


First introduced in 1985, the ISA microphone preamp holds a reputation for outstanding transparency along with subtle warmth contributed by transformer core saturation. The addition of a variable impedance circuit allows ISA users to perfectly match the preamp with any microphone, or to use different settings creatively to shape the sound of the microphone. Unchanged for decades, the ISA microphone preamp topology offers incredible clarity and the signature Focusrite sound that makes it the top choice among many audio professionals today.


The optional class-leading eight-channel A-D converter option used with the ISA828 is also compatible with the ISA428 MkII. Along with the four mic preamp inputs, four additional inputs allow you to route other line signals through the converter.

Four instrument inputs, instantly accessible on the front panel, allow quick and easy connection without the need for a separate DI box. Extra processing can be placed between preamp and conversion via the units dedicated insert points, featured on every channel. Finally, accurate six-segment LED metering is designed to catch even the fastest transients on every channel.


Over 25 years, Focusrite has earned a reputation for bringing superb audio quality and engineering excellence to an array of audio professionals. Whether you’re looking to preserve the sound of your classic microphones, or searching for the perfect front end for your digital studio, the ISA428 MkII brings legendary tools to your studio at an exceptional new price.

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.


ISA A-D Converters (Optional see #180188)
Just paying for the PCB and circuitry, without any additional casing and metalwork, affords a considerable reduction in cost whilst maintaining an extremely high level of sonic clarity. However, the benefits from installing Focusrite’s optional ADC extend far beyond the obvious financial advantages and space saving issues. As most converters use the same chips, it is the quality of the surrounding analogue circuitry that makes the difference. This is the reason why the ISA ADC is unrivalled in terms of both price and performance.

Focusrite’s R and D team have carefully considered the organisation of the PCB layout and grounding, ensuring that the S/N ratio and linearity of the converter are optimised. The noise floor and THD (Total Harmonic Distortion) are kept to a minimum by the nature of the power supply and the analogue input design, with analogue filtering positively effecting the sound of the digital noise.

In addition, the supporting circuitry around the ADC (dealing with data transmission, bit reduction and incoming word clock) has been carefully structured, as it influences the converter through heat, power supply and data jitter. One of the most important aspects of the ADC is the clock design, as this determines the linearity of the digital output. All Focusrite ADCs are designed to have incredibly low jitter figures (44.1kHz-192kHz < 20 psec) which significantly improve the quality of the sound.

With the converters housed within the analogue units, the signal path is kept as short as possible, having no unnecessary analogue connections to the inputs of recording equipment. In addition, Focusrite units do not contain extraneous clocks or other sources of digital noise, unlike so many digital desks and multi-I/O DSP configurations.

Focusrite ISA428 MKII Features:

Four ISA series transformer-based preamps
ISA series mic preamps have been the producers’ choice on countless hit records over the last two decades.
Eight-channel optional A-D converter
Four additional A-D converter inputs allow eight channels of high-quality conversion, with a dynamic range of 122dB and jitter performance below 250 picoseconds.
Selectable impedance
Ideal for mic matching or extended microphone creativity. Settings include the ‘Vintage ISA110’ setting, providing the original Focusrite signature sound.
Four front-panel instrument inputs
Instant access on the front fascia - no separate DI box required.
Dedicated analog insert points
Switchable insert points are featured on every channel, allowing routing (between preamp and output) through additional external processing.
Variable high-pass Focusrite filter
Provided on every channel, this is the same filter as featured on the original ISA428.
Accurate LED meters for every channel
Clear metering designed to catch even the fastest and most elusive of transients.
Phantom power available on every channel
Trim control allows 20dB of extra gain with fine adjustment
Phase-reverse buttons help to correct phase issues with microphones or cables
Optional A-D converters easily synchronise to external word clock

Focusrite ISA428 MKII Specifications:

Mic Inputs
Frequency response at minimum gain (0dB): -0.35dB at 20Hz and –3dB at 122kHz
Frequency response at maximum gain (60dB): –2.5dB at 20Hz and –3dB 103kHz
Gain range: 0dB to +60dB in 10dB steps, plus 0dB to +20dB continuously variable trim
Maximum headroom: +7.4dBu
THD+N: 0.0009% (measured at 1kHz -20dBu input signal, at 30dB gain setting, with a 20Hz/22kHz bandpass filter)
Noise EIN: -126dB (measured at 60dB of gain with 150O termination and 20Hz/22kHz bandpass filter)
Noise at main output with unity gain: -98dBu (measured with a 20Hz/22kHz bandpass filter)
SNR: 123dB (relative to max output 25dBu)
SNR: 120dB (relative to 0dBFS (+22dBu)
CMRR: 88dB.
Input Impedance, variable: 600O, 1400O, 2400O, 6800O

Line Inputs
Frequency Response at unity gain (0dB): -0.3dB at 20Hz and –3dB at 94kHz
Gain range: -20dB to +10dB in 10dB steps, plus 0dB to +20dB continuously variable trim
Maximum headroom: +25.4dBu
THD+N: 0.002% (measured with +4dBu input signal, 0dB gain setting, with a 20Hz/22kHz bandpass filter)
Noise at main output with unity gain: -91dBu (measured with a 20Hz/22kHz bandpass filter)
SNR: 116dB (relative to max output 25dBu)
SNR: 113dB (relative to 0dBFS +22dBu). Input Impedance: 10kO
Instrument Inputs
Frequency Response at +10dB gain: -0.2dB at 20Hz and 0dB at 200kHz
Frequency Response at +40dB gain: -3dB at 20Hz and –3dB at 38.4kHz
Gain range: +10dB to +40dB continuously variable
Maximum headroom: +11.5dBu
THD+N: 0.006% (measured with –20dBu input signal, at minimum gain (+10dB), with a 20Hz/22kHz bandpass filter)
Noise at main output with minimum gain (+10dB): -95dBu (measured with a 20Hz/22kHz bandpass filter)
Input Impedance = >1MO

High Pass Filter
Roll off = 18dB per octave 3 pole filter
Frequency range: 16Hz to 420Hz (continuously variable, measured at the 3dB down point)

Meters
• 6 LED meter is calibrated 0dBFS = +22dBu (the maximum level which can be correctly converted by the optional internal A/D converter before overload occurs). The meter calibration points are as follows:
Meter panel calibration value in dBFS / Equivalent dBu value
0dBFS / +22dBu
-2dBFS / +20dBu
-6dBFS / +16dBu
-12dBFS / +10dBu
-18dBFS / +4dBu
-42dBFS / -20dBu

Front and rear connectivity
Analog Channel Inputs (Inputs 1-4)
4 instrument 1/4” TS jack inputs on front panel
4 microphone XLR inputs on rear panel
4 line 1/4” TRS jacks on rear panel
4 insert return 1/4” TRS jacks on rear panel
4 ADC XLR inputs on rear panel

Analog Audio Outputs (Outputs 1-4)
4 line XLR outputs on rear panel
4 insert send 1/4” TRS jacks on rear panel
Other I/O
IEC power input connector 100Vac – 240Vac
Front Panel Indicators
8 output meters, 6 segment
4 30-60 mic gain range selection indicator
4 +48V phantom power indicator
4 Phase indicator
4 Insert In indicator
4 Filter In indicator
4 Input selection indicators (Mic, Line, Inst)
4 Z In microphone input impedance selection indicators (Low, ISA110, Med, High) Clock select indicators (44.1, 48, 88.2, 96, 176.4, 192)
Ext clock indicators (Lock, Word clock, 256X)

Weight: 12.2lb. (5.5kg)

Dimensions (W x H x D)
480mm (W) x 88mm (H) x 280mm (D)
19" (W) x 3-1/2" (H) x 11" (D)

Focusrite ISA428 MKII

All Focusrite products are built to the highest standards and should provide reliable performance for many years, subject to reasonable care, use, transportation and storage.

In the event of a Manufacturing Defect becoming evident within 12 months from date of purchase Focusrite undertakes that the product will be repaired or replaced free of charge if the product is returned to the authorised dealer from whom it was purchased. In these circumstances, or if you need an out-of-warranty repair to your Focusrite product, please contact your local Focusrite distributor in your country of residence or business : Find your local distributor here and you will be advised of the correct return procedure.

Alternatively contact the Focusrite Reseller from which you purchased the product. If you purchase a Focusrite product outside your country of residence or business you will not be entitled to ask your local Focusrite Distributor to honour this Limited Warranty, although you may request a chargeable repair.

Alternatively, the unit may be returned at your cost to the dealer you purchased the unit from so that they can organise a Warranty repair with their Focusrite Distributor.

This Warranty does not include cost of shipping to and from the authorised dealer from whom it was purchased. In every case it will be necessary to provide the original invoice or store receipt to accompany the defective product to the supplying dealer. This Limited Warranty is offered solely to the first purchaser of the product from an Authorised Focusrite Reseller (defined as a reseller which has purchased the Product directly from Focusrite Audio Engineering Ltd. in the UK or its Authorised Distributors outside the U.K,) and is not transferable.

This Warranty is in addition to your Statutory Rights in the country of purchase.

Please note: A Manufacturing Defect is defined as a defect in the performance of the product which may be expected from a reasonable interpretation of the published description and performance specifications as published by Focusrite Audio Engineering Ltd. This does not include damage caused by post-purchase transportation, storage or careless handling, nor damage caused by misuse.

A significant proportion of products returned under Warranty (which are very few in number compared to numbers sold) are found not to exhibit any fault at all. Please check that the mains voltage is correctly set for your local supply and that your connecting cables are in good order and correctly connected.

If in doubt about the product functions please read the user guide and if necessary contact your dealer for advice before returning the product to the supplying dealer. You can also contact the Focusrite Tech Support team at the Focusrite Answerbase for general advice.

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