Focusrite ISA428-MKII
4-Channel Preamp


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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. More information about the ISA428 MkII A-D converter option can be found here<=link to ISA428/828 ADC option page>.

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.


  • Four ISA series transformer-based preamps
    ISA series mic preamps havebeen the producers’ choice on countless hit records over the last two decades.
  • Eight-channel optional A-D converter
    Four additional A-D converterinputs allow eight channels of high-quality conversion, with a dynamic rangeof 122dB and jitter performance below 250 picoseconds.
  • Selectable impedance
    Ideal for mic matching or extended microphonecreativity. Settings include the ‘Vintage ISA110’ setting, providing theoriginal Focusrite signature sound.
  • Four front panel instrument inputs
    Instant access on the front fascia -no separate DI box required.
  • Dedicated analogue insert points
    Switchable insert points are featuredon every channel, allowing routing (between preamp and output) throughadditional external processing.
  • Variable high-pass Focusrite filter
    Provided on every channel, this isthe same filter as featured on the original ISA428.
  • Accurate LED meters for every channel
    Clear metering designed to catcheven the fastest and most elusive of transients.


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)

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

Analogue 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

Analogue 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)


Dimensions (W x H x D)
480mm (W) x 88mm (H) x 280mm (D)
19" (W) x 3.5" (H) x 11" (D)


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