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Focusrite ISA 428 MKII Microphone Preamp

4-Channel Microphone Preamp with A-D Conversion

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.

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£1,169.00 £974.17 ex VAT
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£1,169.00 £974.17 ex VAT


Product Description


Focusrite’s ISA428 four-channel microphone preamplifier delivers the Focusrite trademark sound at its lowest cost per channel to date, supplying four original ISA transformer-based preamps in a single sturdy two unit chassis. The four ISA-series transformer-based microphone preamps make use of the same archetypal microphone preamplifier design as the original ISA110 module from Focusrite's legendary Forté console (including the original Lundahl L1538 transformer and bespoke zobel network).

The preamps are supplemented by four line inputs, four line outputs and a further optional four channel 192kHz A-D converter boasting a dynamic range of 122dB. The ISA428 also combines speedily and effortlessly with ProTools HD, and other popular hard disc recorders and mixing desks, because of its widely-compatible 25 pin D-Type connectors.

The microphone input stage also features a variable impedance circuit, providing the user the opportunity to alternate between four well thought out selected input impedance settings. The original ISA110 setting is complimented by three more impedance settings to either faultlessly counterbalance the preamp with a microphone, maximising level, or to use diverse settings creatively to sculpt the sound of the microphone being used.

High and low impedance options are present on the four front panel instrument inputs to deliver a complete and accessible DI solution. If the need arises to position extra processing between the preamp and converter insert switches feature on every channel. Each channel provides vintage design 75 Hz / 18dB per octave high pass filters, making certain that unwelcome low frequencies are essentially eliminated.


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 analogue 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


Additional Information


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 150Ω 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: 600Ω, 1400Ω, 2400Ω, 6800Ω

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: 10kΩ

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 = >1MΩ

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


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 and Weight

480mm (W) x 88mm (H) x 280mm (D)

Weight: 5.5kg


Download ISA 428 Mk11 User Manual