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Focusrite ISA428 Four Channel Preamp


The ISA preamp sound is quintessentially British, well-rounded, and with plenty of detail and punch. The Focusrite ISA sound can be heard on countless records made throughout pop & rock history.

The ISA428 combines this timeless sound with contemporary features to meet modern demands. Rack form factor, clear metering and versatile controls/connectivity - all in one. And that's just getting started...

Key features

  • 4 ISA Series transformer-based mic preamplifiers
  • Optional 8 channels of high-quality analogue to digital conversion
  • Classic ISA mic preamp design featuring Lundahl LL1538 input transformer
  • Switchable input impedance including classic ISA 110 setting
  • Variable cut-off high-pass filter
  • 6-segment user-calibrated LED meter per channel
  • Dedicated per-channel insert point
  • Optional AD card with up to 192kHz, 24-bit sampling via ADAT or AES3
  • 2U 19" rack-mount format

Classic Neve-inspired Specs

The ISA428 combines 4 independent classic ISA microphone preamps, taken straight from the original ISA 110 module. As mentioned earlier, the sound of this module has been a mainstay of contemporary music production for decades. 

The addition of a Rupert Neve-specified input transformer - the Lundahl LL1538 - ensures that the impedance is matched with supreme accuracy. Not only that, but you can be confident of a noise-free, crystal-clear response. Many the LL1538 is widely considered to be "probably the best 1:5 microphone input transformer available."

Versatile Modern Design

Each of the 4 preamps have the options of mic, line or front-panel instrument input to suit every scenario. Along with the four mic preamp inputs, four additional inputs allow you to route other line signals through the converter. High quality Burr-Brown Pro Audio Amplifiers and flagship PCM4220 A-D converter chips offer razor-sharp performance seldom found at this price point. 

A convenient 25-pin D-Type connector means seamless connectivity with DAWs, including Pro Tools. If that wasn't enough, you get a subtle but effective high-pass filter, with a variable cutoff frequency of 16-420Hz - a tonal control that can make a huge difference, particularly with vocal performance. 6-segment LED meters show peak signal levels in dBFS, while a real-panel control lets you tweak calibration to match your A-D converters. 

The final touch is the addition of rear-panel insert points. Easily make use of inline processors like compression - a classic sound updated for the modern studio setup.


Mic input response

  • Gain range: 0dB to +60dB in 10dB steps, plus 0dB to +20dB continuously variable trim
  • Noise EIN: -126dB (measured at 60dB of gain with 150 termination and 20Hz/22kHz bandpass filter)
  • Noise at main output with unity gain: -98dBu (measured with a 20Hz/22kHz bandpass filter)
  • 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
  • Input Impedance, variable: 600, 1400, 2400, 6800

Line input response

  • Gain range: -20dB to +10dB in 10dB steps, plus 0dB to +20dB continuously variable trim
  • Frequency Response at unity gain (0dB): -0.3dB at 20Hz and -3dB at 94kHz
  • Input Impedance: 10k

Instrument input response

  • Gain range: +10dB to +40dB continuously variable
  • Input Impedance (Instrument): 2 MO
  • Noise at main output with minimum gain (+10dB): -95dBu (measured with a 20Hz/22kHz bandpass filter)
  • 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

High Pass Filter

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

Meter panel calibration value in dBFS / Equivalent dBu value

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)

  • 0dBFS / +22dBu
  • -2dBFS / +20dBu
  • -6dBFS / +16dBu
  • -12dBFS / +10dBu
  • -18dBFS / +4dBu
  • -42dBFS / -20dBu

Front and rear connectivity

  • Analogue Channel Inputs (Inputs 1-4)
  • 4 instrument 14" TS jack inputs on front panel
  • 4 microphone XLR inputs on rear panel
  • 4 line 14" TRS jacks on rear panel
  • 4 insert return 14" 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 14" TRS jacks on rear panel

Weight and dimensions

  • 480mm (W) x 88mm (H) x 280mm (D)
  • Weight: 5.5kg
Inc. VAT

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