Focusrite ISA 428 MkII 4-Channel Transformer Mic Preamp

Overview

The Focusrite ISA 428 MkII is a professional four-channel microphone preamp designed to deliver the renowned sound of Focusrite's ISA series in a practical 2U rackmount format. Each channel combines a transformer-based microphone preamp with selectable input impedance, flexible gain control, front-panel instrument connectivity, and analogue insert routing for detailed control during recording.

Designed for microphones, line-level sources, and instruments, ISA 428 MkII provides up to 80dB of total gain per channel, a continuously variable high-pass filter, phase control, +48V phantom power, and accurate six-segment LED metering. An optional eight-channel A-D converter can further expand the unit with digital connectivity for modern DAW and studio workflows.

Key Features

• Four Focusrite ISA transformer-based microphone preamps
• Lundahl LL1538 input transformers for classic ISA character
• Up to 80dB total gain with stepped gain and variable trim
• Four selectable microphone input impedance settings
• Four front-panel instrument inputs for direct recording without a separate DI box
• Four rear-panel balanced microphone inputs and four line inputs
• Switchable analogue insert points on every channel
• Variable high-pass filter from 16Hz to 420Hz
• +48V phantom power and phase reversal on each channel
• Six-segment LED metering for accurate signal monitoring
• Optional eight-channel A-D conversion with up to 24-bit/192kHz operation
• Professional 2U rackmount construction

Four Classic ISA Microphone Preamps

ISA 428 MkII features four of Focusrite's renowned ISA microphone preamps, based on the original ISA 110 design. The transformer-based architecture provides a detailed, open sound with the option to explore additional tonal character through selectable microphone impedance.

Each channel provides up to 60dB of stepped gain plus a further 20dB of continuously variable trim, giving up to 80dB of total gain. This makes the ISA 428 MkII suitable for a wide variety of microphones and recording sources, including sources that require substantial clean gain.

Selectable Input Impedance

Each microphone input offers four impedance settings: Low, ISA 110, Medium, and High. This allows the preamp to be matched to different microphones or used creatively to alter the interaction between the microphone and preamp. The original ISA 110 setting provides the classic Focusrite impedance associated with the ISA design.

Direct Instrument Inputs

Four front-panel 1/4-inch instrument inputs provide immediate access for guitars, basses, keyboards, and other instrument-level sources. Each input feeds its corresponding preamp channel, making the ISA 428 MkII particularly convenient for overdubs without requiring a separate DI box.

Flexible Mic, Line and Instrument Connectivity

Every channel can accommodate microphone, line-level, or instrument sources. Microphones connect through rear-panel XLR inputs, line-level equipment connects through balanced 1/4-inch TRS inputs, and instruments can be plugged directly into the convenient front-panel inputs.

Analogue Inserts and Signal Processing

Each channel features a switchable analogue insert point positioned between the preamp and output stage. This allows external compressors, equalisers, gates, or other analogue processors to be integrated into the recording chain while maintaining straightforward front-panel control.

Variable High-Pass Filtering

The built-in three-pole high-pass filter provides continuously variable control from 16Hz to 420Hz with an 18dB-per-octave roll-off. It can be used to reduce unwanted low-frequency rumble, excessive proximity effect, or other low-end energy before the signal reaches downstream equipment.

Accurate LED Metering

Each analogue channel is equipped with a six-segment LED meter for clear signal-level monitoring. The meters are positioned before the optional A-D converters and are calibrated to show how closely the incoming signal is approaching the digital clipping point when the converter card is installed.

Optional Eight-Channel Digital Expansion

The ISA 428 MkII can be expanded with the optional ISA ADN8 eight-channel A-D converter card. This provides high-quality digital conversion for the four analogue channels plus four additional inputs, with support for Dante, AES3, and ADAT connectivity. The optional card supports sample rates up to 24-bit/192kHz, making the ISA 428 MkII suitable for direct integration with modern digital recording systems.

Professional Rackmount Design

The ISA 428 MkII uses a rugged 2U rackmount enclosure that fits naturally into professional studio racks and recording systems. Its switched-mode power supply automatically accommodates different mains voltages, supporting operation from 100V to 240V AC without manual voltage selection.

Typical Uses

• Multi-channel vocal and microphone recording
• Drum and percussion tracking
• Guitar and bass recording through the direct instrument inputs
• Studio overdubbing and tracking
• Professional front-end preamplification for audio interfaces
• External compression and EQ through analogue inserts
• Multi-microphone recording for acoustic instruments
• Digital studio integration with the optional ISA ADN8 converter

Specifications

• Channels: 4 microphone/line/instrument preamp channels
• Mic Gain: 0–60dB in 10dB steps plus 0–20dB continuously variable trim
• Total Mic Gain: Up to 80dB
• Mic Input Impedance: 600Ω, 1.4kΩ, 2.4kΩ, 6.8kΩ
• Maximum Mic Input Level: +7.4dBu
• Mic EIN: -126dBu
• Mic SNR: 123dB relative to maximum output
• Line Gain: -20dB to +10dB in 10dB steps plus 0–20dB variable trim
• Maximum Line Input Level: +25.4dBu
• Instrument Gain: +10dB to +40dB continuously variable
• Instrument Input Impedance: Greater than 1MΩ
• High-Pass Filter: 16Hz–420Hz, 18dB/octave
• Mic Inputs: 4 × balanced XLR
• Line Inputs: 4 × balanced 1/4-inch TRS
• Instrument Inputs: 4 × 1/4-inch TS on front panel
• Insert Returns: 4 × 1/4-inch TRS
• Insert Sends: 4 × 1/4-inch TRS
• Analogue Outputs: 4 × XLR
• Optional Digital Expansion: ISA ADN8 eight-channel A-D converter
• Optional Converter Sample Rate: Up to 24-bit/192kHz
• Power: 100–240V AC
• Format: 2U rackmount
• Dimensions: 480 × 88 × 280mm
• Weight: 5.5kg

What's Included

• Focusrite ISA 428 MkII four-channel microphone preamp
• IEC power cable
• User manual

Note: The optional ISA ADN8 eight-channel A-D converter is sold separately and is not included with the standard ISA 428 MkII.

Why Choose the Focusrite ISA 428 MkII?

The Focusrite ISA 428 MkII combines four classic transformer-based ISA preamps with flexible microphone impedance, direct instrument inputs, analogue inserts, variable high-pass filtering, and precise LED metering. With its 2U rackmount design and optional eight-channel digital expansion, it provides a versatile professional front end for studios, recording engineers, producers, and multi-channel tracking setups.