Focusrite ISA 428 mkII

Focusrite ISA 428 MkII — a 4-channel microphone preamp built around Lundahl LL1538 transformers, based on the classic ISA 110 design from Focusrite's 1980s consoles.

Built for studios chasing the transparent yet transformer-colored sound of the consoles designed for George Martin's AIR Studios. ISA 428 MkII carries a circuit topology unchanged for decades into a convenient 2U enclosure, adding switchable input impedance and optional digital conversion, making it equally useful as a standalone analog front end or paired with any digital interface.

Matching Impedance to the Microphone's Character

The original ISA preamp was designed around microphones available in the 1980s, limiting its versatility with modern capsules carrying different impedance characteristics. ISA 428 MkII solves this by adding three extra input impedance settings alongside the classic Vintage ISA 110 mode, letting an engineer deliberately choose the tonal character — from transformer-core warmth and saturation to clean, transparent signal transfer — depending on the microphone model and source material.

Expanding to a Full Digital Front End Without New Hardware

Studios bridging an analog preamp chain with a digital workflow need a flexible conversion path that doesn't force a separate converter purchase. The optional eight-channel A-D card fitted inside the ISA 428 MkII converts the signal with 122dB of dynamic range and jitter performance below 250 picoseconds, output over a D-Sub25 connector that interfaces directly with Pro Tools HD and other digital gear. That means a single 2U unit works as a purely analog preamp or a complete digital front end, whichever the session calls for.

KEY FEATURES

• 4 channels of microphone preamp based on the original ISA 110 design
• Lundahl LL1538 input transformer with a custom Zobel network
• Switchable input impedance, including the classic Vintage ISA 110 mode
• Gain up to +60dB in 10dB steps, plus continuously variable +20dB (up to 80dB total)
• 4 front-panel instrument inputs (no separate DI box needed)
• 4 line inputs and switchable insert points on every channel
• Variable high-pass filter on every channel
• 6-segment LED metering for accurate level monitoring
• Optional eight-channel A-D card (122dB dynamic range, jitter below 250ps)
• Includes the Brainworx bx_console Focusrite SC plugin

TECHNICAL SPECIFICATIONS

• Channel count: 4
• Frequency response (0dB gain, mic): -0.35dB at 20Hz, -3dB at 122kHz
• Frequency response (60dB gain, mic): -2.5dB at 20Hz, -3dB at 103kHz
• Gain range: 0 to +60dB in 10dB steps, plus 0 to +20dB continuously variable (up to 80dB total)
• Input transformer: Lundahl LL1538
• Inputs: mic, line, instrument (front panel)
• Input impedance: switchable, 4 settings including Vintage ISA 110
• Optional A-D card: 8 channels, 122dB dynamic range, jitter below 250ps, D-Sub25 connector
• Chassis format: 2U rack