Overview

ISA428 MkII packs four transformer-input Focusrite mic pres into a 2U chassis with per-channel HPF, phase, insert, DI, LED metering and switchable input impedance. With the optional ISA ADN8 card, it adds AES3/ADAT/Dante up to 24-bit/192 kHz for straightforward DAW integration and network workflows[1][2][3]. It remains a workhorse front end in many studios.

Scientific Validity

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Comprehensive third-party lab data are limited, so we anchor to the official specs. Mic input S/N is 122 dB (A-weighted, typical), frequency response 20 Hz–20 kHz ±0.2 dB (10 Hz–110 kHz ±1.5 dB), THD+N −92 dB (0.0025%) @ −1 dBr, and EIN < −123 dBu (A-weighted, max gain)[1] — values near the transparency targets in audible terms. Given the lack of independent measurements across units, we keep a conservative 0.5.

Technology Level

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The design offers four selectable input impedances (600 Ω / 1.4 kΩ / 2.4 kΩ / 6.8 kΩ), per-channel 18 dB/oct HPF and inserts[1]. The ISA ADN8 option adds 8-channel 24-bit/192 kHz conversion with Dante/AES3/ADAT and Word Clock I/O, easing integration into modern hybrid and AoIP setups[2][3]. Solid engineering and expandability justify 0.8.

Cost-Performance

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Current market price of the review unit: 1999 USD[4][5]. The cheapest equal-or-better alternative is Focusrite Clarett+ OctoPre (8 preamps, EIN −129 dBu, THD+N −110 dB, 118 dB DR, ADAT I/O, Air impedance/voicing)[6][7]. Computation: 899.99 USD ÷ 1999 USD = 0.45 → rounds to 0.5. Functionally it exceeds channel count and digital I/O; on measured performance it is equal-or-better under the stated conditions, so it is a valid comparator.

Reliability & Support

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Focusrite provides a three-year warranty on all products[8][9]. As an analog device there are no firmware-maintenance risks. With long studio track records and accessible service, 0.8 is appropriate.

Rationality of Design Philosophy

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Transformer coupling has rational engineering goals (galvanic isolation, handling of common-mode content), and variable input impedance is a practical tool for mic loading and matching. Per-channel HPF/inserts and the clean upgrade path to Dante/AES3/ADAT via ISA ADN8 further support workflow rationality. However, the philosophy favors “character options” over maximizing transparency-per-dollar, so we set 0.4.

Advice

If you need transformer-coupled character and variable impedance for vintage mics, ISA428 MkII delivers proven performance. For modern transparency and higher channel count at lower cost, Clarett+ OctoPre offers better measured specs and more flexibility. Consider ISA428 MkII if you specifically value the transformer sound and impedance options; otherwise, Clarett+ OctoPre provides superior cost-performance and modern connectivity.

References

[1] Focusrite, “ISA 428 MkII User Guide,” p.16 (Performance and Specifications), https://fael-downloads-prod.focusrite.com/customer/test/s3fs-public/downloads/ISA%20428%20MkII%20User%20Guide%201.1%20English%20-%20EN.pdf (accessed 2025-08-19)
[2] Focusrite, “ISA 428 MkII,” https://us.focusrite.com/products/isa-428-mkii (accessed 2025-08-19)
[3] Focusrite, “ISA ADN8,” https://us.focusrite.com/products/isa-adn8 (accessed 2025-08-19)
[4] Sweetwater, “Focusrite ISA428 MkII,” https://www.sweetwater.com/store/detail/ISA428mk2–focusrite-isa428-mkii (accessed 2025-08-19)
[5] Full Compass, “Focusrite Pro ISA 428 MkII,” https://www.fullcompass.com/prod/175734-focusrite-pro-isa-428-mkii-4-channel-microphone-instrument-preamplier (accessed 2025-08-19)
[6] Focusrite, “Clarett+ OctoPre technical specifications,” https://userguides.focusrite.com/hc/en-gb/articles/25435532741906-Clarett-Octopre-technical-specifications (accessed 2025-08-19)
[7] Focusrite, “Clarett+ OctoPre” (price), https://us.focusrite.com/products/clarett-plus-octopre (accessed 2025-08-19)
[8] Focusrite, “Three-year Warranty On All Focusrite Products,” https://us.focusrite.com/articles/threeyear-warranty-on-all-focusrite-products/ (accessed 2025-08-19)
[9] Focusrite, “Warranty,” https://us.focusrite.com/warranty (accessed 2025-08-19)

(2025.8.20)