Focusrite ISA 428 Pre Pack

A quartet of flexible preamps

By Mitch Gallagher
Focusrite has been on a roll, turning out an impressive array of preamps and processors at different price points. On the top end, among the latest releases is the ISA 428, a 4-channel, 2-rackspace model with several interesting features.

The back panel has four sections, one per channel. Each has XLR mic in and line out, as well as 1/4" balanced line inputs and balanced 1/4" insert send and return jacks.

The ISA 428 can optionally be fitted with a user-installable 8-channel analog-to-digital converter card, that provides AES and S/PDIF on 9-pin d-sub connectors at up to 24-bit/192 kHz resolution, and ADAT optical outs at up to 24-bit/96 kHz. The card also has word clock in and out for external sync.

Wait, the ADC card is 8-channel? Correct. In addition to the internal four channels, the back panel of the ISA 428 has four XLR inputs that feed the “extra” four channels. This allows you to use one converter card for two ISA 428s, or, if you’re using the 428 as the front-end for a system, to route an additional four line level signals through the 428 converters.

Up front, the 428 has four 1/4" instrument-level inputs on the left. Next are the four channel sections. Each has a lovely mechanical VU meter, a rotary switch for setting overall gain within two ranges: 0–30 dB and 30–60 dB, and a trim control that provides continuously variable gain from 0–20 dB. There are switches for phantom power, phase, and for putting the rear-panel insert connectors into the signal path. Two switches step through multiple selections:

One selects mic, line, or instrument inputs, the other changes the preamp input impedance (more on this later). Rounding out each channel is an 18 dB/octave highpass filter with cutoff variable from 16 to 420 Hz and an in/out switch.

On the right side of the front panel are switches (with accompanying LEDs) for turning a pre-converter soft limiter on and off, sample rate selection, bit depth selection, and internal/external clock source; you can also lock to Digidesign 256x Superclock. Finally, there are LED ladders indicating output levels.

One of the prime features of the ISA 428 is variable input impedance. Each channel can be independently switched among low (600 ohms), vintage Focusrite ISA 110 (1,400 ohms), medium (2,400 ohms), and high (6,800 ohms). Input impedance affects microphones in two main ways: level (higher input impedance generally means higher level from the mic), and frequency response (higher impedance tends to result in increased high end). In practice, the effect of changing preamp input impedance varies from mic to mic. With a traditional ribbon or dynamic mic, the effect can be dramatic. With well-isolated models, the effect may be subtle.

Matching impedance to your microphones is a powerful tool, and can definitely enhance the sound, especially with ribbon mics. But even aside from impedance switching, the ISA 428 is one fine-sounding preamp. It has a clear “high-fidelity” sound, with generous body, natural highs, excellent dynamics, and wonderful presence. In the past few months I’ve had at least a dozen top-notch preamps go through my studio for review. In each case, the 428 has ultimately been my reference; I like the way it sounds that much. I might choose another preamp for a particular sound, but in general the 428 has become the preamp that I reach for first.

Aside from its reference-quality sound (which is really all that matters), I like other 428 features, particularly the optional converter card. Sonically, it sounds as good as more expensive converters. Plus, if you’re using a digital input card for your DAW or hard disk recorder, it makes a great 8-channel front-end. It’s a real value booster.

Give the ISA 428 a look. It’s an excellent sounding preamp with a number of useful features. But you’ll have to find your own, I’m keeping this one.