Focusrite Blue 315
A mastering equalizer built on rotary switches and the ghost of the ISA 110, with a faceplate that means business.
Overview
The Blue 315 is the equalizer that opened Focusrite's Blue Range, a line aimed at the mastering room rather than the tracking floor. "Isomorphic" here means the two channels share identical form and can be matched precisely, which is the whole point of a mastering EQ. Focusrite's technical director Richard Salter explained the construction choice plainly - the company moved away from the plated-steel assembly of the original Blue Range and adopted a build closer to the Red Range to meet upcoming EU electromagnetic compliance regulations. The result is a unit with the sturdy, solid feel collectors associate with the Red line, wearing the Blue identity because, as Salter put it, the 315 is essentially a re-run of the old-style units aimed at a different marketplace.
Under the hood, the 315 shares circuitry with the ISA 215 equalizer, which itself traces back to the Focusrite Studio Console equalizer. The difference is in the resolution and range of the controls, which were reworked for mastering engineers. Rather than 16dB of boost and cut, the 315 offers ±10dB in 23 steps. The assumption is that it will be used for precise corrective work and general enhancement rather than salvage or effects. All controls except the gain trims use rotary switches. That makes the unit practical in a mastering session, because settings recall repeatably, channel to channel and session to session.
The EQ is four-band per channel, with 11 frequencies in each band. The high-frequency shelf runs 3.3kHz to 22kHz in 11 steps. The low-frequency shelf covers 33Hz to 560Hz across 11 steps. The two center bands are peak types with two ranges apiece. The high-mid spans 560Hz to 3.9kHz and 2.7kHz to 18kHz, and the low-mid spans 39Hz to 270Hz and 180Hz to 1.2kHz. Each has 11 frequency steps and Q adjustable from 0.4 to 2.4 in 11 steps. Overall EQ coverage extends from 33Hz to 22kHz. Both the high-pass and low-pass filters run at 18dB per octave. The high-pass offers OFF, 33Hz, 47Hz, 68Hz, 100Hz and 150Hz, while the low-pass offers OFF, 22kHz, 18kHz, 15kHz, 12kHz and 10kHz. There is EQ bypass, phase reverse on either channel, and both coarse and fine input-level adjustment with overload indication.
Studio Sound (April 1995, p.19) reviewed the 315 as the unit that brought Focusrite's technical standards into the mastering suite for the first time, and the reviewer's verdict was that anyone after the accuracy, sonic quality and build Focusrite had become known for should give the Isomorphic Equaliser a serious look. The same review noted that the design's approach to control — stepped switches, expanded frequency ranges, finer Q resolution and a wider low-end range — was the most effective way to handle the job, even where a simpler scheme might have seemed sufficient. Focusrite's own advertising framed the 315 as the answer to widespread demand for the legendary ISA 110 Equaliser in a mastering format, with the boost and cut designed in small increments near the null point and larger steps at the extremities, and the filters given minimum ripple roll-off.
The 315 did not stay static. By 1997 Focusrite had upgraded it with additional EQ shapes - two bells and two shelves, with the shelves switchable separately into bell mode for a maximum of four simultaneous bell curves - and extended the low band. The Blue Range around it grew too. The Blue 330 mastering compressor-limiter, derived from the Red 3, was sold as its companion, and the range eventually included the Blue 245 A-D and Blue 260 D-A converters and the Blue 300 mastering controller.
For anyone buying today, the appeal is the ISA 110-derived circuitry, switchable repeatability and mastering-grade resolution in a 2U chassis. The caveats are the usual ones for specialist analog outboard. It is a mastering tool first, so its ±10dB range and fine steps suit careful, corrective use rather than dramatic tone-shaping. The rotary-switch construction is also the part that would be costly to service if a switch ever failed.
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