The Focusrite ISA 110 Original Mono Mic-Pre & Equalizer is a limited-edition version of the famed single-channel ISA 110 EQ module. The original ISA 110/130 units were developed by Rupert Neve, who had been commissioned to supply 16 extra inputs for a Neve console at AIR Studios. Around 1986, Focusrite began selling outboard racks containing two, four or eight ISA 110/130 modules, and in 1989 the module became the basis of the Focusrite Studio console (of which only ten were made). The Focusrite Red Range products and the ISA 215 units are also derived in part from the original 110 module.

All the rotary switches are U.S.-made gold-plated switches, the push buttons are Swiss-made, and all the pots are manufactured in France using conductive plastic for maximum accuracy and smooth feel. The equalizer's shelving bands use rotary switches with individual capacitors for each frequency and one op amp for maximum fidelity and minimum distortion. Because rotary switches are much more expensive than pots, this qualifies as a costly "audiophile" design; switching individual capacitors are usually replaced by a cheaper single-capacitor circuit design and another pot.

All circuit functions are switched in and out of circuit by way of relays featuring gold-plated silver levers sealed in inert gas containers (compared to FET switching, relays are best for preventing breakthrough and distortion). The main audio op amp used through out the ISA 110 is the 5534.

The ISA 110 has four sections: mic preamp, highpass and lowpass filters, shelving equalizer and a separate parametric equalizer. The preamp has up to 60 dB of gain, which is switchable via the mic rotary switch in 6dB steps. There is also a gain trim control for additional gain from 0 dB to 10 dB. THD is measured at 0.0008%, and noise is measured at -123 dB with a 150-ohm input termination and 60 dB of gain. With an input impedance of 1.2 kilohms and a maximum input level at 26 dBu, there is not much to complain about. However, I wish the trim control had a center detent at 0 dB and a larger knob like the equalizer sections, which would make it easier to manually "ride" mic gain up or down while recording widely dynamic sources. Individual front panel buttons engage 48 phantom on/off and phase reverse.

FILTER AND EQUALIZER SECTIONS

The highpass and lowpass sections provide 18 dB per octave roll-offs. The rotary switch for the lowpass filter has 3.9, 5.6, 8.2, 12 and 16kHz corner frequency choices and an Off position that switches the section completely out of circuit. Likewise, the rotary highpass has settings for 30, 60, 105, 185, 330 Hz and Off.

The two bands of the parametric equalizer section each feature continuously variable boost/cut controls ( /-18 dB) with center detent. The two selectable frequency ranges overlap, and there is a variable Q (bandwidth) control. Each of the two bands also has a yellow X3 button that triples the indicated frequency. The tripled frequency is read on a yellow-colored scale. The first band is sweepable from 40 to 400 Hz (which triples to 120 to 1.2k Hz). The second band goes from 600 to 6k Hz (triples to 1.8 to 18 kHz). A slightly larger control for EQ frequency would have been a nice touch.
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