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Focusrite ISA220

Session Pack Recording Channel
By Hugh Robjohns

The ISA220 offers the circuitry of the renowned ISA430 in a much more affordable package.

FOCUSRITE ISA220 recording channel.The current flagship of Focusrite's ISA range, the ISA430 Producer Pack, was reviewed in SOS November 1999, and is an impressively equipped and beautiful-sounding machine. Its design was derived in part from the ISA110 mic preamp and EQ developed by Rupert Neve, originally for Focusrite's analogue studio consoles.

The ISA220 Session Pack is the new smaller sibling to the flagship ISA430, with a modestly reduced feature set that still retains all the essential facilities — mic preamp, EQ, compression, de-essing, and so on — but allows the unit to be produced for a more attractive price. The ISA220 employs much of the original circuitry of the ISA430, as well as the optional high-quality internal 24-bit/96kHz A-D converter board, making the ISA220 ideal for direct-to-digital applications.

The ISA220 looks superficially similar to its big brother, occupying a 2U blue and grey rackmount case, and bristling with yellow knobs. A point here though — the new machine's controls are all of the same size and colour, whereas the ISA430 employed different-sized coloured knobs, helping enormously in navigating the panel. However, the layout follows the ISA430's philosophy, with input conditioning and EQ controls across the top, and compression, de-essing and output facilities on the lower level.

Grand Tour

The ISA220 is a very straightforward unit to find your way around, with an entirely logical panel layout and plenty of illuminated buttons to reveal the status at a glance. Starting around the back, XLR inputs are provided for microphone and line sources, with an external input for the otherwise unused half of the optional stereo A-D converter. There are jack sockets for a high-impedance instrument input (replicated on the front panel), a compressor key input, and an 'Int A-D Direct Input' — a break jack which replaces the main signal to the left channel of the stereo A-D converter just prior to the output limiter. This allows the converter stage to be used in isolation if so required, or for some external processing to be introduced between the ISA220's processing and the converter. These jack sockets are all wired to accept both balanced and unbalanced signals. A fourth TRS jack socket on the rear panel provides a compressor side-chain linking facility, enabling a pair of ISA220s to be used for stereo applications.

The single, balanced analogue output is presented on an XLR while digital outputs from the A-D converter card (if installed) are provided simultaneously via AES-EBU (XLR) , S/PDIF (phono), and Toslink optical connectors. External word-clock inputs are catered for in two forms on separate BNC sockets: standard rate and Digidesign Super Clock. A rear-panel switch provides 75Ω termination for the word-clock input, while a second sets the converter's nominal headroom by adjusting the maximum input level between +20 and +24dBu.

FOCUSRITE ISA220Focusrite have paid careful attention to the design of the converter's clock circuits to minimise jitter and improve stability. The front panel buttons allow selection of internal clock sample rates between 44.1, 48, 88.2 and 96kHz, with resolutions of 16, 20 or 24 bits. The fused IEC mains inlet has and integral voltage selector.

Moving around to the front panel, the left-hand side is dominated by both bar-graph and moving-coil metering. The large illuminated VU meter provides input signal metering (after the mic preamp, but before the EQ) or shows the amount of gain reduction applied by the compressor. The 16-LED stereo bar-graph metering above monitors the analogue input to the A-D converter, and consequently only the upper of the two rows of LEDs normally shows a signal. A relay-switched global Bypass button is provided, in addition to comprehensive in-out switching for each individual processor block.

The input section comprises two rotary controls and four buttons. The mic, line or instrument input is selected through repeated presses of a button, with associated LEDs to indicate the selected source. A four-position Gain switch coarsely sets the mic and line levels and is accompanied by a continuous Trim control spanning a 20dB range for both mic and line inputs. The instrument input level is only affected by the Trim control, which provides +10 to +40dB of gain.

Illuminated buttons for phantom power, phase reverse and mic gain are also provided, the last switching the mic input coarse gain selector to span 0-30 or 30-60dB ranges. With up to 80dB of microphone gain available, an EIN rating of -128dB, negligible distortion, and stacks of headroom, this is certainly an impressive input stage.

Processing Facilities

The equaliser is split into three sections: high and low filters, a pair of parametric mid-bands, and high and low shelves. The 18dB/octave filters can be tuned over a very wide range, overlapping each other substantially. Both parametric mid-bands can be switched between two frequency ranges, ensuring no part of the audio spectrum is beyond reach. The width of each band is also adjustable from a vague but useful 'narrow', to an equally vague but musical 'wide', allowing both precise and creative equalisation. The top and bottom shelves each have four carefully selected switchable turnover frequencies, with continuously variable cut and boost.

The compressor employs the same Class-A VCA circuitry developed for the ISA430, and can be switched before the equaliser if required — a facility retained from the preceding model. Controls are provided for ratio, threshold, attack and release, with a switch for an auto-release mode.

A new feature introduced in the ISA220, which is quite unique as far as I am aware, is activated by the Blend button. This replicates a popular dynamic control technique which is usually obtained by mixing direct and compressed signals through a sound desk. In the ISA220, the compressed output from the VCA is passed direct to the gain make-up amplifier and on to the output. However, by pressing the Blend button, the compressor's input signal (reduced by 6dB to minimise the risk of overload) is also mixed with the output from the VCA, as the signals enter the gain-makeup amplifier. This can raise the volume of low-level signals substantially, without squashing louder signals. I'm sure this will prove very popular.

The de-esser section employs exactly the same opto-circuit as that employed in the ISA430, essentially using level-dependent equalisation and phase-cancellation techniques to produce a more subtle but highly effective form of de-essing. Threshold and Frequency controls are provided, along with a side-chain Listen button which allows the user to audition only those signal peaks that will be level-reduced by the circuit. I found this allowed the parameters to be optimised far more quickly and easily than would otherwise have been the case.

A continuously variable Output control adjusts the level of both the analogue output and the A-D input, fading down to silence if required or adding a further 6dB of gain. A stereo peak limiter follows the gain control but precedes the output driver and A-D inputs. Its threshold is fixed at +20dBu and the only control is an in/out button. A red LED illuminates to show how hard it is working. This multi-band limiter uses different attack and release times for each of its three frequency bands, each controlled through its own opto-limiter circuit. The low-frequency band has a moderately fast attack time, while the mid-band is faster and the HF faster still — the idea being to provide accurate peak limiting without the distortion from transients 'punching holes' in sustained bass sounds.

A pair of ISA220s can be combined for stereo applications by interconnecting the Dynamic Link sockets on their rear panels. This ensures stable stereo imaging when the compressors are active, but all controls — gain, EQ, compressor, and de-esser — have to be matched to identical settings. Since the A-D card is stereo, only one unit of the pair needs to have it installed, as the analogue output of the second unit can be connected to the external A-D input of the first unit.


The ISA220 is everything you would expect it to be and, in fact, for most people the new model will provide a more focused and practical combination of facilities than the ISA430, for a more attractive price. The input stage sounds clear, detailed, quiet and clean, yet adds a certain 'richness' to microphones that bolsters their inherent qualities in a very satisfying and musical way. The EQ is musical, sweet, effective and very flexible, with sensibly chosen parameters throughout. Likewise the compressor and de-esser sections are a joy to use with straightforward controls that deliver the goods in a fuss-free manner — and the Blend button is a powerful extra feature. The ISA220 obtains the best possible source quality and then allows it to be gently tailored as required. It really is that simple — everything works in an efficient, professional, but above all, musical way.

When you unpack any box with the Focusrite logo on its side you know you are in for a treat, and the ISA220 proved the rule once again. This is a well-thought-out professional product which will enhance any recording session or facility. Although still relatively expensive in the UK, it is a lot more affordable than the ISA430, but gives little away in usability and must surely appear near the top of anyone's wish-list.

Mini ISA vs Maxi ISA

Since the most obvious point of comparison is the flagship ISA430, perhaps it is worth clarifying exactly what is different between the two models. However, I think it worth pointing out that many of the differences simply reflect the realities of typical studio use, and serve mainly to remove or refine nice-to-have but little-used facilities.

The ISA220's input transformer is shared for both mic and line inputs, whereas the ISA430 had an electronically balanced line input. The Instrument input's minimum gain has been increased by 10dB too, and the moveable insert point of the ISA430 has been dropped altogether. This also means that the rather clever ability of the ISA430 to be reconfigured as a split-path unit (by using the insert points as independent I/O for the dynamics processing) has also been lost.

Leaping ahead to the output side of things for a moment, there is no mic preamp direct output on the ISA220, and the output transformer used in the ISA430 has also been dispensed with. The ISA430 boasted a facility to sum the internal A-D direct input with the main output allowing, for example, an effects return to be mixed directly with the main signal. Unfortunately, this feature is absent from the new model, despite its retaining the same external converter input facility.

The ISA220's equaliser stage is almost identical to that of the ISA430 except that the number of switched turnover frequencies on the high and low shelving equalisers has been reduced from six to four — the extreme top and bottom frequencies have been removed.

Whereas the ISA430 included a flexible expander/gate function, this is absent from the ISA220, and the ability to relocate each section of the equaliser into the side-chain of the compressor (for frequency-conscious compression) has also been removed. However, Focusrite have introduced the new Blend feature which enables a more subtle form of dynamic control.


  • The performance and quality of the ISA430.
  • The innovative and effective Blend button.
  • Comprehensive, musical and ergonomic signal processing.


  • The loss of the ISA430's direct preamp output.


An intelligently cut-down version of the flagship ISA430 which will bring high-end performance to a much wider market. This must rate as one of the best complete front-ends available.


ISA220 £1299; A-D option £323.13. Prices include VAT.

Focusrite +44 (0)1494 462246.

Published March 2002