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Focusrite ISA430 MkII

Recording Channel By Hugh Robjohns
Published January 2004

Focusrite ISA430 MkIIPhoto: Mark Ewing

Focusrite update their successful high-end Producer Pack to keep it ahead of the game.

The Focusrite ISA430 has been one of the most popular of all the recording channels since its launch nearly five years ago. Based on designs by Rupert Neve, it combined a remarkably flexible signal path with the superb sonics we have come to associate with the Focusrite marque.

Like many companies at the forefront of analogue design, Focusrite continue to develop and refine their circuits, and have come up with several significant improvements in the intervening years since the ISA430 was first introduced. The company have now decided to add many of these enhancements and features to an updated ISA430 MkII, further improving its sound quality and facilities. The new model also benefits from enhanced internal signal routing options, to increase flexibility even more, and it is also claimed to be easier and more intuitive to operate.

The ISA430 MkII, like the original, is a 2U rackmount processor designed to process a single audio channel. The control layout is much the same as the original and the unit is instantly recognisable. Like its predecessor, the ISA430 MkII features mic, line and instrument inputs (the last doubled to the front panel as well as the rear), plus a comprehensive EQ stage, a compressor, an expander/gate section, a de-esser, and a soft peak limiter. The original design was based on the classic ISA110 mic preamp and EQ circuits, and the ISA130 dynamics (with a solid-state Class-A VCA), plus a bespoke de-esser and limiter using high-speed opto-resistors. However, the input stage, equaliser and compressor in the MkII version have all enjoyed major overhauls, borrowing features and circuit improvements from more recent Focusrite designs.

Although the ISA430 is intended to serve as a complete channel strip in a box, it can also be used as separate processors, and to this end an additional insert point has been added to enhance accessibility. Used in combination, the main I/Os and two insert points now provide discrete access to the mic preamp, EQ module, and dynamics section (the compressor, expander/gate, and de-esser en bloc) — and if the optional stereo A-D board is installed, that can be accessed independently too!

Round The Back

Whenever I come across a new piece of equipment, the first thing I do is take a look at the rear panel, and the ISA430 MkII boasts impressive connectivity for a single-channel preamp. Starting at the right-hand side, the mic and line inputs are catered for with XLRs, and an unbalanced quarter-inch socket provides a high-impedance instrument input. A TRS socket below the mic input can be used to independently link the compressor and gate side-chains with those of a second unit, for stereo working. Two more TRS sockets provide balanced line-level key inputs to the compressor and gate — a useful facility.

Next along the top row is a male XLR providing a direct output from the preamp section of the unit, and this is followed by two pairs of XLRs forming the two balanced insert sends and returns. This is probably the most complicated aspect of the entire unit — have a look at the 'Insert Possibilities' box to see how much flexibility is on offer.

ISA430 MkII rear connections.ISA430 MkII rear connections.Photo: Mark Ewing

The last connector on the top row is another male XLR, providing the main balanced output after all the signal processing. Below this are two more female XLRs which access the optional A-D card inputs, via the soft limiter. A pair of front-panel buttons independently select these rear-panel inputs in place of the normalled internal signals (processed output to channel one and preamp direct out to channel two). In this way, both clean and processed versions of the input signal can be recorded easily to a DAW, if required, and I think this is an excellent facility. However, a signal connected to the first ADC input can also be mixed internally with the processed signal, and the combined sum passed on to the main analogue balanced output and A-D converter. To the right-hand side of the unit is a slot for the optional A-D card, as well as the IEC mains inlet with its integral fuse holder and mains voltage selector.

80 Front-panel Controls!

The ISA430 MkII front panel is even more complicated than the original design, thanks to the additional features and facilities which Focusrite have managed to cram on! I counted 80 separate knobs and buttons — so be prepared to spend more time than usual familiarising yourself with the operation of this product. Fortunately, things are mostly logical and intuitive, and the panel is clearly marked and well laid out. The signal-processing sections kick off with the input stage in the top left-hand corner. The transformer-balanced input circuitry features additional switched impedance facilities from the ISA428, and the Air feature from the Platinum-series Twin Trak Pro.

The preamp section includes a new Mic Air function, developed from a similar feature on the Platinum Twin Trak Pro, plus the facility to select different mic input impedances, previously only available on the company's ISA428 four-channel preamp.The preamp section includes a new Mic Air function, developed from a similar feature on the Platinum Twin Trak Pro, plus the facility to select different mic input impedances, previously only available on the company's ISA428 four-channel preamp.Photo: Mark Ewing

The panel controls include two rotary knobs for coarse and fine gain, plus four illuminated buttons which select high-gain mode, phantom power, the Mic Air function, and phase reverse. For the mic input, the switched gain control operates in 10dB steps between zero and 30dB, or 30dB and 60dB in high-gain mode — there is no pad switch. The line input gain can be varied from +10dB to -20dB, and both can be adjusted further, with the continuous fine gain control, from zero to +20dB. If the instrument input is selected, then this is controlled with the fine gain knob, its range spanning +10dB to +40dB.

Two more black buttons cycle through the mic, line and instrument sources and the input impedance options, which are identical to the ISA428: Low presents 600Ω, ISA is 1400Ω, Medium is a more usual 2400Ω, and High presents 6800Ω. The Mic Air feature has been borrowed from the Platinum Twin Trak Pro, but whereas that design used an electronically balanced input stage with an inductor circuit to emulate the 'air' of a transformer input, the ISA430 MkII uses the parallel inductor circuit coupled into the secondary of the input transformer. The idea is to accentuate the inherent HF resonance of the transformer, creating a brighter and more 'spacious' sound — hence the name.

Another change from the original ISA430 design is that the line input now passes through the transformer as well, but should a transformerless sound be required, the entire input stage can be bypassed by using instead the first insert point, which has an electronically balanced input buffer. The only drawback is that this route does not have a gain control, but that may not be a problem in practice.

Optional A-D Card

An optional A-D converter, with essentially the same design as that employed in the ISA428, can be installed to provide a two-channel digital output at up to 192kHz sample rates and with 24-bit word lengths. ADAT lightpipe, S/PDIF optical and AES-EBU outputs are provided, and the AES-EBU output can be switched to give a coaxial S/PDIF-formatted signal if required. However, to access the AES-EBU or S/PDIF coaxial outputs, a special breakout cable is required to interface with the nine-pin D-Sub socket. This cable is not supplied with the A-D card, and has to be purchased separately. Sadly, the pin configuration is not given in the handbook, making it difficult to construct your own breakout cable.

The ADAT port supports the SMux mode to transfer 96kHz audio if required, and the AES-EBU output can be configured to operate in either single- or dual-wire (double-wide) modes for all sample rates up to 192kHz. Word-clock I/O is catered for with the usual BNC sockets for external clock-synchronisation duties.

The AES-EBU to S/PDIF format conversion and single-/dual-wire modes are selected via buttons on the card's rear panel, but all the other functions are controlled from a section of the ISA430 MkII front panel. Three buttons toggle through the various options: the first cycling around the six sample rates (44.1, 48, 88.2, 96, 176.4, 192kHz); and the second selecting the word length (16, 20 or 24 bits). Another button determines the clock reference (internal, external or external x256 Super Clock).

Comprehensive Equalisation

Moving on across the front panel, another pair of illuminated buttons configures the first insert point, and this is followed by the equaliser section, which is divided into three separate elements. The first provides high-pass and low-pass filters; the second a pair of parametric mid-sections; and the third high and low shelf sections. All three elements can be independently bypassed or switched into either the compressor or gate side-chains for frequency-selective dynamics processing — such as reducing mic pops from wind blasts, or implementing creative dynamic EQ. The entire equaliser section can also be bypassed with another illuminated button.

The flexible EQ section incorporates separate filtering, parametric EQ, and shelving EQ blocks, each of which can be switched independently into the dynamics side-chain if required.The flexible EQ section incorporates separate filtering, parametric EQ, and shelving EQ blocks, each of which can be switched independently into the dynamics side-chain if required.Photo: Mark EwingThe two filters span 20Hz-1.6kHz and 400Hz-20kHz, respectively, with 18dB/octave slopes, and allow excellent control of out-of-band signals, as well as providing useful band-pass filtering for dynamic side-chain applications. The mid-band parametrics are as comprehensive as ever, with ±18dB boost or cut ranges, fully variable Q (bandwidth), and usefully wide and overlapping frequency ranges. The lower band can be adjusted from 40Hz to 400Hz, or 120Hz to 1200Hz if the x3 option is switched on. The upper band spans 600Hz-6kHz, or 1.8-18kHz with the x3 mode.

The shelving section boasts two new frequency positions to make the ISA430 MkII a little more flexible, but without compromising the fundamental ISA110 circuit design. The low shelf turns over at 20, 56, 160 and 460Hz in standard mode, and 33, 95, 270 and 655Hz in high range. The high shelf is provided with frequencies of 1.5, 3.3, 6.8 and 15kHz in standard mode and 2.2, 4.7, 10 and 18kHz in the high range. Again, these are well-chosen and flexible options.

Dynamics Controls

The dynamics facilities are all arranged along the bottom half of the control panel, starting off with three illuminated buttons to configure the second insert point. Next along is another button to determine the position of the dynamics processing within the machine's signal path. The default is post-EQ, with options of pre-EQ and post-sum. The former should be self-explanatory, while the latter means that the dynamics section is moved right up to the output of the unit, immediately prior to the soft limiter, but after the output level control and, more importantly, after the external ADC input, which can be summed to the final output. This mode enables the dynamics section to control the mix of internal and external signals, if required.

The compressor section has been updated with two alternative gain-control elements. The default mode uses the familiar Focusrite VCA design, employing discrete Class-A circuitry, but the alternative provides a high-speed optical element for gain reduction which brings a distinctive vintage sound quality to the dynamic control. Furthermore, the circuit topography of these two modes is entirely different. The VCA circuit uses a feed-forward side-chain which controls the VCA from the input signal, whereas the optical circuit uses a feed-back side-chain, controlling the light-dependent resistors from the output signal. These provide very different sonic characteristics, adding to the colour and character of this unit.

The compressor controls comprise the usual set, but the control ranges change between VCA and optical mode. For the VCA configuration, the threshold spans -28 to +12dBu; the ratio ranges from 1.5:1 to 10:1 with a soft knee; the attack is variable between 100µs and 100ms; and release varies from 100ms to 7s, with an optional automatic programme-dependent mode. Up to 20dB of make-up gain can also be added. In the Vintage optical mode the threshold range is the same, but the attack and release times are fixed. The ratio is controlled in two stages: 1.5:1 to 5:1 in compressor mode with a soft knee, or 5:1 to 20:1 in limiter mode with a hard knee.

Two illuminated buttons bypass the compressor and allow the side-chain to be auditioned (useful when trying to fine-tune an equaliser in the chain), and a third activates the external key input for voice-over or auto-ducking applications, or frequency-conscious dynamic control using an external equaliser or some other control signal. However, the external key cannot be used with the vintage optical compressor mode.

Other new features added to the ISA430 MkII are Blend and Mix controls — borrowed from the ISA220. Pressing the illuminated Blend button causes the Mix control to mix the compressed signal with the original input signal, in any proportion. The idea is to make it easy to implement 'parallel compression' — a particularly subtle form of gentle 'bottom-up' compression widely used in classical music recording circles. However, because the expander/gate section shares the same gain-control element, using the Blend control will inherently reduce the effectiveness of the gate process, since a proportion of the original signal is being mixed back into the output signal. In general, though, I think it unlikely that anyone will want to use the gate at the same time as applying subtle parallel compression, so this conflict of interests is unlikely to be a problem.

The dynamics section comprises compressor/limiter (offering VCA and optical compression flavours), expander/gate, and de-esser.The dynamics section comprises compressor/limiter (offering VCA and optical compression flavours), expander/gate, and de-esser.Photo: Mark EwingThe expander/gate is equipped with controls for Range (0-80dB); Threshold (+10dBu to -40dBu); Hold (20ms-4s); and Release (0.1-5s). In addition, illuminated buttons select Bypass, Expander mode (the default is to operate as a gate), side-chain Listen, External Key, Fast Attack, and Hysteresis. The last increases the level difference between the gate opening and closing, which helps prevent the gate from 'chattering' on slowly changing signals. A row of five green LEDs shows the amount of gain reduction being applied.

The de-esser section is next in line, and this features just Frequency (2.2-9.2kHz) and Threshold controls (+10dbu to -20dBu). As always, illuminated buttons bypass the de-esser and enable the side-chain to be auditioned to make tuning easier. A red LED illuminates when gain reduction is applied, the intensity increasing with the amount of level reduction. The Focusrite de-esser is unusual in that it works as a subtractive process — the sibilant signal is detected in a side-chain and the unwanted element is then subtracted from the main signal so that the latter is affected only at the moment of sibilant reduction, resulting in a more transparent process.

The output section boasts a large output level control spanning +6dB to -60dB, with a calibrated unity gain mark (but no detent to hold the control in position). An illuminated button activates the External Sum mode to mix a signal connected to the rear-panel ADC Input 1 with the internal processed signal. The level of the external signal can be controlled with a smaller rotary knob, calibrated from zero to -60dB. A Mute button kills both the main and post-mic analogue outputs, but not the insert sends or the digital output. A Bypass button tucked away in the corner switches out the entire EQ and dynamics processing to enable a global processed/unprocessed comparison, to check whether quality improvements really have been made or if the source is just louder and brighter!

The last stage of signal processing is the soft limiter, activated by a button in the metering section to the right-hand side. The limiter circuitry is the same as that used in the ISA428, and has a preset threshold of -6dBFS (+16dBu) with a gradually increasing ratio, which starts at 1.5:1 and rapidly increases to infinity. This provides very effective overload protection for the A-D card, as well as clamping the analogue output to +22dBu.

Metering Facilities

The right-hand side of the unit carries a pair of vertical bar-graph meters which default to showing the output and input, respectively, of the ISA430 MkII signal-processing chain. The output meter shows the signal level at a point immediately prior to the A-D converter (if fitted), while the input meter shows the level at the output of the preamp (the input to the EQ and dynamics chain). Alternatively, pressing a pair of buttons below the meters, allows the A-D card inputs to be monitored instead.

Over to the left of the unit is a large VU meter, the calibration of which can be configured with a button such that 0VU equates to either +4dBu or +18dBu — the latter being more appropriate when monitoring the hot levels associated with professional digital recorders. A second button selects the metered source, with options of Input (in other words, preamp output); either insert return; and amount of compressor gain reduction. A fifth option is called up automatically when any of the dynamics-processor Listen buttons are pressed.

Should the signal level exceed +20dBu (or 6dB below clipping at any point in the circuitry), then the red overload LED below the meter illuminates. The signal is monitored at five key points in the signal path to ensure that excessive peaks in one part of the unit are not masked by lower levels in another. Unlike the ISA220, the status of all of the switches on the 430 MkII is remembered when the unit is powered up, minimising the frustration caused by accidental power outages, or when returning to a session the next day.

Insert Possibilities

Whereas the original ISA430 had a single, moveable insert, the MkII benefits from an additional insert point which has been introduced to provide more signal routing flexibility and to enable some extra operational configurations. As a result, the unit now boasts one fixed and one moveable insert, making the possible splits and configurations far more versatile.

When operating in standard mode, insert one remains before any of the signal processing — the equaliser and dynamics, regardless of their order — and after the preamp section. However, the same insert I/O ports can be linked to the equaliser section input and output using the EQ Split button in insert one's control block on the front panel. In this mode, the entire equaliser section is separated from the rest of the signal path (although individual elements can be switched to the dynamics side-chains, of course), and the output from the preamp section is routed directly to the dynamics processor.

The second insert point can be switched to any one of four different positions in the signal path. It can operate as an insert point between the EQ and dynamics sections, or after the dynamics and immediately prior to the output control. Alternatively, it can also be used to provide the I/O to the separated dynamics section with the Dynamics Split button.

Further flexibility (or complication, depending on your point of view) comes from the ability to move the dynamics section pre-EQ or post-sum. In the post-compressor insert position, if the dynamics block is routed post-sum, then the insert falls back to post-EQ (but still before the output section). On the other hand, if the dynamics have been switched pre-EQ, the insert moves to a position between the dynamics and equaliser sections. It may all sound very complicated, but the handbook explains the options well and it results in fantastic flexibility, allowing virtually any signal path to be constructed internally or externally.

On The Session

As with other Focusrite models, the pedigree of this unit is immediately evident in the clear, transparent, open and yet full-sounding signal path — the hallmark of a very high-quality preamp and processor. The addition of switchable input impedances will be appreciated by many, as a useful means to tailor the sound of a microphone without resorting to the equaliser. This facility impressed very much in the ISA428, and its translation over to the ISA430 is most welcome. Likewise the Mic Air facility, which I found extremely useful on almost everything! It is a powerful treatment which seems to add the illusion of a natural space around the sound, and I found it particularly useful when close-miking instruments, to restore some of the lost brilliance and 'air' — I can't think of a better word! — which is inherent in such an approach.

Next to the output-level controls are buttons allowing you to set the sample rate and bit depth of the optional A-D converter board. You can also activate the soft limiter from here.Next to the output-level controls are buttons allowing you to set the sample rate and bit depth of the optional A-D converter board. You can also activate the soft limiter from here.Photo: Mark EwingThe equaliser is essentially unchanged from previous versions, apart from the addition of a couple of extra shelf frequencies. The ISA110 equaliser is rightly acclaimed as a classic, and the pedigree shows again here. High-end equalisers like this seem to be able to make subtle tonal changes without destroying the homogeneity of the source in the way that many equalisers do. The provision of the three separate sections — filters, parametrics and shelves — and the ability to split them and allocate them to the dynamics side-chains, makes this an immensely flexible unit.

The compressor was always a key element in the 'sound' of the original unit, and although described as transparent by Focusrite, I always found the compressor had a subtle character all of its own. In his review of the original ISA430, Paul White described the compressor as sounding 'effortless and professional', and I think that sums it up admirably — except that it just became a whole lot better with the addition of the Vintage mode, providing a rich alternative to the VCA sound. Optical compressors have a very recognisable character, especially when worked hard, often ideally suited to drums and basses. To have the option of both here is pure luxury — and this heavenly state of affairs has been made even better with the inclusion of the Blend facility. I've always been a big fan of the parallel compression technique, and the arrangement offered here is a joy to use. Although I only had the ISA430 MkII for a week or so, I found I used this facility a great deal.

The gate is one of the nicest available, as well as being one of the easiest to use — it really just does the job asked of it without fuss, and operates in a reliable manner, especially with the Hysteresis mode engaged to reduce the possibility of false triggering on slowly decaying sounds. The ability to quickly flip the filters or parametric equaliser into the side-chain to tidy up the control signal is also very useful. I am always amazed at how hard the de-esser can be made to work with minimal side effects, and the provision of the side-chain listen facility makes finding the offending sibilant frequencies very straightforward.

The inclusion of the external input and summing mode will be useful to many, but personally I think the provision of external inputs to the optional A-D card is benefit enough. The soft limiter did the same sterling job here as it did on the ISA428. I tend to leave a modest headroom when I'm working with digital gear, but this optical limiter does a fantastic job of neatly controlling transient peaks without sounding heavy-handed about it. For the red-light junkies out there, this is your saviour — and best of all, most people will never know that the concept of headroom is completely lost on you!

A Channel To Die For

There was never any doubt in my mind (or I suppose anyone else's) that the ISA430 MkII was going to be something special. Taking the features of the original unit and adding all the new tweaks and technologies that the Focusrite back-room boys have developed subsequently has resulted in just about the best possible recipe for a producer channel. This is the kind of device that mere mortals will gladly die for, but only after they have enjoyed using it for a few decades, naturally! Although we are still talking serious professional gear prices, this is a very attractively priced unit in the UK, offering even better value for money than the original, with a stunningly high-quality, comprehensive and extraordinarily flexible package.

The ISA430 MkII has been designed to provide the most complete and superb-sounding channel path available, and it does that in spades. While a large percentage of sales will be to pro freelance producers and engineers as well as studios and remix rooms, I should think a similar volume will be sold to home and project studios that need just one really good front-end unit — and with the optional A-D card, the ISA430 MkII has to be one of the best and most versatile units on the market today. I can't think of anything negative to say about the ISA430 MkII — it really is that impressive. Committed recording engineers now have a new British standard-bearer when it comes to high-end analogue recording channels.


  • Switchable input impedances.
  • Mic Air inductor.
  • More usable EQ and additional insert options.
  • Vintage optical compressor mode and Blend feature.
  • Upgraded A-D card and I/O options.
  • Even more flexible and even better sounding!


  • A-D card breakout cable at extra cost.
  • More complicated to understand, if not to operate.


This unit is the British standard bearer when it comes to high-end analogue channel units for the committed recording engineer. The MkII enhances the original model in many ways, improving its sonic quality and flexibility with switchable input impedances, a Mic Air mode, and an alternative optical compressor topology.


ISA430 MkII £1874.13; optional A-D card £323.13; digital breakout cable £25. Prices include VAT.

Focusrite +44 (0)1494 462246.