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Focusrite ISA428

Four-channel Mic Preamp By Hugh Robjohns
Published May 2003

Focusrite's new preamp offers variable input impedance and an optional multi-channel A-D converter board.

The ever-expanding Focusrite range of preamps has just been swelled further with the arrival of the new ISA428 Pre Pack. This is a surprisingly heavy (7kg) rackmount affair which occupies 2U of rack space and extends roughly 250mm behind the front panel. The case is painted black, while the front panel is finished in the classic Focusrite blue colour scheme with grey control sections, yellow knob caps and plenty of LEDs. Dominating the front panel, though, are four moving-coil meters which have been designed with a very fast reaction time and a slow decay to provide an almost peak-reading ballistic, and these show the available headroom.

Focusrite ISA428Photo: Mike Cameron

The meters are scaled from -26dB to +2dB with the 0dB point equating to +22dBu at the analogue outputs. An overload LED built into each meter flashes when the signal exceeds this level, although the analogue circuitry actually clips 6dB higher at around +28dBu. The 0dB meter level also equates to the maximum input level for the optional A-D converter card.

Each of the four preamp channels is equipped identically and the rear panel bestows each with a balanced mic input XLR socket, a balanced line input TRS socket, a further pair of balanced TRS sockets to provide an insert send and return, and an XLR to carry the balanced channel line output. In addition, the front panel carries four more quarter-inch sockets which accept unbalanced DI instrument inputs, one for each channel. The impedance of these instrument inputs is over 1MΩ.

Controls & Facilities

Moving on to the channel controls now, three illuminated buttons arrayed to the left of each channel's meter engage phantom power, activate a polarity inversion and enable the insert return. Below the meter is one rotary switch and a pair of continuous rotary controls. The switch sets the coarse gain, with the range depending on whether the mic or line input is selected, and another illuminated button increases the gain of the mic amp by a further 30dB. The course gain can be set in 10dB steps from zero to 60dB for the mic input, and from -20dB to +10dB for the line input. To allow finer adjustment of the gain, a continuously variable Trim control provides additional gain from zero to +20dB. The combination of maximum course and fine gain settings therefore provides a whopping 80dB of mic gain!

Focusrite ISA428£1299
  • Classic Rupert Neve mic preamp design.
  • Transformer input with 80dB mic gain.
  • Variable high-pass filter.
  • Variable mic input impedance.
  • Optional eight-channel 24/192 A-D card with transparent limiter mode.
  • Resolution adjustable to 16, 20 or 24 bits.
  • ADAT lightpipe with SMUX mode for 96kHz rates.
  • AES outputs with single-wire and dual-wire modes.
  • Erm... it's quite heavy, I suppose!
All in all, a very nicely thought out package that deserves to do very well.


Beneath the Trim control another button toggles through the three input options — mic, line and instrument — with LEDs to indicate the current selection. With the instrument input selected, the coarse gain control becomes inactive and the Trim control adopts an alternative gain structure which spans +10dB to +40dB.

A second button below the Trim control provides one of the more unusual facilities of the ISA428 — a variable input impedance for the microphone stage. Changing the loading here affects different mics in different ways and is somewhat unpredictable, but adds to the range of tonal colours which can be obtained without resorting to EQ or dynamics. In general, the higher impedance settings tend to provide more signal level and a more open and slightly smoother sound, while lower settings tend to emphasise frequency response peaks and reduce the signal level from the microphone. The button cycles through four options: Low (600Ω); ISA110 (1400Ω); Medium (2400Ω); and High (6800Ω). These different impedance values are obtained by selecting different taps on the microphone input transformer.

The microphone preamp circuitry is the same Rupert Neve design developed for the original Focusrite consoles and the early ISA110 modules. Unlike many of Mr Neve's more recent transformerless designs, this circuit uses a microphone input transformer which provides 20dB of the entire stage gain. The specifications suggest a low noise floor (EIN of -128dBu at 60dB gain with a 150Ω terminating impedance) and a typically very wide bandwidth (-3dB at 30Hz and 120kHz). The final channel control is a variable high-pass filter with a usefully steep 18dB/octave slope. The corner frequency can be set anywhere from 16Hz to 420Hz and there is a Filter In button to engage or bypass the filter so that its effect on the material can be easily auditioned.

Optional A-D Converter

Although the ISA428 is available as a straightforward four-channel analogue-only unit, I suspect many purchasers will also choose the optional eight-channel analogue-to-digital converter card, since this adds considerably to the functionality of the unit with a relatively small financial penalty. Analogue-only units can be field-upgraded at any time by simply purchasing the converter card and plugging it in — a simple five-minute DIY operation.

Focusrite ISA428Photo: Mike Cameron

It may initially seem odd having an eight-channel A-D card in a four-channel unit, but there is some sense in the madness! Many multi-channel digital interfaces work in eight-channel blocks (such as the ADAT interface), and the ISA428 makes the 'spare' channels of A-D conversion available for four analogue line-level inputs, allowing the ISA428 to serve as an eight-channel interface for a DAW or digital recorder. For those who require eight mic inputs to a digital system, all you need is a second ISA428 and some XLR patch cables to hook the line outputs of the first unit to the line inputs of the second unit's converter card.

The four ISA428 preamp channels are routed through the first four digital outputs, while channels five to eight are used to encode four external fixed-level balanced analogue line inputs via dedicated XLRs on the rear panel. The input sensitivity for these line-level inputs is set such that a +4dBu input corresponds to -18dBFS on the digital output (and 0dBFS equates to +22dBu). The digital outputs are presented on duplicated ADAT lightpipe ports and two D-Sub connectors carrying AES-EBU or S/PDIF data streams. There are no TDIF facilities.

I skipped over the right-hand side of the ISA428 front panel earlier, because the controls located there are only operational if the converter card is installed. This section comprises four buttons and two rows of four bar-graph meters which show the digital output levels of all eight A-D channels. If the card is not installed, the top row of bar-graph meters are used to display the analogue output levels of the four channels instead. Once again, the 0dBFS LED corresponds to +22dBu on the analogue output, and the -18dBFS lamp equates to +4dBu.

Three of the four further buttons have associated LEDs to indicate their status, while the fourth is an illuminated button. The bottom button selects the word-clock source for the A-D card; the options being external word clock or an external 256x Super Clock. Each mode is indicated by an LED, while a third illuminates when the unit's clock is stable and locked. The converter card carries a pair of BNC sockets on the rear panel. One accepts an external word clock or Super Clock, and the other outputs the machine's own word clock.

Focusrite ISA428Photo: Mike Cameron

The next button selects the A-D output bit depth with options for 24, 20 and 16 bits. It is nice to have the comprehensive range here which allows the ISA428 to be used with a wide range of current, future and, more importantly, legacy equipment. A lot of converters seem to omit the lower resolutions these days, restricting the equipment with which they can be used. The third button selects sampling frequency, with all the standard rates from 44.1kHz right the way up to 192kHz, each with its own indicator LED. While standard and double-speed rates are in common use, the quad rates are still relatively rare, Pro Tools HD being one of the better-known DAWs to offer this facility. Not all digital interfaces can accommodate such high rates, and various bit-splitting and sample-splitting modes have to be used, of which more in a moment.

The final button in this section engages a soft limiter across all eight A-D channels. This is based on an opto-limiter circuit and is intended to prevent digital peak overloads by progressively increasing the compression ratio over the last 6dB. The compression starts at 1.5:1 between -6dBFS and -4dBFS, progressing on to 2:1 for the next four decibels and to infinity:1 for anything over. Thus it is impossible to overload the A-D converter input. The digital outputs are presented on a pair of ADAT lightpipe sockets and a pair of nine-pin D-Sub sockets which carry AES and S/PDIF signals — a couple of buttons adjacent to these sockets reconfigure their operating modes.

When operating with 44.1kHz and 48kHz sample rates, both ADAT lightpipe ports carry all eight channels simultaneously, and can be used to feed two eight-track recorders, for example. At 88.2kHz and 96kHz sample rates these ADAT ports cannot carry eight channels of high-rate data, and so are automatically switched to operate in the standard SMUX mode, where the first port conveys the first four channels and the second carries the remainder. There is no standard for ADAT lightpipe interfaces at 176.4kHz or 192kHz sample rates, and so these outputs are muted when these rates are selected.

The two nine-pin D-Sub sockets are intended to output AES data, although the first socket can be switched with a small button to carry S/PDIF data instead. The second D-Sub connector always outputs AES signals. A special cable harness is required to access the S/PDIF data stream, and is fitted with phono connectors instead of the AES harness's XLRs — these harnesses are not supplied with the converter card and must be purchased separately. Unfortunately, Focusrite didn't supply one with the review machine either, so I was unable to test the AES ports or the 192kHz sample-rate modes.

A second button located between the two D-Sub ports selects either single- or dual-wire operation when working at elevated sample rates. In the single-wire mode each connector outputs all eight channels at any sample rate up to 192kHz. However, not all equipment is capable of working with such high-speed data and some requires the data to be distributed across two cables, thereby halving the raw data rate. Since a lot of users will probably acquire the ISA428 to use with a Pro Tools HD system, it is worth noting that the latter employs the single-wire AES mode for sample rates up to 96kHz, and the dual-wire mode for sample rates over 96kHz.

The ISA428 In Use

As with most Focusrite equipment, the ISA428 does exactly what you expect it to, in the way you expect it to. I am a fan of the Focusrite Red 1 preamp and (from memory) this new unit seems to share the same qualities, with the same rich, warm kind of sound that also manages to be detailed and transparent at the same time. The sound is not entirely neutral — it has a subtle but recognisable sonic character — but it's more musical than coloured, if that makes any sense, and it seems somehow to complement the sound of whatever is passing through it: voices, acoustic instruments, guitar pickups, or whatever. It passed the twelve-string guitar test with consummate ease, where many mic preamps fall foul of the complex harmonics and challenging string interactions.

All the inputs and outputs maintain absolute phase, and there is a massive amount of headroom in normal use. The line input is clean and bulletproof and the instrument input seems to work well with guitar pickups and piezos, with a sensible gain range, although I found the socket a little loose with Neutrik jack plugs. I didn't have any problems with the connection, its just that there wasn't a particularly positive 'clunk' when pushing the plug fully home. The rear-panel line and insert sockets were far better in that respect.

The mic amp is all that you would expect. It's quiet, clean, dynamic and detailed. In a direct comparison with my reference GML mic preamps it was a little thinner at the bottom end, but only very slightly, and it had very similar levels of detail and resolution. I think the GML might have had the edge when it came to noise at gain settings around 60-70dB, but again there wasn't much in it.

However, the GML is a no-frills preamp and, good though it certainly is, it doesn't include such real-world necessities as polarity inversion and high-pass filtering, and this is one area where the ISA428 scores well. In fact, the variable filter on the ISA428 is very useful indeed, although I would have preferred a smaller frequency range to make the control easier to set accurately. Likewise, the ability to insert an external equaliser or dynamics processor in each channel before the A-D converter is another welcome facility. The insert send is active at all times, by the way, providing a second analogue channel output, if required.

I've come across a few mic preamps with adjustable input impedance, and I find this an intriguing facility. The cynic in me says it's just a fashionable accessory, but there is an audibly creative aspect to it, bringing subtle tonal changes from most microphones. Selecting the higher impedance settings generally increases the output level from most microphones by as much as 4-5dB, simply because of the reduced loading on the microphone's output transformer or electronic drivers. Dynamic and ribbon mics seem to benefit most from this, although I also noticed a level increase with some condenser mics. The other very noticeable effect with higher impedance settings was an increased high frequency response, making the ambient sound of an acoustic space a little more obvious, especially when distant mic techniques or long cables were used.

Selecting the lower impedance settings almost always seemed counter-productive, reducing the signal level and producing a slightly more boxed-in, coloured and congested sound — particularly with typical dynamic mics (the Shure SM58 seemed particularly sensitive to the loading impedance and its character changed quite dramatically). However, I should say that, while the changing sonic qualities can be strikingly obvious when cycling through the impedance options, they are fairly subtle when heard in isolation.

Hooking the ADAT lightpipe up to a Yamaha digital mixer provided a high-quality digital signal, and the bit resolution and sample rate switching all seemed to work well. In terms of technical performance, the card at least equals — and in most cases out-performs — other comparable converters. For example, it boasts a signal-noise ratio of 120dBA and the distortion is below 0.0004 percent. These impressive figures are achieved in part by the Class-A analogue circuitry of the ISA428 and the very short signal path of the A-D stage.

The converter card appears to have a good clock system, as synchronising the ISA428 to an Aardvark Aardsync master clock generator made little if any difference to the sound quality. Personally, I'm not a huge fan of soft limiting on A-D converters and I never use the limiter on my Apogee PSX100, for example. I prefer to leave a safe headroom margin and under-record by a few decibels rather than have squashed transient peaks! However, a lot of people like to illuminate all the bar-graph LEDs when recording, and for those users the limiter function will be very useful, preventing serious A-D overload and the horrendous aliasing distortion that comes with it. The optical limiter circuit Focusrite have used in this converter card seems to be pretty bombproof, holding even seriously overdriven signals under the 0dBFS threshold. It also sounds remarkably clean while doing it, and if used in a more normal manner to catch the odd over-enthusiastic transient it is completely transparent.


I liked the ISA428 very much indeed. It looks good, sounds fantastic, is completely bombproof and is a joy to use. It's significantly cheaper in the UK than the Red 1, yet boasts a couple of unusual facilities and gives little if anything away in sonic quality. The only criticism I could level at the review unit was that it became pretty hot around the power supply. However, it turned out that the unit I had was an early production model and that all new units are fitted with a heat sink to help keep the temperature down. As a stand-alone mic preamp, this unit is very capable indeed, but with the optional converter card installed it makes a very competent and cost-effective eight-channel (four mic, four line) front end for a DAW or digital recorder.

infop.gif ISA428, £1299; optional A-D converter board, £499. Prices including VAT.
infot.gif Focusrite +44 (0)1494 462246.

The Technology Behind The ISA428

Focusrite microphone preamps have a reputation for transparency and, when used with an appropriate source, warmth. In the case of the ISA428 this is obtained by using a circuit designed by Rupert Neve back in the early '80s, which demonstrates his unique awareness of subjective as well as objective qualities in audio. A lot of audio equipment rolls off frequencies outside the recognised audio bandwidth, but this can have an audible effect on the higher-frequency harmonics of a signal. Therefore, to capture subtle musical nuances in the source the ISA428 operates over a very wide bandwidth. By pushing the high-end roll-off well outside the audio band the phase response across the audible range is far more linear, and this ensures the required accuracy.

All of the main inputs to the ISA428 are transformer coupled and, although this is an expensive option, it has the added advantage of providing signal gain without active devices. Lundahl 1538 transformers are used in the ISA428, which were specified by Rupert Neve, along with a bespoke 'Zobel network' to control the inherent high-frequency resonance of the transformer. This Zobel network is also used in the RED preamps and Rupert Neve tuned it by ear, which is probably an important element of the 'Focusrite sound'.

Although the preamp design draws a lot from Focusrite's heritage, the soft limiter within the optional A-D converter board is a new design. It uses a feed-forward opto-circuit designed specifically for the ISA428, incorporating a new generation of fast-reacting custom optical devices unique to Focusrite. The circuit works by squashing the last 12dB of headroom into a 6dB space, with instant attack and instant release time constants. Whereas many protection circuits can sound harsh, this optical design sounds particularly pleasant and transparent, even when working hard.