For the first time, Focusrite have married their analogue heritage and their digital audio expertise in a single product.
Rupert Neve’s Input Signal Amplifier circuit has been a constant throughout Focusrite’s 41‑year history. Developed to meet the exacting requirements of Sir George Martin, it formed the front end of the Forté and Studio consoles, and the heart of numerous standalone preamps and channel strips. During the same period, the company have become a household name for their computer audio interfaces, thanks to ranges like the Saffires, Claretts and Scarletts. Until now, though, the two strands of Focusrite’s business have never crossed.
Most current ISA products have digital connectivity, but it’s a secondary feature, and in some cases a cost option. The fourth‑generation Scarletts have some features derived from the ISA line. However, there has never previously been an audio interface with Focusrite’s premium preamp circuit built in. The new ISA C8X is thus a significant product for the British manufacturer, and one into which they’ve poured a huge amount of development resources.
True Blue
Like the ISA 828 and 428 rackmounting preamps, the C8X is a 2U box, providing double the front‑panel space of the Scarlett or Clarett interfaces. The ISA range’s royal blue colour scheme is enhanced here with a subtle fine‑grained sparkle and chunky contoured face plate. The other dominant colour on display is gold, which is used for most of the indicator LEDs, while the meters light up in a paler blue colour. There are lots of further nice visual touches, too, such as the illuminated ff logo that is echoed in the perforated side panels. This really is a very attractive device indeed — and the C8X doesn’t only look good. It also feels good, thanks to the beautifully weighted encoders with LED halos. With six encoders, 10 meters and no fewer than 30 buttons, it’s a busy front panel, but not crowded or confusing.
The bulk of this panel space is taken up by controls relating to the inputs, which are divided into two groups. The ISA C8X has eight analogue inputs, all of which have mic preamps. But whereas the preamps for inputs 3‑8 are similar to those in the Scarletts, the first pair implement Focusrite’s ISA design in its full glory. Its fullest glory yet, in fact, since the complement of features on offer here is greater than on any other ISA preamp to date. Compared with previous ISA units, there are two really big developments: complete digital control, with an auto gain‑setting function, and an entirely new feature called Console Mode. These complement numerous existing sound‑shaping features, including variable input impedance, high‑pass filters, an ‘Air’ option, and switchable, balanced insert points.
The left‑hand side of the front panel presents meters for both ISA channels, plus a complete set of controls for one, with buttons that let you select which you’re adjusting. Pressing both together links the channels. Like many of Rupert Neve’s preamp designs, the analogue ISA units have a main gain knob that is stepped, augmented by a continuous trim control. That paradigm is carried over to the C8X, but instead of rotary switches, the coarse and fine gain controls are now stepped encoders. This delivers a clear ergonomic improvement over the analogue versions: there’s no need for an additional button to switch between high and low gain ranges, and because the encoders are endless, you can set gain using only the fine control if you want.
The C8X is the first Focusrite product to feature digitally controlled ISA preamps.
A similar control paradigm is employed for the other six input channels, whereby you choose which preamp(s) you want to adjust and then use a single set of controls to make changes. The options here include switchable Air mode and variable Harmonic Drive. To adjust the amount of drive, you hold the button until its LED turns blue, then turn the gain encoder. Again, each of the six ‘standard’ preamp channels has its own input level meter.
The rightmost third of the front panel is given over to monitor control, with endless encoders for the main volume and both headphone outputs. There are dim, mute and mono buttons, plus a talkback toggle that works like that of the Scarlett 18i20. Unlike those of the Scarlett, the headphone outputs have individual mute buttons as well as dedicated volume controls. It’s also possible to assign up to two pairs of alternate main monitors, with buttons available for switching.
Round The Back
The ISA C8X’s digital backbone is similar to that of the Scarlett 18i20. It connects to the host computer over USB, and in addition to its eight analogue inputs it sports 12 analogue outputs, stereo S/PDIF I/O and two pairs of ADAT optical sockets. The second pair of ADAT sockets can’t be used at the same time as the S/PDIF I/O, so the total I/O count is 26 on the input side and 28 on the output side at base sample rates.
What is rather different from the Scarlett 18i20 is the way in which all this I/O is arranged. The only front‑panel sockets are two pairs of quarter‑inch jacks. Those at the left offer high‑impedance inputs for direct injection of electric guitars, and the pair to the right are headphone sockets. All the other sockets are on the rear panel, and the C8X’s more professional orientation is apparent from the complete absence of combi sockets. The eight analogue inputs all have separate XLRs and balanced jacks, as does the first output pair, the remaining outputs forming another bank of 10 jacks.
The point of having dual sockets for all the inputs is to allow the ISA C8X to live in a rack and meet the outside world through a patchbay. For obvious reasons, you don’t want to be plugging and unplugging cables around the back of the interface every time you switch between mic and line sources, and the separate XLR and jack sockets allow both types to be permanently connected. The C8X design also routes the line inputs direct to the A‑D converters without passing through the mic preamps, so the gain setting does not affect them.
Compared with the largest Scarlett interface, the C8X has an extra pair of analogue outputs, and it also has additional options as to what to do with them. You can specify two pairs of alternate speakers rather than just one, and multiple outputs can be ganged under the control of the main volume encoder for basic surround monitoring. However, there’s no provision for speaker calibration, so the delays and EQ necessary for immersive audio will need to be handled elsewhere in the chain. The complement of rear‑panel I/O is completed by BNC word clock in and out, MIDI in and out on five‑pin DIN, a single USB Type‑C socket, and an IEC mains inlet.
The ISA C8X’s comprehensive digital control means that almost every parameter is both editable and recallable from software, as well as from the front panel. Software editing is done from the same Focusrite Control 2 app that partners the Scarletts, and which supports remote editing from a tablet on the same WiFi network as the host computer. This is also where up to six low‑latency monitoring mixes can be set up, and where features such as alternate speaker switching and loopback routing are configured. Focusrite Control 2 is by now a very slick and mature piece of software, and if it’s less powerful than something like RME’s TotalMix FX, it is also much less intimidating.
Air Conditioning
When the ISA circuit was introduced in the mid‑’80s, it marked a step forward compared with previous console preamp designs. It was cleaner, lower in noise, and less prone to distortion. At the time, that seemed like progress! Today, we have easy access to transformerless preamps that offer an even closer approximation to ‘straight wire with gain’, and perversely, much of the appeal of the ISA design now stems from the ways in which it doesn’t quite achieve this ideal. In the C8X, Focusrite have added some creative new ways to extend and exploit that character.
The principal source of distortion in the ISA circuit is the Lundahl LL1538 input transformer. Originally chosen by Rupert Neve as being the most sonically transparent option, it nevertheless exhibits some non‑linearity, especially at low frequencies. Since the transformer is the first thing that your mic ‘sees’, its contribution is independent of the gain setting. Turning up the gain doesn’t give you more transformer saturation: it just makes the output hotter, and eventually drives the op‑amps into clipping, which probably isn’t the kind of coloration you want. So, although existing ISA products offer a form of tone‑shaping courtesy of their variable input impedance, they don’t allow the amount of saturation to be varied. Whatever reaction your signal provokes from the input transformer is pretty much what you get; and in my experience as a long‑term owner of an ISA 828, that tends to be very subtle.
With the C8X, Focusrite have introduced a new way of exploiting the character of the ISA circuit. Console Mode, as the name suggests, is intended to mimic the cumulative saturation effects that stack up as a signal passes through multiple transformers and amplification stages in a large‑format mixer. That coloration is emulated here using an additional circuit that adds harmonic distortion, with a drive setting that varies gain on the way in and attenuation on the way out to keep the subjective signal level constant. Console Mode is available for all three input types, not only microphones. It should also be noted that instrument signals also pass through the input transformer in this implementation of the ISA circuit, which is not the case on the ISA 828 for example.
All eight of the C8X’s analogue inputs feature separate XLR and jack inputs, equipping it for easy use with a patchbay.
The ISA C8X input path offers the same choice of four impedance settings found on the 828. Recent Scarlett models, and the ‘clean’ preamp channels on the ISA C8X, additionally feature an option labelled Air, intended to recreate the ‘mic air’ option from Focusrite’s classic ISA 430 MkII channel strip. The C8X’s two ISA channels have a button labelled 430, which introduces a similar 10kHz boost — but here it’s achieved in the same (expensive!) way as on the original 430 MkII, using an inductor.
Weights & Measures
The extent to which the ISA design pushes the limits of what’s possible in a transformer‑based circuit is apparent from the C8X’s specifications. The two ISA channels even marginally outperform the other six in terms of dynamic range (117dB and 116dB respectively, A‑weighted) and equivalent input noise (‑128dBu and ‑127dBu respectively, again A‑weighted). THD is significantly greater on the ISA channels, even with Console Mode switched off, but then THD is part of the reason you’re buying an ISA device anyway. The ISA channels offer a 79dB gain range, while the others span 69dB, in both cases configurable in 1dB steps.
In terms of conversion and line‑level circuitry, the C8X offers exemplary measurements. The line outs deliver a whopping 125dB dynamic range, with THD+Noise barely measurable at ‑113dB or 0.00023%, and the line inputs are no slouches, measuring 120dB and ‑105dB respectively. If, like me, you think that A‑D and D‑A conversion is basically a solved problem and has been for a while now, the ISA C8X certainly won’t change your mind! Unlike the Scarletts, the C8X operates at a professional +24dBu line‑up level, with the exception of the insert points on the two ISA channels, which are aligned such that 0dBFS=16dBu. This is the same as the maximum input level on the mic inputs, but may mean you need to dial in lower threshold values than you’d expect when patching in a compressor.
One possible issue that might arise in a unit featuring two different input types is unwanted variation between them. You wouldn’t want or expect the ISA channels to sound exactly the same as the other six — otherwise what would be the point? — but equally, you wouldn’t want other discrepancies creeping in. My tests revealed no timing or phase offsets between the two different input types, and Oblique Audio’s RTL Utility reported identical round‑trip latency figures through both. On recent Apple Silicon machines, it should be practical to achieve round‑trip latency of around 4ms; on my older Intel Mac, I got 6ms at 44.1kHz with a 32‑sample buffer size. Typically, when identical gain settings are used, a mic signal captured through the ISA input is 3 or 4 dB hotter than the same signal captured through the other inputs, but this is consistent throughout the gain range and depends on the impedance setting for the ISA channel. Focusrite’s Auto Gain algorithm also works flawlessly on both input types, regardless of Console Mode settings (although they haven’t implemented the Clip Safe element on the C8X). You wouldn’t want to mix and match the two input types across a stereo pair, but otherwise, it’s all good.
And, most importantly, it sounds good. I have other preamps with variable saturation built in, but in practice, I rarely use it. Turning it up to the point where it makes an audible difference usually leaves me worrying that I might have pushed it too far and broken my recordings in a way that I’ll later regret. And whilst the Air option on Scarletts is undeniably appealing, it often strikes me as just a little too much. By contrast, Console Mode and the 430 option on the C8X’s ISA channels are spot on. It’s not that you can’t push them too far, although they rightly tend to the subtle. I just somehow felt confident about dialling in the right settings, and knew instantly whether they were working for a given source. At one end of the scale, Console Mode is practically imperceptible. At the other, it adds a welcome thickness and forward quality to miked signals, and begins to get a little bit hairy on sources with lots of low‑frequency content, but you never run into that situation where the setting that sounds right on quiet bits starts to clip audibly on loud transients. And 430 mode has a glorious sweetness that I don’t think the Air option quite captures. It’s not appropriate for everything, but on the right vocal, it’s magical.
The C8X can lay good claim to featuring the most comprehensive implementation of the ISA preamp design yet.
All in all, thanks to the development of digital control and Console Mode, plus the inclusion of the 430 option, insert points and variable input impedance, the C8X can lay good claim to featuring the most comprehensive implementation of the ISA preamp design yet. The only obvious omission is the polarity button found on the 828 and other all‑analogue designs.
The Great Divide
When I reviewed SSL’s 18 last year, I noted that there’s a fault line in the market for audio interfaces. On one side, we have the ‘prosumer’ or serious home studio market, which is dominated by USB interfaces. These are typically expandable using ADAT, but only up to a point, and can’t usually be cascaded. On the other side, Thunderbolt and Ethernet interfaces such as Focusrite’s own RedNet range cater for huge channel counts, and to users who are serious about immersive monitoring.
Given the pro heritage of the ISA preamp design, some might be surprised that Focusrite have positioned the C8X on the former side of the divide. It has the usual upper limits on I/O count common to interfaces using the XMOS platform, which are no problem in most home and project studio scenarios, but which mean it won’t be a suitable choice for a pro studio running a large console. But in practice I think this makes sense. Pro studios typically don’t want their mic preamps built into their interfaces, and if you’re working in one, you’ll have no trouble integrating an ISA 828 or similar. The serious home studio market is much bigger, and the C8X ticks pretty much all the boxes. The two ISA channels rival any ‘boutique’ mic preamp on the market, the other inputs are superbly specified too, and unless you’re working in Atmos, it has all the monitor control and cue mixing functionality you’re likely to need in such an environment. It also supports standalone operation, so you can use it as an ADAT expander too; and on macOS, multiple Focusrite USB interfaces can be aggregated using Core Audio.
The Focusrite Control 2 software is very slick and mature, and boasts the useful capability of being remotely controlled from a tablet on the same Wi‑Fi network as the host computer.
The only criticisms I can muster are very minor. Some would prefer the insert points to operate at the same +24dBu line‑up level as the rest of the I/O, and the meters and halos on the encoders pursue a stylish ‘looking a bit like plasma meters’ aesthetic at the expense of being rather vague. Personally I’d rather have plain multi‑segment LEDs, but this is hardly a deal‑breaker. And considering what’s on offer, I think the C8X represents very fair value for money. You could assemble a cheaper rig with similar channel count and two ISA inputs by pairing the Scarlett 18i20 with Focusrite’s ISA Two preamp, but in practice, it really wouldn’t be the same. On the preamp side, you’d miss out on Console Mode, digital preamp control and the 430 option, and as an interface, the C8X betters the Scarlett in numerous ways.
When I first heard about the C8X, I wondered whether it would turn out to be a generic USB interface with a couple of ISA preamps bolted on. It absolutely isn’t. Both from my own testing and from conversations with Focusrite’s engineers, what really stands out is the attention to detail that has gone into its creation. From the weighting of the encoders to the bevelling of the front panel to the implementation of the converters, everything has been thought through with immense care. It’s almost as though the designers have felt a responsibility to go the extra mile on a product that brings together the two strands of Focusrite’s heritage after four decades. The ISA C8X proves that it’s not only small boutique manufacturers that can design hardware with passion, imagination — and love.
Alternatives
Audio interfaces with ‘boutique’ mic preamps built in include Heritage Audio’s i73 Pro Edge, Neve’s 88M and 1073 SPX‑D, and the Icon/Harrison 32Ci, but all are very different from the C8X. Perhaps the most obvious rival is Universal Audio’s Apollo x6, which features two of UA’s Unison modelling preamps in a 1U multi‑channel interface, albeit connecting via Thunderbolt rather than USB.
Pros
- Focusrite’s ISA preamp gets full digital control for the first time.
- Console Mode and the 430 circuit provide a broad palette of great‑sounding tone‑shaping options.
- I/O configuration designed for permanent installation and use with a patchbay.
- Slick and easy‑to‑use software front end with remote control.
- Built‑in talkback, speaker switching and monitor control.
Cons
- The choice of USB for connection and ADAT for expansion helps keep the C8X affordable, but limits I/O count.
Summary
It’s taken a long time for Focusrite to bring to market an audio interface with their ISA preamps built in, but the wait was very much worth it.
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