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Solid State Logic SiX

Analogue Mixing Desk
By Hugh Robjohns

Solid State Logic SiX

It may be the smallest mixer they've ever made, but the SiX shares more than just heritage with SSL's legendary large-format consoles.

To many, the large-format SSL mixer represents the very quintessence of the recording console. But although still a very important part of SSL's business, the market for such consoles has changed — the majority are probably now sold to educational institutions and very well-heeled personal studios, rather than commercial music facilities. Smaller project studios now drive much higher volumes of pro-audio gear sales, of course, and these typically have very different requirements. SSL have catered for this market for quite some time, with several more affordable, compact, but very high-quality mixers and outboard designed for contemporary DAW-based workflows.

More recently, however, SSL embarked in an interesting new direction, with their impressive Fusion Stereo Analogue Processor (reviewed in SOS December 2018). This was a novel product for SSL, not only because it introduced new functionality and addressed new applications, but also because it was the first to be manufactured wholly in China. Most of SSL's product lines are built from sub-assemblies manufactured in Britain but, since SSL's acquisition by the Audiotonix group (in which SSL now sit alongside Allen & Heath, Calrec, DiGiGrid, DiGiCo and Klang:technologies) the company have had access to a very high-quality manufacturing facility in China. As a result, SSL are now developing a range of products that wouldn't have been viable if built with their established manufacturing techniques. This promises to bring some seriously high-quality gear within reach of project-studio users. The Fusion has already proved very successful, and now SSL have embarked on their second overseas adventure: the new SiX console.

SiX Of The Best

Apparently, the SiX (so named due to its input channel count) was conceived a couple of years ago, but initial build costs meant it was forecast to retail at over £2000 (about $2500), which was thought to make it commercially unviable. Building it in China has allowed a much lower manufacturing cost without sacrificing component or build quality so, after a year in production development, the SiX is now a welcome reality.

The design is classic SSL, with ultra-clean 'SuperAnalogue' circuitry throughout, mostly derived directly from existing SSL consoles. DC servos are used to manage inter-stage coupling, so there are no electrolytic capacitors in the signal path, and the SiX's high-end performance is clear from its impressive technical specifications — which I confirmed though tests with an Audio Precision analyser. For example, the maximum output level is +27.5dBu, and the worst-case noise floor (ie. with all channels routed to the mix bus) is better than -85dBu, giving a dynamic range of around 112dB. With just a single channel routed to the output (such as when recording a single vocal mic) the noise floor is better than -90dBu, giving a dynamic range of more than 117dB. The THD+N figure is equally impressive, at 0.0015 percent, and the console's -3dB bandwidth extends from 4Hz to well over 80kHz, which should ensure precise transient and time-domain behaviour with minimal in-band phase-shifts. So there's clearly been no corner-cutting here — the SiX delivers genuinely superlative, professional, high-end performance.

Opening the smart Apple-esque shipping carton reveals a mini-console that resembles other compact desktop mixers, but even at first glance there's a distinct air of purposeful professionalism and quality here, with five long-throw faders and a generous sprinkling of knobs and buttons, all in SSL's familiar style. The SiX is wedge-shaped and half rack-width (kits are available for rackmounting a single SiX, or two of them arranged side-by-side), with two mono mic/line and two stereo line input channels to deliver the six inputs — but there's much more to this little mixer than that!

The SiX features several channels of compression, including two one-knob channel compressors, a 4:1‑ratio stereo bus compressor, and a listen-mic compressor for the talkback input — which, incidentally, can be used with a little creative routing as another mic input channel at mixdown.The SiX features several channels of compression, including two one-knob channel compressors, a 4:1‑ratio stereo bus compressor, and a listen-mic compressor for the talkback input — which, incidentally, can be used with a little creative routing as another mic input channel at mixdown.

The headline features start with the two mono mic/line channels, which both also have DI capability, one-knob compressors, two-band EQs, balanced insert points (the sends doubling as direct recording outputs), and two stereo cue sends. Alongside are two stereo line channels. While these also have access to both cue sends, they lack the EQ, compressors and inserts. Two external stereo line inputs are also provided and are routable into the main mix bus A, the foldback outputs or the monitor bus.

In addition to the stereo mix bus A there's an alternate stereo mix bus B, a simple but capable monitoring section with main and alt speaker feeds, and talkback (complete with SSL's infamous Listen Mic Compressor, or 'LMC') to the two stereo foldback outputs. In total, this mini-SSL can accommodate 12 inputs for analogue summing, and there is even a simplified G-series bus compressor for a bit of 'mix glue', plus balanced inserts on the mix bus A path.

Most of the physical inputs are, conveniently, on the top panel, while the outputs, inserts and ancillaries are accessed from a recessed panel at the rear. The mixer is powered from an external universal 'line-lump' PSU, and is convection-cooled through vents under the front and at the rear. In use, it gets noticeably warm, with the top of the rear panel reaching a toasty 41 degrees Celsius after a few hours.

SSL's design team have clearly tried to make the SiX as flexible and versatile as possible for the widest imaginable range of applications. They see these as including tracking and analogue summing in project studios, small location recording sessions, general mixing and monitoring duties for small-scale audio-for-video post-production, voiceover, podcasting and broadcast/streaming facilities, and even on-stage source mixing and monitoring for musicians in live-sound applications.

For me, an obvious elephant in the room is the SiX's lack of direct computer interfacing — the SiX is 'just' an analogue mixer. That will doubtless disappoint some potential customers, and could limit its market in the face of properly integrated preamp/DAW interface/monitor-controller products from the likes of Apogee, Audient, Focusrite, UA, and some less high-end small mixers. On the upside, though, I can't think of any other compact desktop mixer that matches the SiX's sound quality and feature set, let alone its gorgeous styling. And who knows? A combined mixer/interface combo is well within SSL's capabilities for the future... A SiX-Extra perhaps? Meanwhile, you'll need an audio interface that has at least a stereo input and output if you want to hook this mixer up to your DAW software.

In Depth

A mixer's functionality can usually be inferred from its connectivity, and that starts at the rear of the SiX in a recessed vertical panel. Located here are the main stereo mix bus A outputs on XLRs, with mix bus B's outputs on quarter-inch TRS sockets. All are electronically balanced — there are no audio I/O transformers in the SiX. Two more pairs of TRS sockets provide the balanced main and alt monitoring outputs, while another two pairs deliver the two balanced stereo foldback outputs.

For the uninitiated in 'SSL-speak' I'll explain, so that we're all clear on the terminology, that what you may think of as the channels' aux sends are called 'cue sends' here. These cue sends don't actually have dedicated outputs of their own, but become a selectable source for the artist foldback outputs, which would normally be used to feed the performers' headphone amplifiers or stage monitors. And in case you were wondering, these foldback outputs can, of course, also be used for effects sends or clean feeds (mix-minus) and so forth, if required.

The external line-lump power supply connects using a five-pin XLR — an interesting choice, since only two of the pins are actually needed — and delivers 15V DC at 3.3A (which is why the console runs slightly warm). DC-to-DC converters inside the mixer generate all the required symmetrical power rails and phantom voltages. The screen terminals of the audio connectors are grounded directly (less than 0.5Ω) to the safety earth connection at the PSU's IEC mains inlet, and a spectral analysis of the mixer's residual noise floor showed it to be very clean, with a few (mostly mains-related) spikes all below -120dBu. An on/off button is tucked away immediately above the XLR but doesn't isolate the PSU from the mains supply, obviously.

The rear panel plays host to the various analogue outputs as well as the inlet for the external DC power supply.The rear panel plays host to the various analogue outputs as well as the inlet for the external DC power supply.

Various other I/O connections are accessed through a pair of 25-pin D-sub sockets wired to the familiar AES59 (Tascam) standard. The output socket carries insert sends for stereo mix bus A as well as the two mono input channels (the latter being usable as direct record feeds). There are also (parallel-wired) duplicates of both the mix bus A and main monitor outputs, which could be useful for feeding hardware meters, perhaps. The input D-sub connector receives insert returns for mix bus A and the two mono channels, and also accepts 'alternative' (mono) line inputs, which are selectable sources for the two mono channels' stereo cue sends (see below).

If you've been counting, you'll realise that this particular input socket has two unused channels — and I was surprised that SSL haven't seen fit to make constructive use of them, since several practical opportunities spring to mind. For example, direct I/O access to the talkback section's LMC compressor might have been nice, or a dedicated stereo PFL output for external monitoring (always handy in a broadcast/post-production application), or even duplicating the mix bus B outputs to make it easier to sum bus B into bus A for clean-feed (mix-minus) applications. Of course, these potential ideas would break the convention of all analogue AES59 channels operating in the same direction... but rules are made to be broken after all, and it wouldn't be for the first time!

Moving to the front panel's connectivity, the two mono channels are each provided with an XLR for the mic input and a quarter-inch TRS socket for the line/high-impedance DI input; individual phantom power and the appropriate input type are selected on adjacent buttons. The two stereo channels are equipped with pairs of TRS sockets, cross-normalled to ensure that a single left‑channel input automatically appears as a dual-mono signal in the centre of the stereo image. Two more pairs of TRS sockets accept the two external stereo line inputs, with independent level controls located in the master section.

Solid State Logic SiXSurprisingly, there's no internal talkback mic, but a third XLR on the top panel (with independently switched phantom power) accepts an external talkback microphone signal, and it's positioned perfectly for a gooseneck-type mic to plug straight in. There's also a conventional quarter-inch stereo (unbalanced) 'engineer's headphone' output socket here, too.

Signal Flow

Working through the signal paths, the mono channels adopt the familiar practice of padding down the line input when selected (by a nominal 9dB in this case) and routing it through the mic preamp. The gain range for the mic input is marked +6 to +72 dB. My bench tests revealed the range to be 0 to +71 dB, but this was measured from the input to the main output, with the faders at zero and the pan-pot in the centre position — the range would be offset 4.5dB higher with a hard pan, or if measured at the insert send. Although there's no input pad, the maximum mic input level is a very healthy +20dBu before things start to sound edgy, and the incremental gain spread around the control's rotation is relatively uniform. I noticed a slight 'gain rush' at the clockwise end but it was barely perceptible compared with so many budget preamps and it wasn't a problem in practice at all.

Bench tests revealed that the line input is actually padded down by 8dB, so that the gain control then ranges from -7 to +61 dB, and that applies to the DI mode, too, which simply raises the input impedance of the line socket from 10kΩ to 1MΩ (the mic input presents the traditional 1.2kΩ). Another button activates a second-order (12dB/octave) high-pass filter with a corner frequency of 75Hz, but there is no polarity reverse option, sadly. At maximum preamp gain, the residual noise floor from the mic preamp measured -55.5dBu, so the EIN figure comes out just under -127dBu, which is very good.

A bypassable two-band EQ comes next. Each band offers standard ±15dB low- and high-shelf equalisation, with the corner frequencies at 60Hz and 3.5kHz, respectively. However, each band can be switched separately into a bell mode, for which the centre frequencies are 200Hz and 5kHz. This ingenious arrangement provides considerable versatility, and I found the options to be very effective in real-world applications.

Following the equaliser is a bypassable 'one-knob' compressor, which, though inspired by the channel compressors of SSL's large-format consoles, is actually an entirely new design. Equipped with a 'traffic-light' gain-reduction meter, the single threshold knob spans a range of +10 to -20 dBu to determine how much 'squash' is required, while the release time and ratio are fixed at benign settings of 300ms and a hard-knee 2:1. The attack time, though, is programme-dependent and varies between 8 and 30 ms, which is fast enough to exert reasonable control over the dynamics without being overly aggressive. An automatic gain make-up system preserves the output level, adding up to 10dB of boost for the lowest threshold setting.

Next comes the balanced insert point (post-EQ/post-compressor) and the send is always active, as already mentioned. Since the high-pass filter, EQ, and compressor can all be fully bypassed, the insert send can deliver the preamp's direct output, if required, for ultra-clean direct-record tracking. The balanced return is selected by a button alongside the channel fader, and the post-insert/pre-fade signal is displayed on an eight-segment bar-graph peak meter scaled from -21 to +24 dBu. The fader is configured with 10dB of gain above the unity position, and the channel pan pot applies 4.5dB attenuation at the centre (relative to each end). By default, the panned channel output is routed to the main stereo mix bus A. A large button near the bottom of the fader routes it instead to mix bus B, if desired, but also serves as a channel mute if mix bus B is not being used. A smaller, latching button activates PFL (with a yellow status LED).

The two stereo line channels are much simpler affairs, offering just a level trim control (-10 to +20 dB) at the top of the strip, a stereo bar-graph peak meter alongside the stereo fader, the bus B/mute and (stereo) PFL buttons, and a balance control instead of pan.

I previously mentioned the two stereo cues, which can be accessed from all four input channels, with individual level controls and 'on' buttons. The two mono channels also feature pan controls for each cue send, and 'Alt' buttons for cue 1, which replace the channel's signal with that alternative external line input arriving through the D-sub connector. The cue sends are all normally derived post-insert but pre-fader, but they can be changed to post-fader using buttons in the foldback master section, which reconfigure all cue 1 and/or all cue 2 sends globally.

Published April 2019