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Solid State Logic Fusion

Stereo Analogue Processor
By Hugh Robjohns

Solid State Logic Fusion

This classy stereo processor aims to bring analogue colour to your digital recordings.

SOS Gear Of The Year 2018 Editors PickNeatly summed up by its ‘Stereo Analogue Colour’ front‑panel slogan, the Fusion has been two years in the making, the result of careful analysis by SSL’s engineers of various bits of highly regarded equipment to determine just what creates the desirable characteristics associated with high‑end analogue gear. This was followed by painstaking development and refinement with the help of a team of trusted mix and mastering engineers.

The substantial 2U rackmount box’s front panel has a brushed‑aluminium finish, and its spread of fluted knobs with brightly coloured caps, and square illuminated buttons, is classic SSL. A mains inlet on the rear, with on/off switch for the internal linear power supply, accepts 115 or 230 Volts AC. A label declares the Fusion to have been ‘Engineered in the UK, made in China’; both the engineering and building are to very high standards, with most circuitry taking the form of surface‑mount devices on a large motherboard.

Two pairs of XLRs carry the stereo balanced line‑level inputs and outputs, and there’s a set of balanced insert sends and returns, also on XLR. The intention is for the Fusion to be used for mastering or stereo mix‑bus processing, whether hooked up to a console or to your DAW and audio interface. The Fusion’s insert loop makes it easy to add further analogue processing (such as a SSL’s mix‑bus compressor) to the signal path, and provides some further useful options, as I’ll explain below.

There are eight processing stages, and most are bespoke new tools. The design builds on SSL’s extensive experience, of course, but while the stereo input and output trim and high‑pass filter stages are familiar enough, there’s no repurposing of ‘legacy’ circuitry. Both trim controls have over‑sized, centre‑detented knobs with a ±12dB range, and a pair of tri‑colour LEDs indicate the signal levels. A vertical stereo bar‑graph peak‑hold meter shows the output level, calibrated from ‑21 to +24 dBu, and when the processor’s bypass is engaged this shows the input level. A four‑position rotary switch sets the 18dB/octave high‑pass filter to 30, 40 or 50 Hz, or off.

The most innovative part of the signal path comprises five distinct processes: a Vintage Drive harmonics generator, a ‘Violet’ two‑band shelving EQ, an HF compressor, a stereo image adjustment tool, and an output transformer. Each can be bypassed using the illuminated buttons below the respective controls. There’s also a master bypass, and a couple of buttons to engage and position the insert point.

Relay clicks are heard when buttons are pressed, but all of the internal signal‑path switching is actually performed with solid‑state devices. I can’t think of any other product that works this way, but it’s very neat — it provides feedback to the user that switching has occurred, and is especially helpful when not looking at the Fusion while making adjustments (such as when switching a section in and out). This ‘sounding relay’ can, if you prefer, be turned off via the settings mode, accessed by holding down the transformer and bypass buttons. In this mode, a number of other functions can be customised, including the brightness of the buttons and meters. A factory reset can be performed here, you can identify the firmware version, and there’s even a Simon Says game (dull studio hours simply fly past!). Various other short‑term configuration modes are available by holding down other buttons. For example, pressing and holding bypass moves the bypass point post the input trim, while doing the same with the insert button configures the insert point to work in Mid‑Sides mode.

Fusion Reactor

Designed to introduce harmonic complexity and gradual saturation, or soft‑compression, SSL suggest that the Vintage Drive section can be used to bring “cohesion and strength to the mix”. The effect is to add thickening and density to the mix at lower settings, building to more obvious musical edginess and eventual distortion at more extreme settings. It’s very analogue‑sounding, and brings a recognisably vintage air to proceedings.

The Vintage Drive section is designed to add harmonic complexity to the signal, in similar fashion to SSL’s VHD preamps.The Vintage Drive section is designed to add harmonic complexity to the signal, in similar fashion to SSL’s VHD preamps.

Its green‑capped Drive and Density knobs each have a tri‑colour LED to indicate how hard the circuitry is working, and an illuminated In button engages the processor. Drive is scaled, in Spinal Tap fashion, from 1 to 11, and the output level is reduced a little as the knob is advanced, to compensate for the added harmonic content and perceived increase in loudness. Density adjusts the level and balance of harmonics, with mainly even‑order content at the anticlockwise end and mainly odd‑order at the other. The perceived loudness and the RMS level increase as the Density knob is advanced, and while there’s no automatic compensation, the input and output trim controls allow you to maintain comparable levels with the unprocessed signal. A combination of high Drive and very low Density settings seems to produce a kind of dynamic expansion, which can be useful for emphasising transients in already compressed sources.

Hitting The Highs & Lows

Next up, the Violet EQ is SSL’s first new analogue EQ circuit for more than 25 years! It features a classic minimum‑phase shelving EQ with adjustable gain and corner frequencies. It’s intended for gentle tonal shaping of a finished mix; for adding a low‑end solidity or high‑end gloss, perhaps. The two sections offer a sensibly restrained gain range of ±9dB. The purple‑capped low section is switchable for corner frequencies between 30, 50, 70 and 90 Hz, and the pink‑capped high section between 8, 12, 16 and 20 kHz. Another illuminated In button enables/bypasses the EQ.

The HF compressor’s threshold and crossover frequency controls have orange caps, there’s an illuminated In button, and a tri‑colour LED indicates gain reduction. The idea is to tame and smooth the high end of a source, somewhat like the effect of recording to analogue tape, but also to engender a smoothly rounded vintage quality to the mix. The compression is very controllable and surprisingly transparent. The crossover control allows you to determine precisely the frequency range that’s affected, and the threshold the amount of HF rounding to reduce harshness, brittleness, or edginess. Coming, as this compressor does, after the Violet EQ, it’s possible to boost the high‑end deliberately before taming it dynamically with the compressor — a technique which can enhance brightness without making things sound too harsh.

Wide Load

The elaborate stereo image section has blue knobs for Space and Width, and the ubiquitous In button. It converts the signal from Left‑Right to Mid‑Sides for processing, then back to L‑R for the ongoing signal path. Width alters the overall level of the Sides signal over a ±6dB range to narrow or widen the image width, of course, but the Space control may be less familiar. This adjusts a Sides‑only low‑frequency EQ, as used long ago by Alan Blumlein and later by EMI as part of a ‘Stereo Shuffling’ technique. It’s also used in the form of an ‘Elliptical Filter’ to constrain out‑of‑phase LF information when cutting vinyl.

The stereo image section is much more than an M‑S balance circuit — it also includes an implementation of Alan Blumlein’s Stereo Shuffler circuit.The stereo image section is much more than an M‑S balance circuit — it also includes an implementation of Alan Blumlein’s Stereo Shuffler circuit.

Boosting the low‑end width relative to the highs can give a greater sense of spaciousness and depth, and that’s the main intention here, but this processing can also be used to sharpen and improve the focus of the stereo image obtained from simple coincident mic techniques. I wonder if providing up to +12dB of low‑end boost was the optimum choice, as +6dB often seemed way too much to me — an asymmetrical range of, say, +6 to ‑18dB might have allowed for greater adjustment precision. And since I’m already being very picky, a crossover control to determine the frequency range affected would have been a nice bonus!


The final section is controlled by a single illuminated button with a transformer image on the cap. This routes the stereo signal through a custom‑designed drive stage and a couple of under‑damped miniature 600Ω 1:1 transformers, designed specifically to introduce subtle low‑frequency saturation and a progressive high‑frequency phase shift. This combination imparts a nice, analogue sheen and sparkle, some gentle low‑end thickening and weight, and some further reduction of sub‑sonic frequencies, for a slightly tighter and ‘cleaner’ sound. There’s no adjustment available and since it has its own drive stage it doesn’t respond much to the input level. So it’s really just a case of switching this circuit in or out and choosing which sounds best for whatever material you’re working on.

Send & Return

The remaining buttons activate the rear‑panel send/return insert point and determine its position in the signal path. The default mode (in which the button lights blue) puts the insert post the HF compressor and pre the stereo image section, working with normal L‑R stereo signals. Pressing the Pre EQ button moves the insert point after the Vintage Drive section but before the EQ. But pressing and holding the insert button (changing its colour to a bright white) activates the M‑S mode. Since this makes use of the stereo width section’s matrixing circuitry, the M‑S insert is normally after the stereo image stage and before the transformer, but pressing the Pre EQ button moves it ahead of the stereo width process, to the same position as the default mode, while still operating in M‑S. Configuring the insert for M‑S operation allows you to use an external single‑channel signal processor to manipulate just the centre (Mid) or edges (Sides) of the stereo image — a very versatile and powerful means of polishing and finessing a mix.

The insert points can be configured for L‑R or M‑S operation, and can be moved to different points in the signal chain.The insert points can be configured for L‑R or M‑S operation, and can be moved to different points in the signal chain.

In Use

The Fusion can be installed in a wide variety of systems easily. Some will want to use it with a DAW as an external analogue processor, using spare I/O on their audio interface to allow its use on individual tracks, stereo stems, or final stereo mixes. Die‑hard analogue mix engineers can plug the Fusion into the stereo mix‑bus or group inserts of their analogue console. And mastering engineers will probably hook it into a processing loop of their mastering console — which is mostly how I tested it, connecting it to the Analogue Insert 3 loop of my Crookwood M1 mastering console, and processing a variety of commercial mixes and my own work.

Technically, the Fusion’s I/O has a generous headroom margin, easily accommodating up to +27.5dBu, a plentiful 3.5dBu over and above the peaks of the hottest of normal converters. The signal bandwidth through the unit (with all processes turned off) is equally expansive, with the low‑frequency ‑3dB point at 5Hz, and the high‑frequency one well beyond the capabilities of my Audio Precision test set — I can easily believe SSL’s claim that it’s around 180kHz. The THD+N was also impressive, at below 0.004 percent (without engaging the Vintage Drive, compressor, or transformer stages). The noise floor measured a touch over ‑90dBu, giving a dynamic range of around 117dB (which matches the performance of most high‑end converters), and the crosstalk at 20kHz was exceptional (around 110dB). Even the ganged input and output trim controls track extremely closely across most of their ranges. In short, the Fusion’s technical performance is exemplary — as you’d expect of a premium SSL product.

Conceptually, the Fusion is pretty straightforward and the various processing tools are all clearly set out across the panel, with plenty of space between the controls, and clear status pointers and lamps. I find it easier to focus on what I’m doing to the sound when I can close my eyes and tweak the knobs and buttons by touch alone, rather than having to coordinate my mouse with an on‑screen pointer, and the Fusion is great for that tactile way of working: grab the control, close the eyes, listen and tweak! I liked the relay‑click feedback, even though I knew it was fake — it provided just the right sonic reassurance that the Fusion had done what I wanted.

As a device for introducing ‘stereo analogue colour’, the Fusion hits the mark very nicely indeed. The highlights are undoubtedly the Vintage Drive and HF compressor, and I found myself using these tools often. That said, the transformer does some very nice things, and the EQ and stereo image controls are perfect complements in this overall package.

For my own small‑scale mastering work, I often use Drawmer’s DC2476 processor. This includes almost all the facilities of the Fusion but it processes exclusively in the digital domain, so it was interesting to set up similar processing chains for the two boxes and switch between them. While I could achieve very similar overall results in most cases, I almost always ended up preferring the sound of the Fusion... a fact that doesn’t bode well for my bank account! I’m talking subtleties and musical emotions here, of course, and the differences were often almost imperceptibly small. The Fusion doesn’t do quite as much and isn’t quite as flexible as the Drawmer, but it is significantly easier and much faster to use, and it does all the important things in a subjectively nicer, smoother and, yes, a more ‘analogue’ way. I’ve always thought the EQ and Tube drive effects in the Drawmer were amongst the best I’ve heard from a digital processor, but the Fusion’s analogue circuitry definitely has the edge; dialling in subtle amounts of Vintage Drive and Density often delivered a more emotionally appealing result.


I found the Fusion very useful as a multi‑function processor in my mastering chain. It was a delight to use it to add most welcome and attractive tonal and sonic enrichments to a wide range of mixes in wildly different styles and genres.

It’s a refined tool, which seems to me to give its best when used with a light touch — it serves to enhance the good and desirable aspects of a stereo mix or stem, but usually does this in a delightfully subtle, transparent way, while simultaneously gently suppressing the bad and undesirable. That’s not to say it can’t add crunch and grit, exaggerated smiley response curves, and excessive stereo image widths if that’s desired — it most certainly can do that, especially when working on individual sources rather than final mixes. And with that in mind, it’s probably most versatile when integrated into a DAW system, where it is relatively easy to deploy on individual tracks, stereo stems, or final mixes, as required.

In short, then, the Fusion represents an interesting new direction for SSL, and it’s one they’ve embarked upon with confidence and accomplishment. It’s a very well‑built and expertly developed ‘colouring’ tool that exudes quality and class, both physically and sonically.  


The Fusion provides a unique combination of features. That said, it’s broadly comparable with the less expensive SPL Tube Vitaliser, and much more expensive high‑end mastering devices like the Kerwax Replica and the BlackBox HG2.


  • A very classy, versatile signal processor.
  • Exemplary technical performance.
  • Six elegant, intelligently specified and extremely effective signal-colouring tools.
  • Insert loop configurable for L-R or M-S.
  • Very versatile stereo-image enhancement.
  • Good price, given the sound and facilities.


  • None.


SSL have evidently applied great skill and intelligence to produce this elegant and effective ‘colouring’ tool, which is equally well suited to mastering, mix-bus processing, and applying polish to in-the-box projects.


£1798.80 including VAT.

Sound Technology +44 (0)1462 480 000

Published December 2018