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Solid State Logic The Bus+

Dual-channel VCA Compressor & Dynamic EQ By Matt Houghton
Published May 2022

Solid State Logic The Bus+

Other manufacturers: “Hey SSL! We made your bus compressor even better.” SSL: “Hold my beer...”

If imitation is the sincerest form of flattery, Solid State Logic (SSL) should feel very flattered: the stereo VCA ‘bus compressor’ which first featured in their 1980s G‑series consoles and went on to earn an enviable reputation as a ‘mix glue’ compressor has to be one of the most widely cloned audio circuits. In fact, if we discount mic preamps, it is arguably the most copied of them all. But most of those ‘tributes’ aren’t direct copies. They ususally implement at least a couple of design tweaks, and often many. Some products compete on sonics, whether that means better specs or recreating an authentic original version, some on the sheer number of additional functions and settings, and others feature trade‑offs that enable keener pricing. The ‘SSL bus compressor’, then, has become a genre all of its own, with an option for every user and pretty much any budget.

Curiously, while others augmented this design, SSL themselves did relatively little to it. They’ve tweaked things a little to take account of component availability and manufacturing methods, and they’ve made the processor available in more accessible form factors: SSL made a 19‑inch rackmount unit for years and continue to offer both a 500‑series module and their Bus Compressor 2 plug‑in. Other than adding more ratios and a side‑chain HPF to the 500‑series module, the functionality remained pretty much unchanged for decades — but with their new Bus+ they’ve not only caught up with the pack, they’ve raced right past it and pulled away with the front‑runners!

The Bus+ retains the classic SSL gain‑reduction circuit based, as with most such devices, around THAT 2181 VCA chips. The merest glance at the front panel, though, tells you that it has been massively augmented — and that’s before you even consider how many controls have secondary functions. In fact, this thing is overflowing with interesting and useful features, some of which will be familiar to anyone who’s checked out the competition, and others which I’ve not seen on a device of this type before.


The audio and sidechain signal paths are entirely analogue, yet everything is digitally controlled. SSL have evidently decided not to use software remote control; rather the aim seems to be to ‘exploit digital’ to deliver a standalone device that offers great precision (for instance, MDACs are used to apply the makeup gain) and more extensive functionality than a 2U faceplate could usually accommodate without becoming fiddly and confusing. They’ve done a very slick job of it too: the Bus+ feels very much like good analogue hardware should. The build quality is solid, and there are no screens or annoying menus. All the knobs are rotary switches, with each detent being a precise, replicable configuration of the processor. They have both a physical start and end to their travel and a clean white line pointing at the legends; no endless rotary encoders or LED rings here. Gain reduction is indicated for each channel by a reassuringly old‑school needle meter.

Because each control has an obvious primary function, basic operation is pleasingly intuitive, but you can quickly dive deeper, since lots of buttons have secondary functions (courtesy of long button presses) and some knobs also act as push switches. There’s good use of button backlighting (brightness and colour) for status indication, which means none of this really gets in the way of using this thing like you’d use a ‘normal’ processor. Put more simply, despite the immense sophistication, everything feels very natural in use.

Around the back, it’s pretty standard too. An IEC power inlet with associated switch is joined by two quartets of XLR connectors: a main electronically balanced in and out for each channel, and another pair each for the sidechain. One sidechain XLR sends a mult of the input signal out, and the other is an external sidechain input. So you can key this device from external sources by connecting only the latter, or you can combine the two to use external hardware as, effectively, sidechain insert processors. Nice!

As well as the balanced audio I/O and external sidechain input, there’s an Ext Send output: a mult of the input signal which means you can use external processors to fine‑tune the Bus+’s response.As well as the balanced audio I/O and external sidechain input, there’s an Ext Send output: a mult of the input signal which means you can use external processors to fine‑tune the Bus+’s response.

Controlling Behaviour

Each channel has an almost identical control set and, between them, a Mode button determines how the channels are linked: ‘classic’ L‑R stereo, with each channel reacting to the highest of the two control signals at any given time; summed L‑R stereo (the two audio signals are summed to create the control signal); Mid‑Sides; or dual mono. A short press steps left‑to‑right through these modes, while a long press takes you in the opposite direction. For the two L‑R stereo modes, channel 2’s meter and controls become inactive and the channel 1 control set governs both processors; obviously, in M‑S and dual mono modes, each control set operates on its own channel.

As with so many aspects of the Bus+, the two‑band dynamic EQ is surprisingly configurable, with deeper functions accessed by long button presses, and knobs that double as buttons. Even the meters are cleverly pressed into service for parameter switching!As with so many aspects of the Bus+, the two‑band dynamic EQ is surprisingly configurable, with deeper functions accessed by long button presses, and knobs that double as buttons. Even the meters are cleverly pressed into service for parameter switching!The knobs are helpfully colour‑coded, with those for the compressor being blue‑capped, while black‑capped knobs operate the two‑band dynamic EQ. For each channel’s compressor, the expected Threshold, Ratio, Attack, Release and Make‑up Gain controls are joined by a knob for the sidechain’s 12dB/octave high‑pass filter (off when fully anti‑clockwise, then 0‑300 Hz in 10Hz steps) and a Mix control (for the compressor only, not the dynamic EQ). Two buttons cater for different distortion modes (of which more later) and another toggles the point in the signal path from which the internal sidechain signal is derived, switching the compressor between the conventional feed‑forward topology and a more laid‑back feedback architecture.

There are many additional settings for the ‘standard’ bus compressor’s familiar controls too. For example, there are more Attack times: 11 in total, ranging from 0.1 to 40 ms. There are more Release times, with nine options from 0.05 to 1.2s and two Auto settings; the additional Auto setting is twice as fast (50 ms short, 6 seconds long) as the standard one and a very handy addition. There are also lots more ratios on offer, including lower ones that should hold appeal for mastering compression (1.3 and 1.5:1) and, intriguingly, negative ratios that go beyond limiting — these actually bring the output level down beneath the threshold, and can yield some interesting rhythmic effects on the right material.

Some simple but cool ‘hidden’ functions can be found here. Press and hold the Make‑up Gain knob, for example, and it switches to a Fine mode: by default it applies ‑10 to +20 dB of gain in steps of 1dB, but in Fine it’s ‑5 to +10 dB in 0.5dB steps. And a short press of the same button swaps the order of the compressor and dynamic EQ in the signal path, with LEDs above the knob/button indicating the status of these ‘hidden’ settings.

Start Me Up

Yet another interesting compressor feature is accessible only during the startup sequence (a flashy display of blinking buttons and clicking relays that lasts a couple of seconds): you can change the Mix knob’s behaviour from the default Wet/Dry crossfade control (with a 50:50 mix in the 12 o’clock position) so that it leaves the dry signal at unity gain and turning the knob sets the wet level. That’s similar to the way you’d manage parallel compression with faders on a console and is how I generally prefer to work, since it allows the untreated transient peaks to breathe while I ride up a ‘crushed’ wet signal for more sustain, energy or ‘detail’. It’s a really thoughtful and very welcome feature.

Startup mode is entered by pressing and holding the Mode button during startup, and allows various other things to be configured: the brightness of the LEDs, for example, and whether you hear the relays click. A power‑saving Auto‑Sleep mode can be set up (you can enter Sleep mode manually at any time too) to come on after a user‑specified period of control inactivity or of the signal remaining below a threshold; if you wish, it can auto‑wake when a signal rises above a threshold. You can also set the input level trim (0dB default, or +10 or ‑5 dB), engage a 26kHz low‑pass input filter (to remove the HF hash that some converters can spew out above the audio band) and make changes to the way Sides Solo is handled, as well as setting the channel 2 meter to remain in action when in the L‑R stereo modes.

Unfortunately, the only power button is on the rear panel. While the sleep modes mean this isn’t a major problem in terms of energy consumption, it does mean that if this device is mounted in a rack you’ll need to power down/up the whole rack to access the startup functions. I’d prefer at least some of them to be accessible during normal operation. A front‑panel power switch would solve this, of course, but I wonder if these functions could be assigned to unused press‑hold buttons; I passed that idea on to SSL who say they’re considering the possibilities.

Perhaps the biggest headline feature is the two‑band dynamic EQ, which is relatively rare in hardware.

Dynamic Duo

Perhaps the biggest headline feature is the two‑band dynamic EQ, which is relatively rare in hardware. Though similar to multiband compression/expansion, there are no crossover filters involved and it tends to sound cleaner; it’s just two parallel EQ bands with dynamic gain, so the two bands don’t interact unhelpfully. The inner two controls for each channel (the one exception to the mirror‑image layout) activate the LF and HF dynamic bands when pressed. Turning them anti‑clockwise introduces gain‑reduction and clockwise expansion; the threshold is lowered as you turn the knob and LEDs above light/change colour to indicate the amount of boost/cut being applied.

The LF and HF controls have 31 steps, and at step 16 (12 o’clock) the dynamic bands are inactive. The third knob (LF Gain on channel 1 and LF/HF Gain on channel 2) is a static ±10dB band gain control. The LF Gain knob is always an LF‑band makeup gain control, whereas channel 2’s LF/HF Gain changes role according to the selected mono/stereo mode: in M‑S and dual mono modes it is an LF‑band gain, while in the two L‑R stereo modes it becomes an HF gain. Sadly, this means there’s no static HF boost/cut available for either channel when the device is configured for dual mono or M‑S operation. Each of these two knobs also doubles as a button to switch the channel’s LF shelf between 6 and 12 dB/octave; a simple but useful change that can make a crucial difference to the sound when processing something like a stereo a drum bus.

The three adjacent buttons all have an obvious primary function: the top one toggles the HF filter between (first‑order) shelf and bell types. The next toggles the HF attack time between 3 and 1 ms (the release remaining at 50ms in each case), and the third toggles both the attack and release for the LF band, the default being a 30ms attack and 100ms release and Fast being 10 and 50 ms, respectively. A press‑hold of either button turns it a vivid magenta to indicate that you’ve selected the Auto release setting and a 10ms attack.

There’s yet more to explore here... Press and hold either the LF or HF knob and all the button lights dim and the meter backlights start gently pulsing. In this mode, the left meter’s needle position indicates a frequency preset for that band, and you can step through the presets using the left pair of HF/LF Fast buttons, the needle moving accordingly. The right‑hand meter and button pair is used similarly to set the desired range for the dynamic EQ band. It’s a neat trick that allows you much more control over this processor than first appearances might suggest.

Clean Or Coloured?

‘Analogue warmth’ has been fashionable for years, and it’s common for bus compressor clones to offer a means to inject some warmth or energy into a source. The Bus+ approach to this is one of the most versatile and controllable I’ve encountered.

First, there’s the 4K button. With this mode disengaged, the Bus+ processor sounds very clean, and similar to SSL’s Superanalogue devices. Press the button and you introduce harmonic distortion. It’s mostly even‑order harmonics, so it adds a brightness and generally sounds ‘musical’; think triode‑valve distortion. Press and hold 4K and the two adjacent buttons can be used to scroll through eight degrees of distortion, ranging from clinically clean, indicated by the 4K button’s backlight turning a brilliant white, to downright dirty (red), via various in‑between hues of cream, yellow and orange.

Press‑hold 4K again and you’re returned to the normal operating mode, in which one of those other two buttons engages Low THD mode. This reduces LF distortion, with a marked drop in odd‑order harmonics, resulting in a tighter, clearer‑sounding bottom end: think more ‘transparent’. Note, though, that unlike the 4K mode, this doesn’t exert a great influence when there’s no gain reduction; it seems to be all about the sound of the compression. As mentioned earlier, the other button, F/B, toggles between feedback and feed‑forward compression, and while technically that refines the compressor’s action it is typically perceived as a change in tonality: eager and edgy versus smoother and more relaxed. The bottom line is that by using this trio of buttons in combination, you can access a vast range of different tonal characters.

Sound Opinion

I’ve explored most of the functions to give you a feel for the Bus+ and whether it might appeal, but for chapter and verse it’s worth diving into the PDF User Guide. I’ve not said a huge amount about the sound yet, or what material this processor works best on, but there’s a reason for that: this thing is so precisely controllable and so versatile, and its sonic character so configurable, that its use really isn’t limited to specific genres and tasks. It can range from ultra‑clean to warm and musical, aggressive to chilled‑out. It’s all about supporting your choices.

Despite the immense sophistication, everything feels very natural in use.

I tried it on a range of material and in various roles, including as a stereo bus and mastering processor for a couple of rock and electronic dance mixes as well as some singing‑guitarist material, and as a processor for individual sources including drums, bass, acoustic guitar and vocals. It always impressed. For what it’s worth, it’s absolutely capable of doing ‘the mix glue thing’ (tickle the meters at 2:1, with auto release...) but it’s capable of much greater subtlety too, and that second Auto mode is really handy. I’d have no qualms using this for mastering‑type jobs, even if, ideally I’d have had access to the HF EQ band in M‑S mode. It can be used incredibly assertively, particularly in low THD mode, which seems to allow you to get away with more gain reduction on LF‑heavy sources like bass and drums. And while it’s never going to give you quite the same je ne sais quoi for lead vocals as a nice opto or vari‑mu device, it’s certainly capable of a decent job on such sources, and the feedback and dual mono modes make it more useful than the classic bus compressor. You can patch channel 1 into channel 2, for example, to give you a huge amount of control over a vocal.

The dynamic EQ is a real treat, and a much more useful companion to the compressor than I’d initially imagined it would be, particularly when using it on a drum bus. Compression can often have unhelpful side‑effects: take the glue too far and things can become flat and lifeless; back off too far and you lose the cohesion. The dynamic bands can restore some life and bounce, while still getting everything to gel nicely, and they can alternatively be used to tame a rogue element before hitting the compressor. Perhaps my favourite touch, though, is the ability to perform true parallel compression, just riding the wet signal up and down with a single knob.

So, while there were a couple of mild frustrations (the inability to access HF EQ in M‑S and dual mono modes, and some functions being tucked away in Startup mode), the Bus+ impressed the hell out of me. It strikes me that all the talk of button presses might have put you off, but it really shouldn’t. I’m not always keen on secondary functions and on guitar pedals particularly they often remain ‘invisible’, necessitating guesswork or constant reference to the manual. But SSL have done a cracking job of indicating on the front panel exactly what’s going on, with wonderfully considerate use of the brightness and colour of the backlighting and clear legends. So it’s top marks on sound, top marks on ease of use and a big list of ticks when it comes to features. Given all that, the price isn’t unreasonable either.  


The closest competition is undoubtedly from the Wes Audio ngBusComp. Like The Bus+, that device is digitally controlled, has separate control sets for each channel and can be configured to operate in L‑R, M‑S and dual mono modes. It also offers controllable distortion. It boasts a preset system and DAW plug‑in remote control, neither of which are offered by The Bus+, but the Wes device lacks the SSL’s dynamic EQ and while it does offer an external sidechain input for each channel, it lacks the sidechain send. Both are excellent devices and well worth auditioning.


  • At heart, still the classic SSL bus compressor.
  • Lots of additional compressor settings/functions.
  • Includes a two‑band dynamic EQ!
  • Pleasingly precise control.
  • Sophistication never spoils the ‘analogue’ user experience.
  • No software updates to worry about.


  • Some useful facilities only accessible during startup.
  • No HF EQ band in M‑S or dual mono modes.


A stunning overhaul of SSL’s bus compressor, the Bus+ goes much further than most imitators without sophistication undermining ease of use.


£2038 including VAT.

Sound Technology +44 (0)1462 480000


Solid State Logic Eastern Region (NY) +1 212 315 1111.

Solid State Logic Western Region (LA) +1 818 643 7040.