The front panel of The Bus+ is that familiar grey brushed aluminium finish, which sits perfectly alongside Fusion, and the rest of the recent SSL range. It comes in the form of a 2U, 19” rackmountable chassis.
You'll notice there’s not one, but two sets of controls mirrored across the front panel, including a pair of the distinctive gain reduction meters (where there's usually only one).
As many of you might know, the classic Bus Compressor design usually has one dB reduction meter, and one set of controls for both channels, as it is a linked stereo compressor. The Bus+ has two sets because you can now split the two channels to process individually – either in dual mono, or mid / side. We’ll circle back to this a bit later back to this!
On the back of The Bus+ you’ll find two pairs of XLR inputs and two pairs of outputs. The main inputs are for channels 1 & 2 - aka Left/Right, Mid/Side or CH1/CH2 depending on how you wish to use it. The outputs beneath those are exactly as you'd expect - the affected signal out from The Bus+
The other pair of XLR inputs are for the external side chain. This allows you to use external audio to trigger the compressor(s). The corresponding set of outputs labelled ‘EXT SEND’ duplicate the signal from the input, pre-compression. The idea being that you can take a clean feed from the EXT SEND, run it through another processor, and then return it to the side chain. An example of this in use: Sending the pre-compressed audio through an EQ, focusing in on a specific frequency area, and using that frequency to trigger the compressor - thus making the compressor more sensitive to that frequency.
This signal path of The Bus+ is all analogue through & through. However, all the parameters within the compressor are actually digitally controlled - but what does this mean for the way this compressor functions?
Digital control produces precise values for all the parameters which would otherwise be continuous (such as makeup gain or threshold.) This means that all the knobs on The Bus+ are indented. The accuracy of these digital controls means you can accurately recall and match settings with ease.
Because there is no audio passing through the pots that would otherwise be analogue, the performance of The Bus+ isn't subject to pot tolerances. This makes The Bus+ reliably precise, even over a long period of time.
The great thing about digital control is that the knobs and buttons are just passing information, rather than affecting the signal directly. This has allowed the engineers at SSL to cram in additional functions and features, without having to increase the number of controls on the front panel - saving you money, confusion and finger space! Some of the knobs have push functions to activate different modes, and the settings menu doubles up the front panel controls with other settings within The Bus+ such as input filters and dry level mixing.
The core design of The Bus+ remains faithful to the original Bus Compressor. Without any other colouration options engaged, the performance is closest to the Bus Compressor found in the centre section of modern SSL consoles. It utilises high performance 2181 THAT VCAs that give the Bus Compressor its distinct sound.
The signal through the VCAs is balanced, allowing a good amount of headroom and therefore a clean output. Typically, the Bus Compressor features a "feed-forward" design, meaning the cue for gain reduction is taken from before the gain reduction stage, making it snappy as it reacts to audio creeping over the threshold.
These are the basic collection of features that make the Bus Compressor sound the way it does. The Bus+ covers all these bases, but it can do a whole lot more. SSL have crammed in quite a few colouration options and useful features that make it hugely versatile for a great range of dynamic processing tasks.
First of all, The Bus+ has a Mode switch, allowing you to appropriately apply this compressor to a multitude of audio sources. There are 4 main operation modes to choose from:
This can be thought of as the ‘default’ mode of stereo compression for the bus compressor. The circuit analyses both the left & right channels, applying the same settings to each channel and compressing them individually.
Rather than compressing the L/R channels individually, this mode sums the two together, and applies compression based on summed mono signal. This means that the compressor becomes more sensitive to mono sources within your mix or stem, however, the output is still stereo. It’s definitely worth experimenting with this mode, as you may find this actually sounds better for particular mixes or stems
The Bus+ lets you split the left and right channels for completely separate compression of either channel - turning your stereo compressor into two individual mono compressors, with the second meter & set of controls now active.
Like the SSL Fusion, The Bus+ has a Mid / Side encoder & decoder. This allows you to break out your mix into mid & side signals, then apply different compression settings to each. Processing the mid and side channels separately, instead of L/R stereo, can yield some really well balanced results. Once processed, the audio leaving The Bus+ returns to left & right channels.
As well as the different compression modes on The Bus+, there are also a few different flavours to choose from. As we’ve already mentioned, the standard compression style for The Bus+ is similar to a modern centre console Bus Compressor – fast & punchy.
But we can change all that with the 3 buttons at the left & right edges of The Bus+: Low THD, F/B & 4K. Lets explore what each of these settings do.
Firstly, we have the ‘Low THD’ button. If you didn’t feel like the standard bus compressor was clean enough for you already, the low THD button reduces the amount of total harmonic distortion added by the compressor even further.
As with most compressors, you normally find that the low frequency content can be prone to some distortion artefacts as you hit the compressor harder - especially when using fast attack times. This can be a desirable effect to the sound, but if you’d rather keep it as clean as possible, you can enable Low THD for vastly cleaner performance.
As we mentioned earlier, an SSL Bus Compressor features a feedforward design - giving the compressor its classic ‘grabby' performance.
With F/B enabled, the trigger for the compressor’s gain reduction circuit comes from after it has already passed through the gain reduction section. This results in a much more ‘relaxed’ style of compression, with slower reponse times. This is great for material that you want to apply more gentle or transparent dynamic processing to. We found this mode particularly effective on acoustic guitar.
Nowadays we associate 4K with higher resolution. Whilst this is true for screens, the same can not be said for this Bus Compressor flavour. It's designed to mimic the sonic characteristics from the earlier Bus Compressor found in the SL4000 series consoles (hence '4K'). It achieves this by making the normally balanced signal path through the compressor unbalanced - reducing the headroom and adding more harmonic distortion. 4K mode allows you to get some really 'grungy' compression from an otherwise clean unit (and without the noise floor we might add!)
The level of harmonic distortion is fully controllable on The Bus+. Press and hold the 4K Mode button until it starts flashing, and you can then use the F/B and Low THD buttons to set the level of distortion through the VCA. There are 9 different levels of harmonic distortion, denoted by a colour coding system.
Aside from all the different flavours you can squeeze out of The Bus+, there are also a few useful extras that make The Bus+ an even more versatile tool for modern mixes.
Powerful low frequencies in modern productions can sometimes send a mix bus compressor into overdrive. Whilst trying to keep dynamic balance, you may find you're losing low end, and the mix can become a little unbalanced.
For this reason, SSL have added a Side-chain High-pass Filter to The Bus+, allowing lower frequencies to pass through without triggering the compressor. This is great for mixes and stems with a lot of low frequencies that would otherwise push The Bus+ a bit too far. This feature can also be found most modern versions of the Bus Compressor.
The Dry / Wet Mix control on the Bus+ allows you to blend between the compressed and uncompressed audio. This control doesn't normally feature on hardware Bus Compressors, but it's a feature that makes the popular technique of Parallel Compression really easy to achieve without additional audio paths or patching.
Parallel compression is a technique where you mix a heavily compressed audio with the uncompressed audio. By doing so, you can achieve a consistent dynamic range without losing those all important transients and dynamics. The result is a thicker, more natural sound.
Compressor ratios determine how much reduction is applied to a signal as it exceeds the set threshold. For example, if a 2:1 ratio is set – then for every 2dB that goes above the threshold, 1dB comes back out. Simple enough, sort of!
SSL Bus Compressors of the past used to have 3 fixed ratios – 2:1, 4:1 and 10:1. However, The Bus+ features quite a few more between that. If that still wasn't enough for you, it also features negative ratios: -2.5:1, -1.5:1 & -0.5:1. If you’ve not come across them before, SSL explains in simple terms how they work in The Bus+ manual:
“A negative ratio is a setting whereby the output signal level is reduced once it exceeds the threshold, to a level that goes below that of the threshold.”
Negative ratios result in a very aggressive style of compression, which is great for controlling loud sound sources, or creating the 'pumping' effect thats popular in dance music or heavy drum compression.
Another common attribute of the SSL Bus Compressor is the ‘Auto’ release setting. Normally, the release of a compressor is set to a single fixed time intervals, such as 0.05s or 1.2s. Auto is actually a 2-stage release, which uses a combination two time intervals time. This allows the compressor to recover quickly from short loud transients, whilst keeping consistently loud sources under control.
The Bus+ features 2x Auto release modes. The first is the same as most SSL Bus Compressor models: 0.1s (short) & 12s (long). 'Auto 2' is 0.05s (short) & 6s (long), which provides a more rapid response time to the signal running through it.
Now we come to the section of The Bus+ that’s completely new to this hardware – the Dynamic EQ section. More commonly seen in software, a dynamic EQ is a frequency specific form of compression & expansion. The hardware version on The Bus+ is a 2-band shelving EQ, with single controls for expansion through compression. The HF shelf can also be switched to a parametric band for more focused dynamics processing. The Dynamic EQ can be applied pre or post compressor.
The Dyn-EQ frequency bands complimented by low frequency & high frequency cut / boost controls, which help to compensate for the dynamic processing applied to these bands. The LF band is switchable between standard and G-Series modes, which changes the behaviour of the EQ band from a 1st order band, to a 2nd order band with an overshoot/undershoot characteristic. This feature adds an alternative behaviour which can be quite pleasing on certain stems and mixes
The bands of the Dyn-EQ on The Bus+ have a few extra parameters that you can adjust and this is where the digital controls come in very handy!
You set the frequency and range of each band by long pressing the compression / expansion controls. You’ll then notice the gain reduction meters begin to pulsate light, and flip to a fixed position - becoming indicators for both Frequency and Range. You can then use the HF/LF Fast buttons on either side to set the frequency of the shelf on the left, and the maxium level of compression or expansion (known as Range) on the right.
The Dyn-EQ frequency bands have switchable response times, giving you even more processing flexibility. This is set by the HF / LF fast buttons we just mentioned when in normal operation. There are 3 preset time constants that you can set for each:
HF | LF | |
Nominal (Off) | 3ms Attack / 50ms Release | 10ms Attack / 50ms Release |
Fast (White) | 1ms Attack / 50ms Release | 30ms Attack / 50ms Release |
Auto (Magenta) | 10ms Attack / AUTO Release | 10ms Attack / AUTO Release |
The dynamic EQ is a great additional too for sculpting your sound that little bit further. It can be great for adding definition, taming rogue frequencies, and all the while remaining very musical to the ear.
As we’ve given you a rundown of the dynamic processing tools that you’ll find onboard The Bus+, you begin to see where the '+' is coming from. It is most definitely an SSL Bus Compressor at heart, complete with all the standard features you’d expect to see. But it can also behave more like a vintage SSL Bus Compressor, with adjustable grit and grunge. Further still, it can be an ultra clean or gentle compressor with a more natural or transparent character. Plus the Dynamic EQ for more frequency focussed shaping. The Bus+ really has a wide range of compression characteristics to fufill a variety of roles – whether gluing a mix together or smashing the bajeesus out of some stems.
We think you’ll find a multitude of way to make The Bus+ work for your sound, whatever that might be!
This new blog is presented by the team at Sound Technology Ltd, a leading distributor of musical instruments and pro audio equipment in the UK and ROI.
