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Safe Sound Audio Dynamics Toolbox

Stereo Compressor-Limiter
Published October 2008
By Paul White

This may look like a standard compressor, but there's plenty beneath the surface to get excited about.

Safe Sound Audio Dynamics ToolboxPhoto: Mike CameronSafe Sound Audio are a small West Yorkshire-based company who build high-end outboard gear. Their main focus is on compressor-limiters, and they believe that their approach addresses some of the limitations they can see in existing designs. Although I've never used one, the company's P1 analogue preamp and compressor/expander seems to have been well received by those studios who've bought one, and their Dynamics Toolbox, reviewed here, builds on the experience gained in designing it. All of Safe Sound's products are entirely built and tested in the UK.


At first glance, the Dynamics Toolbox appears to be a conventional two-channel, solid-state (VCA) compressor, but closer examination reveals more interesting features. The two channels may be used in mono or can be stereo linked, with channel one's controls applying to both channels (other than the master output gain). This isn't particularly unusual in itself, but there's also a choice of Peakride or Dynamic-tracking compression — and I suspect these are terms that need some explanation...

Peakride compression, when engaged, uses three linked side-chains acting on the VCA, so it's almost like having three compressors, each with different ratio, attack, and release settings. This makes a lot of sense, because engineers often have to set up two or more separate compressors to tackle different aspects of the same signal. For example, you could have a fairly slow, low-ratio compressor looking after overall level, teamed with a faster, higher-ratio compressor to pull down transients. Trying to do this with a single compressor can often leave a signal sounding over-processed but, according to the designers, Peakride was designed to even out levels — primarily during tracking or mixing — without robbing the sound of life or compromising transparency of sound. The user only has to adjust the usual compressor controls, because the Peakride circuit takes care of its own operation behind the scenes. In some ways it is reminiscent of the way Aphex's Compellor works, combining longer-term levelling with control over more rapid changes, but the beauty of exotic compressors of this type is that every designer brings something new to the party.

Dynamic-tracking compression is more conventional. It was developed with mastering or stereo bus compression in mind, although when the Auto Release button is engaged, the release time adapts to the programme dynamics. This isn't original, but it's a sensible idea, as many mixes change their dynamics so much that no single release setting would work for all parts of the song. Using low to medium ratios in combination with the Auto Release function, the compressor can be used to make a mix sound smoother, denser and more homogenous... which is usually the main reason to use a compressor on a complete stereo mix. There's also adequate scope for creative abuse if you dial in higher ratios combined with faster attack and release times, at which point you can get audible gain-pumping — an effect that can add energy and excitement to rock or dance music elements, especially drums.

Parallel Lines

In our technique articles in SOS, we occasionally discuss the subject of parallel compression — the art of mixing a compressed signal with a dry version of the same signal, in order to bring up low-level sections without affecting higher-level material in any significant way. This 'bottom up' type of compression can be very effective, and used subtly it works across all kinds of genres — including classical and acoustic music — but it's surprising how few hardware compressors have a wet/dry mix control built in. Safe Sound call this technique 'New York Compression' (because it was engineers there who developed a technique where you mix some rather heavily compressed signal back in with the dry signal to beef up drums and guitars) and have included that mix control, which can be used in either the dual-mono or stereo-linked mode of operation.

In addition to the type of gain-control element — which, in this case, seems to have been designed with transparency in mind — the sonic character of a compressor has a lot to do with the way the side-chain reacts to the incoming audio, because it's in the side-chain that the crucial gain-control signal is generated. In the Dynamics Toolbox you'll find sweepable high- and low-pass side-chain filters which, when active, skew the way the side-chain 'hears' the signal being processed. For example, if you dial in less high-end at the side-chain, the compressor will perceive the high frequencies as being quieter than they really are, so those frequencies will be less likely to trigger the compressor. Similarly, if you cut the low end going into the side-chain, less compression will be applied when low frequencies are present in the input. In many cases, the addition of these filters takes away the need to patch in separate equalisers via a side-chain insert point — but Safe Sound have also included balanced side-chain inserts, and listen facilities are included to cover those situations, such as de-essing, where more precise or more radical side-chain EQ is needed.

Because a compressor has adjustable attack, release and ratio controls, it can't respond quickly enough or hard enough to limit short-duration, high-level peaks when set up for typical musical applications. To do this you need a separate limiter, so the designers have included a separate limiter side-chain that controls the same VCA. This limiter adjusts its attack and release times dynamically, depending on the nature of the incoming transients. It has a 100-microsecond minimum response time, so any clipping resulting from anything sneaking through during this period should be inaudible. As with the compressor section, the limiter can be abused in a creative way by deliberately driving it into heavy limiting, but under normal use you'd only expect the limiter to come into action briefly on transient events such as snare-drum hits. You can tell when the limiter operates because a red LED comes on behind the meter dial.

Inside & Out

The front panel of this 2U processor is fairly plain, with an off-white finish and grey plastic knobs. Mains power comes via an IEC socket on the rear (there's no power switch), but a less conventional feature for a pro piece of gear is the use of balanced TRS-jack sockets (which may also be used unbalanced) for all the audio connections: there's not an XLR in sight. Personally I prefer jacks, as they're easy to hook up to a patchbay, but I know some pros who may not feel the same way.

There's a generously rated analogue power supply inside, which is based around a torroidal mains transformer feeding a pair of linear voltage regulators. Because modern digital audio systems have so much headroom, high signal levels (typically around 18dB) are often necessary to achieve 0dBFS, and with that in mind the electronically balanced output stage has been designed to handle levels of up to +27dBu. For users who prefer the sound and true galvanic isolation of audio transformers, an optional Lundahl output stage is available (Sowter, Jenson and Cinemag options will also become available in the coming months), and a pair was fitted to the review model. The transformers are mounted on plug-in cards for user installation and have exactly the same output capability as the electronically balanced version, but they allow the engineer to add in some authentic transformer flavouring.

The Dynamics Toolbox includes two output jacks per channel: the electronic output comes as standard, and this review unit came with the optional Lundahl transformers on each channel. Further transformer options are planned.The Dynamics Toolbox includes two output jacks per channel: the electronic output comes as standard, and this review unit came with the optional Lundahl transformers on each channel. Further transformer options are planned.Photo: Mike CameronWhere transformers are fitted, you get separate output jacks for the transformer balanced and electronically balanced signals, which is a nice touch, and the balanced insert send and return jacks can be bypassed from the front panel, so you can leave them permanently connected to a patchbay. Separate ground-lift switches are provided on the inputs and the outputs of both channels. Another useful switch sets the range of the moving-coil VU meter so that it can operate at nominal levels of +6dBu or +18dBu — so, when it's feeding a converter during mastering, you're able to see the correct VU level relative to 0dBFS set at +18dB. As is commonplace, the same meter can be switched to read input level, output level or gain reduction, and there are separate bypass buttons for the compressor and limiter, as well as an overall bypass and an Insert switch to bring the insert points into circuit.

Despite the clever tricks going on inside the box, the compressor controls are quite conventional, including attack, release, ratio and gain make-up controls, plus a separate knob to set the limiter threshold anywhere between +6 and +24dB. Further buttons engage the auto release and Peakride functions, as well as bringing the EQ into the side-chain circuit. All bypass functions are via relays, and a failsafe bypass operates if the unit is switched off or suffers a power failure. Two knobs control the high- and low-pass filter cutoff frequencies, which cover the ranges 20Hz-2.5kHz and 1.3kHz-15kHz respectively. Finally there are master output level controls and blend controls (with a Blend switch) for each channel, for mixing the dry and processed signals, and a central link button for stereo operation.

While the Dynamics Toolbox definitely deserves 'boutique' status, it is actually very realistically priced. Some of the cost has been saved in sensible packaging rather than the sculpted 'unobtainium' front panels some companies use to bolster the looks of their products, although the Yorkshire origins of the unit make themselves evident when you first switch it on: you're greeted by a panel full of illuminated buttons that look for all the world like a packet of back-lit wine-gums!

Powerful Performer

Hooking up via the jacks is easy enough, although I found the sockets rather tight for my cables, so perhaps they need to be used for a while to loosen up? As an audio processor, though, the Dynamics Toolbox turns out to be extremely versatile and can certainly be used to lay on considerable amounts of gain reduction without obtrusive side-effects. In Dynamic mode, the unit takes on the role of bus compressor very effectively, gently knitting together the parts of a mix, but with the additional advantage of the Blend control and the side-chain EQ for 'bottom up' processing or indirect tonal tweaking. At modest settings this processor adds the required density without killing the high end or introducing audible pumping, but when I brought the threshold right down and turned the ratio right up, I was easily able to get it to kick back in a way that was musically very useful — remember, though, that when you provoke deliberate gain pumping it is important to adjust the attack and release times so that the pumping effect doesn't fight against the beat of the music.

I managed to get some interesting effects by compressing harder than I'd normally do and then mixing in the dry sound using the Blend control, but found that setting extremely fast attack and release times could introduce audible distortion, as the compressor responded to the shape of the incoming waveform itself (not just the overall loudness envelope). I also achieved some dramatic results when using the Dynamic mode to beef up a sampled drum kit, where getting the compressor to the edge of pumping added a lot of excitement and attitude to the sound. Similar benefits were audible on some bass-guitar tracks.

Peakride really is a very powerful compression mode, and I found it particularly useful for evening out the level of vocals that were recorded with excessive performance dynamics. Used conventionally, this will often be enough to get around the problem, but for singers who have a habit of really dropping their level on some phrases, the Blend mode will also be useful, as you can leave the peak levels more or less where they are and bring up the low-level material underneath. In a practical situation I'd probably use mix automation to iron out the worst of the level changes before bringing in the compressor, but for smoothing levels without bludgeoning the sound to death, the Peakride mode works really rather well — and combining three sets of side-chains certainly pays off. In either mode, the limiter caught peaks in a very unobtrusive manner and I had to hit it very hard before it made its presence audible.

There's little obvious subjective difference between the solid-state outputs and the transformer outputs. Critical listening shows up a slightly smoother transient quality with the transformers, and the individual components of a mix gel rather better. It is hard to put your finger on what the difference is in purely technical terms, but the effects are real and (more importantly) musically satisfying, whatever the underlying cause.

A Fine Blend?

Wrapping all this up, it seems that the Dynamics Toolbox is very aptly named: it's certainly flexible enough to meet most compression requirements, from tracking to buss compression to mastering, yet it is also as easy to use as any standard compressor. It can control dynamics in a subtle and transparent way, but it's able to stamp its sonic identity on a signal with the authority of an overweight yeti sporting lead diving-boots. The Blend control, the Peakride mode and the side-chain EQ have to take much of the credit, but the Dynamics Toolbox also boasts a very clean and sweet-sounding audio path. In short, this is a truly professional compressor and well worth checking out. 


Many of these features, or something like them, are available in competing products but this is genuinely the first unit I've come across that offers them all in the same box.

Published October 2008