Audient Sumo

Photo: Mark Ewing

Can analogue summing really make a difference to the sound of digital mixes? We test out a new unit which offers this controversial facility.

There are many technical and philosophical arguments both for and against audio summing. Although adding anything extra into a signal path must degrade it in some way, the subjective improvements that can be achieved when mixing DAW audio streams externally clearly indicate that the introduction of an analogue element has some psychoacoustic benefit. Furthermore, where the DAW uses an imperfect digital summing technology, using an analogue mixer may conserve some aspects of the signal resolution more effectively.

In theory at least, digital summing must always produce a result that has an inferior resolution to that of the individual source signals, as some data reduction has to take place in order to scale down the mixed signal to prevent the summed signals exceeding digital full scale. As you might imagine, the more streams are summed, the greater the loss of resolution, as the mix level needs to be scaled down more. Having said that, modern DAW systems with internal 32-bit (or better) audio streams have resolution to spare, but there are still those who say analogue summing sounds noticeably better.

Using a full-scale studio console for such a purpose makes little sense if you do all your processing and automation within your DAW system, so a number of companies have introduced high-quality, but basic, summing mixers to do only what is necessary to combine multiple mono and stereo streams into a stereo mix.

Audient Mixing In A Rack

Audient are in the fortunate position of having a very strong reputation in analogue mixing circles, and the rackmount Sumo reviewed here is a spin-off of their existing console technology, sharing the same balanced stereo mix buss to minimise noise and distortion. Overall the frequency response is flat within ±0.1dB from 22Hz to 22kHz, and is only 3dB down at 135kHz. In addition to its ability to mix eight stereo analogue audio input streams, the Sumo also includes the stereo master compressor and peak limiter taken from Audient's ASP8024 recording console.

A stereo monitor control section is built in, while a 192kHz digital output converter (AES-EBU and S/PDIF with word-clock sync socket) can be bought as an option. Where eight stereo inputs are insufficient, a linking system allows up to three more units to be used as slaves, the whole functioning as a single 64-channel mixer in just 4U of rack space. If you need more still, you could use the link inputs on the slaves to add even more units. Usually stereo submixes would be fed from the DAW's audio interface into the Sumo's input channels, but for those occasions where mono streams also need to be added, channel pairs 1+2 and 3+4 can also be used as mono inputs.

In keeping with Audient's design philosophy, the inputs all have better than 24dB headroom, while the variable gain differential mix amp has more than 27dB headroom. Having plenty of mix-buss headroom is particularly important, because adding signals adds their levels, meaning that the peak output level is likely to be significantly higher than the individual peak input levels.

The stereo buss compressor and peak limiter can be used to process the mixed stereo output, but there are also balanced insert points (switchable before or after the dynamics) that allow other processors to be used on the stereo mix where necessary. The monitor control section is a simplified version of what you'd expect to find on an analogue console, offering control-room volume and mono controls, plus the facility to monitor an external input. Extremely precise stereo metering (26-step LED) is provided on the front panel, measuring from -36dBu to +24dBu, which for most professional digital systems equates to -60dBFS to 0dBFS.

If you're wondering how Audient crammed a 16-input mixer, a compressor, a limiter and a digital option card into a 1U rack, the answer is that that the mixer section has no controls other than Mix Gain and Mix Master. Mix Master provides the final gain control before the mix leaves the unit or enters the digital card after the dynamics section. All the inputs arrive on three sets of 25-pin D-Sub connectors (two for inputs, one for linking to subsequent units) wired according to the Tascam system, with the analogue main mix outs and monitor outs on balanced XLRs. A further XLR and RCA phono are fitted to carry the AES-EBU and S/PDIF digital output signals, while word-clock input is handled by the usual BNC bayonet coaxial connector.

The balanced mix insert points are on four TRS jacks at one end of the rear panel. Two front-panel buttons allow these inserts to be switched before or after the dynamics section, or to be bypassed entirely so that the jacks can be left permanently connected to a patchbay. Two further switches allow channel pairs 1+2 and 3+4 to be individually set for mono operation, where the input is panned centre instead of the hard left/right that pertains in stereo mode.

Because of limited rear-panel space, most of the analogue connections are accessed via three 25-pin D-Subs. Separate sockets are given only for mix and monitor outputs, mix-buss insert sends and returns, and the optional digital interface.Photo: Mark Ewing

The centre section of the front panel is dominated by the controls and meters for the soft-knee buss compressor, which can be bypassed when not in use (to allow the limiter to be used on its own) by means of a switch built into the Ratio control. A separate Bypass button bypasses both compressor and limiter. Controls include the usual Threshold, Ratio (1.5:1 to 20:1), Attack, Release, and Make-up Gain knobs, with eight-section LED metering for both the compressor and limiter gain-reduction read-outs. A separate Peak Limiter Threshold control allows the limiting level to be set anywhere between +10dB and +24dB, which is adequate to provide overload protection when feeding either -10dBV or +4dBu systems, though the Sumo is optimised for use in +4dBu systems.

That leaves the master section, which, as explained earlier, is fairly basic. There's no speaker switching, talkback, phones output, or even monitor dim button, but you can switch to monitor an external source, mono the control room output, and adjust the control-room level. To the right of this is the digital section, which only operates when the digital output option is fitted. Here you can select all six standard sample rates, from 44.1kHz to 192kHz by stepping through six status LEDs denoting the options. All the buttons on the unit have internal LED illumination, and the digital button doubles as a lock LED when an external clock is being used via the word-clock or digital-signal inputs. To select external sync mode, you simply hold down the button for a couple of seconds — the switch LED flashes until sync is achieved, after which it glows solidly.

In The Ring With The Sumo

Testing the Sumo proved to be most revealing, and using it to remix a track I'd originally mixed within Apple Logic showed up a noticeable difference in what might best be described as 'smoothness'. To make sure the difference wasn't just psychological, I opened up both song versions, one mixed via the Sumo and one mixed internally, so that I could switch between them fairly quickly. The difference was most noticeable on the DI'd acoustic guitar, which when mixed within Logic showed up its piezo honk and scratchiness rather more than when it was being mixed via the Sumo. However the overall Sumo mix sounded more refined, with more stable stereo imaging and a better ability to separate instruments within the mix. Even when I soloed the acoustic guitar parts in both mixes, the sound was better from the Sumo, which is slightly curious given that there's no mixing going on in the case of a soloed sound!

This lead me to my next test, which was to go back to mixing within Logic, but then to pass the stereo mix through the Sumo to see if it sounded any different. If the Sumo was simply avoiding summing problems in Logic 's mixer, I should hear the sound get worse, but as it turned out I could detect little or no difference between the sound of a mix done in the Sumo and that of a Logic mix passed through two channels of the Sumo. Why this should still sound better than listening directly to the output of my audio interface isn't clear, especially as its converters were being used throughout. It's also unlikely to be a case of analogue distortion adding something magical, as Audient's signal path is extremely clean.

Audient's compressor, if used in moderation, is good for smoothing and fattening mixers, though I'd have preferred to be able to get down to ratios of 1.1:1, rather than the minimum 1.5:1 on offer here, especially for mastering purposes. I also found the gain-reduction meters rather confusing, as the amber LEDs in the lower meter tend to show through the windows of the top meter, leading you to believe that there is some limiter gain reduction going on where in fact there may not be. The peak limiter works very well, but the manual indicates that this comes before the Mix Master level control, which seems to defeat the object of having the limiter as a final stage of protection.

Fat Sounding Or Fat Headed?

Despite a couple of minor design whinges, there's no getting away from the fact that the Sumo helps make your mixes sound more 'analogue' without adding noise or obvious coloration. Personally I could hear very little difference between using the Sumo as a mixer and using it to process a stereo Logic mix. This being the case, perhaps there's a potential market for a two-channel unit for those of us who can't justify the cost of a Sumo? As to why external analogue boxes of this calibre have such a profound effect on the sound, I don't feel I know enough to offer an opinion, but I'm convinced that it isn't simply to do with getting the summing away from the DAW. I'm equally convinced that it's not just a case of the emperor's new clothes either. Maybe it's just that the inherent flaws in analogue and digital, at least from a psychoacoustic viewpoint, are in some sense opposite and so tend to cancel out when the Sumo is introduced into the system?

In the Sumo, Audient have produced a compact and sweet-sounding summing box that, while not exactly cheap in the UK, is realistically priced given the quality of circuitry on offer. The master section isn't a replacement for a fully featured desktop master controller, but it offers the essentials. To be realistic, nobody is really going to want to do all their master control work from the front of a rackmount box anyway — they'll probably have a dedicated desktop unit for that purpose. Nevertheless, a headphone output could have been helpful. Whatever the real reason, the Sumo does bring about a subjective improvement in sound quality, and it seems to do so on typical project-studio converters as well as when working with esoteric ones. If your mixes are missing that magical something, then the Sumo may help you wrestle with the problem.