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PreSonus Quantum

Thunderbolt 2 Audio Interface
Published September 2017
By Sam Inglis

PreSonus Quantum

The PreSonus Quantum offers spectacularly low latency and a simplified way of working. Is it the best Thunderbolt interface you can buy?

If you’re buying a multi-channel audio interface for recording to a Mac or Windows PC, it’s not easy to know who to wave your credit card at. There is a great number of products on the market, and to the untrained eye, it can be hard to differentiate them. Convention seems to dictate that mid-priced interfaces come in a 1U rack format, with eight mic preamps and a similar quotient of line-level outputs, plus a couple of headphone sockets and perhaps some extras such as monitor control.

PreSonus’s new Quantum adheres firmly to this particular orthodoxy. Elsewhere, however, it makes a decisive break with convention. For as long as I can remember, audio interfaces with more than a couple of inputs and outputs have included digital mixing and routing facilities that allow input signals to be auditioned at the outputs without passing through the computer’s input and output buffers. It’s a way of dodging the issue of latency that works, up to a point; but it complicates the whole recording process, and is no use if you want to hear soft synths or plug-ins in real time.

Too often, in my opinion, manufacturers have trumpeted the features of these built-in digital mixers in the hope of obscuring the reason for their existence, which is that said manufacturers haven’t been able to code efficient drivers. The last PreSonus interface I reviewed in Sound On Sound was a case in point: the Studio 192 offered good sound quality and a comprehensive feature set, but if you didn’t want to use its much-hyped internal mixer and clunky UC Surface control software, it was a struggle to get the round-trip latency low enough for real-time input monitoring.

So it’s to PreSonus’s great credit that, with the Quantum, they have not only acknowledged the elephant in the room: they’ve ushered it into the corridor, handed it the pearl-handled revolver and locked the door behind it. So confident are PreSonus that the Quantum’s drivers will deliver negligible latency that they have binned the internal mixer altogether. In other words, if you want to hear what’s coming into any of the Quantum’s inputs, you can only do so via your recording software.

Quantum Theory

Whereas the Studio 192 is a USB3 interface, the Quantum is PreSonus’s first Thunderbolt device. Apart from the different computer ports and colour schemes, though, the two are almost identical in terms of features. There are eight of PreSonus’s digitally controlled X-MAX preamps, which are configured either in software or using a front-panel encoder. These are presented on combi XLR-jack sockets: plug a jack in instead of an XLR, and your line-level signal bypasses the preamp, enabling it to reach the A-D converter with no loss of fidelity. The line inputs can be configured to operate at +4dBu or -10dBV, and inputs 1/2 can also be switched to act as high-impedance sockets for plugging in guitars. On the output side, more quarter-inch jacks make available a pair of main monitor outputs, eight additional line-level outputs and two headphone outputs, all of which are independently addressable in software.

The Quantum’s rear panel is jam-packed with twin Thunderbolt 2 ports, MIDI I/O, optical I/O and a host of balanced audio inputs and outputs.The Quantum’s rear panel is jam-packed with twin Thunderbolt 2 ports, MIDI I/O, optical I/O and a host of balanced audio inputs and outputs.

Digital I/O is also plentiful. As well as stereo coaxial S/PDIF and BNC word clock I/O, the Quantum boasts two ADAT optical inputs and outputs, furnishing up to 16 additional channels at base sample rates. It also inherits the Studio 192’s LED ladder meters, monitor control and talkback features, of which more presently. And, unlike the 192, the Quantum has 5-pin DIN sockets for MIDI In and Out, which is a welcome addition. On the down side, however, the Quantum doesn’t have an internal power transformer. Instead, like the 192, it uses an external laptop-style power supply, which locks securely into place.

A glance at their respective specifications reveals that, although the Quantum and the Studio 192 boast an identical complement of audio I/O, there are differences beneath the skin. The Studio 192 already offered better-than-decent audio performance, but the Quantum uses even more highly-specified converters and thus improves on this in most respects. For example, the 192’s line and monitor outputs have a dynamic range of 112dB, but the Quantum raises this to 118dB, and offers the same range on the line ins, up from the 192’s 114dB. Quoted dynamic range for the A-D and D-A converters is 120dB to the 192’s 118dB. (On the other hand, the mic preamp design is the same, so improvements in the quoted EIN figure reflect changes in the test conditions rather than the preamp circuit itself.) The number of situations where you’ll notice a difference in practice is probably small, but it means the Quantum is playing on a level field with other Thunderbolt interfaces like the Focusrite Clarett range.

Like the Claretts and most of its other competitors, the Quantum does not come with a Thunderbolt cable. I know these cables aren’t cheap, but why do manufacturers think it’s acceptable to ship a product costing four figures in such a way that it is basically a doorstop until you’ve paid a visit to the Apple Store? Unlike the Claretts, though, the Quantum sports two Thunderbolt 2 ports rather than just one, and with the help of a few more Thunderbolt cables, this should allow up to four units to be daisy-chained, for a total of 104 inputs and 128 outputs at base sample rates.

Now UC It...

The UC software’s launcher window is where you can set the Quantum’s sample rate and clock source.The UC software’s launcher window is where you can set the Quantum’s sample rate and clock source.Although there’s no internal mixer to control, it’s still necessary to install software before the Quantum can be used. This includes the Thunderbolt driver, of course, along with a touch-friendly utility called Universal Control. This initially greets you with a ‘launcher’ window that offers to update the Quantum’s firmware, if needed, select sample rate (on Windows) and choose a clock source. Once this is done, you can click on or touch the picture of the Quantum to enter the main UC panel. (The launcher remains accessible in the background.)

Compared with the UC Surface utility that is used to configure the Studio 192, the Quantum’s UC utility is blissfully simple and intuitive. That’s partly because there’s a lot less for it to do, but it’s also, I think, down to better layout and design. Though it shares some of the same grey-themed visual cues, it’s much more pleasing on the eye, and I never found myself having to hunt around for some feature that ought to have been somewhere but wasn’t! The one exception to this concerns clock source selection. For some reason, it’s only possible to choose a clock source from the launcher window, and although the Quantum’s power button turns a reassuring blue when a valid clock is received, there’s nothing in the main UC panel to show what clock source is selected, nor any means of changing it. Perhaps this could be added in an update.

In practice, it’s perfectly possible to use the Quantum without needing to visit the UC window very often, which is exactly as it should be. One of the main irritations with digital mixers built into audio interfaces is that you’re constantly having to tab between your DAW and the mixer control panel, which slows everything down, duplicates effort and risks confusion. Since the Quantum doesn’t contain such a mixer, its UC utility mostly reflects features that can also be set from the unit’s front panel, such as preamp gain. However, there are also various options that can only be configured in software. For instance, although the headphone sockets each have their own stereo output paths and D-A converters, convenience dictates that quite often you’ll want them to mirror the main output, and PreSonus have thoughtfully provided the option to permit this.

The UC software can also be used to set preamp gain. The UC software can also be used to set preamp gain.

As on the Studio 192, the level of the main outputs is governed by a large front-panel knob. This applies attenuation in the analogue domain rather than digitally, which helps preserve sound quality at low levels, and means it should still be operational in the event of some digital calamity spewing noise into your speakers. Within UC, it’s also possible to bring any or all of the other output pairs under the control of this attenuator; so if, for instance, you want to use four of the line outputs to connect the subsidiary speakers in a surround system, you can have global volume control over all six 5.1 channels. Mute, dim and mono fold-down are also accessible from UC as well as from the front panel, but there is no provision for alternate speaker switching (which, personally, I’d find more useful than the dim control).

The absence of an internal mixer means that the built-in talkback mic is, in effect, just another input to your DAW. The front-panel Talkback button switches it on and off — pressing and releasing the button latches it on, or you can hold it down and have it disengage when you let go again — but routing and distribution needs to be handled within the DAW. This is fine with me, and makes it easy to slate the talkback mic to a recorded track if you wish.

UC’s simple but effective functionality is rounded out by a couple of nice bonus features. There’s a real-time spectrum analyser, which can be set to inspect any input or output pair (and, as I failed to spot at first, there's also a phase meter in the bottom corner!). And, as on the Studio 192, if you choose to expand the Quantum with PreSonus’s own Digimax DP88 preamps, their gain settings are also accessible from UC. Meanwhile, if you want to store the Quantum’s own preamp settings within your DAW projects, you can add it as a MIDI peripheral and adjust the gain and phantom power settings using Continuous Controller messages, which is pretty neat. In PreSonus’s own Studio One DAW these gain settings show up directly in the mixer [and are saved with the Song project - Ed], as do the Quantum’s main monitor control functions, so you almost never need to open UC.

...Now U Don’t

Ideally, a report on the experience of working with an audio interface should be pretty short. What you’d hope to read is that it was plugged in, recording software recognised it without fuss, and that audio came into and went out of the appropriate holes without noticeable latency, sounding great. In the real world, alas, it seldom works out like that, because a built-in mixer that’s supposed to make things easier ends up making everything far more complicated than it needs to be.

The front panel is slightly less busy, but still hosts a pair of mic/line inputs and two headphone sockets. The front panel is slightly less busy, but still hosts a pair of mic/line inputs and two headphone sockets.

So I’m very happy to report that working with the PreSonus Quantum really is that easy. Want to have four different cue mixes and route talkback to each of them at a different level? Fine: use aux sends to set it up in the DAW project you’re recording into. There’s no need to learn a separate mixing utility, no duplication of effort, no endless tabbing between applications searching for the rogue channel that’s muted or soloed in the ‘other’ mixer. Want to hear your audio input processed with plug-in effects in real time? Go right ahead and use your favourite VST, AU or AAX plug-ins, just as you would at mixdown. Sound quality is exemplary, digitally controlled mic preamps are inherently a Good Thing in my view, and the built-in talkback mic and mono button are both very handy features not found on all of the Quantum’s competitors. All this is made possible by low-latency performance that not only matches that of rival Thunderbolt interfaces but beats them hands down (see 'Latency? What latency?' box).

To be fair to PreSonus’s rivals, I think it’s arguable that a similar level of simplicity can be achieved without completely binning the built-in mixer. For instance, MOTU’s 1248 offers extremely comprehensive mixing facilities, but allows them to be bypassed completely if you want to do everything within your DAW, while the Focusrite Control utility that you get with the Claretts provides very simple cue mixing if you need it and stays well out of the way if you don’t. And the lack of any in-built mixer means that the Quantum can’t be used as a stand-alone A-D converter without the computer attached. However, what PreSonus’s decision means is that you don’t end up leaning on these crutches through sheer force of habit. I don’t know about you, but I’m so accustomed to relying on built-in cue mixers that I tend to use them just because they are there. That temptation is not available on the Quantum, and the user experience is all the better for it.

Thundering Typhoons

Given the Quantum’s launch price, you could be forgiven for comparing it with the Studio 192 and asking why we are, in a sense, paying more for less. After all the 192 has almost the same complement of I/O as well as a sophisticated digital mixer! Both are 24-bit/192kHz devices. However, this disparity in price between Thunderbolt and USB interfaces with similar features is hardly unique to PreSonus, and no doubt reflects the additional cost of developing drivers and chipsets for a new protocol. To my mind, the Quantum’s remarkable low-latency performance and vastly improved user experience are more than worth the extra cost, and of course there’s the improved audio specs to factor in as well.

The Quantum also compares well with competing Thunderbolt products. It’s more expensive than the Zoom TAC-8 or the Focusrite Clarett 8Pre, but the additional outlay gets you digitally controlled preamps, two banks of ADAT ports, built-in talkback and the ability to daisy-chain up to four interfaces. Further up the scale, the Quantum is a little more affordable than the MOTU 1248 and Apogee Element 88. Each of these has distinctive highlights, such as the 1248’s AVB expansion and the Element’s very superior mic preamps, but neither outguns the Quantum on overall features. In short, if its selection of I/O meets your needs, there is absolutely no reason not to buy this interface.

PreSonus have made a bold move by ditching the built-in mixer to focus on low-latency operation, and in my view, it’s exactly the right one. They’ve achieved low-latency performance that, with the exception of PCIe cards, is currently unrivalled by any interface I know of, so if you have a computer recent enough to have Thunderbolt 2 ports, you’ll surely have no need of monitoring workarounds. There can be no better way to respond to the criticism that was levelled at the Studio 192 than by producing an interface that inherits all of that product’s many good qualities and does away with its faults at one stroke. Bravo.

Latency? What Latency?

At least half a dozen Thunderbolt interfaces have now crossed my path, and so far, there has been little to choose between them in terms of low-latency performance. On Mac OS, all offer buffer sizes down to 32 samples, which, at base sample rates, usually translates into a real-world round-trip latency figure of about 4 milliseconds, give or take half a millisecond or so. I was fully expecting the Quantum to offer the same sort of performance — but it doesn’t. It’s way better.

The lowest available buffer size is still 32 samples, but when I chose this setting at 44.1kHz, Reaper reported that the input latency was 0.9ms and the output latency 1.0ms, giving a reported round-trip latency of just 1.9ms. I tested this figure by looping an output back to an input, and found that a re-recorded audio file lined up perfectly with the original, confirming that it is accurate. Even raising the buffer size to 64 samples yields a round-trip latency of just 3.4ms, which is better than any other Thunderbolt interface I’ve tested can manage at 32 samples! In short, then, the Quantum doesn’t just equal its Thunderbolt rivals under Mac OS: it significantly improves on them. What’s more, I formed the distinct impression that it was also more resource-efficient than many rivals, allowing me to run larger and more complex Pro Tools sessions before the dreaded CPU overload warnings appeared.

Not having a Thunderbolt-equipped Windows machine to test it with, I wasn’t able to confirm whether this superb performance is matched on that platform, but as a Mac user, I take my hat off to PreSonus’s engineers.

DC Coupled Outputs

PreSonus chose to launch the Quantum at this year's Superbooth event in Germany. Why? The eight line outputs and two main outs are all DC coupled, which means that they can provide analogue triggers and control voltage (CV) signals to external analogue equipment, such as modular synths. This feature can be used with any DAW that supports it.

Published September 2017