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Universal Audio Apollo X

Thunderbolt Audio Interfaces & DSP Platforms
By Sam Inglis

From top to bottom, the Apollo x6, x8, x8p and x16.From top to bottom, the Apollo x6, x8, x8p and x16.

Universal Audio’s new Apollo X range sees the company pull out all the stops to best the competition — have they succeeded?

Over the last two decades, computer‑based recording and mixing has become completely ubiquitous. This trend has brought both challenges and opportunities for equipment manufacturers — and few companies have grasped the opportunities as fully as Universal Audio.

The original Universal Audio was established by studio owner, engineer and designer Bill Putnam, and produced many items of classic studio kit such as the 1176 compressor. The modern company of the same name was set up by Bill’s sons Jim and Bill Jr, originally with the goal of re‑making this vintage analogue equipment, alongside new designs such as the 6176 channel strip and 4‑710D mic preamps. Many software‑based studios have found a role for UA’s analogue gear alongside their Macs and PCs, but what has really put the company on the map is their own contributions to the computer recording revolution.

Jim and Bill Putnam realised early on that there would be a market for plug‑in emulations of vintage analogue outboard, and began recreating devices like the 1176 and LA‑2A for the Pro Tools TDM format. The next step was to develop their own DSP platform: a PCI card that repurposed a graphics chip to run audio DSP algorithms. This was superseded 10 years ago by a newer and more future‑proof product range employing Analog Devices’ SHARC DSP chips. Over the years, these UAD‑2 processors have been made available with PCI, FireWire, ExpressCard and USB connectivity, while the current range is dominated mainly by Thunderbolt devices.

Universal Audio’s ‘Powered Plug‑In’ platform has also been central to their hugely successful Apollo range of audio interfaces. Introduced in 2012, the original silver Apollos were 1U rackmounted FireWire devices, while a second‑generation of black units offered improved specifications and connected using Thunderbolt 2. UA have also introduced Thunderbolt and (Windows‑only) USB Twin and Twin MkII desktop interfaces, which can serve as monitor controllers in a multi‑Apollo system, along with the miniature, bus‑powered Arrow.

The Arrow and Twin MkII remain current products, as do — for compatibility with legacy systems — the original FireWire Apollo and the blackface Apollo 8 Quad. However, the other ‘blackface’ rackmount Apollos have now given way to a third generation of interfaces with grey front panels. The new Apollo X interfaces offer more DSP grunt, Thunderbolt 3 connectivity, even better audio specs and additional functionality, and to cap it off, UA have added a completely new model to the family.

The x8 has four standard XLRs for the mic preamp inputs, and eight separate quarter‑inch jacks for line‑level connections.The x8 has four standard XLRs for the mic preamp inputs, and eight separate quarter‑inch jacks for line‑level connections.

X Marks The Spot

The Apollo x8, Apollo x8p and Apollo x16 are direct replacements for the Apollo 8, Apollo 8p and Apollo 16 respectively, all of them featuring broadly the same list of upgrades over their immediate predecessors. For many people, the most significant of these will be that all three devices are now available only in a ‘Hexa core’ configuration, with six SHARC chips, offering 50 percent more DSP power than the previous Quad versions. This is particularly significant in the case of the x8p, with its eight Unison‑enabled mic preamps; some of UA’s Unison plug‑ins are so resource‑intensive that using them across all eight preamps would pretty much max out the Quad version, but you can now have eight emulated Neve 1073s running alongside other plug‑ins.

Universal Audio have also pulled out all the stops as regards sound quality. There has never been a bad‑sounding Apollo that I know of, but other manufacturers have edged ahead of the silver and black units in the race to achieve ever‑better specifications. UA’s response is impressive: the Apollo x16, for example, boasts dynamic range figures of 124dB on its line inputs, 127dB on its line outputs, and a staggering 133dB on the stereo Monitor output pair. UA have also catered for professional users by adding the option to align these inputs and outputs to +24dBu rather than their predecessors’ +20dBu, and by making it possible to leave all the mic and line inputs connected simultaneously (see 'No More Sharing' box). And whereas most interfaces derive all clock signals from a single crystal, the Apollo Xs use independent crystals for 44.1kHz and its multiples, and for 48kHz and its multiples. This has made it possible to reduce jitter to less than 10 picoseconds.

The Apollo x16 boasts dynamic range figures of 124dB on its line inputs, 127dB on its line outputs, and a staggering 133dB on the stereo Monitor output pair.The Apollo x16 boasts dynamic range figures of 124dB on its line inputs, 127dB on its line outputs, and a staggering 133dB on the stereo Monitor output pair.

The other major changes in these third‑generation Apollos concern ‘master section’ features. The monitor volume control in previous Apollos was a digitally controlled analogue device, which could be bypassed if the interface was being integrated into a studio with its own monitor controller. On the Apollo Xs, the volume control has been switched to the digital domain. In theory, this can have an impact on sound quality, because at low levels, you’ll be hearing what is in effect a word‑length‑reduced version of the output signal. However, given the amazing dynamic range figures quoted for the monitor outputs, you’d have to have a very odd monitor signal chain for this to be apparent in practice, and the key benefit of the change is that the Apollo Xs will all be capable of surround monitor control in formats up to 7.1, with speaker calibration and fold‑down. (I employ the future tense because this functionality was not implemented at launch: a software update should add it before the end of the year.) UA have also equipped all the Apollo Xs with a built‑in talkback mic.

The x8p offers eight Unison‑enabled mic preamps.The x8p offers eight Unison‑enabled mic preamps.

All of the new models boast two Thunderbolt 3 ports, but this time around, they all employ USB‑C connectors rather than the mini‑Display Port sockets used on the previous generation. Windows operation is supported only over a Thunderbolt 3 connection, but Mac users can connect (as I did) from a Thunderbolt 1 or 2 socket using the appropriate wildly overpriced Apple adaptor. Needless to say, neither cables nor adaptors are supplied; this seems to have become the norm with Thunderbolt interfaces, but I’m going to carry on complaining about it until it isn’t!

The Only Way Is x6

The new addition to the range is the Apollo x6. In essence, this is a sort of Apollo ‘greatest hits’ package. It offers a carefully cherry‑picked selection of features from the other Apollos, designed to appeal to the project studio owner who rarely needs to do multi‑mic recording, but requires more flexibility, DSP power and analogue I/O than is available on the Apollo Twin. Like the Twin, it has two Unison‑enabled mic preamps, but these are joined on the rear panel by four further line‑level inputs on balanced quarter‑inch jacks, and on the front panel by two high‑impedance sockets for connecting guitars. On the output side, the stereo Monitor outs are augmented by six further balanced quarter‑inch line‑level outputs, and a pair of front‑panel headphone outputs. There are two optical digital inputs and outputs — although the second of these is present only to maintain the maximum eight‑channel count at 88.1 and 96 kHz (four channels are supported at 192kHz) — plus word‑clock I/O and stereo coaxial S/PDIF outs. And, like the rest of the Apollo X range, it includes a six‑core DSP engine.

The x6 has two Unison‑enabled mic preamps and four further line‑level inputs on balanced quarter‑inch jacks.The x6 has two Unison‑enabled mic preamps and four further line‑level inputs on balanced quarter‑inch jacks.

Build quality seems excellent, as has been the case with all the previous Apollos I’ve tried, and I only have two minor niggles concerning the design of the Apollo X hardware. One is that, like previous Apollos, the new interfaces all require external power supplies; these at least are reasonably substantial devices that employ locking four‑pin XLR connectors to ensure they stay plugged in, but I’m sure most of us would rather use a good old‑fashioned IEC cable. The other is that although the 10‑segment LED ladders used to meter signal levels are admirably clear, I’m less keen on the circular displays that surround the Preamp and Monitor gain controls. These look as though they should offer a continuous measurement, but in fact are quite crudely stepped, and although the luminance of the topmost active step is varied to add some finer graduation, the result is that the readout just looks vague and imprecise.

As before, it’s possible to cascade multiple Thunderbolt Apollo and Satellite units of any generation, the limit being four of the former and six in total. For review purposes, UA supplied all four Apollo X interfaces in a single flightcase, cabled up and ready to go. On the plus side, this gave me a first chance to check out how UA’s drivers and Console utility handle an aggregated system with a large total I/O count. On the down side, if I require a hernia operation in the next month or so, I’ll know who to blame. Racked all together, the Apollo Xs also get hot, to the point where I couldn’t comfortably touch the front panels for any length of time; but this didn’t affect performance as far as I could see, and there are no internal cooling fans to kick in and ruin your takes. (Although they didn’t supply the review units this way, UA recommend that users leave a rack space between units.)

In any multi‑Apollo rig, one unit defaults to being the ‘master’; if you like, you can select a different master unit in the Settings dialogue of UA’s Console utility. Only the master Apollo can be used for monitor control and talkback. On slaved units, the stereo monitor outputs become conventional line‑level outs, and the ALT and FCN buttons are disabled. You can have up to two sets of alternate monitors, but these must be fed from line outs 1‑2 and 3‑4 respectively on the master unit, and cannot be reassigned to other outputs. The dedicated ALT button switches to the first of these alternates, and the assignable FCN button can be used to switch to the second.

By default, however, the FCN button is used for talkback, routing the talkback mic on the front panel of the ‘master’ Apollo to the Console utility’s Talkback bus. The button latches on if pressed and immediately released, and otherwise acts in momentary mode, which is pretty much how I’d want it to behave. It also automatically dims the designated monitor output, which isn’t necessarily what I’d want it to do, but at least the dim amount is configurable and can be disabled. The talkback mic can be routed into your DAW for slating takes and so forth, and even has its own Console channel with insert points for UAD plug‑ins. What’s not possible, however, is to designate any other source as the talkback mic; nor can you use a footswitch or MIDI controller instead of the FCN button. (The preferred solution is to buy an Apollo Twin MkIl to act as monitor controller and talkback source, since this can sit conveniently under your hand and has a dedicated talkback button.)

O Console Mio

No matter how many Apollo devices you have in your system, they are all controlled from a single instance of UA’s Console utility, which expands to accommodate everything. The review system sported around 70 physical inputs, and each Apollo also provides eight Virtual channels; in this sort of configuration, the Console channel count can become intimidatingly large, and the driver can present a huge number of possible inputs and outputs to your DAW software. This last is a particular problem for Pro Tools, which can only access a maximum of 32 ins and outs on a non‑HDX system, and is dealt with through the I/O Matrix page of the Settings dialogue. Here, you can choose whether you want the Apollo driver to present eight, 16, 32, 64 or 128 inputs and/or outputs to the DAW, and freely map these DAW inputs and outputs to physical inputs and outputs. This means you never need run into that awkward situation where Pro Tools can’t ‘see’ a particular input because it happens not to be one of the first 32 presented by the driver.

UA’s Console software.UA’s Console software.

DAW output streams don’t have Console channels, so if you need the ability to adjust their playback level within Console, you’ll need to route the relevant DAW returns to Virtual channels, rather than directly to physical outputs. This can be essential when you’re overdubbing to previously recorded material, especially as there is no positive gain available on the input channel faders, only attenuation. Console channels themselves are routed to the master Monitor bus by default; this routing can be changed, but only to another physical output on the same Apollo. You can’t, for instance, route the first analogue input on one Apollo to the line outputs on another, nor to a Virtual channel. Unusually, it’s not possible to address the headphone outputs directly from DAW software. Instead, a Console setup can contain up to four Cue busses, fed either from the main mix or from sends, and it is these Cue busses that are routed to the headphone outputs.

To help you cope when the size of the Console threatens to get unwieldy, UA have added a number of helpful management features. You are free to name channels, inputs, outputs and entire Apollos according to the scheme of your choice, and there’s also a Show/Hide function that can be used to make unused channels disappear from the mixer. An overview window at the top, which looks a little like the Universe view in Pro Tools, displays all non‑hidden channels, complete with miniature meters, and you can drag around a grey frame representing the main viewing area to quickly focus on the channels of your choice.

Console I/O Matrix page.
  Console I/O Matrix page. This all works well, but to my mind, there’s more that could be done to make large Console setups manageable. It would be useful if the Show/Hide function could be made to operate on an entire class of input channel, so that you could for instance hide all ADAT channels or all Virtual channels with a single click. It would also be nice to have the option for the Console to hide channels that don’t have an active routing destination in the I/O Matrix, as would the ability to have names entered in one area mirrored elsewhere. As it is, it’s quite easy to end up in a situation where a given input has different names in the Console, in the I/O Matrix, and again in your DAW.

Finally, I often found myself wanting the ability to reorder Console channels across different Apollos so that I could put all the mic inputs in a single group, all the ADAT inputs in another, and so on, though I can see that this would introduce potential for confusion.

Unison & Harmony

The opportunity to play with a fully expanded four‑unit Apollo system doesn’t come along every day, and to take full advantage, I arranged a live tracking date with a band. Now, I am not one of those people who fetishises mic preamps, and nor would I usually process signals on the way in, but for this session I decided to embrace the UA way, and turned the Apollo rig into a virtual Neve desk by slapping a Unison 1073 on every input. It was easy to do, it seemed to add a certain je ne sais quoi to the sound, and it didn’t undermine the convenience and flexibility of DAW recording in any way. In essence, the Unison technology allows you to use the Console in pretty much the same way that people use real Neve or API desks as a ‘front end’ for their digital recording setups, and on this sort of project, it felt exactly right. It also resulted in a multitrack that required less work at the mixing stage than I expected, which is always a good thing!

Any mixing utility needs to strike a balance between ease of use and flexibility. UA’s Console leans more towards the former than, say, RME’s TotalMix FX or the internal mixers in MOTU’s AVB interfaces, but I never felt it was preventing me from achieving anything I wanted to do. The only issue I encountered is that, occasionally, putting my Mac to sleep rather than shutting it down caused clocking aberrations on waking. Restarting always fixed this and I’d recommend doing so before any session with the Apollos.

The X Factor

In most respects, the Apollo X range represents refreshment rather than reinvention. It sees Universal Audio’s range of rackmount interfaces leapfrog the competition in the never‑ending race for better specifications; it offers more DSP to run more of their highly desirable UAD Powered Plug‑ins; and in a world where immersive audio is becoming A Thing, the promised surround support negates an increasingly significant reason for choosing the competition. Along with the option to align line‑level I/O to +24dBu and the ability to leave line and mic cables permanently connected, this will help to increase the Apollos’ appeal to professional users, while the introduction of the x6 fills an obvious home‑studio‑shaped gap in the range between the Twin MkII and the Apollo x8.

Admittedly, there are ways in which Universal Audio could have gone further to appeal to the upper end of the market. In terms of connectivity, your choices are still limited to Thunderbolt or Thunderbolt, and while the Apollo x16 offers line‑level A‑D and D‑A conversion of the highest quality, it doesn’t feature DigiLink connectors that would allow it to be integrated into a Pro Tools HDX rig. Nor is there currently any way to use an Apollo as a bridge into the networked audio world, as you can with products like Focusrite’s Red 16 Line or MOTU’s 16m. I’m sure this is a deliberate choice, and that UA considered these possibilities before deciding to retain the focus on project‑studio recording that has served them so well to date; but it does restrict the ways in which an Apollo system can be integrated into wider professional setups.

I don’t think the advent of the Apollo X series will see existing owners rushing to replace their second‑generation units, but really, that’s because UA got the formula right first time around — and the combination of new features, upgraded specs and backwards compatibility will surely tempt anyone who was already thinking of adding extra Apollo units to an existing rig. For those setting up a studio from scratch, meanwhile, the fact remains that when you buy an Apollo, you buy into a particular way of working. It’s a decision that makes the most sense for those who are willing to embrace the Console, Unison and UAD‑2 plug‑ins as core elements of the recording process. If your ideal studio experience is tracking real instruments through analogue consoles, preamps and processors, the Apollo X will get you closer to it than almost any other all‑digital system.

Unison, Plug‑ins & Metering

The Apollos come with separate PDF manuals for the hardware, the Console software and each of the UAD‑2 plug‑ins, which are models of clarity and consistency, and which are extensively hyperlinked so that you can find your way around without constantly having to return to the contents. However, there is one aspect of the Apollo design that presents a few gotchas, and which isn’t documented as fully as I’d have liked.

All of the Apollo Xs’ analogue inputs have front‑panel input meters, each consisting of a ‘ladder’ of 10 coloured LEDs. Input metering is also available in the Console utility, courtesy of the overview at the top, which doubles as a miniature meterbridge, and the detailed meters within each input channel. When no plug‑ins are used in the input path, the readings on the hardware and software meters coincide. However, when UAD plug‑ins are inserted into the standard insert slots, the Console meters now report the level at the end of the signal chain (they can also be switched post‑fade, if you prefer). This situation is unaffected by the channel’s Rec/Mon switch, so if you leave that in the default Mon mode in order to record the unprocessed signal, inserting a plug‑in means the Console meters no longer report either the true level coming into the A‑D converter or the level being passed to your DAW. As a consequence, clipping at the input doesn’t show up at all in the Console.

Things become even more interesting when you insert one of UA’s Unison plug‑ins, which have the ability to reconfigure the gain staging in the analogue domain. These seem to reduce the headroom of the analogue front end to the point where this is driven into saturation before it can clip the A‑D converter; so, although drums and other transient sources can make the front‑panel Clip LED illuminate, less dynamic signals don’t, no matter how hot they are. And although the Console channel clip light can still be provoked into action, this merely indicates an over within the Console processing, not excessive level at the input. The upshot is that it’s possible to have a grossly distorted signal coming into an input with a Unison plug‑in enabled, without seeing any red lights come on. Even keeping the Unison plug‑in interface open is no guarantee that you’ll be able to see when something is amiss; the Neve 1073 plug‑in, for example, has no metering or overload indication at all.

What this all boils down to, of course, is that you need to use your ears. As Joe Meek famously said, if it sounds right, it is right; and as long as you like the way it sounds, the Unison gain structure allows you more freedom to drive the inputs hard without running up against the 0dBFS ‘brick wall’. In that sense, the Apollo behaves more like an analogue recording device than any other digital audio interface I know of. And that, I suspect, is exactly UA’s intention.


The Thunderbolt interfacing protocol is an externalised version of PCIe, allowing compatible devices to hook into the computer’s architecture at a lower level than is possible over USB. One consequence of this is that Thunderbolt audio interfaces typically have very good low‑latency performance. UA’s design philosophy is intended to make round‑trip latency a non‑issue for audio recording, since all monitoring of live inputs will typically be handled through the Console, but of course it is still relevant for playing virtual instruments and so on.

The latency of the Console itself is variable. Some UAD plug‑ins incur latency, and there is a global Console delay compensation setting that can ensure sample accuracy is retained across all inputs on the way in. The lowest round‑trip latency for the system as a whole is achieved when this delay compensation is switched off and no plug‑ins are used in the Console.

When I reviewed the second‑generation Apollo 8p in our SOS December 2015 issue, the lowest buffer size on offer was 32 samples. At a 44.1kHz sample rate, this delivered an output latency of 1.5ms and a minimum input latency of 3ms, for a total round‑trip latency of 4.5ms. Those figures are maintained in the Apollo Xs, but UA have also now added a 16‑sample buffer size option to their Mac driver. Selecting this knocked around half a millisecond off both figures, delivering a reported output latency of 1.1ms and input latency of 2.5ms. These figures also varied slightly depending on how many of the four review Apollo Xs were switched on. On my relatively old MacBook Air, I wasn’t able to achieve glitch‑free playback at all at the 16‑sample setting, and the 32‑sample setting was marginal; the system load seems to rise as more Apollos are added, which is hardly surprising. I did not have access to a Windows machine with Thunderbolt 3 for testing on that platform, unfortunately.

Even if, like me, you are unable to use the Apollo at the very lowest buffer size, this is unlikely to be a problem in practice. Audio entering the system from outside can be monitored at low latency in the Console, while audio generated in the computer — such as the output of software synths — only passes through the output buffer.

It’s So Last Century

Universal Audio Apollo XUA’s plug‑in developers usually focus on emulating classic studio hardware, but the latest version of the UAD software includes a new processor that takes a slightly different tack. Century Channel Strip is designed to have a retro, valve‑flavoured sound, but does not recreate a specific piece of equipment. Instead, the idea is to offer users — particularly those who are new to the world of recording — a simple, foolproof tool for tracking and mixing.

There are three components, which can be switched in and out of circuit as you please: a ‘tube’ preamp with two gain stages, allowing you to take advantage of UA’s Unison technology, a simple three‑band equaliser, and an even simpler one‑knob, opto‑style compressor. These are followed by an output gain control with a VU meter.

UA have done their best to make it all as forgiving as possible, and they’ve also aligned the VU meter in such a way as to encourage the user to leave plenty of headroom, which has to be a good thing. The compressor is generally benign, and particularly effective for levelling vocal tracks, while the EQ is designed for restrained, broad‑brush tonal shaping rather than radical surgery. It is possible to drive the tube preamp into obvious saturation, and this can sound very nice on instruments such as bass guitar, but the controls are arranged to make gentle warmth a more natural outcome.

Whilst experienced engineers will sometimes want a bit more control than is available here, I think Century Channel Strip is a well‑judged tool for those who are just starting out and perhaps finding some of UA’s other plug‑ins intimidatingly complex or confusing.

No More Sharing

The Apollo x8 and x8p provide basically the same I/O complement as their second‑generation predecessors, but the analogue inputs and outputs are implemented differently.

  • Whereas the Apollo 8’s analogue inputs comprised four ‘combi’ mic/line sockets and four line inputs on quarter‑inch sockets, the x8 has four standard XLRs for the inputs with mic preamps, and eight separate quarter‑inch jacks for line‑level connections.
  • The x8p, meanwhile, still offers eight analogue inputs on ‘combi’ XLR/jack sockets, and stereo Monitor outputs on quarter‑inch jacks; but the remaining analogue outputs now emerge on a D‑sub connector rather than quarter‑inch jacks, leaving space for a second D‑sub to duplicate the eight analogue line inputs.
  • The new x6 also features separate XLR and jack sockets for its first pair of inputs.

The point, in all these cases, is to allow both mic and line inputs to be permanently connected and switched in software, which will be welcomed by anyone who needs to plumb their Apollo into a studio rig where a patchbay is involved.

Surround Sound

One long‑standing limitation of the Apollo interfaces is that they have never supported surround monitoring. The switch to fully digital monitor control in the Apollo X series lays the groundwork for its introduction, which should happen before the end of the year, and looks set to be suitably comprehensive.

The Apollo x6 will support formats up to 5.1, and the other X‑series units will offer 7.1 monitoring also. The surround features will include not only ganged level control but the facility to mute, solo and trim individual speakers, and fold‑down of surround mixes to stereo or mono. Even those who don’t need to monitor in surround, meanwhile, will benefit from the ability to calibrate their monitor system to a dB SPL reference level.

Watch our AES show video interview below to learn how surround will be implemented in Apollo X.


  • Excellent subjective sound quality, with probably the best audio specifications of any current Thunderbolt interfaces.
  • With all the Apollo X interfaces featuring six DSP chips, there are no limitations on Unison processing even in the Apollo x8p.
  • Built-in talkback and the promise of surround monitor control.
  • The new Apollo x6 offers a well-judged balance of features for solo recording artists and smaller project studios.
  • Mic and line inputs can be left permanently connected and switched in software.
  • New Century Channel Strip is a worthy addition to UA’s plug-in catalogue.


  • The Apollo range is probably a less good fit for users who don’t buy into the entire UA philosophy of using Console and plug-ins for input conditioning.
  • The lack of networked audio and Pro Tools HDX connectivity limit the options for integrating Apollo X units into some professional environments.
  • No Thunderbolt cable supplied.


Universal Audio’s goal is to make ‘in the box’ recording look, sound and feel more like working with a top-class analogue front end. With the new Apollo X interfaces, they are closer than anyone else to achieving that goal.


Apollo x6 £1800, x8 £2250, x8p £2700, x16 £3150. Prices include VAT.

Apollo x6 $1999, x8 $2499, x8p $2999, x16 $3499.

Published November 2018