Pro-Ject Pre Box S2 Digital - "SoC" (and "TiC"*) Review & Measurements

Pro-Ject

Pre Box S2 Digital - $399

Per my recent status post, while I was on hiatus, I received literally NO requests, every week, for a review, impressions and measurements on this little unit. Okay, so @Luckbad claims he asked me to write something up on it, but that spoils the above line so I’m sticking to my story.

However, I was quite interested in its combination of specs and features (dual ESS 9038 build and hardware MQA support especially), particularly at its $399 asking price, so I bought one to play with sometime before the holidays. Despite having had ample time to listen to, and evaluate, this unit I am going to post my thoughts on it in more of a "stream of consciousness" or "episodic" style, albeit with some structure.

Initial impressions were quite positive, including sonically (notably the classic "Sabre glare" is absent here), and, despite some mostly minor firmware issues (two of which DO need to be resolved), they remain so. I still have the unit. I think it's competitive for its price - and actually pretty decent even if you only wanted to use it as a DAC. Obviously the value proposition there falls off somewhat. Specifics will follow, of course, including thoughts on relevant alternatives.

But before things get silly here, let me just say that it's a unit that's well worth checking out if the feature set and price are in-line with what you're looking for and you don't need to drive super-demanding cans directly.



Excuse the phone-quality “photography”. This thing is so diminutive that I probably should have pulled out a proper camera and a macro lens, and made sure I was shooting straight-on, but I’m lazy so this will have to do. The grey, painted, aluminum exterior is not quite as flat and dull as it seems here … the front panel almost looks brushed in proper light … but it’s not as pretty as the black unit.

So despite the fiddly naming, what is this really?

Pro-Ject describe it as a “Digital micro preamplifier with MQA and DSD512 support” or, in other words, the “Pre Box S2 Digital” is a combined DAC, headphone amp, and digital pre-amp with an aggressive spec, feature set and pricing.

The highlights there are three digital inputs (including XMOS-based USB, and COAX/TOSlink S/PDIF connections) and single-ended pre-amp/DAC output, full-size 6.3mm headphone output with a 68mw/32 ohm headphone “amp” driving it. DAC duties are handled by dual ESS 9038 converters, supporting PCM up to 32 bit/768 kHz and support for up to DSD512, with full MQA hardware decoding, with user-selectable digital filters.

There’s also a small-but-useful display and remote control (with input select and volume etc.) and a dual-power option that allows you run the thing entirely of the USB bus, use an (included) external SMPS if you're only using it with S/PDIF sources and to replace the included wall-wart with a better unit of your choice.

Additionally, various claims are made about clocking, power regulation, and the USB interface, and we'll see how those pan out as I complete posting on this.

Measurements

In the exceedingly rare situations which I've bothered to measure audio gear I've never done so before listening. And since I'm not at all practiced at it, and am not using "proper" test gear**, I don't post them. However, since measurements, particularly by people apparently ill-equipped to do them consistently, seem to be a big thing suddenly, especially in other corners of the internet, I thought I’d jump on the bandwagon and do some measurements of my own. No reason I can see that they should have all the fun ...

As you can see below, this thing is ruler-flat from 10 cm to 200mm.



The unit is 103mm wide, 103mm deep, 36mm tall and other measurements as well ... namely it weighs 366g***. As a result it feels pretty solid and sits nicely on the desk without moving around all over the place every time you turn your head or the volume nob.

Just to be sure about those, I measured the unit twice, using different instruments each time to attempt to eliminate any major deviances between their respective calibration. In the picture above the engineering scale I'm using is a cheap plastic job and any "calibration" is, I'm sure, entirely accidental.



The capacitive caliper above IS calibrated however - for reasons that are unlikely to be come clear for the moment.

...

Okay, time to be a bit more serious ... as I do not want to give the wrong impressions of this unit.

I'll start, with some actual measurements. These are not going to be the same vein that's usually seen. I was interested in this purely to see if there was any measurable difference between the selectable digital filters - as there is certainly an audible one.

Note that they're not very granular, were done with a very simple tool (introduced to me by a friend of mine who's visiting in the area at the moment - who also has one of these boxes - note that he's posted a variation of this data [different physical source and ADC] on another site that you might well come across - the plots agree.). Not that it really matters, but source was USB, 24/48 signal, and a basic AKM based ADC was used for the capture.

Effects of the Selectable Filters/Frequency Response for Each Filter:



Roll off starts fairly early, if usually gradually, with all of the filters. This might be what's responsible for the general lack of "classic Sabre glare". The "Optimal Transient" filter rolls off very early and unusually quickly and while it might do good things for transient response it isn't how I'd want to listen to this unit. My preference was for the "Fast Roll-Off" setting, which is what it's named in the menus, but the documentation calls it "Linear Phase, Fast Roll-Off" I believe.

Note that with MQA content there's no choice of filter.

There are some operational "modes" that interact with filter settings. In the menu there are "Best" and "Test" modes. ALL these do is pre-select a specific filter and distortion compensation setting (on or off). If you manually change the filter or the distortion compensation setting the mode switches to "User". The modes themselves have no effect anything ... they're just pre-sets/names for specific filter/compensation settings - so the same settings in "User" mode as are defaulted in "Best" mode will sound, and BE, the same.

...

To come ... operation, interfaces, quirks/bugs/issues, and commentary on some of the more interesting aspects fo the overall spec and features.

After that will be the audible bits ....

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*"TiC" means "Tongue in Cheek". Aspects of these comments are intended to be some light relief from other areas of measurement focus. Specifically the "ruler flat" casework and the calibrated caliper measurements. The remainder of my comments are 100% serious and the unit is definitely worth checking out if you're looking in this price range for these features.

**Ths is likely to change. But first I have to choose and buy a suitable analyzer. Doing the work with test files and high-end general purpose DSOs is quite tedious.

***The previous 600g figure was because I had looked up the weight and apparently that was the whole package. Actually weighing it gives a different number! Apparently I can't do "measurements" by reading the spec-sheet!

 

Okay, the first set of impressions actually had me giggling like a kindergarten brat, and the second set were entirely unexpected. Thanks for both! Kinda curious what bugs you were referring to in the preface, but since neither of them seem to be dealbreakers I'm still cautiously optimistic.

Looking forward to subjective impressions too, measurements be damned (/s)

 

I almost hate to ask, but what does the remote look like?

 

Typical CR2023 powered, "credit-card" type remote. Battery is not pre-installed, so make sure you don't lose it when you unpack the thing.

​

Note that the transport controls you can see feedback to the player application if a) you're using USB and b) that application supports such things.

 

Nice micrometer!

Also, dang that's a lot of filters!

Are you going to cover how it does as a pre as well?

P.S. "10cm to 200mm" had me twitch, as intended.

 

Well, the "pre" part is really just that you can select, remotely if you so choose, between the three digital inputs - and, of course, alter the volume and other settings. There's no analog inputs/switching or anything like that.

As I don't have a conventional analog speaker setup (the Linn Exakt system I run would put this units output through an ADC), other than feeding the pre-outs into one of my headphone amps I don't have a good way to evaluate that.

 

I would appreciate an opinion on the transparency as a preamp, if you can squeeze it in

 

Thanks for taking the time look forward to your further thoughts. There is a lengthy thread over at CA with complaints about customer support and problems with firmware upgrades.

 

Operation:

First, let’s look at powering the S2 unit. You have two fundamental options here: a) USB bus power with a regular USB cable into the USB socket or b) an external 5v USB-terminated PSU (such a unit is included as part of the package).

Contrary to what the manual claims, if the 5v power-input is used then USB bus-power is disconnected internally in the DAC.

The included PSU is really intended to be used when not connecting the S2 via USB. But, per above, you can use the USB input and still get the benefit of a better outboard PSU.

I tried several different outboard units, including the one that comes in the box, a Teddy Pardo unit, iFi Power 5v and a custom battery-based supply (linear … not a CoTS USB power-pack. Initially I thought I was getting a better result using the included SMPS vs. letting it run off bus-power. More listening, including some assisted blind(ish) testing made me reconsider that position.

I did find an improvement with the iFi Power 5V and my battery supply. I had to be using it to drive one of my “proper” headphone amplifiers and TOTL headphones to discern this, however. With the internal headphone amp/connection it all sounded the same. Regardless, I would not personally go any further than using the iFi Power 5V here.

Interfaces & Sources:

While the unit has three inputs, the USB is the one you’re probably going to want to use. Ye,s you read that correctly. Sacrilege I know. But … for one thing, if there was a difference between using it and the S/PDIF inputs it was not apparent coming through the internal headphone output. For another, only the USB input supports the full range of bit-rates and formats (up to 32/768 PCM and DSD512 via DoP); the S/PDIF connections are limited to 24/192 PCM.

The USB input is both actively and passively filtered, as well as being asynchronous. This is on top of the jitter/timing isolation capabilities of the XMOS chipset and the native timing management features of the ESS chipset. The combination of the these seems to work. It took a very high-end S/PDIF source to find any shortfall with the USB connection … not the sort of thing it’s going to be paired with in normal use.

Short version … no audible reason with this unit to use anything but the USB input.

User Interface:

The full-color screen is very small (reference in comparison to the 6.3mm socket), but quite readable. Resolution isn’t very high. It’ll show you the selected input, attenuator level, input format/rate and indicates if the currently playing content is MQA or not. It, of course, also used for making configuration changes to the unit.

The screen on mine has half a vertical line of pixels on the left side of the display that do not work. Wasn’t a big enough issue for me to bother returning it over as it was only visible if headphones were connected directly to the unit.

Buttons are a bit scratchy and squidgy. The knob is an stepped encoder and usually takes two or three detents to actually respond, excepting when changing volume. The knob is clickable, so most options are selected by turning it and then pressing it when you find the value you want. Hitting the menu button takes you into the limited set of options which you can scroll through.

Nothing amazing, or surprising. It’s clean. It works. You won’t be fiddling with it much.

Firmware:

Firmware Updates:

The unit has user-upgradeable firmware. This is a good thing as there are still some outstanding issues with the unit, but I’ll come back to those. When I bought it the only way to do this was from a Windows-based machine. A week or so ago Pro-Ject also released a firmware updater for OSX/macOS. Some people have reported the Mac updater not finding the DAC (even though it plays properly for them) – but it is found correctly for me. I’m assuming they’ve got the device in “Exclusive” mode and still have their player software running.

When I bought my unit almost the first thing I did was update it to firmware v2.12. The next thing Pro-Ject did was remove that version of the firmware! It’s available again now, though apparently is the same as it was before … with a few bugs you want to be aware of:

Firmware Issues:

Some updates here, largely courtesy of "Left Channel" - and the support work in the thread they've been diligently attending to on CA.

There are three principal issues I’m aware of right now, with some in a potential state of flux:

• Some people find their settings aren’t saved if they power the unit off. I have not seen this myself, but it wouldn’t surprise me.

I have not been able to reproduce this and have been informed that it is fixed.​

• The “Distortion Compensation” option works backwards currently. Turning it “On” in the menu increases distortion. Turning it “Off” reduces this. It is known to be a simple flag-inversion and will no doubt be fixed in the next update.

There might be more to this than the simple description suggests. And there's some disagreement if this is actually what's happening. I will do some more testing to see if this is really still the case.

This issue appears to be fixed in firmware v2.12.​

• If the Optimal Transient filter is selected, after playing an MQA track (which disables the filters, per Meridian requirements) and returning to a regular PCM track, the filter will not reengage properly and the music will sound "thin" and weak.

This can be fixed by selecting the next filter in the list, and if you go all the way through the list back to Optimal Transient that filter will be working again too. Better yet, just don't use Optimal Transient (it's not my preference anyway and has a comparatively aggressive roll-off).​

• V2.12 was supposed to fix an “MQA dropout” issue. It has improved it, but not eliminated it. What happens is that you’ll be playing an MQA file and all of a sudden the MQA indicator will go out, the volume will drop significantly, and then after a few moments the MQA indicator will come back on and everything will return to normal. You can go hours without it happening, and then have it happen three times in as many tracks.

I have NOT seen anything like those MQA dropouts with ANY other MQA-enabled DAC (and I own four of them, and have had five here back to back at one point).

Those are things to be aware of if you’re considering buying this unit.

 

I'll make sure this is inserted in the listening notes ... but I measured the headphone output ...

It is, indeed, 6.35mm or 1/4" in diameter.

And since that gag has probably worn thin at this point, I measured the output impedance too which is effectively 2 ohms.

 

Updated the firmware issues notes.

 

Under a factor of 68 margin of error, correct?

 

Again, it's NOT a pre-amp in the analog sense of the word ... it's just an input-selectable DAC with variable output. You can't plug analog sources into it at all, so "transparency" in that sense doesn't really apply. So if you feed it multiple digital sources, you're really just hearing the DAC (plus whatever variation between input types, of which there isn't much).

While it sounds better via it's RCA outputs than it does via it's headphone output, you have to consider that any such comparison has to include the contributions of the then-necessary headphone amplifier. And every headphone amplifier I have, or have had access to, is a significant improvement over the built-in one.

Pair it with something like a Magni 3 or a Vali 2 and that utterly annihilates the built-in headphone output.

Uhuh!

1% margin of error in this case.

 

@Torq right. I just have been severely sleep deprived the first time you mentioned it, as it just didn't parse. Apologies.

I've been eyeing it for my bedroom 2ch setup, which has a power amp I'd keep, but my pre/dac is way too big. Looking forward to impressions with headamps then

 

Updated the firmware issues notes.

Distortion compensation setting appears to be working correctly.

 

Sound:

General:

I’ll tackle the unit as a DAC first, so unless otherwise stated listening impressions are based on output via the RCA connections into proper headphone amplifiers (tube, solid-state and hybrid). The direct headphone output will be addressed separately (and briefly).

Also note that some comparisons to some specific units are based on listening notes and not direct A/B comparison.

Settings:

My filter preference was consistently “Linear Phase, Fast Roll-Off”. I settled on this for all critical listening. “Distortion Compensation” was on. This will result in the “Mode” being displayed as “User” (as previously discussed).

Content/Format:

Most of my listening is, as ever, from ripped 16 bit/44.1 kHz CDs. Comparisons are generally against other units using content in this format. However, it is worth noting up-front that this thing definitely performs better when fed multi-rate DSD content, even if it’s just converted on the fly (by your player/source) from regular RBCD PCM files.

In other words, if you’re willing to drive the Pre Box S2 Digital with DSD 512, via player-driven up sampling, it’s audible performance improves to a useful degree. This is not something I find with most units. Whether it’s down to the updated ESS 9038 chipset’s performance with DSD content or something else is hard to say.

On Inputs:

As already mentioned, there’s not much, if anything, to be gained by worrying about which input you use on this device. I’d start with the USB input, if your source allows this, as it’ll give you the most flexibility. If you need to use the additional inputs (for multiple sources), then I’d use whatever is most convenient. And if you have ground-loop issues, either power the thing via a USB power-pack or use the TOSlink connection (if you can).

Thus impressions are via USB using the included PSU.

Big, Bad, Broad and (Probably Controversial) Comparisons:

Without getting into specifics, or regard to price/value, and given that some thoughts here are based on prior listening/notes rather than direct comparison, some common and/or recently hyped, DACs/sources that I think the Pre Box S2 Digital delivers a more enjoyable listen than include:

LKS Audio MH-DA004, Gustard X20 Pro, Auralic Aries Mini, Chord Mojo, Schiit Modi 2, Mytek Brooklyn, FiiO X5iii, A&K KAAN and A&K 3XX series, AudioQuest Dragonfly Red​

This is not to say that the Pro-Ject box necessarily outperforms all of these units in every technical category, but overall if you stuck all of those in front of me, I’d pick the Pre Box S2 Digital to actually listen to.

In contrast, some DACs/sources that I prefer, sonically, to the Pre Box S2 Digital, also without regard to price/value:

iFi Micro iDSD BL, Schiit Modi Multi-bit, LH Labs GoV2+ Inf, Arturia AudioFuse, Sony WM1x, RME ADI-2 Pro AE, Chord Hugo 2​

Yes, many of these comparisons are nonsensical, price-wise. They are what they are.

The ESS “Sabre Factor”:

Outside some ridiculously over-built ESS 9018-based units (e.g. things like the Merging NADAC) the Pre Box S2 Digital is one of the rare ESS-based unit (in my experience) that doesn’t immediately shout SABRE!!!! Well, to me at least. Whether this is down to the 9038 chipset itself, or the filters (with their attendant HF roll-off) being applied I cannot say.

The treble lacks the sense of artificial detail I’ve experienced with even some of the more serious ESS units (e.g. Auralic Vega) and the fatigue that tends to ensue with prolonged listening to same is absent here.

MQA Playback:

For various reasons, I am done with thinking about, or exploring, MQA. However, since full-hardware MQA decoding is one of the features of this unit that caused me to buy it in the first place, it’s worth covering here.

Using native MQA material, and in comparison to other MQA-enabled DACs (either full decoders, or just renderers that need software to do the first unfold), the Pre Box S2 Digital has delivered the best output I’ve come across so far. The ranking for DACs with hardware rendering/decoding support is, for me, as follows:

Sony WM1Z > Pre Box S2 > Mytek Brooklyn > Meridian Explorer 2 > Audioquest Dragonfly Red​

All of these support full hardware decoding except the Dragonfly Red, which requires the player software/source to perform the first unfold.

This is NOT to say that MQA sounds better than other formats, simply that with MQA content, the little Pro-Ject box yielded the best rendering vs. other MQA-DACs.

Actual Listening:

Also known as “get the smeggin’ point” …

There was no “wow” factor when first listening to this, nor anything that stood out in particular, good or bad, merely an even handed and pleasant presentation. So it’ll probably seem a bit arse-about-face that that was the MOST noticeable thing about it initially. There were no obvious areas of emphasis or reticence to that drew attention away from the music. This is generally a good thing as “wow” factors have a nasty habit of become irritations or problems with more listening.

Tone is neutral and balanced. Overall presentation is pretty smooth. Bottom end lacks some slam. Top end lacks a sense of air and space. But overall the result is coherent and pleasant, and this isn’t warm/rounded in the lower registers like, say, typical data-sheet AKM4490 based units. Nor is it noticeably grainy or uncouth in the upper registers like a lot of ESS and CS based units.

Macro dynamics are pretty decent (perhaps a slight lack of weight is evidenced here in some cases) and better handled than micro-dynamics. Some things don’t resolve as well as I’d like, with one common example I use being aspects of the “gravel” to Leonard Cohen’s voice (particularly in “Show me the Place”). This is less apparent an issue when listening via planar designs than when using highly resolving dynamic cans. In particular it falls behind things like the LH Labs GoV2+ Inf and the Schiit Modi MB in both realms of dynamics.

Actual detail resolution is pretty impressive. D/S designs seem to fair pretty well in detail in budget implementations compared to other musical factors and converter architectures. You can discern subtle aspects of a piece without trouble, such as where a drum-skin has been hit.

Background is impressively dark/quiet, and not just for a unit being fed via unmodified USB (only internal clocking/filtering is present, otherwise it’s a straight to-spec cable to the source). No lack of blackness, which is good for both transient perception (notes emerging from nothing) as well as general reproduction. It beats Modi MB in this regard, as well as the Meridian Explorer 2, and the LH Labs unit.

Presentation generally has a sense of being a little laid back (or maybe “relaxed” is a better term), though not to the point where it harms the sense of pace. There’s a bit less “excitement” evident than I’ve found typical of ESS-based designs in the past, but at the same time this seems to yield a lower incidence of fatigue. The combination of which made my session with “Carmen” entirely enjoyable. Not a standout delivery, but I wasn’t reaching to turn down the volume or failing to complete the entire performance.

Layering and separation are competent but not stellar. You can pick things apart easily enough, but doing so results in a less vivid sense of the individual components of a performance than with, say, Modi MB or the RME units.

There’s not much going wrong here. But also not much that really calls attention to itself. As such I’d say it’s “character” is less pronounced, for better or worse, than many other units in, or around, it’s price-range (taken just as a DAC). It’s competent if un-exciting, but as a result it essentially gets out of the way and plays music.

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This is quite nice, overall, for a unit at this price point and this level of functionality. The raw DAC performance was actually something I preferred to a number of much more expensive, and bizarrely-hyped, units (which perhaps says more about them than it does about the Pre Box S2 Digital). It doesn’t need a bunch of messing about to fix USB issues, can be used as-is without any meaningful compromise in performance.

If the headphone output was on the same level as the DAC portion of the unit it’d be an easy recommendation for an all-in-one or budget-starter system. As it is, unless you’re using low-impedance, sensitive, cans then it just doesn’t have enough grunt to drive headphones in a manner that let the better aspects of the DAC shine through.

With something like the ATH-M50x it does a pretty commendable job, and I imagine it’d pair VERY nicely with the TH-X00 line of headphones. Maybe even the Edition X. But HD6XX or something similar? It’s just not there.

And with such headphones, that are closer to neutral, the bottom end suffers quite a bit (in other words, it’s quite limp), and this is accompanied by an increasing sense of grain the further one progresses up the frequency range

Additionally, the headphone output takes away from the generally smooth presentation of the DAC somewhat and things feel, if not “lumpier”, at least less “even”. There’s also some grain evident, increasing as one goes up the frequency range

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Pair this with a decent headphone amp, such as a Magni 3 or a Vali 2 (etc.) and issues about headphone drive ability go away. And it is not a bad basis for a system in that context. And an even better one if you want things like remote control, multiple digital inputs, and care about either DSD or MQA support (again, it fairs better when fed with DSD).

For me, where DSD is interesting but not essential, and MQA-support has now rendered itself irrelevant in my listening, then if it was me, spending my money, for something to use in (or as) my primary headphone rig, I’d likely pick the Modi MB/Magni 3 combination ($50 cheaper) or the iFi Micro iDSD BL ($150 more expensive) instead.

 

..though for the Modi Multibit, I guess you'd need to budget in a DDC of some kind, if you were looking to drive it from a computer. From what you've said, it seems like the receiver in this unit is pretty competent.

It looks quite neat- if some better firmware eventually turns up. Good to see another decent bang for buck option.

 

I'd say Modi MB is a bit more sensitive to USB issues than the Pre Box S2 Digital and I generally prefer bypassing USB with the Modi MB, be that with a DDC or just using a source that can output COAX S/PDIF.

So, yes, sure, you'd get an even better result with a DDC + Modi MB + Magni 3 than you would with just the Pre Box S2, but you'd also get a better overall result, in my opinion, even without using a DDC with the Modi MB combination. In other words, where the Pre Box S2 falls short is not attributable to use of a specific input, nor does it's USB input benefit much, if at all, from upstream fannying about.

 

It's much easier to recommend a device like that to a civilian if it doesn't require upstream fannying about, so this is a good thing.

 

Anyone considering buying a Pre Box S2 for MQA remember the S2 only processes MQA through USB. You can't use the S2 with a Schiit Eitr and stream MQA using Tidal. You would have to use a Wyrd or iFi Micro iUSB 3.0.