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Pro-Ject Pre Box S2 Digital DAC/Preamplifier

Pro-Ject Pre Box S2 Digital DAC/Preamplifier

Pro-Ject, which hails from Austria, has been in existence since 1990. And while it isn’t the first audio firm to specialize in small modular components, it has been especially focused on miniaturized high-performance audio. Currently Pro-Ject has five different “lines” of components—the RS Line, DS2 Line, S2 Line, Classic Line, and Elemental line.

All of these Pro-Ject boxes were designed to be used in conjunction with one another to form complete miniature systems. The Pre Box S2 Digital is from the S2 Line, which also includes a streamer (the Stream Box S2 Ultra), a disc player (the CD Box S2), and a stand-alone DAC (the DAC Box S2 ), as well as three just-announced external power supplies for S2 devices. Pro-Ject also has a Phono Box S2, Bluetooth Box S2, AD Box S2 Phono (to digitize phono sources), Tuner Box S2, Remote Box S2 (with iOS and Android Apps), and to control them all the DS/RS remote.

How small is the Pro-Ject Pre Box S2? Official measurements are 4″ by 1.4″ by 4″, which by any standard of measurement is small. The Pre Box S2 is also relatively light at only 500 grams, which is slightly more than one pound. During my time with the Pre Box S2 I held it in place on my desktop with a VPI brick (the brick is a 4″ by 4″ by 2″ wooden box filled with metal plates that weigh several pounds); otherwise it was far too easy to reach for the volume control and accidentally push the unit far away from its original location. If you lack a brick, the Pre Box S2 also comes with a remote control, so you don’t have to touch it. The price for this miniature marvel? $399.

Technical Tour
As its name implies, the Pre Box S2 is a digital preamplifier, but it also includes a headphone amplifier with a full-sized ¼” headphone output based around an ESS9602 chip. The Pre Box S2 includes three digital inputs; coaxial SPDIF, TosLink optical, and USB B. The Pre Box S2 DAC section utilizes the ES9038Q2M DAC chip, which is based on the ES9038PRO chip but designed with the lower power capabilities of a mobile device in mind. This allows the Pre Box S2 to be powered either by a USB connection or by a beefier 5-volt external power supply. Support for DSD up to DSD512 (DSD over PCM) and PCM up to 768/32, as well as MQA decoding, is built into the Pre BOX S2. Besides MQA decoding, the Pre Box S2 also includes seven user-selectable PCM filter settings that can be accessed via a single button on the front panel or remote. With its own proprietary clock circuitry, organic polymer capacitors, and thin-film mini-MELF resistors, the Pre Box S2 may be budget-priced but is not limited in features or performance.

One area where Pro-Ject devoted additional technical focus was the USB input. According to the company, “one of the major problems for USB DACs is the transmission of unwanted noise coming through the USB output. A lot of research of Pre Box S2 Digital went into how to make this noisy connection quieter. After a good deal of head-scratching trial and error, we found the solution in active and passive filtering of the output.” Pro-Ject claims its two-step process reduces noise by up to 75dB at 1kHz.

If you open up the Pre Box S2, you will discover four-layer, gold-plated PC boards fully enclosed inside its thick aluminum case. While I would not recommend dropping a Pre Box S2 out a second floor window to check its ability to withstand shock, I suspect it might very well survive the fall and retain full functionality. It is an extremely sturdily built micro-component.


Ergonomics and Setup
Considering how limited the Pre Box S2 front-panel space is (the entire surface is 1.4″ by 4″), the area includes a remarkable number of controls and a good deal of display information. On the left side you’ll find the ¼” stereo headphone output. The center area is filled by a 7/8″ by ¾” color display, which is flanked on the left by up/down input buttons and on the right by the filter and menu selection buttons. The right side of the Pre Box S2 holds the volume control knob. The rear panel of the Pre Box S2 has three inputs—USB, TosLink, and RCA coaxial SPDIF—along with one pair of single-ended RCA outputs and a Type C USB 5-volt power connection.

Setting up the Pre Box S2 was as simple as plugging in its wall-wart power supply, inserting a USB cable, connecting the unit to a power amplifier and subwoofer, and finally selecting the Pre Box S2 as my designated sound output device via my Mac’s midi control. Because the Pre Box S2 has only a single pair of variable analog outputs, I resorted to a splitter to supply a second output pair to drive my amp as well as my subwoofer.

Since the Pre Box S2 has its own dedicated, fully featured remote control, which can perform all control functions, you can place the Pre Box S2 almost anywhere within line of sight of the remote. Instead of putting it on my desktop I placed the Pre Box S2 under my desk, right on top of a power amplifier, connected to the amp via a six-inch length of Kimber KCAG cable.

Although the centrally located display is small, it transmits plenty of useful information. Not only does it display the current volume settings of the 80-step volume control, but it also tells you the current bit-rate, current input source, whether the signal is MQA, and finally, whether the Pre Box S2 is connected to an external power supply (as opposed to being powered by your computer’s USB connection).

I used a multiplicity of playback apps and audio sources via my Mac Pro desktop with no hiccups. I tried Pure Music, Amarra 4, Audirvana+, iTunes, the Tidal app, Internet Radio URLs via Safari and Firefox, YouTube videos, AudioTest frequency sweeps, and Roon. Even switching back and forth from MQA to non-MQA tracks via Tidal generated no drop-outs.

I connected a wide variety of headphones to the Pre Box S2. With the most sensitive in-ears, such as the 119dB Empire Ears Zeus, there was no detectable hiss, and at normal listening levels the Pre Box S2’s volume setting read -57dB. Going to the opposite extreme I connected the Beyer Dynamic DT-990 600-ohm version. To get the same volume levels the Pre Box S2 volume was turned up to -14dB. With the HiFIMan HE-1000 V2 the volume setting ended up at -18dB. Given that the vast majority of headphones and in-ears will fall somewhere between the 119dB-sensitive Zeus and the Beyer Dynamic DT-990s, I feel secure in saying that the Pre Box S2 will work successfully with most headphones.

I mentioned the Pre Box S2’s remote earlier. It’s “credit-card” sized but within its confines are all the adjustments you need to control the component. The layout shows some thought, with the mute button at the top center where even a first-time user will find it instantly. Volume up and down are also easy to locate, which is not always the case with many third-party, off-the shelf remotes.

The Project components were designed to be used together as the building blocks of an audio Erector set. By adding the Stream Box S2 Ultra to the Pre Box S2 you gain a streaming source, so you can use the Pre Box S2 without being tethered to a PC or a laptop. Add the CD Box S2 and you can play all your “legacy” CDs. Soon-to-be-available external power supplies—the Accu Box S2, Accu Box S2 USB, and Power Box S2—promise even lower-noise options for the expanding S2 eco-system, which as I indicated earlier, has miniaturized audio devices for almost every audiophile need.


The primary fault I hear with budget-priced audio components is their lack of inner detail, dynamic life, and overall musical definition. I’ve often heard these sonic under-performers described as sounding “gray.” That’s not an issue with the Pre Box S2. When I placed the Pre Box S2 in my nearfield system, it replaced the $999 Mytek Liberty DAC/pre. Even upon initial listen, without any run-in time, it was obvious that the Pre Box S2’s performance was not a big step downward, but more like sideways.

Time for the audiophile equivalent of a warning label: If you are an especially tweaky, compulsive audiophile, do not begin to explore the sonic differences between the various PCM filter options on the Pre Box S2, for that way leads to madness. Well, maybe not total madness, but certainly the sort of wrinkled-brow consternation brought on by too many subtly variable choices. The Pre Box S2 filter options include optimal transient (Pro-Ject preferred), fast roll-off (linear phase), slow roll-off (linear phase), minimum phase fast, minimum phase slow, linear apodizing, hybrid filter, and a brickwall filter.

It would be so easy if only one PCM filter sonically “ruled them all,” but I did not find that to be the case. I found myself vacillating between the optimal transient, linear apodizing, and hybrid filters as my “most often settled upon” after multiple blind A/B listening sessions. Given the physical location of the Pre Box S2 in my nearfield setup, my filter choices were true blind tests (I could not see the front panel while listening, so I didn’t know which filter I preferred until my choice was made). Never, in all my tests, did I choose the brickwall filter. Also, with some cuts I could not reliably pick a favorite. That’s where the consternation part set in.

So, how does the Pro-Ject Pre Box S2 sound? Well, let’s assume you have chosen the optimal filter for a particular track. At that point you would be hearing a clean, well-defined, neutral/natural harmonic presentation that tries to be as “not there” as it can. Depending on your transducer choice, a system based around the Pre Box S2 could be tilted in virtually any sonic direction you choose. Pick a component with a noticeable sonic “personality”—be it your headphones, loudspeaker, or power amplifier—and the sonic palette will be dictated far more by that link of the chain than by the Pre Box S2. And as if that weren’t enough sonic flavoring options, the EQ functions available on all the top playback apps now allow precise sonic tailoring, so end users can tweak the sound of a Pre Box S2-based system to a fare-the-well. The answer to the question “What kind of sound has the Pre Box S2 got?” is “What kind of sound do you want?”

Since $399 isn’t a huge sum when it comes to high-performance DACs, you would think the Pre Box S2 would have little in the way of serious competition. You would be wrong. The $399 IFI xDSD delivers a similarly high level of sonic performance and offers several features the Pre Box S2 lacks. Depending on your primary usage-location and -situation, one will be more useful to you than the other. If you will mainly be using a DAC connected to a computer in a desktop system, the Pre Box would be my first choice due to its superior ergonomics, including the dedicated RCA outputs and separate headphone output, easier-to-read display, MQA decoding, and greater expansion options. But if I required a DAC that would primarily serve as my portable device’s DAC, spending more time in transit, located in a pocket rather than on a desktop, I would choose the xDSD since it has a built-in long-life battery, supports Bluetooth devices, and can easily be operated “blind” while in your pocket. You can’t go wrong with either one, as both offer excellent sound and features at entry-level prices, but each has particular situations where it will excel.

For audiophiles with a slightly larger budget looking for a complete, one-box home-based electronics system solution, I would recommend the $599 PS Audio Sprout100 over the Pre Box S2 Digital because you will be hard-pressed to find an amplifier for $200 to add to the Pre Box S2 that provides the same amount of power and sonic quality as the Sprout100. The Sprout is not expandable like the Pro-Ject S2 system, but it delivers great sound and elegant ergonomics within its compact package.

The Pro-Ject Pre Box S2 Digital offers audiophiles a very high-value DAC/digital preamp at an almost ridiculously low price. Not only does it include a plethora of important features, but it sounds good, has an elegantly designed control surface, and is expandable. As an audiophile’s needs grow, Pro-Ject micro-systems have the components to support nearly every potential source and format available through various accessory units designed to perform specific functions. Got vinyl? Need a device to convert your turntable’s raw output into a digital stream? Pro-Ject has a box for that, too, and at a similarly low cost.

The Pro-Ject Pre Box S2 isn’t just for audiophile newcomers. Perhaps you’ve been involved in high-performance audio for a while and already have a great-sounding DAC, but it lacks USB and MQA capabilities. Just add the Pro-Ject Pre Box S2 Digital to your system, connect it to a computer (or smartphone) via USB, and you can enjoy all the wonders of modern computer-based audio without a major monetary commitment. If you like what you hear, you can “upgrade” later, perhaps adding the Stream Box S2 Ultra to liberate the Pre Box S2 from being tethered to your computer.

Perhaps the best way to view the Pro-Ject Pre Box S2 (and most of the components in the Pro-Ject S2 line) is to consider it a sonic building block or Lego. You can acquire Pro-Ject components to do exactly what you need, no more and no less. The Pre Box S2 Digital can be the beginning piece in a system that can grow and change as you do. While not quite bespoke audio, the Pro-Ject Pre Box S2 is one small part of an elegant system that gives even audiophiles with limited means a way to assemble a first-class audio system tailored specifically to their requirements. If you are looking for a first step to begin your computer audiophile journey or a small addition to your more traditional audio mega-system, I recommend the Pro-Ject Pre Box S2. Small, in the case of the Pro-Ject Pre Box S2 is, indeed, beautiful.

Specs & Pricing

Type: DAC/pre/headphone amplifier
Inputs: USB, TosLink, coaxial SPDIF
Formats supported: PCM, 32/44.1/48/88.2/96/176.2/192/352.8/384/768 kHz; DSD, DSD64, DSD128, DSD256, and DSD512
Output: One ¼” stereo headphone, one pair variable RCA
Dimensions: 4″ x 1.4″ x 4″
Weight: 1.1 lbs.
Price: $399

Sumiko (U.S. Distributor)
2431 Fifth Street
Berkeley, CA 94710
(510) 843-4500

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