PRO-JECT PRE BOX S2
digital micro preamplifier
This dinky DAC and headphone amp is nicely priced at $580 and yet offers high levels of performance whatever your format preference.
Pro-Ject terms this product a ‘digital micro preamplifier’... we would be more prosaic, and call it a DAC/headphone amp, although with three inputs and an output under volume control, preamplifier it certainly is as well. It’s a small thing, larger than life in our picture above, being just 103mm wide, 37mm tall and 122mm deep… which includes the volume/control knob on the front. It weighs just 366 grams. Yet my goodness, it does enough for both its size and its price. Some might even call it overkill — but it overkills in the right way, and does so without the usual monetary penalty.
Equipment
Pro-Ject is, of course, best known for its turntables, and primarily for the quality of those turntables and their value for money at whatever given price level. But it has also run a nice line in little boxes like this, and in fact, the Pro-Ject Pre Box S2 Digital was designed by John Westlake, former designer of products for Cambridge Audio, Peachtree Audio and Audiolab. It seems that this was largely a labour of love for him, with Pro-Ject taking this device the rest of the way to industrial production.
In addition to the volume/control knob on the front panel there’s a 6.35mm headphone output, four small control buttons and a small — 25mm diagonal — colour display. Two of the control buttons are for selecting inputs, which brings us around to the rear panel (shown opposite). In addition to the USB Type-B socket, which you use to connect to a computer for USB audio, there are also coaxial and optical digital audio inputs. Analogue output is via standard RCA sockets.
Via the optical ro coaxial inputs, the unit supports PCM digital signals up to 24 bits and 192kHz sampling. Used as a USB audio device, the unit supports PCM right up to 32 bits and 768kHz sampling, as well as Direct Stream Digital signals up to DSD512. That is eight times the standard SACD rate of DSD64.
That’s what Pro-Ject says, anyway. Despite my assiduous collection of test clips, I could provide material only up to 24 bits and 384kHz in PCM, and DSD256. All of them worked perfectly.
And if you wanted to add a swing-tag saying ‘buy me!’ the Pre Box S2 Digital also supports MQA and is Roon approved. There’s one last extra: a credit-card infra-red remote control.
The DAC section uses two ESS Sabre ES9038Q2M 32-bit DAC chips, the lowpowered versions of ESS’ premium DAC chips. Headphone output is driven by an ESS Sabre ES9602Q headphone amplifier. Also incorporated is the ES9311 dual regulator to clean up the power supply for low-noise work.
Performance
Installation went smoothly; pretty much plug and play, with set-up only just a little more
complex on Windows than it is with a Mac. The driver installs a little control panel which allows you to tweak a few things, such as changing the ASIO buffer size (if you get dropped samples, you might be able to fix it here).
As I have already noted, I found that the Pro-Ject Pre Box S2 Digital DAC worked perfectly well with all my PCM and DSD tracks up to 24-bit/384kHz and DSD256. I could be certain of that because not only did it sound fine, but also the front panel display showed the format that was being decoded (along with the volume, the source and whether external power was connected).
There were no weird anomalies in playback. Perhaps the only criticism is that sometimes when switching from one DSD track to another there was a very low-level ‘click’ audible at the transition between tracks… but that was it.
I listened and listened and listened for weeks. I used the line outputs with high-quality powered speakers, plus several different kinds of headphones. The headphones I used most were the Blue Ella over-ear headphones reviewed elsewhere in this issue (used here in passive mode), the Oppo PM-3 planar magnetic over-ear headphones, the Audiofly AF-140 in-ear monitors, and my old Sennheiser HD 535 over-ear open-backed dynamic headphones. I kept coming back to the Sennheisers both because they are tricky to drive, and because they sounded glorious being driven by the Pro-Ject Pre Box S2 Digital unit.
The Sennheiser HD 535 headphones have a variable impedance curve, a middling high impedance (around 120 ohms) and fairly low sensitivity, but despite them being a difficult load, there were no perceptible frequency response anomalies with the Pro-Ject Pre Box S2 Digital, and plenty of volume range, even with material recorded at a low encoding level. It was a delight.
All the other headphones I used are significantly easier loads, with lower impedances and higher sensitivity. The Blue and Oppo both have flat impedances, but the Audiofly IEMs have a curve which results in a substantially elevated bass when fed by a source with a high internal resistance. None of that was evident when listening through the Pro-Ject Pre Box S2 Digital, which suggested to me that its output impedance is very low.
The DAC/headphone performance was utterly quiet. There was no perceptible noise… nor, indeed, anything else audible that could conceivably detract from performance. If there were any defects in delivery, they were well below the limits of human perception.
The purist in me wishes that the unit had a switch to create a fixed output for the RCA outputs. But that’s the old analogue me. The digital me realises that there isn’t a potentiometer attached to the back of the volume control knob. It is merely an interface to the digital level controller. If there were a switch, it would simply be selecting one of the exact same settings as the round control knob.
The important thing is that the unit maintains separate volume level settings for the headphone and RCA outputs. So whenever you remove your headphones from the unit the line output activates and returns to whatever volume you left it on… typically maximum.
The provided remote control has buttons for input selection, volume, muting, and filters (to which I will return), as well as other settings. But the remote had one big surprise in store, because it also has play/pause and track skip keys… and they all worked — at least they did with Foobar2000 running on Windows. This was a new one to me: I hadn’t realised DACs were capable of sending control signals back up the wire to the computer. Small bananas? Not really, as it makes the whole device far more usable when plugged into a computer. You don’t need to be sitting right on top of the computer for basic control.
As with many DACs, though often more expensive ones, there are a variety of filters available, which you can play with all day trying to hear the difference. The most fascinating one, at least from a technical point of view, was the ‘Optimal Transient’ filter. This is the filter recommended by Pro-Ject and it is one weird filter. Rather than being a low-pass filter, it seems to be a slow notch filter centred at 44kHz. How slow? It’s already down by 0.8dB at 10kHz and 3.5dB at 20kHz. But it bounces up again shortly afterwards, and at the Nyquist point of 22.05kHz it’s only down by 3.1dB. Audible effect? Nothing that I could hear. So…. why? I guess it’s in the name. Of all the filters fitted, it really does produce the optimal transient. There’s no doubt about it, ‘Optimal Transient’ produces far cleaner-looking 1kHz square waves than any of the other filters. All of them displayed the customary ‘ringing’, some more than others, and some biased more to one end or the other of the wave. But if you want the prettiestlooking square wave, Optimal Transient is the one that does the job. Of course, that’s because it isn’t filtering out the higher-than-Nyquist frequencies. At least I doubt that it is, but I await some well-conducted double-blind testing to prove me wrong.
Conclusion
The Pro-Ject Pre Box S2 Digital DAC and headphone amp is a high-performance unit, and versatile too, given you could use it as a preamplifier if all your desired sources fit into its three inputs. It has a strong and highquality headphone output, while the DAC section handles all non-obscure extant audio formats — and is pretty much future-proof given the PCM performance to 32-bit/768kHz and DSD upto DSD512. For this money, it’s something of a ‘wow’. Stephen Dawson