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Review and Measurements of Pro-Ject Pre Box S2 Digital


Patreon Donor
Apr 12, 2019
Coventry, UK
This is a detailed measurement and review of Pro-Ject Pre Box S2 Digital. The unit is on loan to me from a kind member but it is one that I have been eyeing for good few months given the designer, John W, who is active on other forums. It was sold in Europe for a while until becoming available in the U.S. Retail price seems to be $399 and unfortunately not available on Amazon as of this writing.

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As you see above, it is a very diminutive box compared to the Topping DX7/DX7s below it. It does have decent weight for its size and sticky feet so it is kind of supporting the heavy cables I have dangling behind it.

I always like to see a display on non-budget DACs as I like confirmation of what is being played, sample rate, format, etc. as often those can be transformed in software without the user knowing. The display here is small and grainy but good contrast. So for desktop use it is fine but in a rack, you need better eyes than me to see it from your seating location. :)

The buttons have a scratchy feel (rubbing against the case?) but otherwise have a very good feel. There are only a handful of options in the menu but despite that, it manages to create confusion with modes such as "Test" and "Best." Apparently the former generates best measured performance while the latter best sound according to the designer. Most of my tests are in "Test" mode with one exception where I show what "Best" does. There is also a distortion mitigation setting which I could not get to the bottom of as turning it on seemed to increase distortion, not reduce it! I will do more testing on this later but wanted to get this review out before that.

Most of the measurements are through S/PDIF input with signal generated from my Audio Precision Analyzer (see https://www.audiosciencereview.com/forum/index.php?threads/understanding-audio-measurements.2351/ on how I test DACs). USB tests were from my HP Z series laptop. Roon player was used in WASAPI exclusive mode. This worked initially but in later testing I had to set it to poll mode as opposed to preferered Event driven. No driver was necessary under Windows 10 Creator's edition.

Roon reports the following formats supported:

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I tested the device using S/PDIF with USB connected and vice versa but did not detect any degradation in measurements. So isolation is good. For S/PDIF testing, I tested with the external power supply. With USB I tested both ways but it made no difference.

Let's start with simple test of frequency response:

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Even at this magnified scale we don't see any problems here. Response is just 0.2 dB down before the cut off which is a bit low at 18.6 Khz.

Next let's look at Jitter and noise over S/PDIF:

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There are a few ticks here and there but they are down below -130 dBFs so really not a concern. Importantly there is no low frequency noise like there is in Exasound (see the green peaks on far left). Surprisingly clean for a device that uses switchmode power supply as there is no leakage current visible. Well done!

Next I compared USB to S/PDIF:

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We see a rise in noise floor with some peaks and valleys. Nothing of audible concern but a bit disappointing.

NOTE: in re-testing for effect of power, etc. I could NOT duplicate this degradation. USB was just as clean as S/PDIF above. So not sure what causes this condition to show up. But again, not an audible or even a measurement concern due to very low levels.

Next let's look at how linear/faithful the DAC is as levels get smaller and smaller:
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I am showing a comparison to Topping DX7. We see that while both reproduce "19 bits" faithfully according to my 0.1 dB of error cut off, the Pre S2 DAC is smoother than Topping DX7 before getting there. Mind you, we are at the limits of my analyzer here at nearly -120 dB so not an audible concern. But fair is fair and the Pre S2 does pull ahead a bit here.

Along the same lines let's see how a 24-bit file plays a -90 dB sine wave that is at the limit of 16 bit audio files:

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Sorry about bolding of the graph on the right. Did not realize I had selected the graph when I captured it. Both are similar here and produce a very good sine wave. Only the Exasound E32 is superior (not shown) at 9X the cost.

Let's look at intermodulation distortion:
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As we see here, Topping DX7 and especially Exasound E32 do better here.

Let's look at THD+N versus frequency:
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Similar picture emerges with low frequencies having higher noise/distortion in Pro-ject Pre S2 Box.

Finally let's look at what happens with a 1 Khz tone when we filter it out and look at what is left (i.e. distortion and noise):

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You can't see it but the S2 has higher second harmonic and that probably causes its measurements to suffer above. Further out as you see, it is competitive to better than Topping DX7. In all cases though the levels are very low and not an audible consideration for the most part.

As I promised, here is the difference in above test when selecting "Best" versus "Test" in the menu:

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We see that "best" sharply raises the noise floor by some 25 dB! In doing so the noise now masks all of those distortion products. Seeing how there is some 32 dB of gain in the above measurements, the "best" mode likely has less signal to noise ratio than needed for 16 bit audio. When I have time I will do some listening and see if there is an audible difference. For now, I would probably use the DAC in "Test" mode. :)

The Pro-Ject Pre S2 Digital Box has an awful name but competently designed. I don't detect any aberrations in its performance. While it loses to Topping DX7 in a couple of tests, I don't think that will impact its audible performance. As a result the Pro-ject Pre S2 Digital DAC goes on my recommended list!

Note: The Topping DX7 is discontinued. I have the replacement DX7s here which I will review soon.

Edit: See the hardware teardown here: https://www.audiosciencereview.com/...-ject-pre-box-s2-digital-dac.2393/#post-67549
As always, I welcome comments, corrections, questions, etc.

If you like this review, please consider donating funds for these types of hardware purchase using Patreon (https://www.patreon.com/audiosciencereview), or upgrading your membership here though Paypal (https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054).
Hi Amir, just registered. Whatever you did so far is quite commendable, I shall donate :)

Question, why didn't you measure the 'Jitter' on S2?

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