Pro-Ject Pre Box S2 Digital D/A headphone amplifier Measurements

Sidebar 3: Measurements

I measured Pro-Ject's Pre Box S2 Digital with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). I used both the Audio Precision's S/PDIF outputs and USB data sourced from my MacBook Pro running on battery power and playing WAV and AIFF test-tone files. Apple's USB Prober utility identified the Pro-Ject as "Pre Box S2 Digital" from "Pro-Ject," and confirmed that its USB port operated in the optimal isochronous asynchronous mode. Apple's AudioMIDI utility revealed that, via USB, the Pre Box S2 Digital accepted 32-bit integer data sampled at all rates up to 768kHz; its TosLink and coaxial S/PDIF inputs accepted PCM data sampled at up to 192kHz.

Pressing the Menu button revealed that the review sample was set to Distortion Compensate Disabled and the Audio Quality to User. I left these settings alone for the measurements and mostly used the Hybrid reconstruction filter, except where indicated. With the volume control set to its maximum, the output levels were 2.02V from the unbalanced jacks, 2V from the headphone output. The voltage varied very slightly according to which filter had been selected. The maximum levels with the Optimal Transient and Linear Apodizing filters were 0.05dB higher than with the Hybrid filter. The output impedance was a very low 0.5 ohm from both outputs, both of which preserved absolute polarity (ie, were non-inverting).

The Pro-Ject's impulse response depended on which of its reconstruction filters had been selected. With 44.1kHz data, the Brickwall, Fast Rolloff, and Linear Apodizing filters' impulse responses were all conventional linear-phase types, with time-symmetrical ringing (fig.1). Slow Rolloff is a much shorter linear-phase filter (fig.2), Minimum Phase Fast a minimum-phase filter with all of the ringing following the single high sample (fig.3), and Minimum Phase Slow a shorter minimum-phase filter (fig.4). The Optimal Transient filter had a perfect transient response (fig.5; the small amount of linear-phase ringing in this graph is due to the Audio Precision's A/D converter operating at a 200kHz sample rate). The Hybrid filter had some slight Nyquist-frequency ringing, with considerably more ringing after the single high sample than before (fig.6).

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Fig.1 Pro-Ject Pre Box S2 Digital, Brickwall filter, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).

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Fig.2 Pro-Ject Pre Box S2 Digital, Slow Rolloff filter, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).

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Fig.3 Pro-Ject Pre Box S2 Digital, Minimum Phase Fast filter, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).

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Fig.4 Pro-Ject Pre Box S2 Digital, Minimum Phase Fast filter, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).

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Fig.5 Pro-Ject Pre Box S2 Digital, Optimal Transient filter, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).

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Fig.6 Pro-Ject Pre Box S2 Digital, Hybrid filter, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).

With white noise sampled at 44.1kHz, the ultrasonic rolloff again depended on the reconstruction filter and mostly fell into three categories. The Fast Rolloff and Minimum Phase Fast filters rolled off the output above 20kHz, reaching full stopband attenuation by 24kHz (fig.7, magenta and red traces), and the aliased image of a 19.1kHz tone at 0dBFS (cyan, blue) was suppressed by 94dB. The Slow Rolloff and Minimum Phase Slow filters had, as expected, the same slower ultrasonic rolloff (fig.8), with just 27dB attenuation of the image at 25kHz. The Hybrid, Linear Apodizing, and Brickwall filters behaved identically, with a null at the Nyquist frequency of 22.05kHz (fig.9). As is usual with a filter that offers perfect time-domain behavior, the Optimal Transient filter offered a very slow rolloff with just 3dB attenuation of the aliased image at 25kHz, and nulls at 44.1 and 88.2kHz (fig.10).

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Fig.7 Pro-Ject Pre Box S2 Digital, Fast Rolloff filter, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan), with data sampled at 44.1kHz (20dB/vertical div.).

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Fig.8 Pro-Ject Pre Box S2 Digital, Slow Rolloff filter, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan), with data sampled at 44.1kHz (20dB/vertical div.).

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Fig.9 Pro-Ject Pre Box S2 Digital, Linear Apodizing filter, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan), with data sampled at 44.1kHz (20dB/vertical div.).

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Fig.10 Pro-Ject Pre Box S2 Digital, Optimal Transient filter, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan), with data sampled at 44.1kHz (20dB/vertical div.).

Fig.11 shows the Pro-Ject's frequency response with the Hybrid filter at sample rates of 44.1, 96, and 192kHz. The response rolls off relatively quickly above each Nyquist frequency (half the sample rate) from both the line and headphone outputs. The Pro-Ject's channel separation (not shown) was superb, at >120dB below 3kHz, and still 112dB in both directions at 20kHz. The low-frequency noise floor was very low, and commendably free from power-supply–related spuriae (fig.12). The low noise and the superb channel-separation measurement are extraordinarily good, considering that the Pre Box S2 Digital is powered by a tiny wall-wart supply. Someone at Pro-Ject knows how to optimize a printed-circuit-board layout!

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Fig.11 Pro-Ject Pre Box S2 Digital, Hybrid filter, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel gray, right green), 96kHz (left cyan, right magenta), 192kHz (left blue, right red) (1dB/vertical div.).

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Fig.12 Pro-Ject Pre Box S2 Digital, spectrum (0Hz–1kHz) of dithered 1kHz tone at 0dBFS (left channel blue, right red) (20dB/vertical div.).

Increasing the bit depth from 16 to 24 with a dithered 1kHz tone at –90dBFS lowered the noise floor by almost 20dB (fig.13), which implies a resolution of 19 bits. With undithered data representing a tone at exactly –90.31dBFS (fig.14), the three DC voltage levels described by the data were well resolved, with a symmetrical waveform. With undithered 24-bit data, the result was a clean sinewave (fig.15).

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Fig.13 Pro-Ject Pre Box S2 Digital, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with: 16-bit data (left channel cyan, right magenta), 24-bit data (left blue, right red) (20dB/vertical div.).

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Fig.14 Pro-Ject Pre Box S2 Digital, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit TosLink data (left channel blue, right red).

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Fig.15 Pro-Ject Pre Box S2 Digital, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit TosLink data (left channel blue, right red).

With a full-scale 50Hz tone, the second harmonic was the highest in level in the right channel (fig.16, red trace), at –94dB (0.002%), and the third harmonic the highest in level in the left channel (fig.16, blue), at –110dB (0.0003%). These low levels of harmonic distortion were maintained into the punishing 600 ohm load.

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Fig.16 Pro-Ject Pre Box S2 Digital, spectrum of 50Hz sinewave, DC–1kHz, at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

While intermodulation products were always extremely low in level, how the Pro-Ject handled a full-scale mix of tones at 19 and 20kHz depended on the filter selected. With the Brickwall, Fast Rolloff, Minimum Phase Fast, and Hybrid filters, the aliased images of the fundamental tones were all suppressed by 90dB or more (fig.17), though the left channel (blue trace) had more low-level audioband spuriae present than the right (red). As expected from their slower ultrasonic rolloffs, the Slow Rolloff and Minimum Phase Slow filters suppressed the aliased images by around 20dB (fig.18). With its poor ultrasonic rejection, the Optimal Transient filter produced aliased images of the fundamental tones at very high levels, with many higher-order aliased images appearing in the audioband, even when I used the volume control to reduce the signal level by 3dB (fig.19). (The improved performance at the lower level implies that the Pro-Ject's volume control operates in the digital domain.)

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Fig.17 Pro-Ject Pre Box S2 Digital, Brickwall filter, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 100k ohms, 44.1kHz data (left channel blue, right red; linear frequency scale).

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Fig.18 Pro-Ject Pre Box S2 Digital, Minimum Phase Slow filter, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 100k ohms, 44.1kHz data (left channel blue, right red; linear frequency scale).

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Fig.19 Pro-Ject Pre Box S2 Digital, Optimal Transient filter, HF intermodulation spectrum, DC–30kHz, 19+20kHz at –3dBFS into 100k ohms, 44.1kHz data (left channel blue, right red; linear frequency scale).

When I tested the Pre Box S2 Digital for its rejection of word-clock jitter using undithered 16-bit J-Test data fed to the TosLink input, the odd-order harmonics of the low-frequency, LSB-level squarewave were all at the correct levels (fig.20, sloping green line), though a sideband pair of unknown origin can be seen in the left channel at ±2.2kHz. These sidebands were also present with 24-bit TosLink data (fig.21), but were absent when I repeated the analysis with 24-bit USB data (fig.22).

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Fig.20 Pro-Ject Pre Box S2 Digital, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit TosLink data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

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Fig.21 Pro-Ject Pre Box S2 Digital, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit TosLink data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

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Fig.22 Pro-Ject Pre Box S2 Digital, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit USB data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

I was pleasantly surprised by the Pro-Ject Pre Box S2 Digital. This tiny, inexpensive box offers almost state-of-the-art measured digital performance. I note that KM didn't say definitively which of the eight reconstruction filters he preferred, mainly using the Optimal Transient filter. However, I suspect from the measurements that the Hybrid filter offers the best balance between time-domain and frequency-domain performance.—John Atkinson

COMPANY INFO
Pro-Ject Audio Systems, Audio Tuning Vertriebs GmbH
US: Sumiko Audio
6655 Wedgwood Road N. Suite 115
Maple Grove, MN 55311-2814
(510) 843-4500
ARTICLE CONTENTS

COMMENTS
Lorenzo-Italia's picture

Ladies and Gents
With 399$ You get “almost state of the art...”
If you really need more than “almost”, maybe “full” state of the art just multiply almost by a factor of 100!

Is this the way you manage your everyday life ? Think before you buy
Best
Lorenzo from Italy

allhifi's picture

Lorenzo (from Italy): What exactly are you saying ?

Your comment is difficult to decipher/decode ! lol

It's been my long-standing experience to note that 'Digital' source's
(when competently engineered) ALWAYS sound excellent. Unlike 'Analog' gear, very short signal paths and very near-by, 'controlled' circuit power supply 'draw'appears to reward handsomely. Consider 'Chord' Electronics for example.

In any case, the Pro-Next Pre Box S2 appears a thoroughly engineered product. I suspect it will offer up superior SQ. At the very least, it can be used as an evaluation tool with the very desirable option of the multitude of Digital Filter choices.

Presently, I'm using a couple mid-level Network Player's/Streamer's and as far as Internet Radio is concerned, my two separate SBC's (Rasp. Pi, Odroid C2) running Rune Audio/MPD/Linux via USB cable to DAC sounds considerably better than the 'commercial' Player's ;even using the modest Schiit Modi-2 Uber DAC.

later this week, I'll have one to evaluate (PB S2), and shall report back.

Seeing as Pro-ject's Pre Box S2 has garnered favorable reviews, it's surely a fine DAC. It certainly has earned its retail price by offering the latest premium DAC chip (32/768 KHz), careful USB implementation, Display Window, Filter Choice -gotta love that feature/option, MQA capability, premium parts/circuit board along with a pair of spdif Inputs along with USB.

Shall report back,

peter jasz

YankeePhile70's picture

I just bought this little beauty and I can attest to its CLASS A rating! After having owned Audioquest, Grace, and Chord dacs, this one is a keeper. Voices in particular offer outstanding clarity. None of the typical, "venetian blind" affect one gets, where voices don't lock to the center of the soundstage. Subtle shifts in soundstage perception are very audible. The highs are clean and liquid, without sibilance, typical for dacs in this price range. Dynamics are ELECTRIFYING! THIS DAC IS A REAL WINNER!

Kblade's picture

The fact that this reviewer only used the Optimal Transient response filter. Shows me he had no clue of what he was doing. It is the worst filter on the project. This filter pushes the midrange way to forward causing it to sound very dry with no air no sparkle. Awful review! ONE HELL OF A DAC FIVE STARS!

Bogolu Haranath's picture

Another DAC/headphone amp to consider is Chord Mojo ($570) ......... Stereophile Class-A listed ........ Mojo is self powered with built-in battery :-) ........

er1c's picture

I have jumped back and forth between them as well. I too did not hear much difference scanning through the filters at first but now, 6 months in, with great NOS tubes in my Rogue Sphinx 2, I can clearly hear differences between a few. My preference, a significant preference, is the Linear Apodizing filter. With this filter mids on my LS50s are richer and have more weight. Close second (in a mood, a preference) is Hybrid, which on this gear gives some edge to the transients and I hear more energy in high frequency sounds. I did buy it for MQA on Tidal, for which I can offer no strong opinion at this time, just share that it is case by case, sometimes exciting but other times my experience is that I feel something in the music like the way you might get a feeling from food that's too processed, a sense that something is added that's subtly uncomfortable. S2 also doubles as DAC for my Cambridge CD transport, a very enjoyable detailed sound from my CD collection, though I plan this year to move up the food chain with a more sophisticated DAC. Film at eleven. Nice review thanks.

ken mac's picture

for your comments.

rt66indierock's picture

This review is an example of why you measure first and listen second. I measured it, Audio Science Review and John Atkinson measured it. We all came to the same conclusion that it measures well. Amir and I both found second harmonics which would have been worth discussing.

But if Tom Petty sounded louder in the MQA version you didn’t volume match. This renders many of your sound comments invalid. Finally, MQA like any format seems to be normally distributed with the majority either no difference or a different sort of different (thanks Kal).

ken mac's picture

you measure and listen independently. There are many many instances where gear measures poorly and sounds great and vice versa. A story as old as the hills. MQA sounded louder and I adjusted volume. And MQA sounded better on every available track. Your comments are invalid. Cheers.

rt66indierock's picture

I appreciate you documenting your lack of understanding of volume matching. Fortunately, someone I’m frequently at odds with can help, your new editor Jim Austin.

Ken you measure first to simply verify the component is quiet enough to take full advantage of the 96 dB on a 16/44.1 file. If it doesn’t, I box it up and return it as unacceptable.

As for every MQA track sounding better, those of us who tuned live systems with The Nightfly find the MQA version to be worse.

As for your comments about measurements and sound I think you are misinterpreting what Daniel von Recklinghausen meant so I’m going to write an article about it.

ken mac's picture

Which began with J. Gordon Holt, continued through the first JA and is now upheld by the current JA is to listen first, then measure. Sorry if our methods don't agree with your's. Plenty of mags/sites/forums where your views are in vogue...hey, you can even submit your article...Nightfly as barometer? No wonder.

allhifi's picture

Ken Mac is right; your comments (vol.level matching) are naively invalid.

AND, as Ken correctly points out again, listen, measure -comment. Only with considerable experience can one come to the realization that impressive "specifications" and SQ are not particularly related -again as Ken Mac has stated).

pj

Graham Luke's picture

...model worthy of consideration is the excellent Gilmore Lite Mark 2 by Headamp. The Gilmore does not have a built in DAC but pairs incredibly well with a Dragonfly Red. The beauty is that you can mix and match DACs and upgrade in the future.

RaimondAudio's picture

There are many other very good cheap DAC's like: Topping DX7s, Topping DX3PRO, Topping D50, Aune X1s, Khadas Tone,SMSL SU-8V2 etc. Over 1.000$ we have Okto DAC, RME ADI-2, Gustard X26, Yulong DA10, SMSL D1, etc.

dc_bruce's picture

"Class A," they said.

If these classifications have any meaning, then this little box is just as good as, say, the Chord Quetest or the Schitt Yggdrasil. Admittedly, the test scores are nice; but the aural comparisons in the review are with other similarly priced budget DACs, like the Dragonfly. And, it was not even a clear winner, subjectively, over the Dragonfly.

For that matter, after Mr. Atkinson's glowing, enthusiastic review of the Quetest, I was surprised it was not in the "A+" category. Would that be because the much more expensive Chord DAVE is in that same group?

Having never had the opportunity to compare a slew of DACs against each other, I can't say whether meaningful (i.e. audible) differences between DACs exist. But it seems to me that if Stereophile is going to be in the business of ranking them-- Class A+, Class A, Class B, etc. -- then some kind of explanation is in order . . . and more than the vague generalities that precede the rankings.

Or, maybe I should just risk a few hundred $$$ and find out for myself.
So, is it just going to be about the numbers from now on?

allhifi's picture

dc_Bruce: For someone as yourself who doubts any "meaningful" distinctions exist between DAC's has no place here (experience/interest) to cast opinion.

Why in the world would you be interested in Stereophile's rating grade when you yourself don't even own a DAC and quake in your shoes at the thought of shelling out/"risking" a few hundred dollars.

Stick to your 'Gun's' and Sears stereo.

pj

helomech's picture

based on JA's measurements. I had been wanting to try a more modern architecture but figured I'd wet my feet before shelling out 3X as much for a Benchmark or Mytek. After spending a few days with the Pre Box S2, it's hard to imagine much can be gained with those pricier units. While it may not be a giant slayer, it's quite an upgrade over my old Topping PCM DAC. The Pre Box also outperforms my Line Magnetic 502 CA when compared to the latter's SS mode. That DAC had an MSRP of $1800 just a few years ago.

allhifi's picture

Great to hear you are enjoying the S2 -it wouldn't surprise me at all. I also have no doubt it easily outperforms the other DAC's noted.

It would be informative to hear from owner's (or qualified listener's) experiences between the S2 and the Mytek 'Liberty', 'Brooklyn +, Benchmark 2/3 or even the Chord 'Qutest'.

pj

Kblade's picture

What is a qualified listener. Do you have to have dog years or do you have to write for a magazine. What a dumb comment

allhifi's picture

Yoh, Chooch: For 'normal' (i.e. no-moronic) folk it's rather self explanatory.

Perhaps you should spend more time listening to your Candle (brand) radio as opposed to imposing your 5th grade education/mentality here. Good luck -not really.

pj

allhifi's picture

helomech: It's unlikely you/we have "specs" for 'cabling' (or are looking at the right ones -lol) but you be'd really surprised at the SQ distinctions noted if you exchange/experiment with the 5V Walmart power cord/USB-Micro cable -providing the 5V DC power to S2.

AND, without exception, give your DAC it's own space/ shelf/ space -as opposed to perched upon something else.

Finally, if possible feed your 'Digital' gear AC-Regen power or at the least Balanced/Symmetrical AC power; gains in SQ/performance are considerable.

Enjoy,

pj

helomech's picture

had this to say about the Pre Box S2:
https://www.audiocircle.com/index.php?topic=158442.0

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