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Hardware Teardown of Pro-Ject Pre Box S2 Digital DAC

amirm

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#1
This hardware teardown is a follow up to my review of the Pro-Ject Pre Box S2 DAC. The DAC performed well in objective measurements. Let's see how it does from overall design point of view.

Here is the high level diagram/major integrated circuits:

Pro-Ject Pre S2 Box Teardown pictures.jpg


We see one of the nicest layouts given the small box constraints. Digital side is fully isolated on the left with a moat between it, and the right side analog/mixed signal. The asynchronous interface is implemented using ubiquitous Xmos.

Extracted digital audio samples from USB or other inputs is then routed on top across an isolation moat and board to a pair of ESS Sabre ES9038Q2M parts. ESS is secretive with its specs so I don't have a link to provide for it or the rest of the ESS parts used here.

Precision reference voltage for the DACs is provided by ES9311. http://www.marketwired.com/press-re...h-to-simplify-audio-system-design-2085996.htm

upload_2018-2-25_10-29-38.png


Measurements showed absence of typical power supply noise so seems like ESS has done a good job here as has the designer by selecting this part.

Headphone output is provided by Sabre 9802Q: https://www.businesswire.com/news/home/20150108005016/en/ESS-Technology-Introduces-SABRE9602-CES-2015-World’s

upload_2018-2-25_10-35-31.png


Alas with no heatsink and no higher than 5 volt supply, it does not provide much drive capability as noted in my review of Pro-Ject S2 Pre DAC.

Another not so good news is use of Lelon electrolytic capacitors indicated by the "OCRZ" notation on top of them. Lelon is a fourth grade brand of capacitors and is known to fail in power supply circuits. Fortunately the analog side of this board runs cool and parts are not under stress so that choice to save money is probably OK. Not so good is the same choice for the switchmode power supply on the digital side to drive the XMOS. Keep this unit cool and away from other hot running products for better longevity.

Interestingly they did use much higher quality WIMA caps elsewhere.

Here are the "clean" pictures of digital and analog sections:

Pro-Ject Pre S2 Box Teardown pictures digital.jpg


Pro-Ject Pre S2 Box Teardown pictures analog.jpg


Notice the nice board providing isolation between the two sides:

Pro-Ject Pre S2 Box Teardown pictures isolation.jpg


Mechanical design is quite sturdy for such a small unit. Notice the hefty bolt here to hold the PCB with the front panel switch:

Pro-Ject Pre S2 Box Teardown pictures mechanical.jpg


There is no flex as a result and the switch feels solid when pushed in and out.

Same routine is followed in the back with connectors feeling very sturdy:

Pro-Ject Pre S2 Box Teardown pictures connector.jpg


The external power supply is a typical switchmode unit:

Pro-Ject Pre S2 Box Teardown pictures Power Supply.jpg


Summary
The Pro-Ject Pre S2 Digital is a well-built, and well-thought out DAC. Excellent attention is paid to isolation of digital and analog. Layout is very good for such a small unit. Mechanical construction is also quite solid and overbuilt compared to much lessor designs (I am looking at you Schiit).

The only thing that raises the eyebrow is use of cheap electrolytic caps from Lelon. For this price range, spending another $10 on better caps would have been nice to see.

So this is a competently designed DAC both in measurements and electrical/mechanical design.

Note: I should say that this tear down was very difficult due to very tiny footprint of the surface mount parts, and lack of documentation from ESS. I think I got all the part numbers right but I am open to being corrected. :)

Much thanks to forum member who loaned me this unit for measurement and allowing me to get my sticky fingers inside for this teardown. :)
 
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#2
Overall this looks like a really well designed DAC at it's price point compared to the rest of the market. I always like seeing nice well organized looking PCBs. I don't have the expertise to really tell if it's well organized, but it LOOKS good.
 
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#3
This DAC really looks well designed, congrats to John Westlake.
The caps should be easy to fix. Or is there more to it than changing the parts list?
Would the headphone amp perform better with a higher supply voltage (12V)? A "high power mode" would of course require an external PSU. Some USB power based DACs have that kind of feature (e.g. Grace m9xx/m900).
The Grace m9xx/m900 in my view is an excellent piece of kit, a bit more expensive than the Pro-ject though.
 

amirm

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#4
It is a well designed DAC.

Alas, you can't just increase the voltage. That can lead to instant destruction of the unit. Even if it is capable of running at higher input voltage it would need heatsinking, etc. to manage the extra heat which it does not. So this is really a design limitation likely driven by small amount of space they have in there.

On capacitors, they take a while to fail so I would use it during the warranty period as is. After that, open it up and see if any of the capacitors are bulging or anything leaking out of them. If so, then replace them immediately with a Japanese brand capacitor. As a precaution you could replace the cap on the digital side alone after that warranty period.

How much is that Grace design amp?
 

PuX

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#5
How much is that Grace design amp?
Grace m9xx was sold on Massdrop for ~$400 (I think). And the regular m900 is $500.

What bothers me about all these DAC/amps is that they all have switching power supplies or even combined USB power+data.
What's your opinion, do devices with proper toroidal transformers measure/sound better? My assumption is they should.
 

amirm

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#6
That is usually the case. But somehow the problems I typically see with switchmode supplies are NOT visible in Pro-Ject S2. I suspect they selected a switching wal-wart that doesn't have mains leakage.

In general in this price range I like to see a larger box with a linear power supply in it.
 
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#7
That is usually the case. But somehow the problems I typically see with switchmode supplies are NOT visible in Pro-Ject S2. I suspect they selected a switching wal-wart that doesn't have mains leakage.

In general in this price range I like to see a larger box with a linear power supply in it.

Amir, What's your take on Benchmark's claims about switching power supply noise versus linear power supply? They discuss these issues on their site here https://benchmarkmedia.com/blogs/ap...audio-myth-switching-power-supplies-are-noisy
 

amirm

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#8
I agree with them as long as one does NOT buy off-the-shelf power supplies. They have an internal unit there and can make sure there is no mains leakage. The ones sold as wal-warts opt for ease of regulatory compliance by leaking mains into output of the supply which travels through your entire system!
 
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#9
I agree with them as long as one does NOT buy off-the-shelf power supplies. They have an internal unit there and can make sure there is no mains leakage. The ones sold as wal-warts opt for ease of regulatory compliance by leaking mains into output of the supply which travels through your entire system!

Thanks for the reply. I figured you were primarily talking about the cheap wall warts but didn't know if this might play into any of what benchmark was doing.
 
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#10
Am I reading these pictures correctly? The DACs are in the analogue side?

DACs/ADCs normally straddle the analogue/digital split, with digital signal return path going to digital ground, and analogue signal returns going to the analogue ground
 
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#11
Am I reading these pictures correctly? The DACs are in the analogue side?

DACs/ADCs normally straddle the analogue/digital split, with digital signal return path going to digital ground, and analogue signal returns going to the analogue ground
We used to think that back in the beginning of digital audio design. Now look at the ESS dac which has everything, including the kitchen sink, in one chip!
It has SPDIF receiver, filter dsp, clock PLL and sigma-delta converters all part of the chip.
It's important to keep digital signal routing confined to the digital input side and analog to the output side with all supply bypasses located accordingly.
 

Jimster480

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#12
Am I reading these pictures correctly? The DACs are in the analogue side?

DACs/ADCs normally straddle the analogue/digital split, with digital signal return path going to digital ground, and analogue signal returns going to the analogue ground
That is actually a good observation there.... the DAC's are on the analog side... which makes little sense lol
 
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#13
It has SPDIF receiver, filter dsp, clock PLL and sigma-delta converters all part of the chip.
But they should have partitioned the silicon in the conventional manner, to minimise the coupling of digital to analogue. Whilst the ESS website isn't forthcoming any information beyond the home page, I would suspect that the silicon floorplan and pinout of the chip are designed to support board-level partitioning.
 

DonH56

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#14
Maybe Amir could rename the analog side the mixed-signal side and obviate the debate... For devices like this it seem logical to isolate the (noisy) SMPS and incoming high-level digital stuff (which includes that durned USB interface that always seems to be a noise source) from the DAC and its support circuitry. Pretty sure based on the measurements that they didn't mess up the digital signal, power, and return routings for the DAC. I would bet the digital and analog power and ground (return) pours (planes, traces) on the board keep the digital signals and power/gnd separate from the analog side of the DAC chip.
 
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#15
Pretty sure based on the measurements that they didn't mess up the digital signal, power, and return routings for the DAC.
Agreed.

I think that isolation barrier is keeping the really noisy PSU and some of the external digital away from the analogue. If the DACs are driven by a cleaned, differential signal, and there is a small digital area at the top of that 'analog' side, purely driving the DACs, then there shouldn't be too much coupling to the analogue side.
 

DonH56

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I think that isolation barrier is keeping the really noisy PSU and some of the external digital away from the analogue. If the DACs are driven by a cleaned, differential signal, and there is a small digital area at the top of that 'analog' side, purely driving the DACs, then there shouldn't be too much coupling to the analogue side.
Completely agree and was the point I was trying to make.

I was wondering if the signals to the DAC were differential or not. It makes huge sense for isolation CM/power noise rejection, plus limiting radiated noise, but doubles the layout area so I do not know if that is done or not. Even single-ended they can be laid out so as to minimize coupling to the analog side, and again they appear to have no problems either way.

In my world clock coupling was a huge deal and it was hard to keep the clock out of the signal path. Not as big an issue at lower frequencies where it doesn't blast through the parasitics of the switches and traces like it does at X-band or where ever.
 
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#17
For devices like this it seem logical to isolate the (noisy) SMPS and incoming high-level digital stuff (which includes that durned USB interface that always seems to be a noise source) from the DAC and its support circuitry. Pretty sure based on the measurements that they didn't mess up the digital signal, power, and return routings for the DAC. I would bet the digital and analog power and ground (return) pours (planes, traces) on the board keep the digital signals and power/gnd separate from the analog side of the DAC chip.
Exactly. When questioning John Westlake's design, we need to keep in mind constraints like physical size and budget (and others).

The size is in line with Pro-ject's S2 lineup of devices, so not much wiggle room there, as is the retail price.

John's done a fantastic job.
 

amirm

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#19
Maybe Amir could rename the analog side the mixed-signal side and obviate the debate...
I noted that in my text: "Digital side is fully isolated on the left with a moat between it, and the right side analog/mixed signal. "

The output of the DAC is analog so sticking it in the digital section then causes the reverse argument. From what I have seen, it is more common to stick it in the analog section than digital. Indeed Schiit calls their upgrade board for Yggdrasil DAC "Analog Gen 2."
 
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#20
The output of the DAC is analog so sticking it in the digital section then causes the reverse argument.
That's why a DAC or ADC usually straddles the boundary between the two sides, as I said. Look at the app notes from any mixed signal manufacturer, and you'll find PCB layout guidance showing how to do this, and the possible grounding arrangements. Silicon and pinout layouts are also designed to support this arrangement.

The DAC/ADC is the interface between the two domains, so has to straddle the boundary.
 
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