The Soekris R-2R DAC: Technical Details

[rb2013]

A new true R2R Ladder DAC!  The first reasonably priced Ladder DAC to come along in a long time!
 
I'm very excited about the potential for the implementation of this keen new piece of gear.  Now to the particulars.
 
Here is the link to the Soekris website: http://soekris.com/products/audio-products/dam1021-01.html
The official name of the R2R DAC board is "The Soekris dam1021 Sign Magnitude R-2R DAC"
It's priced at $325 for a single board.  I will be ordering one soon for my new ground up DAC build project.
 
I will be sharing much of the excellent and very detail information from the outstanding Audio Engineering blog 
HiFiDUINO. https://hifiduino.wordpress.com/2015/01/30/building-soekris-r-2r-dac/
 
WHY R2R DAC'S?
First why the older design R2R DAC?  Well the R2R stand for 'resistor to resistor'.  Versus the newer Sigma-Delta DAC design - and the newest offshoot the multibit segmented Sigma-Delta DAC.
 
Read this for a basic understanding of the different DAC designs : http://www.mother-of-tone.com/conversion.htm
 
And this for an explanation of the newer designs: http://funwithaudio.blogspot.com/2012/01/today-in-electronics-everything-is-made.htm
 
And from MSB's excellent website: http://www.msbtech.com/support/How_DACs_Work.php
 
MY DACS:
I have had many of both and have to say I'm a big fan of the R2R kind.  The outstanding rich natural tone has sold me on this design.  So much so - I have taken a low cost stock unit using the famed PCM1704 24bit R2R DAC chips and done a series of very extensive mods - the results have blown away my wildest expectations. 
See: http://www.head-fi.org/t/740362/lite-dac60-pcm1704-r2r-tubed-dac-mod-project
 
My other DAC is a very special implementation of the AKM 32 bit DACs in a 6 DAC per channel laddered configuration.  The design of this DAC is proprietary  - but I believe harnesses the R2R portion of these excellent AKM  32bit mulitbit S-D DAC's.  The APL features: tube output (ECC99), Lundahl transformer coupling, advanced clocking, etc.  This is the little sister to the acclaimed APL NWO DAC - {http://www.6moons.com/audioreviews/aplhifi/nwo30go.html).  These are not being made any longer - APL has used this similar topology in it's newest $20K iteration the DSD-M. http://www.aplhifi.com/products/dsd-m
 
OTHER EXCELLENT R2R DAC's AVAILABLE TODAY:
 
The outstanding and very expensive Total DAC. http://www.totaldac.com/principles.htm
My favorite -being a big tubed DAC fan - http://www.totaldac.com/D1-tube-eng.htm
 
Bill Hobba's 'Killer DAC' http://killerdac.com/forum/index.php?topic=932.20
 
OK on to the new Soekris R2R DAC board.
 
Specs and photos (again taken from the HiFiDUINO blogsite)



 
Specs (from HiFiDUINO):
INITIAL FEATURES- Price $325 for a single board (quantity discounts available)
The initial release of the firmware together enables the following features:

• I2S input up to 384KHz sample rate (tested to 192KHz)
• SPDIF input up to 192KHz (tested to 96KHz)
• Automatic De-emphasis for 44.1KHz material
• Built-in set of simple FIR filters for all sample rates
• Digital volume control through simple potentiometer
• Automatic input selection
• Data reclocking: s/w PLL with 0.02 Hz low pass filter
• S/W interface (serial interface) allows:
  • Volume control (e.g. with Arduino)
  • Input selection (e.g. with Arduino)
  • Loadable FIR filters including bypass filter for NOS support (s/w utility included)
  • Firmware update/upgrade

Upcoming features (through user-performed firmware upgrade)

• DSD support
• Filter cascading for digital crossover
• Filter cascading for balanced use
• Master clock output

From the Soekris website:

The dam1021 is a DAC module based on a discrete R-2R sign magnitude DAC design, with FPGA based FIFO buffering/reclocking and custom digital filters, < 1 ps jitter clock generator, with 28 bit resolution so there is headroom, oversampling up to 3.072 Mhz. Up to 24 bit / 384 Khz input from SPDIF, I2S and USB (via USB to I2S interface board), with isolation on the I2S interface. The board is very flexible, with digital volume control and filter parameters that can be downloaded. The board is fully firmware upgradable over a simple serial connection, which enables new features later on. For now we plan to add digital crossover filters soon.​

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The basis R-2R network has an output voltage of 1.4V RMS and output impedance of 625 ohm and can therefore drive a lot of things directly. There is also onboard balanced output drivers that can drive high impedance (>= 300 ohm) headphones directly. The power supply is also onboard, just add a 5W+ toroid transformer.​

Very close tolerance resistors - Standard Configuration: 0.01% resistor version
 
More info from DIY forum http://www.diyaudio.com/forums/vendors-bazaar/259488-reference-dac-module-discrete-r-2r-sign-magnitude-24-bit-384-khz.html
 
I will be sharing more technical details soon.

 

[Benny-x]

Just what I need, more hi-fi DIY...
 
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[rb2013]

  Just what I need, more hi-fi DIY...
 
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+1 LOL I hear you.  If somebody builds one with a 6922 tube output - with 384K hdmi input compatible with the Gustard U12 - USB input to 384k (not mandatory but would be nice) - and excellent components and ps filtering. I'm interested!

 

[rb2013]

And a build has begun over on HiFiDUINO
 
https://hifiduino.wordpress.com/2015/02/10/soekris-dam1021-build/

 

[rb2013]

The price of this board is very competitive with the Twisted Pair Buffalo II board/DAC

ESS Sabre32 Reference (ES9018)​

$379 with Trident Shut Reg's - $299 w/o.
http://www.twistedpearaudio.com/digital/buffalose.aspx

 

[abartels]

  Just what I need, more hi-fi DIY...
 
Subscribed.

Me too!
 
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[Anda]

  WHY R2R DAC'S?
First why the older design R2R DAC?  Well the R2R stand for 'resistor to resistor'.  Versus the newer Sigma-Delta DAC design - and the newest offshoot the multibit segmented Sigma-Delta DAC.

 
Very interesting! Just want to point out that it is not 'resistor to resistor', but a description of how the ladder is designed:
 

 
http://en.wikipedia.org/wiki/Resistor_ladder

 

[Currawong]

I have an 0.01% board and am awaiting seeing what the results are with a good PSU vs. just a transformer before I build anything. The inbuilt PSU is supposed to be "good enough" though.

 

[rb2013]

   
Very interesting! Just want to point out that it is not 'resistor to resistor', but a description of how the ladder is designed:
 

 
http://en.wikipedia.org/wiki/Resistor_ladder

+1 Thanks - you are correct.  I guess just the nomenclature of R2R. 
 
I think this is a good description - I believe is accurate:
Quote from
Mother of Tone:

R2R

In the CD-format chapter you have seen that digital numbers are translated into a staircase signal, and by rounding the steps, we get something that pretty much resembles the originally captured signal.
This translation process from numbers into voltage steps, is what happens in a R2R DA converter which is sometimes also called a "ladder-DAC" or "multibit-DAC", as resistors (the R's in R2R) are configured as an ascending series of voltage dividers.
Such a R2R converter is a static device, in that it is able to generate a clean voltage (with 16 bits resolution there are 65536 different voltages) and hold that voltage until another number is converted, or if numbers repeat - for an infinitely long time, while maintaining an extremely low noise level.
In order to achieve a certain level of performance, the resistors (R's) inside the converter chip must be precisely trimmed, which can make a device very expensive to manufacture, especially when true 24-bit resolution is required.

Versus the regular Sigma Delta design - the newer Multi-bits S-D go along way to solving this - but introduce their own issues (on chip opamps).
Quote from Mother of Tone:

In order to avoid the required precision in the manufacture of R2R converters (and save manufacturing costs), another conversion technique became very popular. In this conversion technique, a single switch replaced the precision resistors of the R2R DAC, and it was believed (and shown to a certain extent) that if only the switch was toggled fast enough, one could also achieve many different voltages.
If for example the switch is more often switched to 5V than to 0V, and if that switch-signal is run through a low-pass filter then the output voltage will also be closer to 5V, than to 0V, plus lots of noise.
This fast switching technique is employed in sigma-delta conversion and it is also the technical foundation of the newly introduced SACD or DSD format.
As a 16 bit R2R DAC is able to generate 65536 different static voltages, and a single switch can only generate 2 different voltages (hi and low), a high switching frequency (usually in the range of a couple of MegaHertz, and achieved through oversampling) is necessary for proper operation and thus, much noise will be produced by the switching process in a sigma-delta converter.
In fact, for any practical application, the noise-level is much higher than the signal that is to be reproduced.
Therefore, this inacceptable high noise-level is shaped into higher-frequencies, as it is believed that high-frequency noise is outside the human listening range and will not degrade sound quality.
As a side effect of this high-order noise-shaping even higer noise levels are generated, residing in the high-frequency region.
To our surprise, in the technical specification of those sigma-delta DACs very low noise-levels are mentioned, and by reading the data-sheets, one could have the illusion, that a decent sound quality would be attainable by such a device.
In fact, the total noise-output is never stated in the data-sheets, and measurements only mention in-band-noise up to 20kHz, suggesting that higher frequency noise has no effect on fidelity.
Of course with this eye-closing practice only those can be fooled that also have closed ears.
In plain English: sigma-delta DACs are coarse noise-generators and when measured the way they should be measured they never make it to 16-bit resolution, don't even think about 24 bits.
As some engineers realized that the el-cheapo method did not lead to satisfying fidelity, because with 1-bit switching performance the quantization noise level was just too much, new converters came up, being called "multi-level sigma-delta".
By increasing the number of voltages from 2 (1-bit) to 5 or more (multi-level), the quantization noise could be reduced.
As this was still not satisfactory, some companies started introducing more levels and coined the term "multibit sigma-delta), as they now use 32 or 64 voltage- or switching-levels, which make up 5 or 6-bits.
Now imagine how much money was spent in research & development to make DA-converter chips cheaper, and in the end just finding out, that more and more levels - more resolution - has to be introduced in order to make them listenable.

 

[rb2013]

  I have an 0.01% board and am awaiting seeing what the results are with a good PSU vs. just a transformer before I build anything. The inbuilt PSU is supposed to be "good enough" though.

I plan on doing the same - as you know building a quality PSU is not easy.  Your help here is greatly appreciated. 
 
I'm also wondering about the buying a .02% board at lower cost for just that purpose.  To just get a general idea of SQ.  I'll contact Soekris about US availability.  He mentioned it in the DIY announcement post - but I didn't see it offered on the his website.  The price he quoted there was $235 for the .02% board vs $325 for the .01% board.

 

[sfo1972]

Just as I got my last DIY project out of the way, with good outcomes, you start this thread and get me all geeked up again to start thinking about a new build! Intrigued as hell...

Subscribed.

 

[rb2013]

Just as I got my last DIY project out of the way, with good outcomes, you start this thread and get me all geeked up again to start thinking about a new build! Intrigued as hell...

Subscribed.

Thanks looking for a PS as we speak. And a decent tube (6922) output board.
 
Here is a Class A board - a bit expensive - http://www.ebay.com/itm/Brand-New-Stereo-Class-A-tube-buffer-using-ECC88-6922-OR-12AU7-ECC82-tube-X-10-/271211226400?pt=LH_DefaultDomain_0&hash=item3f2572e520

 

[rb2013]

Ordered my .02% board today -$235
 
Now looking to start building the PS.  Will need two transformers - one toroidal 5W for the board and another for the tube board.

Originally Posted by soekris
Power is either +- 7-15V DC, preferable 9-12V DC or 2x 7-8V AC. It goes though a diode bridge so polarity doesn't matter. Connector is MTA156 type.

 
This one looks interesting: http://www.digikey.com/product-detail/en/62030/1295-1059-ND/3881371
 
 

 

[RudeWolf]

I have one built. At its current state the design is mostly held back by a suboptimal digital filter. I just hope that someone will come up with a better one for everyone to use.

 

[rb2013]

  I have one built. At its current state the design is mostly held back by a suboptimal digital filter. I just hope that someone will come up with a better one for everyone to use.

Hey thanks!  Do you have the popping issue on turn on/off (from the HiFiduino blog an imbalance in the drain on the neg rail vs pos rail)? 
 
What do you think of using a Acopian linear regulated Gold Box PS like the B12G100 - it's 12VDC 1 amp - 1 amp should be enough
 
http://www.acopian.com/store/9-(7).aspx?min=12&max=16

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