I/V + output discrete stages or transformer for DSD-1796

[Tino_diy]

Dear digital DIYers,
I have no experience in DAC modding, but would like to improve the OPamp based I/V + low pass buffer of my SONY XA5400ES SACD players (that you can see here:
http://digilander.libero.it/agostino..._xa5400es.html ), which is based on a Burr Brown DSD-1796 DAC, with maximum balanced output current of 4mA PP (about -3mA of DC bias?). Since I have not enough experience to develop a good board (e.g. layout grounding), I'm focusing on universal I/V + buffer boards like the following (price in 200-400 Euro range):

- Erno Borbely EB-108/435 or EB-604/417 ALL-JFET I/V converter, Filter and Buffer for DACs.
http://www.borbelyaudio.com/pics/Web435DAC.pdf
- Audial Module LAB (by the good DIY member Pedja Rogic)
http://diy.audialonline.com/modulelab/
- ZAPfilter MK 2
http://www.lcaudio.com/index.php?page=6
- New Class D Gold pre
http://www.newclassd.com/index.php?page=50
- Universal Stereo Tube Output Stage
(from Hong-Kong)
So my first question is if some of you have tried one of these with the BB 1796 DAC or compared the sound performance of some of them.

My second question is if some of you have tried a simple transformer coupling with this specific DAC. If I connect the I+ and I- outputs of the DSD-1796 directly with the two primaries of a 1:2 transformer and connect the single secondary say to a 400 Ohm resistor then the DAC output will be loaded by a 100 Ohm resistance. The maximum 4mA PP output will create a maximum voltage output of 40mV, which should be low enough for the DAC protection. Anyone have tested how much is the DC voltage compliance of this DAC?
The maximum output of the transformer will be 0.8V, which is 8dB less than usual... not a big problem, despite the big simplicity.
The transformer must handle about 3mA of DC current bias, which will be multiplied by the transformer winding resistance to get the voltage offset, so care must be used to chose the transformer. Eventually this DC current can be stopped adding a capacitor in the primaries central tap, if that will not degrade the sound performance or load too much the current DAC outputs.
The output impedance of the player is 400Ohm, so care with the cables.
I think that something similar is done by the Reference Audio Mods company (CA, USA), using Audio Consulting (CH) transformers, e.g.:
http://www.referenceaudiomods.com/Me...oduct_Count=36

Has anyone experience with that topology on the 1796 (used also in the Denon 3910ci) or very similar DAC (e.g. 1798)?
Best regards and sorry if I will not be prompt in answering, but I'm going away for work in the next 2 weeks...
Ciao!
Tino
http://digilander.libero.it/agostino...dio/audio.html

[SoNic_real_one]

Looks like the DAC is the "smaller" brother of DSD1794 (7.8mA output, 129dB)? Nice parts.

[dddac]

I did this with my sony sacd player SCD777

not exactly same dac (1738) but very similar set up

you can read the article on my website www.dddac.de

http://dddac.de/at04.htm is the direct link

[Tino_diy]

Hi Doede,
if I understood well, your I/V stage was done with simple resistors (150 Ohm?) and the low pass with a capacitor (10nF Auricap) in parallel with the Sowter 8347 primary. I was told that a simple resistor I/V would not be optimal for the DSD-1796.
Thanks anyway,
Tino

[Calvin]

Hi,

the Borbely stage is truely fine stuff. With the PCMs of TI/BB you can either use them as active I/V-stage feeding the current to the negative input, or use a resistor as passive I/V converter and using the Borbely as amplifying stage -feeding the signal into the positive input. Seems that all TI/BB DACs feature protection diodes at their outputs, so the value of the I/V-resistance must be quite small <100Ohms to keep the voltage swing low enough for the diodes not to go into conduction.

jauu
Calvin

[Tino_diy]

Hi Calvin,
did you tried the EB-108/435 or EB-604/417 with a PCM179x yourself? Is it the extra cost of the 435 worth for you? Have you compared it with the others?

If at the DAC outputs there are diodes which will go into conduction at 0.7V, then 4mA*100Ohm = 0.4V should be safe enough, but I would like to know from someone who has tried it. It seems that Doede (dddac) has tried 150 Ohm without problems, but the DACs were quite different.

Thanks,
Tino

[jan.didden]

Originally Posted by Tino_diy

Hi Calvin,
did you tried the EB-108/435 or EB-604/417 with a PCM179x yourself? Is it the extra cost of the 435 worth for you? Have you compared it with the others?

If at the DAC outputs there are diodes which will go into conduction at 0.7V, then 4mA*100Ohm = 0.4V should be safe enough, but I would like to know from someone who has tried it. It seems that Doede (dddac) has tried 150 Ohm without problems, but the DACs were quite different.

Thanks,
Tino

That 0.7V is a relatively arbitrary number for 'good conduction', whatever THAT is.
Even at 200 or 300mV, diodes do conduct, if only a little, but that does eat into your linearity of course.

jd

[Tino_diy]

Thank you Jan for pointing out the importance of the true BB DSD1796 diode characteristics.
Even at low voltages there is a small conduction, but the slope of the I-V curve is very different and one should check how much that can influence the output linearity...
What I hope to find is someone who has actually tried it on a DSD-179x DAC and -possibly- who have made some measurements.
Thanks again,
Tino

[Calvin]

Hi,

EB108 and PCM1794A and passive conversion outperformed anything I heard before. Till just very recently.....but thats a whole different story.
For the I/V-R. Diodes conduct much earlier than the 0.7V stated in datasheets.
So keep R below 100Ohms (with 1794/1792).

jauu
Calvin

[jan.didden]

Originally Posted by Calvin

Hi,

EB108 and PCM1794A and passive conversion outperformed anything I heard before. Till just very recently.....but thats a whole different story.
For the I/V-R. Diodes conduct much earlier than the 0.7V stated in datasheets.
So keep R below 100Ohms (with 1794/1792).

jauu
Calvin

Actually what would be needed is a current transformer presenting a very low impedance to the DAC, and a voltage output at the secondary. I know there are current xformers for measurement purposes.
Any xformer geeks around?

jd