NOS DAC based on AD1865

[Giordano]

Dear All,

I want to make a good NOS dac based on AD1865 fed by a CS8414. I do not know what PSU would be the best for it. I'm thinking about shunt regulators based on LM431s, mainly for the analog supply.

Do you have experience with different regulator/psu circuits ?

The filter at the output is temporary, a transformer will follow the DAC chip than a 6072 fed with a 6X4.

Thanks,

JG

[kevinkr]

Hi Giordano,
I looked at your schematic and having used related current dacs from AD they work better with 20mV or less of audio on their outputs to prevent linearity issues. (They are designed to drive the virtual earth represented by the input of the I/V converter circuit - which is a transimpedance amplifier.) That 270 ohm resistor should be more like 10 - 20 ohms, check the specification for the 1865 to see how much voltage can be tolerated on the outputs before performance starts to degrade.

Transformer coupling works well as long as the impedance reflected back to the dac current output is within an acceptable range.

Oddly enough many of the newer current dacs no longer specify this value, I guess the expectation is that everyone will use an I/V converter with an input that looks like a virtual earth.

[Giordano]

I did not focus on the oputput yet, but thank You very much for the info about that.

What do you think about the power supplies ? (series resistor values not set yet)

Regards,

JG

[dsavitsk]

Originally posted by kevinkr
That 270 ohm resistor should be more like 10 - 20 ohms, check the specification for the 1865 to see how much voltage can be tolerated on the outputs before performance starts to degrade.

Kevin, my experience with the 1865 is that up to about 200R is good for the I/V. Sowter even recommends this value when using their I/V transformer: http://www.sowter.co.uk/specs/9762.htm



Things like the 1541, on the other hand, don't like higher than about 33R.

[kevinkr]

Hi Doug,
Have you actually measured distortion with various I/V resistor values to see whether or not there is a correlation between load resistance and distortion? I expect that measurements would probably indicate that lower resistance values are better in this regard. Since this thing is a mix of r2r ladders on the low order bits and binary weighted current sources on the high bits (segmented dac) there may well be an issue or not, I'd be curious to know either way.. Sorry to be such a pain..

http://www.analog.com/en/prod/0,2877,AD1865,00.html

[dsavitsk]

No measurements, but it is NOS, so I'm not sure how much good they would do. I'll try to do some in the near future, though to see what it looks like. I believe AudioNote uses ~122R or so (actually, the schematic I've seen has a 380, a 221, and a 1K all in parallel.)

[rubydac]

Through all layer audio note dackit 1.1 but I changed 12au7 output to 6dj8 srpp output and upgraded high volt power supply to matisse fantasy tube preamp (I cloned from my friend PCB)

[Giordano]

do you connect analog and digital ground together through a 10ohm resistor for example ?

Thanks,

JG

[Giordano]

One more important question I would have :

On all the drawing I have seen so far, the "analog" supply of the 8414 is supplied from the digital supply, but the analog ground of the 8414 with the filter is connected to the analog ground.

Is this normal ?

I do not feel good about it, since it makes current from the digital psu through the anlog ground, analog-digital connection and than digital ground (or the way back with the phisical current flow).

A less important thing, I see a suggested reset circuit in the datasheet of the 8414, but nowhere implemented, as well as some D/A converter requires that at powerup, the analog supply comes up first, this is not guaranteed anywhere either - in the other hand I thrust it is not necessary. The first thing makes me more nervous.

Please let me know how you do and why.

Thanks,

JG

[Giordano]

One more thing :

How do you switch between digital sources. I need 3 input, minimum 2, a toslink and a coax.

I plan to use an Omron G6K to switch both lines and use the differential input of the 8414. I think AND gates should be better, but than I can not use the differention input of the 8414 and I have to make a differential receiver for the coax. G6K is a small signal relay, it was giving acceptable performace at a volume control, the best I have tried so far, but on base band signal.
It is in the 10MHz range, so I'm not sure it is good any more.

So, what would you use ? Small sigla relay is good ?

Thanks,

JG