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Old 1st May 2017, 04:12 PM   #1
lykkedk is offline lykkedk  Denmark
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Default AD1856 I/V (I/U) converter

Hello

In my adventure with my new "old" AD1856 R2R ladder DAC, i finally get to listning to a temp. setup here at home. -If someone here had read about my adventures in the Salas SSLV thread you got the idea, me saying adventure. Well i had some minor explosions setting up the power supplys for this little DAC, but most survived through! ... Anyway the DAC and PSU's are all working now.

Me and our oldest (boy 19years), sat down (as usual when pa' got new equipment), and did some listning test.
My current setup is a buildin iqaudio dac, buildin along with an RPI in my Salas DCG3 linestage, so doing A/B test is very easy now.
I started my tidal and my logitech squeezebox interface on my PC; from there i can easily switch between the two dac's (ofcause i also have to switch the input on the DCG3 ) ... So first number we played i did not tell my son, which dac, was playing; but after 5-10sec. he told me that everything sounded better than everything else he heard. -Which dac? yes the new one was playing ... Well we tested ofcause, and besides some crappy recordings, there was no doubt that this new dac is smoother and less sibiliance-aggresive.

What i donno for now, is how much the Reflector-D's and SSLV's has to do with this smooth and listning richable dac?; but as long as it sound so good i can live with that quistion (for now )...

Okay, back to topic, and why i started this thread at all !

I bought this DAC from diyinhk, and they wrote on theire site, iout is possible.

Quote:
For Diyer requires more deeper customization or customized IV stage, the Iout directly from the DAC chip is connected to a 2.54mm spacing socket, the latest version of AD1856 datasheet is available in the official analog device website.
...

I have been searching for a I/V circuit to tryout, but i am having trouble figuring out what to use, so maybe someone here can help me get started.

I see, that there a diffent setup's regarding how much current the dac delivers... according to the AD1856 datasheet, this one is +/-1mA.
I have been reading the zen iv converter article, but this converter need batterypower, and i like to integrate everything with psu's. I also have been reading about the Fetish-zator Fet-ishizator, but i am in doubt howto calculate the right values for the AD1856 (The Fetish-zator article is about TDA1541, with much higher current output)...

I am really looking forward to some inputs here, thanks all

Jesper.
Attached Images
File Type: jpg ad1856-r2r-isolated-nonoversampling-nos-audio-dac-with-fifo-reclock-v2.jpg (30.5 KB, 316 views)
File Type: jpg DAC_TEST.jpg (51.9 KB, 318 views)
Attached Files
File Type: pdf AD1856.pdf (865.4 KB, 30 views)
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Old 2nd May 2017, 01:20 AM   #2
Calvin is offline Calvin  Germany
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Hi,

You could try a common base circuit like the one in this thread
In a common base stage the 'Input transistor' functions like current buffer and performs impedance matching only.
As it doesn't follow a exponential Working curve but simply performs current-in --> current-out its highly linear without relying on feedback .... and as such it s also very fast
It provides the DAC with the low impedance load it requires and looks like a high-impedance source for its own load, the I/V-resistor.
One can easily scale this resistor to generate the required signal voltage (+-1mA x R).
This Voltage can be used to drive either the following device or a output buffer stage (recommended).
Sonically such a simple discrete I/V stage can perform way better than any OPAmp equipped circuit I heard.
It puts music back into HiFi

jauu
Calvin

ps. I probabely cannot reply over the next two days
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Old 2nd May 2017, 03:39 PM   #3
lykkedk is offline lykkedk  Denmark
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Quote:
Originally Posted by Calvin View Post
Hi,

You could try a common base circuit like the one in this thread
In a common base stage the 'Input transistor' functions like current buffer and performs impedance matching only.
As it doesn't follow a exponential Working curve but simply performs current-in --> current-out its highly linear without relying on feedback .... and as such it s also very fast
It provides the DAC with the low impedance load it requires and looks like a high-impedance source for its own load, the I/V-resistor.
One can easily scale this resistor to generate the required signal voltage (+-1mA x R).
This Voltage can be used to drive either the following device or a output buffer stage (recommended).
Sonically such a simple discrete I/V stage can perform way better than any OPAmp equipped circuit I heard.
It puts music back into HiFi

jauu
Calvin

ps. I probabely cannot reply over the next two days
Hi and thanks for answer...

I have been studing the thread; seems nice.
In post #7 of the thread PCM1796 I/U stage for CD/DAC

Did anyone actually build that ?

Jesper.
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Old 2nd May 2017, 03:49 PM   #4
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For me the built-in IV sounded best.
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Old 3rd May 2017, 06:04 AM   #5
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Hi Jesper,
My conclusions from a small diy adventure perhaps could be helpful. When it comes to I/V conversion we have to consider signal level, DC offset and load presented at the DAC chip's output. The most common solution is to use a single resistor from current output to ground. Then feed a gain stage. That's exactly what fetishizator -and similar- is. The bigger the I/V resistor, the higher the signal level but there are limitations to this. The chips don't like to see high impedance. Bad for performance and maybe for longevity. I use a PCM 1792 chip which delivers 7,8 mA. With 82 ohm I/V it could move directly my super sensitive amps. But the sound was not good. Fetishizator was my entry build on this adventure. Easy to build and the designer is very helpful answering to queries. I also once found a variation by T. Loesch with different transistors somewhere in the internet. The Zen I/V is going further. It presents an ideal load to the chip to squeeze the juice out of it. I never test it though. I moved to tube gain stage after the I/V resistor. Very convinient as gain is usually high and grids do not care that much about the presence of a little - or a little more - DC offset. In my case, I ended up using 2 ohm resistor. The DAC likes this very much. And DC offset is down to 11 mV. Next move was to connect it directly to DCG3. It doesn't seem to care about this small DC offset. Sound is a dream. I have to tell that DCG3 is set to 6x gain and my amps also have more than enough gain. If you need higher output signal from the DAC, meaning raising the value of the resistor and consequently the DC offset, you could use coupling capacitors.
Another I/V conversion scheme is with transformers. See DDDAC. I never test this.
Kostas

Last edited by MagicBus; 3rd May 2017 at 06:14 AM. Reason: fixed typos
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Old 3rd May 2017, 03:10 PM   #6
Calvin is offline Calvin  Germany
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Hi,

Quote:
Did anyone actually build that ?
Yes, I did .... and many more built such grounded-base or grounded-gate style circuits (see the JockoHomo or OPCīs MOSFET circuit for example)
The ZEN is a symmetrical grounded-gate JFET circuit which is even better suited for the AD1856 but where the sourcing of parts may become critical and a suiting power supply may be not so easy (except for a battery supply).
I showed a bipolar Version of the ZEN rather early in that same thread, where the parts are easier to source.
Then thereīs a very similar Nelson Pass I/V utilizing Mosfets.
Common to all these circuits is that the active devices simply pass the DAC current into external I/V resistors ... which means they only do impedance matching.
impedance matching or precise impedance mismatch is the key word when using a I/V resistor directly at the DAC output.
A current output DAC wants to see a as low as possible (ideally 0R) load.
They donīt like to see a higher impedance ... the more so the lower the DACīs internal impedance is (only 1.7k for the AD1856).
Also the signal voltage dropping over the I/V resistor may trigger internal protection diodes ... which limits the output voltage to less than ~300mV before the diodes start conduction.
The direct I/V conversion thus requires a voltage gain stage to get the output voltage level to the desired value.
I donīt recommend that way of I/V conversion.
A active grounded-something stage feeding a I/V resistor and maybe a simple output buffer stage are giving the best results with least hassle.

Attached are two symmetrical bipolar I/V stages.
The first beeing simpler, but due to the connection of the I/V resistors (Rivx) towards the supply lines with low PSRRR --> hence highquality supplies required
The second with current sources shielding against the supply lines and the Rivs connected to gnd (the bipolars can be sourced very cheap as matched Duals ... BC847/857, BC807/817 ... and even matched Quads ... THAT 300 series for example).

jauu
Calvin
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Old 3rd May 2017, 03:12 PM   #7
lykkedk is offline lykkedk  Denmark
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Hi MagicBus, and welcome

Quote:
Next move was to connect it directly to DCG3. It doesn't seem to care about this small DC offset. Sound is a dream
...

So is this connection directly from the I/V resistor, or through a buffer?

There are a lot of things i cannot figure out yet regarding i/v conversion, but things come slowly to my mind now!

Anyway, i have ordred a chassis (yes ordred) for this DAC project, giving me space for all the psu's, and space for other dac in time.

So by some time i hope i can figure out howto create an i/v converter

Jesper.
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Old 3rd May 2017, 03:47 PM   #8
lykkedk is offline lykkedk  Denmark
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Quote:
Originally Posted by Calvin View Post
Hi,


Yes, I did .... and many more built such grounded-base or grounded-gate style circuits (see the JockoHomo or OPCīs MOSFET circuit for example)
The ZEN is a symmetrical grounded-gate JFET circuit which is even better suited for the AD1856 but where the sourcing of parts may become critical and a suiting power supply may be not so easy (except for a battery supply).
I showed a bipolar Version of the ZEN rather early in that same thread, where the parts are easier to source.
Then thereīs a very similar Nelson Pass I/V utilizing Mosfets.
Common to all these circuits is that the active devices simply pass the DAC current into external I/V resistors ... which means they only do impedance matching.
impedance matching or precise impedance mismatch is the key word when using a I/V resistor directly at the DAC output.
A current output DAC wants to see a as low as possible (ideally 0R) load.
They donīt like to see a higher impedance ... the more so the lower the DACīs internal impedance is (only 1.7k for the AD1856).
Also the signal voltage dropping over the I/V resistor may trigger internal protection diodes ... which limits the output voltage to less than ~300mV before the diodes start conduction.
The direct I/V conversion thus requires a voltage gain stage to get the output voltage level to the desired value.
I donīt recommend that way of I/V conversion.
A active grounded-something stage feeding a I/V resistor and maybe a simple output buffer stage are giving the best results with least hassle.

Attached are two symmetrical bipolar I/V stages.
The first beeing simpler, but due to the connection of the I/V resistors (Rivx) towards the supply lines with low PSRRR --> hence highquality supplies required
The second with current sources shielding against the supply lines and the Rivs connected to gnd (the bipolars can be sourced very cheap as matched Duals ... BC847/857, BC807/817 ... and even matched Quads ... THAT 300 series for example).

jauu
Calvin
Hi Calvin...

I somewhat posted at the same time you did

Well you wrote
Quote:
The ZEN is a symmetrical grounded-gate JFET circuit which is even better suited for the AD1856 but where the sourcing of parts may become critical and a suiting power supply may be not so easy (except for a battery supply).
; do you meen the Nelson Pass Zen, or the Sen -> Cen -> Zen battery powered unit. -

I have a good part stock of 2sk170bl / 23j74bl / 2sk170gr, so if you meen that it could be a problem, it is not at moment.

I also read the Pass article, but i though that having enough voltage swing (preferbaly cd level ~2vrms), would not be easy with the AD1856's +-1mA current out???

Jesper.
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Old 3rd May 2017, 04:12 PM   #9
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Quote:
Originally Posted by lykkedk View Post
Hi MagicBus, and welcome
...
So is this connection directly from the I/V resistor, or through a buffer?
...
Jesper.
No buffer, directly from I/V resistor. But... I use a dac with high current output so I managed to use a very small resistor to address all issues explained by Calvin. Your chip is different thus I think you should follow Calvin's suggestions. Just keep in mind that you allready have a great preamp that can be set for higher gain and it can't tolerate DC at its input.
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Old 3rd May 2017, 04:25 PM   #10
lykkedk is offline lykkedk  Denmark
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Quote:
Originally Posted by MagicBus View Post
No buffer, directly from I/V resistor. But... I use a dac with high current output so I managed to use a very small resistor to address all issues explained by Calvin. Your chip is different thus I think you should follow Calvin's suggestions. Just keep in mind that you allready have a great preamp that can be set for higher gain and it can't tolerate DC at its input.
Yes... i see, and i got it also i guess ...

I also think Calvins solutions must be good, but i am not right capable of figuring enough out to build one myself, without any help.

I ofcause like the Pass design, as i know most of it works for me, but still i think it will deliver way to little voltage-swing for my DCG3 as for now. -Lets see how it goes... now i am just waiting for my chassis to arrive here in DK.

Jesper.
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