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-   -   AD1955 I/V stage/tube version (http://www.diyaudio.com/forums/digital-source/107538-ad1955-i-v-stage-tube-version.html)

SunRa 26th August 2007 07:59 PM

AD1955 I/V stage/tube version
 
Hello,

I have some questions regarding the AD1955 I/V stage presented in the datasheet here , page 18. I'm intersted in learning the design principles of this stage in order to try and design (adapt in fact) my own vacuum stage. However I think the AD1955 has some particularities that need do be adressed. I hope the questions are not too dumb as I have no studies in electronics and little experience with digital and signal conversion.

The first question is regarding the Vbias pin of the I/V stage. The datasheet indicates this is to be conected to the Vref (2.39V) from the dac or to the Vrefa (2.90) obtained with a voltage divider from the AVDD (the analogic voltage line). It seems to me that this vbias conected to the non-inverting pin of the op-amps acts in creating a voltage reference diferent from the ground with 2.39-2.9V. My question is why is this needed, what are the tehnical considerations for this?

Other questions will come for sure. Regarding the output stage I'm tinking to adapt: I'm intersted in the advantages of transformer-coupled, differential circuits using triodes and parallel feed. Some examples are here on Mr. Dave's Davenport site here for the general topology and here for the parafeed. Another great source are Lynn Olson's articles.

What to do in order to adapt this circuit (in my opinion):
1. find a way to rise the ground potential to the 2.39-2.90 specified in the datasheet. I would probably use a Jung regulator for this.
2. design a current source for the ad1955 as it has a current sink output. The same superregulator will be used for the voltage suply, as for current conversion I would probably try first a simple resistor just as in the datasheet.
3. I have no clue if a triode can act as an inverting amplifier as in the datasheet output stage either.

Well, I'm still not sure if this isn't just a bunch of pseudo-tehnical non-sense I'm thinking of, but that's why I wait your opinions.

Thank you

aparatusonitus 26th August 2007 08:08 PM

Re: AD1955 I/V stage/tube version
 
Quote:

Originally posted by SunRa


What to do in order to adapt this circuit (in my opinion):
1. find a way to rise the ground potential to the 2.39-2.90 specified in the datasheet. I would probably use a Jung regulator for this.

Where are you gonna find a opamp that can work (preferable at unity gain) as error amplifier at Jung SupeReg on 2V39-2V9?

SunRa 26th August 2007 08:15 PM

Hello aparatusonitus,

thanks for the comments. I gave the Jung example more to ilustrate I'm after a low noise source. I didn't give too much thought to the Jung limitations. I guess I could just use a voltage divider as in the datasheet, with the jung circuit suplying the analog voltage line as a last resort.

aparatusonitus 26th August 2007 08:21 PM

I would try a CCS loaded TL431 shunt regulator (aka super-shunt regulator) before anything more compex;)

SunRa 26th August 2007 09:24 PM

If I ever get to build this I'm sure I'll try something simpler where I feel I can make a compromise for the begining. So the ideea with the shunt regulator is a viable option.

I think the main design guidelines are differential triode operation with active plate loading, single gain stage, parafeed operation (as in the western electric derivation)... and this should be all I guess for proof of concept..

analog_sa 26th August 2007 09:47 PM

Re: AD1955 I/V stage/tube version
 
Quote:

Originally posted by SunRa
The same superregulator will be used for the voltage suply, as for current conversion I would probably try first a simple resistor just as in the datasheet.


Just as in the datasheet? Are we looking at the same datasheet? The one i have specifically warns agains using a "simple resistor".

No idea what is the max compliance voltage at output but it's probably really low. I don't have first hand experience with the 1955 but it doesn't seem like a great choice for resistor/tube I/V.

SunRa 26th August 2007 10:01 PM

Quote:

Just as in the datasheet? Are we looking at the same datasheet? The one i have specifically warns agains using a "simple resistor".
I was refering to the current suply of the I/V stage. The AD1955 dac sinks current so it needs an active conversion. The datasheet states that you need the I/V stage able to supply the signal current. And I supose that the 2K R2 and R3 in series with the 12V suplly is doing just that.



Quote:

No idea what is the max compliance voltage at output but it's probably really low. I don't have first hand experience with the 1955 but it doesn't seem like a great choice for resistor/tube I/V.
Could you explain a little more? For example the PCM1794 has almost the same current capabilities of the ad1955 (in fact a little less even) and it seems it's suitable for this (the rakk dac is a very good example) . Of course the main difference between these two (AD1955 sinks current) is the one that complicates everything :)...

analog_sa 26th August 2007 10:26 PM

Quote:

Originally posted by SunRa


The AD1955 dac sinks current so it needs an active conversion.


In my understanding whether it sinks or sources current is irrelevant. The datasheet recommends that instead of virtual ground the output is connected to a 2.8v potential, so the IV stage has to be biased at this voltage but as far as AC current is concerned it still has to flow in a dead short.

There may be a small ac votage at output before distortion really takes off but i have no idea of its value.

Is passive IV possible? No idea. The 1794 datasheet does not specifically warn agains it but the 1955 does. It may simply mean that a low enough iv resistor will eventually degrade the S/N.

SunRa 26th August 2007 10:35 PM

Quote:

In my understanding whether it sinks or sources current is irrelevant.
Yes, you are right, I didn't payed atention to my explication. My english is not so good and I have some dificulties to properly express what I want.

Quote:

There may be a small ac votage at output before distortion really takes off but i have no idea of its value.
Well I think I get it now. But in order to understand better, how exactly the op-amp circuit works? I understand it's not a simple non-inverting amplifier with gain... but then? If you have a link to some references would be great, because one reason for this thread was the fact that I can't find some resorces on the net about this..

analog_sa 26th August 2007 11:00 PM

Quote:

Originally posted by SunRa
But in order to understand better, how exactly the op-amp circuit works? I understand it's not a simple non-inverting amplifier with gain... but then?

Google transimpedance amp. The textbook version has the positive input grounded, the input current applied to the negative input directly (virtual ground) and the feedback resistor develops the output voltage. Vout=-Iin*Rfb. In our case the positive input is biased to 2.8v which automatically biases the negative input to the same.


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