Using the AD844 as an I/V - diyAudio
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Old 8th January 2013, 08:21 PM   #1
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Default Using the AD844 as a zero feedback I/V

I posted this by accident on another thread anyway here it should be.

Funny you should bring up the AD844 Jan.
I found this circuit of Pedja Rodic's (attached) and used it as is (NO FEEDBACK) and it worked a treat, best opamp based I/V so far by a long way, it blew the OPA627 completely away I though it would never be beaten except for a discrete I/V of some sort. (And I’ve tried just about every opamp out there)
I used it without feedback as per the diagram and was very very happy. Used with TZ as he has it 1.5K with 1000pf to ground, I think this gives a 1st order rolloff –3db @100khz.
Does the 1.5k resistor have a bearing on what input resistance the dac (PCM1704 in my case) sees?
Because I changed the resistor for 2.2k to get some more gain but this time with a 260pf cap –3db @ 270khz (still all nice and stable with a bit more gain) still very very good.
But I had the feeling it was puchier and more dynamic with the 1.5k resistor even though I had less gain. I feel that the input loading for the PCM1704 is better with the 1.5k than the 2.2k resistor or am I imagining things?.

Cheers and thanks for your input George
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File Type: jpg AD844.JPG (73.5 KB, 2694 views)
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Last edited by georgehifi; 25th March 2013 at 01:30 AM.
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Old 9th January 2013, 07:15 AM   #2
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Quote:
Originally Posted by georgehifi View Post
Does the 1.5k resistor have a bearing on what input resistance the dac (PCM1704 in my case) sees?
Because I changed the resistor for 2.2k to get some more gain but this time with a 260pf cap –3db @ 270khz (still all nice and stable with a bit more gain) still very very good.
But I had the feeling it was puchier and more dynamic with the 1.5k resistor even though I had less gain. I feel that the input loading for the PCM1704 is better with the 1.5k than the 2.2k resistor or am I imagining things?.
Probably you're not imagining things but no that resistor won't change the loading on the DAC which is determined by the input (common-base) pair. I guess you heard lower noise modulation from the AD844 with the lower valued resistor. if I was playing with this circuit I'd experiment with different grounding of those decoupling caps, or preferably using shunt regs for both supplies.

Also worth trying a ferrite bead prior to the AD844 -ve input (with shunt resistor for damping - say 100R) to help reject some of the glitches. I use MPZ2012S102. Do this when removing C1 and see what you hear.
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Old 9th January 2013, 08:44 AM   #3
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Hi abraxalito, I am using it a little different, as per the attached.
I am still using your dc offset trimmer for the dacs. Input is direct from the PCM1704. And I found only a little dc offset comming from the AD844 so it is direct coupled to the active LP opamp a OPA627.

Cheers George
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Old 9th January 2013, 08:44 PM   #4
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Quote:
Originally Posted by georgehifi View Post
I posted this by accident on another thread anyway here it should be.

Funny you should bring up the AD844 Jan.
I found this circuit of Pedja Rodic's (attached) and used it as is (NO FEEDBACK) and it worked a treat, best opamp based I/V so far by a long way, it blew the OPA627 completely away I though it would never be beaten except for a discrete I/V of some sort. (And I’ve tried just about every opamp out there)
I used it without feedback as per the diagram and was very very happy. Used with TZ as he has it 1.5K with 1000pf to ground, I think this gives a 1st order rolloff –3db @100khz.
Does the 1.5k resistor have a bearing on what input resistance the dac (PCM1704 in my case) sees?
Because I changed the resistor for 2.2k to get some more gain but this time with a 260pf cap –3db @ 270khz (still all nice and stable with a bit more gain) still very very good.
But I had the feeling it was puchier and more dynamic with the 1.5k resistor even though I had less gain. I feel that the input loading for the PCM1704 is better with the 1.5k than the 2.2k resistor or am I imagining things?.

Cheers and thanks for your input George
I am wondering about the AC voltage at pin 2 of the opamp. The SK170 CCS is basically the equivalent of a really large impedance resistor between the DAC output pin and the +15 volt supply (which is an AC ground). It seems to me that this CCS would act as an IV resistor, but don't know for sure.
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Old 9th January 2013, 09:52 PM   #5
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Hi Dirk, I don't use the SK120 ccs circuit for dac dc offset nulling. I use the one Abraxilito gave me below. I don't know what proplems it creates but it works fine, as my PCM1704K's gave out 17mV and 50mV dc offset and this nulled it to zero and holds perfectly. And then the AD844 (i/v) as I have used it above gives out only a couple of mV, direct coupled all the way through an active (LP) filter OPA627 also direct coupled to an LME49710 as a (single to balanced convertor) then a two fet output buffers on each phase to the balanced or single ended outputs.
Everything is direct coupled from dac to outputs.

Cheers George
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Old 12th January 2013, 12:59 AM   #6
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I I have to spread the joy on this Pedja Rogic's way of implementing the AD844 I/V with no overhaul feedback, it is a game changer.
I just for interest I put on the cd's that I love the music of yet never played, you know the ones, that are massed on another shelf marked never to be played.
Well I got out an old Crosby, Still's & Nash the one where they're sitting on an old lounge on the porch. Every time this was played with any of the other i/v opamps with feedback, it was thin, shrill and smeared. Now with this Pedja Rogic AD844 way of using it, it's like the first time you heard it and loved it on the best analog setup you had back then, gone are the nasties and it's almost like it was done yesterday but now with body and detail and dynamics that you never though possible.
I'm not going to say it's the equal of the discrete I/V stages that are being developed, but it's hard to believe it can get better than this.
And it's been said that the output buffer in it is not that good in the 844 and the output should be taken from the TZ output. Can someone say what the output impedance is at that point, and if it able to drive my next stage which is the opamp based LP filter at the moment a OPA134.

Cheers George

Cheers George
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Old 12th January 2013, 01:39 AM   #7
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Originally Posted by georgehifi View Post
Can someone say what the output impedance is at that point, and if it able to drive my next stage which is the opamp based LP filter at the moment a OPA134.
The impedance at pin 5 is equal to the resistor value 1.5k .
You have to post the circuit based in the opa134 , for a more accurate answer.
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Old 12th January 2013, 03:25 AM   #8
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Hi Serg, I mean what is the output impedance at TZ without the resistor, the data sheet seems to me to say it could be 1.3megohm to 1.5megohm!!!!! That would mean the input stage of the OPA134 LP filter would have to be at least 10megohm or more as not to load the i/v input stage down.
I have no circuit of the LP filter I assume it's typical of what CDP manuafcturers use, as it is made by Cary. CD-303/200

Cheers George
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Old 12th January 2013, 03:31 AM   #9
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I am going to try to get hold of the designer of the AD844 Barrie Gilbert of ADI to see if he is allowed to give us the whole circuit diagram of this little beauty if he's allowed.

Cheers George
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Old 12th January 2013, 03:46 AM   #10
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You're right that the DC output impedance will be the output impedance of the current mirrors in the chip, however its audio so the AC output impedance is the one to consider and that will be determined by the shunt capacitor. You're not doing yourself a favour by following up a NFB-free stage with a common-or-garden opamp!
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