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Get a 9V battery, and a pot. Reverse the direction, to test for equal swings.

Jocko

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"Because, I hate you guys. I hate you guys so very, very much.

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Sorry Jocko,
but I don't understand how can I have a balance output with battery and pot? Maxpou

I was talking about how to see if you can hook up a servo, and see if it will work.

Jocko

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"Because, I hate you guys. I hate you guys so very, very much.

Yours,
General Cartman Lee"

Ok Jocko for servo, but for my inputs + and - I don't know how if I don't have a generator with a output balanced? Maxpou

Wouldn't the first test to be to see if the servo can zero the offset generated in the circuit by parts mismatches (i.e with grounded inputs)? Zero in should equal zero out.

Hi,
unfortunately it does't work, if I put the positive or negative voltage I don't have movement at output. I don't know where I can put my servo in? Maxpou

Hi Guys,
I found where put the input of my servo, between R5 and R6 of CCS. I don't tried with a opamp but with a 9V battery and pot. Sorry I forgot to say thank you to Knewton, Calvin and erik_s for your comments. Maxpou

Hi Guys,
I have two questions for you: What's the limit of dc servo for correction of output?
I would like to know if I have a -35mV and +19mV at the input it is dangerous for my DAC? Thank you! Maxpou

I have 2 questions:

Why are you wasting a 'J74, for a follower, and an AD820, for a stinking lousy servo? You guys seem to love making stuff harder than it needs to be. That is not automatically a promise of good sound.

Jocko

_________________
"Because, I hate you guys. I hate you guys so very, very much.

Yours,
General Cartman Lee"

Hi Jocko,



if you want, it would be much appreciated to have an answer to my questions in my other post and this post. Thank you! Maxpou

Maxpou, you will found answer HERE. But I could be wrong.

I think Jocko is more referring to the point that the AD820 is overkill for the job. And the fact that the speed of the AD820 could get you in trouble (I think).

Why not use an OP07?

I can heartily recommend the AD743 for the job. Main ingredients:
1. JFET input, precision
2. lowest voltage noise for a JFET op-amp from 0.1 to 10Hz, where the servo 'gain' is important - probably this is why they choose to pack a single op-amp in the wider SOIC-16 package, to help with the thermals (it is also idling at a highish 10mA because of the paralelled FET diff. input stage)
3. not too much SR and bandwidth
4. very good PSRR when sourcing from the V+ side

Probably there are others suitable...this one is not really cheap.

A 'local' servo loop with an inverting op-amp and a pnp instead of the J74?...this one is a really cheap shot , personally I never tried it.

You can use a J74 for a follower but there are others that are cheaper, will work just as well or better; BF861/BF862.

And what sidiy said too...

Hi Guys thank you for reply,
Stormsonic: I read this document many years ago and i forgot this phrase: However, in certain applications such as
filters, this can cause distortion..

erik_s: I have in my hand many opamp as: opa2134, tl071, ad817, op27 and lm833 but i don't have a ad743 as sidiy propose to me but I read the tread of universal servo of carlosfm and he propose a opa2134 I will try it, maybe a good choice or another one?

sidiy and mikeg: I will try with 2 2sk170 as in my other schematic and with a bipolar CCS. I wanted to use a J74 because I have about 75 of them in stock with many matched.

Maxpou