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Old 31st October 2012, 12:45 PM   #551
smms73 is offline smms73  Portugal
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Quote:
Originally Posted by staccatiss View Post

LTspice cannot be doing something wrong with JFETs?


Ok, now i know where i was wrong, i was not counting with drain gate capacitance. It is not a ltspice problem.
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Old 5th November 2012, 07:35 PM   #552
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As the pcm1794 has a high output impedance i thought about using the 2sk170/2sj74 pair in the input. I have bought 4 matched pairs from Spencer Cheung, they are not cheap, it cost the equivalent of 1 Kilo gram of BC327 . but i am very curious to try them, and this project seems to be perfect for them. So there it is, the version that I going to build.
I have found an elegant soluction to connect the dc servo, instead of connecting the servo at the input as usual, the servo will be connected to the input of current sources, any noise that the servo generate will be filtered by C2 and C3 (I have already test this, and work very well). So this I/V converter will not have output capacitor.
In the schema is absent the ccs of 6,2ma for balance the pcm1794 dc_offset.

I will do also a version with BC327/337 transistors in the input. And i am working in a I/V for the ESS9018 .
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Old 5th November 2012, 07:40 PM   #553
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The Dc servo will be one of this, the first is recommended by Bob Cordell and only use 1 capacitor. Can you guys recommend a jfet opamp with good dc precision for this, no need for low noise.
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Old 5th November 2012, 08:06 PM   #554
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Just for curiosity, the fft of the output with 2Vrms out , 7,8ma p-p.

The impedance of the source is 100k with lower impedance the distortion will be worse. But pcm1794 has a output impedance higher than that, so no problem .

in the schematic , Q10, Q12, Q13, Q3 will be the BC550c for npn and BC560c for pnp. not bc857c bc847c as this are smd.

edit: This is a ltspice simulation,i will do real measurements when i receive the jfet.
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Last edited by smms73; 5th November 2012 at 08:26 PM.
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Old 5th November 2012, 08:45 PM   #555
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I have more faith in the BJT version.. I have made phono-stages with Jfets, they always sound nice, but also wailed and cloudy compared to the Bjt's

Like your Servo- very much, the same way I designed one in for the paradise phonostage--there we use the OPA134.
I would make a jfet current source instead of the two resistors setting the current. and then insert the servo with two 100 kohm resistors on each side of the CCS.
I would then use the 6.2mA countercurrent to trim the offset to 0 before engaging the servo.
Then you know the servo is just dealing with thermal drift.

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Old 5th November 2012, 08:51 PM   #556
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I only have experience with the second example (one op amp).
For this I would use OPA211.

Quote:
Originally Posted by smms73 View Post
The Dc servo will be one of this, the first is recommended by Bob Cordell and only use 1 capacitor. Can you guys recommend a jfet opamp with good dc precision for this, no need for low noise.
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Old 5th November 2012, 09:04 PM   #557
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why 211...?? too low input impedance to my liking...
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Old 5th November 2012, 09:17 PM   #558
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MiiB I will do also a bipolar version, that i will post tomorrow, just need to do some final tests tonight. I want to try something with this jfet. And i realy think the jfet version will be very good.

The current source use in this schema is very good and generate very low noise, no need for improvements.
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Old 5th November 2012, 09:20 PM   #559
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OPA211 is a bipolar input opamp , i prefer jfet input.
Miib i also use opa134, but there are others with lower offset voltage.
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Old 5th November 2012, 11:06 PM   #560
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OPA827 has only 0.1mV.
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