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Old 1st October 2012, 08:11 PM   #7451
MiiB is online now MiiB  Denmark
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To me hard is the amount of correction current needed, compared to the driving current. Here the needed current is so low, that the transistors working points is so minutely affected that it's insignificant.
If you lover the resistors to 22K or even 10K the servo will have much more power to adjust. Making this circuit without the servo is not possible. Well it is if you AC-couple it with at small cap before the buffer.
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Old 1st October 2012, 08:22 PM   #7452
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Quote:
Originally Posted by MiiB View Post
To me hard is the amount of correction current needed, compared to the driving current. Here the needed current is so low, that the transistors working points is so minutely affected that it's insignificant.
If you lover the resistors to 22K or even 10K the servo will have much more power to adjust. Making this circuit without the servo is not possible. Well it is if you AC-couple it with at small cap before the buffer.
I respectly disagree with you on both points:

1) if current needed is small doesn't mean that servo is not working hard. If the voltage that needs to adjust is 10V of constant drifting, then servo is working hard. But that might be debatable it's like the glass half full or half empty.

2) Circuit can't work with HP filter if the drifting is high and frequency involved are pretty high, also because you will find bias of your gainstage sometime at +10V and some other time at -10V which deeply affects distortion's performance.
Also, if the variation is so significant that not only involves frequency below 2Hz or so, then bandwidth for the servo has to be wider which in turns definitely affecs sonic performance as the servo will go correct audio signal as well by a small amount

I am hope that Joachim or some other guys who took part to the development of this design could comment on it, especially considering that the servo has marked up a resistor value of 150K which means that it should work properly with that value which suggest that something is wrong with my implementation or this aspect might have been overlooked a bit.
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Old 1st October 2012, 08:42 PM   #7453
MiiB is online now MiiB  Denmark
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But the drift is not jumping up and down....variations are slow, but due to the high impedance of the mirrors voltage may be quite high-ish.
I made the house keeping to work with a servo. But in a way so the servo is taken extremely out of the signal loop. mind you that the 22K or what you need is way way higher than the current setting resistor. The servo is only dealing with DC. or app 0.1 Hz.

Last edited by MiiB; 1st October 2012 at 08:47 PM.
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Old 1st October 2012, 08:47 PM   #7454
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Originally Posted by MiiB View Post
But the drift is not jumping up and down....variations are slow, but due to the high impedance of the mirrors voltage may be quite high-ish.
I made the house keeping to work with a servo. But in a way so the servo is taken extremely out of the signal loop. mind you that the 22K or what you need is way way higher than the current setting resistor. The servo is only dealing with DC. or app 0.1 Hz
In that case it would be ok.
In my build the variation are pretty fast and that is why I think the servo doesn't handle the offset.
I am wondering why I would experience this strange offset drifting.

Could it be caused by low hfe BJTs on the mirror? I don't have an explanation to that.
Matching should affect DC drifting little IMHO.
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Old 1st October 2012, 08:49 PM   #7455
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the drifting is mostly thermal... that why it's slow... just touch the input and watch. pretty fast, 10 sec movements..??
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Old 1st October 2012, 09:01 PM   #7456
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I mean drifting is not pretty slow in my opinion and yes is mainly thermal as always.
However I need to know a reference to understand if that is normal somehow.

Like if you trim the offset either by changing current on the mirrors or unbalancing top against bottom size, what would be your thermal drift? let's say 10mV is 1 seconds 100mV in 5 seconds?

Also does anybody here have a screen shot of the residual noise with the scope?
I have a little residual noise that doesn't look like it is broadband noise.

I am thinking if the resistors I am using at the top of the 2 current mirrors which sees a current of 35mA is rated for 1/4W or 1/8W.
I am using a small SMD resistor there and if rating is 1/8W it could possibly cause more residual noise and more drifting.
I will also look into that later.
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Old 1st October 2012, 09:32 PM   #7457
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Stefanoo, i was not watching the thread for some time. First you say you run the Paradise on plus-minus 12V from a battery. That is not enough, it needs plus-minus 18V.
Then you say you use an AD797 as servo. That does not work because it is a bipolar. You need a high impedance Fet like OPA134.
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Old 1st October 2012, 09:38 PM   #7458
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Quote:
Originally Posted by Joachim Gerhard View Post
Stefanoo, i was not watching the thread for some time. First you say you run the Paradise on plus-minus 12V from a battery. That is not enough, it needs plus-minus 18V.
Then you say you use an AD797 as servo. That does not work because it is a bipolar. You need a high impedance Fet like OPA134.
Right I kind of figure that.
Why do I need 18V there? Technically you get enough voltage to bias input devices.
If I increase voltage over 12V I get oscillation, I will try to fix that.

For the AD797 I agree with you.
The discrete module I am using in place of the OPA, though, is JFET input pair.
I am using Burson Audio Discrete op amp (since I am out of opamps now.)
Would you see any problem using that one?

I am going to put on order tonight some OPA134.

Question: how is the offset without servo? Is it possible to run the preamp without servo and use coupling cap just to test the actual effect of the servo?
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Old 1st October 2012, 09:53 PM   #7459
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The offsety without the servo is hard to predict. Some comes from the input stage and some comkes from the buffer and is dependend how well you match the sides. Also there is a high impedance point between the buffer and the mirrors ( where the RIAA is ). Even tiny missmatch in the mirror gives a big offset. Made correctly you can expect around 10mV offset ( with servo ) that needs some stabilizing. Directly switched on it needs some time to come down. That happens also when the elcaps are new. Their leackage current goes down after a while. That behaviour disappears with use. For absolute minimum offset you have to run the Paradise say for one week continous and then trim the servo.
You talk about safety. The only safe way is a coupling cap. This circuit has an enormous gain in only one stage and even our ex professor from Canada that chalenged me to a point where i closed the thread for some time put a coupling cap after his phono stage although on his website it says : DC coupled without caps in the signal path, he simply lied. DC coupling from cartridge to speaker is posibble but NOT SAFE>

Last edited by Joachim Gerhard; 1st October 2012 at 10:00 PM.
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Old 1st October 2012, 10:36 PM   #7460
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I can not say if the Bursons can be used for servo. I only know that they performed poor in the tests that Samuel Groner made.
Download the file : Opamp Distortion
SG-Acoustics Samuel Groner IC OpAmps
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