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Old 1st October 2012, 05:21 PM   #7441
RCruz is online now RCruz  Switzerland
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My actual build drifts from + to - 10mV constantly (with the servo on).... It sounds wonderfull nevertheless.

Afte power on it starts at about 4v but gets lower very fast and settles with time.
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Old 1st October 2012, 05:26 PM   #7442
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Quote:
Originally Posted by Stefanoo View Post

Accordinlgy to what you say, if I understand corretly, if you have a preamplifier AC coupled at the input like I do, you could run it without servo as well, am I right?
If that is the case, the built I have done so far, for some unknown reason, is not even close to the actual desired result.

Thanks for the attention.
Now that is an interesting question..... I guess my build would drift too much to be used without servo and a coupling cap.
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Old 1st October 2012, 05:46 PM   #7443
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Quote:
Originally Posted by RCruz View Post
Now that is an interesting question..... I guess my build would drift too much to be used without servo and a coupling cap.
right you just consolidated my point.

I mean +/-10mV with Servo at the outpu is it too much for a phono amp since the signal will be amplified at leat other 30dB.
Not that 10mV static offset is too much rather the opposite, but how fast drifs is the problem.
It works because it works in your case and in mine too, but it is not optimal.
Which also brings to the point that if you strip the servo off, the circuit wonders really bad which means that servo is working really hard, thus affecting performance.

It would be interesting to get some other people's here experience.

I hope that this aspect wasn't overlooked, but I am pretty sure it wasn't and then I would like to understand how I could fix this issue.
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Old 1st October 2012, 06:37 PM   #7444
MiiB is online now MiiB  Denmark
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To make the drift smaller the servo can be fitted with smaller resistors. The resistor in the back to back pair sets the current, the servo then injects the needed adjustment current, by making the resistors to the servo smaller, it's possible to keep the drift in the low micro-volt range.
I have made some of my paradise variants with mirror and current source formed by quad SMD transistor-bricks. The thermal lock here improves DC performance quite a bit. But makes the PCB more critical as the stray capacitance between the transistors tend to trigger oscillation (cured with ceramic decoupling at each transistor)
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Old 1st October 2012, 06:42 PM   #7445
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Quote:
Originally Posted by MiiB View Post
To make the drift smaller the servo can be fitted with smaller resistors. The resistor in the back to back pair sets the current, the servo then injects the needed adjustment current, by making the resistors to the servo smaller, it's possible to keep the drift in the low micro-volt range.
I have made some of my paradise variants with mirror and current source formed by quad SMD transistor-bricks. The thermal lock here improves DC performance quite a bit. But makes the PCB more critical as the stray capacitance between the transistors tend to trigger oscillation (cured with ceramic decoupling at each transistor)
I know that, you are basically setting the servo authority higher and higher.
Basically the servo would need to work very hard.
My quesiton connected to your observation was: what is your offset before servo i.e. stripping the servo off?

I feel like that if servo has to work so hard something is not quite right and needs to be improved.
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Old 1st October 2012, 07:12 PM   #7446
MiiB is online now MiiB  Denmark
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The servo is working through 100K... looking at the output at the op-amp it.s almost a steady voltage (On my own PCB with SMD) app 2-3V. meaning that the injected adjustment current is is 2/100000=0.02mA-0.03mA. Compared to the 15mA driving the circuit. the servo cant be judged as working Hard.

If your circuit is a tad bright, have you checked for oscillation..?? I mean we have had trouble with the first generations due to sub-optimal PCB layouts.
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Old 1st October 2012, 07:25 PM   #7447
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Originally Posted by MiiB View Post
The servo is working through 100K... looking at the output at the op-amp it.s almost a steady voltage (On my own PCB with SMD) app 2-3V. meaning that the injected adjustment current is is 2/100000=0.02mA-0.03mA. Compared to the 15mA driving the circuit. the servo cant be judged as working Hard.

If your circuit is a tad bright, have you checked for oscillation..?? I mean we have had trouble with the first generations due to sub-optimal PCB layouts.
oh no, absolutely not at all.
The circuit doesn't have bad brights at all. I am saying they are a tad glossy a tad and that is likely have to do with the quality of my build now that is missing the high quality PSU.
There are not oscillation, hoever if I increase the voltage rail over 12V I have oscillation that I will deal with.

My own PCB is SMT too.

The servo is not considered to work hard only on the amount of current injected but, and you have checked this, by the variation.
The sensitivity of that folded current mirror, is very high and few uV from the servo can adjust few volts at the output.

Do you have matched devices on your board?

I am wondering if you have built it with first hand devices and offset was significanlty different.
I am sure distortion wise the matched devices will yeald to a better result, but just from a offset's standpoint, what is the effect of matched devices.

Moreover, if you take your servo off, what is your drifting (not static offset introduced by tolerance and buffer)?
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Old 1st October 2012, 07:37 PM   #7448
MiiB is online now MiiB  Denmark
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I have boxed it, so I can't make that observation. the offset is placed in the center of very very high impedance mirrors and the high impedance input of the Jfet buffer. If you disconnect the servo even micro-volts of current drift will throw the offset way off... that is the basic idea. The close to Ideal mirrors makes the distortion spectra clean.
I don't consider the servo working hard, as the current injection is so tiny compared to the driving current.

My mirrors are matched as they are on the same chip, and my input is HFe matched to app 5%

Last edited by MiiB; 1st October 2012 at 07:40 PM.
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Old 1st October 2012, 07:42 PM   #7449
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Quote:
Originally Posted by MiiB View Post
I have boxed it, so I can't make that observation. the offset is placed in the center of very very high impedance mirrors and the high impedance input of the Jfet buffer. If you disconnect the servo even micro-volts of current drift will throw the offset way off... that is the basic idea. The close to Ideal mirrors makes the distortion spectra clean.
I don't consider the servo working hard, as the current injection is so tiny compared to the driving current.
Ok so you technically answered to my question, due to the very high impedance morrors, the smallest variation of current die to temperature or mis-matching or whatever else, will cause several volts of offset.
Agree with you.
Saying that servo doesn't work hard is probably not corrected just because the amount of current injected is small.
It is determined by how much cirrection you need to apply at the output not at the source.
I will try to see how I can make the servo working properly and set output down to +/-1-2mV which is more than acceptable.
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Old 1st October 2012, 07:53 PM   #7450
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Quote:
Originally Posted by Stefanoo View Post
I will try to see how I can make the servo working properly and set output down to +/-1-2mV which is more than acceptable.

Looking forward for your input
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