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Old 21st February 2007, 01:54 PM   #11
h_a is offline h_a  Europe
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Great! Thanks a lot!

I guess R12 needs to be adjusted as well?

As I see it, Q1 is a variable resistor and its resistance is set by the voltage reference with the aid of Q2.

Seems that Q12 is sort of feedback, set by the voltage divider R10/R11. Since Q12's emitter is on the same potential as the emittor of Q2, I think this controls the voltage drop over Q2 and thus the voltage fed to Q1.

Last I guess R12 and R6/R7 finetune the sensibility of the Q2/Q12 regulator.

Is any of this correct?

Have fun! Hannes

PS: and of course now there's the question wether a 7 V voltage reference that needs to amplified by additional 22% (when compared to the zener) still gives less noise.
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Old 21st February 2007, 02:02 PM   #12
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Without looking at the Pearl schematics, did you include an RC filter after the voltage reference? If not that would be more important than having the lowest noise figure from the reference itself.
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Old 21st February 2007, 02:32 PM   #13
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Well, there's 3300uF attached in parallel and R=221 Ohm in series.

Cheers, Hannes
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Old 21st February 2007, 10:20 PM   #14
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Quote:
Well, there's 3300uF attached in parallel and R=221 Ohm in series.
Well, you have an R and a C closeby, but not the RC filter for the voltage reference!
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Old 22nd February 2007, 12:43 PM   #15
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Well after reading Ron Mancini's article on that topic, one could insert an 50 Ohms resistor in series. However this affects the regulator impedance? But probably not an issue here?

After some extensive searches/readings about that whole noise topic, the summary is terrific...no simple answer to the question what replacement for the zener gives less noise.

I think the lm329 would give less noise even if it has to be amplified by additional 30% (6.9 V vs. 9.1 V of the zener). Maybe connecting additional 50 Ohms in series would yield together with the 3300uF cap a nice RC-filter - but well this is just my impression. The cutoff-frequency of below 1 Hz is indeed impressive.

What about R9 (220 Ohm, connecting to the basis of Q2) then?

Remove it and use these 220 Ohm in series instead of the 50 Ohm?

Since I don't have any noise measurement equipment I cannot check wether all of this would really give some reasonable benefit.

I'm hoping somebody with more brain and experience could confirm my idea.

Cheers, Hannes
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