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Old 5th January 2013, 09:14 PM   #831
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I see what you mean - it's just common sense and general grounding guidelines. It is, as i suspected, irrelevant to the issues some of us have. Thanks for the detailed reply.
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Old 5th January 2013, 09:19 PM   #832
Take5 is offline Take5  Germany
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Quote:
Originally Posted by analog_sa View Post
Take5, how is your offset?
Output Offset, 5mV adjustment on my very old scope:

Click the image to open in full size.

Chris

Last edited by Take5; 5th January 2013 at 09:33 PM.
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Old 5th January 2013, 09:42 PM   #833
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Could you just take a reading with a DVM? Does the oscillogram indicate a couple mV of white noise?
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Old 5th January 2013, 09:57 PM   #834
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You would need to make a FFT measurement and express inetgral noise value over 20 kHz band.
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Old 5th January 2013, 09:59 PM   #835
Take5 is offline Take5  Germany
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Quote:
Originally Posted by analog_sa View Post
Could you just take a reading with a DVM?
DC is floating between +30 / -30 mV.

Quote:
Does the oscillogram indicate a couple mV of white noise?
Yes
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Old 5th January 2013, 10:32 PM   #836
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Thank you. My measurements under the same conditions appear a bit worse, but not by orders of magnitude. Hiss levels with a 103 also appear intolerable.

If this is indeed normal operation, then this device cannot safely be used in a dc coupled system. There should be a coupling cap somewhere and probably best ahead of the buffer in order to minimise its value. Maybe Joachim can suggest an appropriate spot for a cap.
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Old 5th January 2013, 11:28 PM   #837
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Quote:
Originally Posted by Take5 View Post
DC is floating between +30 / -30 mV.
I need to drag the boards back from the cupboard and measure again, but i think mine measured a rapidly fluctuating dc within 120mV. Could it be your results are better as your input caps are 2 x 4700 as opposed to mine 2 x 6800? I seem to recall Joachim mentioning something along these lines. Maybe further reducing these caps and speeding up the servo will bring some improvement.

Any possibility to improve the stability of the front end?

PMA, what do you think?

Last edited by analog_sa; 5th January 2013 at 11:30 PM.
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Old 6th January 2013, 12:18 AM   #838
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Hi,
could one of those in the know (Joachim perhaps) please sum up the latest consensus on optimal hFe?

should i really use the same high hfe throughout the whole board? in the assempby guide input and three other sections are circled in. joachim stated someplace that 400 is enough for the input, but i canot find references to the other sections. there are over a thousand posts in the relevant threads, i`m lost.

i have most matches in the 470-500 range, so if thats fine i stuff with these. (i have a lot of high gain npn, just post me and i may even send some to somebody needing )
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Old 6th January 2013, 11:50 AM   #839
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If you have oscillation did you use the 2sk170's or the 107's ?

Could it be an unforeseen issue with the substitution of these parts? I used the 107's and have the oscillation.

Any advice re grade of 2sk170's to purchase I'll try swapping them out?
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Old 6th January 2013, 12:48 PM   #840
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Don't worry about the k170 substitution - it's not an issue.
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