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Old 11th March 2013, 09:51 AM   #9401
Salas is offline Salas  Greece
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So I am thinking if centering at 12.5mA IDSS then the Rs will be bit higher but not too high for the same gain and able to bring +/- 0.5mA samples together with the degeneration it will provide. Then again when having so many around 11mA they should be the preferred ones not to be adding extra noise from Rs. It would not be an issue if it was not for maintaining some currents in value and proportion so to center it easily always.
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Old 11th March 2013, 09:55 AM   #9402
massimo is offline massimo  Italy
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You can suggest Rs for different groups, i.e. 10.5; 11; 11.5; 12 and 12.5

With the 3.01 ohm resistors I've already bought, which pairs can I use for 2x 2SK369 63dB version? 10.5 > 11.5 ?

Last edited by massimo; 11th March 2013 at 09:58 AM.
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Old 11th March 2013, 09:58 AM   #9403
Salas is offline Salas  Greece
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Most probably will do so. Good thing is that in Tea-Bag's and yours there is an even spread they are not bunched only low or mid or high.
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Old 11th March 2013, 10:04 AM   #9404
Salas is offline Salas  Greece
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Quote:
Originally Posted by massimo View Post
With the 3.01 ohm resistors I've already bought, which pairs can I use for 2x 2SK369 63dB version? 10.5 > 11.5 ?
Your 11.21, 11.21, 11.22, 11.22
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Old 11th March 2013, 10:25 AM   #9405
massimo is offline massimo  Italy
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Yes, those are almost perfect, but I do need another quad for a friend.
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Old 11th March 2013, 01:47 PM   #9406
RCruz is offline RCruz  Switzerland
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Quote:
Originally Posted by Salas View Post
P.S. Another way is to measure their resistance from drain to source. If you will do a check you will notice that same IDSS pairs show the same RDS. The higher the IDSS the lower the RDS also.
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Originally Posted by Joachim Gerhard View Post
i found this :
JFET Matcher
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Originally Posted by Salas View Post
Self explanatory. DMM in Ohm mode and you measure across drain to source.
After measuring my last batch of k170, I found some deviations of Vp even in matched Idss pairs.

I know that we should match for Idss and Vp in order to get the same gain figures.

According to the formula Rds = Vp / (2 x Idss) found in those papers, a 7.55 Idss fet with 0619 Vp should have Rds = 41

I just measured the same specimen with a DMM and got Rds = 39.9 ohm

Admitting the deviations are not significative, I would say we could match those fets for same Idss and Rds using a DMM.
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Old 11th March 2013, 02:32 PM   #9407
Salas is offline Salas  Greece
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Exactly why I mentioned. Biggest deviation in average DMM should come from its mA spec and it mixes along your calc. So your RDS is valid.
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Old 11th March 2013, 02:38 PM   #9408
Salas is offline Salas  Greece
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Yes, those are almost perfect, but I do need another quad for a friend.
Choose next best around the 11 mark.
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Old 14th March 2013, 02:56 PM   #9409
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I can get both bf245 b and c, but a seems harder.

Off topic, but why is always the semiconductors Im looking for out of stock? Is it the universe, or is this a hipster thing audio designers do?
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Old 14th March 2013, 02:58 PM   #9410
Tea-Bag is offline Tea-Bag  United States
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I can get both bf245 b and c, but a seems harder.

Off topic, but why is always the semiconductors Im looking for out of stock? Is it the universe, or is this a hipster thing audio designers do?
I beleive we can use 2SK170 here as well. Salas will confirm though.
Agreed - these are not generally available now.
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