Simplistic NJFET RIAA - Page 941

Salas · 11th March 2013, 09:51 AM

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.

massimo · 09:58 AM

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 ?

Salas · 11th March 2013, 09:58 AM

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.

Salas · 11th March 2013, 10:04 AM

Quote:

Originally Posted by massimo

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

massimo · 11th March 2013, 10:25 AM

Yes, those are almost perfect, but I do need another quad for a friend.

RCruz · 11th March 2013, 01:47 PM

Quote:

Originally Posted by Salas

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.

Quote:

Originally Posted by Joachim Gerhard

i found this :
JFET Matcher

Quote:

Originally Posted by Salas

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.

Salas · 11th March 2013, 02:32 PM

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.

Salas · 11th March 2013, 02:38 PM

Quote:

Originally Posted by massimo

Yes, those are almost perfect, but I do need another quad for a friend.

Choose next best around the 11 mark.

Jallhalla · 14th March 2013, 02:56 PM

I can get both bf245 b and c, but a seems harder.

Off topic, but why is always the semiconductors I´m looking for out of stock? Is it the universe, or is this a hipster thing audio designers do?

Tea-Bag · 14th March 2013, 02:58 PM

Quote:

Originally Posted by Jallhalla

I can get both bf245 b and c, but a seems harder.

Off topic, but why is always the semiconductors I´m 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.

11th March 2013, 09:55 AM	#9402
massimo diyAudio Member Join Date: Dec 2002 Location: Milan, Italy	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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11th March 2013, 09:51 AM	#9401
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	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.

11th March 2013, 09:58 AM	#9403
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	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.

11th March 2013, 10:25 AM	#9405
massimo diyAudio Member Join Date: Dec 2002 Location: Milan, Italy	Yes, those are almost perfect, but I do need another quad for a friend.

11th March 2013, 02:32 PM	#9407
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	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.

14th March 2013, 02:56 PM	#9409
Jallhalla diyAudio Member Join Date: May 2012	I can get both bf245 b and c, but a seems harder. Off topic, but why is always the semiconductors I´m looking for out of stock? Is it the universe, or is this a hipster thing audio designers do?