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diyAudio Member

Join Date: Sep 2009
Location: Tampa, Florida
Quote:
 Originally Posted by Itsmee R26 should be 10K, assuming 60V supply rails -60V ________ x R28 = -19.32V the correct voltage reference for the cascode transistor R26+R28
So for 32 volt rails, R28 should be the same value as R26 (4.75k). That yields about 16 volts for the cascode reference.

Rush

Last edited by Rush; 9th April 2012 at 04:25 PM.

 9th April 2012, 04:16 PM #132 diyAudio Member     Join Date: Oct 2004 Location: At the sea front, Rotterdam or Curaçao If you think 10 Volts on the JFET drains is not good enough. __________________ President Dwayne Camacho says you should drink Brawndo : It's got electrolytes
 9th April 2012, 04:22 PM #133 diyAudio Member   Join Date: Feb 2009 for 1/3 of rail voltage you will have just above 10V. so still R26=10Kohm but there is no problem to rise it to 16V. or higher. Last edited by AudioSan; 9th April 2012 at 04:26 PM.
diyAudio Member

Join Date: Sep 2009
Location: Tampa, Florida
Quote:
 Originally Posted by jacco vermeulen If you think 10 Volts on the JFET drains is not good enough.
Help me out, what is good enough?
I am using 10 IDSS 2sj/2sk.

 9th April 2012, 04:54 PM #135 diyAudio Member   Join Date: Jul 2004 Location: Gone on holiday again. back soon. If you run the jFETs at 70% of Idss and you have 10Vds then each jFET has to dissipate ~70mW. That will be cool to just warm. __________________ regards Andrew T. Sent from my desktop computer using a keyboard
 9th April 2012, 05:30 PM #136 diyAudio Member     Join Date: Feb 2009 For 32V supply rails, I would make R25,R26,R27,R28 10K and leave it at that. If you want to find the sweet spot, you have the option of putting a resistor in parallel with R27 & R28 to increase the cascode reference voltage, or putting a resistor in parallel with R25 & R26 to decrease the cascode reference voltage.
 9th April 2012, 06:18 PM #137 diyAudio Member   Join Date: Dec 2005 Location: São Paulo Are V grade jfets suitable?
diyAudio Member

Join Date: Sep 2009
Location: Tampa, Florida
Quote:
 Originally Posted by Daniel(Del) Are V grade jfets suitable?
These are BLs that I have a bunch of in that range. Papa said they would be too high idss and get hot if you put full voltage on them. So, I am cascoding them and as Andrew pointed out they should be just warm.

Rush

diyAudio Member

Join Date: Dec 2005
Location: São Paulo
Quote:
 Originally Posted by Rush These are BLs that I have a bunch of in that range. Papa said they would be too high idss and get hot if you put full voltage on them. So, I am cascoding them and as Andrew pointed out they should be just warm. Rush
Rush, that's because rail voltage is higher than the jfet breakdown voltage, right?
Perhaps I may have not been specific on my question; actually I'd like to know if replacing blue jfets with violet ones is possible and if it would bring any advantages in terms of gain, noise, sound quality etc.

Many thanks,
Daniel

 9th April 2012, 11:52 PM #140 diyAudio Member     Join Date: Oct 2004 Location: At the sea front, Rotterdam or Curaçao The J74/K170 are 400mW devices, doesn't mean they're supposed to dissipate the full amount. Standard practice is 50% Max, read chapter 6 of the Toshiba semiconductor reliability handbook, referenced to in any Toshiba part datasheet. 50% is a bit conservative. The standard F5 design has J108/K370, 200mW devices. Vds is a little under 20V, derated Idss is 6mA, makes short of 120mW. Or ~60% of the 200mW Max Pd. 50% derated J74/K170 handle 200mW, at 10Vds good for 20mA drain current. High current good : higher Idss, better numbers, e.g. capacitances, lower noise. High voltage bad : more heat, more gate leakage, etcetera. __________________ President Dwayne Camacho says you should drink Brawndo : It's got electrolytes

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