F5 Turbo Builders Thread - Page 14 - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 9th April 2012, 03:08 PM   #131
Rush is offline Rush  United States
diyAudio Member
 
Join Date: Sep 2009
Location: Tampa, Florida
Quote:
Originally Posted by Itsmee View Post
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.

Some one say yes, please.

Rush

Last edited by Rush; 9th April 2012 at 03:25 PM.
  Reply With Quote
Old 9th April 2012, 03:16 PM   #132
diyAudio Member
 
jacco vermeulen's Avatar
 
Join Date: Oct 2004
Location: At the sea front, Rotterdam or Curaçao
Send a message via Yahoo to jacco vermeulen
If you think 10 Volts on the JFET drains is not good enough.
__________________
The buck stops Here
  Reply With Quote
Old 9th April 2012, 03:22 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 03:26 PM.
  Reply With Quote
Old 9th April 2012, 03:23 PM   #134
Rush is offline Rush  United States
diyAudio Member
 
Join Date: Sep 2009
Location: Tampa, Florida
Quote:
Originally Posted by jacco vermeulen View Post
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.
  Reply With Quote
Old 9th April 2012, 03:54 PM   #135
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
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.
  Reply With Quote
Old 9th April 2012, 04:30 PM   #136
Itsmee is offline Itsmee  England
diyAudio Member
 
Itsmee's Avatar
 
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.
  Reply With Quote
Old 9th April 2012, 05:18 PM   #137
diyAudio Member
 
Join Date: Dec 2005
Location: São Paulo
Are V grade jfets suitable?
  Reply With Quote
Old 9th April 2012, 05:58 PM   #138
Rush is offline Rush  United States
diyAudio Member
 
Join Date: Sep 2009
Location: Tampa, Florida
Quote:
Originally Posted by Daniel(Del) View Post
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
  Reply With Quote
Old 9th April 2012, 07:55 PM   #139
diyAudio Member
 
Join Date: Dec 2005
Location: São Paulo
Quote:
Originally Posted by Rush View Post
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
  Reply With Quote
Old 9th April 2012, 10:52 PM   #140
diyAudio Member
 
jacco vermeulen's Avatar
 
Join Date: Oct 2004
Location: At the sea front, Rotterdam or Curaçao
Send a message via Yahoo to jacco vermeulen
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.
__________________
The buck stops Here
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
F5 Turbo Circuit Boards chchyong89 Pass Labs 2347 14th July 2014 02:19 PM
B1 builders thread Bas Horneman Pass Labs 696 14th July 2014 12:57 PM
F5 Turbo is posted Nelson Pass Pass Labs 635 6th April 2014 10:51 AM
F5 turbo problems Rob Dingen Pass Labs 17 11th January 2013 08:38 PM
F5 Turbo ............. if only !!!!!! a.wayne Pass Labs 23 21st January 2012 02:24 AM


New To Site? Need Help?

All times are GMT. The time now is 04:54 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2