Weird F5 bias issue - 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 6th June 2011, 10:38 PM   #1
JZatopa is offline JZatopa  United States
diyAudio Member
 
Join Date: May 2003
Location: Chicago
Default Weird F5 bias issue

So I have my F5 playing using a bench power supply that has 27V rails and can put out up to 10 Amps. I have read 2 different ways to bias the amp (Nelsons and someone else's post). I am using Cvillers V2 boards and TechDIY parts. I do have current limiting resistors and thermistors in the circuit. When I try to set the bias using what is outlined in the Pass manual I can get .59V over R11 and R12 but my DC offset is around 230mv. The other way described has you set bias so it is ok on one resistor and then use the pots to 0 out DC offset. When I do this the highest voltage I can get on R11 and R12 is around .3V. Each time I let the amp warm up and adjusted settings, inputs were shorted and the speaker terminals were left open.

Am I doing something wrong? Could I have put a resistor in the wrong place? If so which location should I check first? It looks like everything is ok visually but some of the resistors are turned so I would have to remove them to make sure the values are proper. I would think that if I did make a mistake I must have made the same mistake on both boards.

Any help would be greatly appreciated.
  Reply With Quote
Old 6th June 2011, 10:49 PM   #2
JZatopa is offline JZatopa  United States
diyAudio Member
 
Join Date: May 2003
Location: Chicago
Would this happen if I put a pot in the wrong way?
  Reply With Quote
Old 6th June 2011, 10:57 PM   #3
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
increase input Jfets drain resistors for , say , 20%

repeat biasing procedure from start

use " one source resistor , offset " procedure
__________________
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa... by Mighty ZM
  Reply With Quote
Old 6th June 2011, 11:38 PM   #4
JZatopa is offline JZatopa  United States
diyAudio Member
 
Join Date: May 2003
Location: Chicago
Zen Mod - Thanks for the help but I need you to break it down into ultra-amp noob speak for me. I think it was your post that explained the bias method but I didn't want to say for sure because I didn't remember.

Which resistor is the jfet drain resistor? What is it's R#?
  Reply With Quote
Old 6th June 2011, 11:49 PM   #5
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
R3 & R4 at Papa's schmtc

go to 2K7
__________________
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa... by Mighty ZM
  Reply With Quote
Old 7th June 2011, 01:13 AM   #6
diyAudio Member
 
Andersonix's Avatar
 
Join Date: Jan 2004
Location: Silicon Valley
Or 3k3
  Reply With Quote
Old 7th June 2011, 11:13 PM   #7
JZatopa is offline JZatopa  United States
diyAudio Member
 
Join Date: May 2003
Location: Chicago
Before I try different resistors I read somewhere that the pots on the board should be mirrored not oriented the same way. Do I have one of the pots going the wrong way on this board? I have measured R3 and R4 and one has the proper V and one is around 3V. This is true for both boards. Should I switch the one on the low V side around?
Attached Images
File Type: jpg image.jpg (191.4 KB, 220 views)
  Reply With Quote
Old 7th June 2011, 11:34 PM   #8
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
having pots oriented is just matter of CW or CCW for increasing resistance

I'm not bothering usually with that - I always use ohmmeter to pre-set them ,prior to powering up ( if needed) .

in case of F5 -just be sure that you have them set to zero resistance , later voltage meters will tell you what's right direction to turning them
__________________
my Papa is smarter than your Nelson !
clean thread; Cook Book;PSM LS Cook Book;Baby DiyA ;Mighty ZM's Bloggg;Papatreasure;Papa... by Mighty ZM
  Reply With Quote
Old 8th June 2011, 01:02 AM   #9
JZatopa is offline JZatopa  United States
diyAudio Member
 
Join Date: May 2003
Location: Chicago
OK then I am not going to worry about trim pots.

Should I be worried about low voltage across the resistor or should I just up the resistance and try to bias again?
  Reply With Quote
Old 8th June 2011, 03:38 AM   #10
JZatopa is offline JZatopa  United States
diyAudio Member
 
Join Date: May 2003
Location: Chicago
Both boards

R3 = 4.08V
R4 = 4.5V

The highest I can bias R11,R12 to and keep ~ 0mV offset is .25V

This is roughly the same for both boards. One side I could bias up to .3V and still keep offset. Are there other Voltages to check?
  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
Weird issue with tube driver. 807tubenut Tubes / Valves 17 15th September 2010 12:19 AM
f5 power up issue specialidiot Pass Labs 54 2nd May 2010 11:27 AM
Sundown SAE-1200Dv1 Weird output issue possible Fet problem? RosDaddy Car Audio 7 11th April 2010 02:12 AM
Weird Bias on Simple SE ixe13 Tubes / Valves 12 9th July 2009 09:53 PM
Weird soundcard issue JacquesK Multi-Way 1 19th May 2008 06:07 AM


New To Site? Need Help?

All times are GMT. The time now is 10:00 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