Problems when starting up F5. - 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 19th December 2014, 11:54 PM   #1
diyAudio Member
 
Join Date: Aug 2009
Default Problems when starting up F5.

Hi!

My friend has copied my F5 builds (CLC input filter, dual mono).
I was over at his house helping him with the start up procedure, and everything was going well until we were starting to turn up the juice (adjusting the bias and offset).

I noticed that turning the variable resistors had little effect on the voltage reading across R7\R8. The offset (measuring voltage across speaker terminals) responds normal to adjustment of the resistors, but when I reached a reading of ca. 30 mV across R7\R8 the resistors had reached their end point.

We have tripple checked every resistor position and value. We have checked all positions and directions of transistors. We were getting tired at the end, but we must be missing something quite obvious?

He has built two amps. Both amplifier sides of both amps behave in the same way.....Help?
  Reply With Quote
Old 20th December 2014, 05:15 AM   #2
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
Idss of input Jfets ?
__________________
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 20th December 2014, 10:07 AM   #3
diyAudio Member
 
Join Date: Aug 2009
He bought grade a or b from the diyaudio store...
  Reply With Quote
Old 20th December 2014, 10:14 AM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
measure the Idss of the jFETs. (a DMM + 9V battery + 100r is all you need to get an approximate value).
measure the voltage across the drain load of the jFETs.
measure the output source resistor Vdrop.
__________________
regards Andrew T.
  Reply With Quote
Old 20th December 2014, 12:23 PM   #5
diyAudio Member
 
Join Date: Aug 2009
Could you give me further instructions of how test this up? Thanks!
  Reply With Quote
Old 20th December 2014, 12:34 PM   #6
diyAudio Member
 
KatieandDad's Avatar
 
Join Date: Nov 2011
Location: UK
If you do a search for "matching fets" you will see how it's done.
  Reply With Quote
Old 20th December 2014, 06:39 PM   #7
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Looks like you need to increase the value of the load resistance between
the supplies and Drains of the input devices. As there is a resistor in parallel
with each pot on stock versions, you can increase the value of those resistors.

  Reply With Quote
Old 21st December 2014, 12:16 AM   #8
haato is offline haato  Norway
diyAudio Member
 
Join Date: Sep 2014
Thank you for your answer, PMA is helping me out. I have changed the resistor (R3 and R4) in parallel with each pot from 2K2 to 4K7. Before the switch I reached 30 mV. I now get 180mV across the R7\R8 resistors before the variable resistors (P1 and P2) reached their end point.

What value should I choose for R3 and R4 to reach the final goal of 590mV?
  Reply With Quote
Old 21st December 2014, 02:44 AM   #9
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
try 6K8

I asked for Idss .... in that case you would be served with precise answer

__________________
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 21st December 2014, 10:26 AM   #10
haato is offline haato  Norway
diyAudio Member
 
Join Date: Sep 2014
The LSJ74A should have Idss min 2,6mA max 6,5mA. The LSK170A should have Idss min 2,6 mA and max 6,5 mA. I bought them from the DIYAudio store.

I`m quite new to DIY, thank you for all your help. Hope I will have the amp`s up and running for Christmas.
  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
Starting to build my first F5 Turbo V3 2bz Pass Labs 94 5th December 2014 06:28 PM
F5 bias problems IZHAKKATZ Pass Labs 14 27th March 2013 02:32 PM
F5 turbo problems Rob Dingen Pass Labs 17 11th January 2013 09:38 PM
F5 problems Flemming J P Pass Labs 15 14th April 2012 02:37 AM
Just starting to learn o-scope, F5 1k square looks funny JZatopa Pass Labs 24 25th March 2012 08:48 AM


New To Site? Need Help?

All times are GMT. The time now is 12:51 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