Class A mosfet idle 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 15th April 2010, 05:59 PM   #1
diyAudio Member
 
east electronics's Avatar
 
Join Date: Jan 2006
Location: Athens GREECE
Default Class A mosfet idle issue

that is an issue i ve never seen before and i need some help

there is a Magnum class A amplifier that has unregulated power supply of 42+42 volts dc to drive 2pairs of mosftes per amplifier at 800ma idle about ...

first stages of amplifier work with regulated psu of 56+56 volts coming from a pair of 317 337

now biassing the amplifier using a ma meter at positive rail fuse and make sure that idle is at 800 ma at both chanels after a long warming up period was simple ...and bias was stable ..

But .... voltage drop on source resistors of chanel A was 80 mv and voltage drop on chanel B was 140 millivolt each and every 0R33 resistor had less than 2% tollerance so that means that semis are closelly matched

it is obvious that in the first measure something is wrong and there is current consumed elsewhere except output stages

have no schematic and seems that UK made machine is a pain in the a@@ to crack open ...

will see though
any input will be very usefull
__________________
SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
  Reply With Quote
Old 15th April 2010, 06:58 PM   #2
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
ask Magnum ;

they were pretty helpful and kind in past
__________________
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 15th April 2010, 07:34 PM   #3
diyAudio Member
 
event horizon's Avatar
 
Join Date: Feb 2008
Location: East Midlands, England
Quote:
Originally Posted by sakis View Post
it is obvious that in the first measure something is wrong and there is current consumed elsewhere except output stages
Are any of the PSU electrolytics fitted to the circuit board?

I'm simply asking as i used to find SMPS electrolytics that leaked onto PCBs tend to make the PCB rather conductive It might not be due to a large electrolytic either, the end result is a useless PCB.

Just a thought & i hope this isn't the case as the only cure is to cut away the area impregnated with electolyte
__________________
"Never let your morals prevent you from doing what is right!" Salvor Hardin
  Reply With Quote
Old 16th April 2010, 12:51 AM   #4
eyoung is offline eyoung  United States
diyAudio Member
 
eyoung's Avatar
 
Join Date: Aug 2007
Location: Sunny SC,USA 15 min south of Charlotte NC
cut the power to the outputs and see what the driver boards are pulling. I use a 1r resistor in place of the fuse and read voltage drop across so you don't have current meandering through your leads and meter...

Regards, Elwood
__________________
"Lead me not into temptation...I can find it myself."
  Reply With Quote
Old 16th April 2010, 06:51 AM   #5
diyAudio Member
 
east electronics's Avatar
 
Join Date: Jan 2006
Location: Athens GREECE
Quote:
Originally Posted by event horizon View Post
Are any of the PSU electrolytics fitted to the circuit board?

I'm simply asking as i used to find SMPS electrolytics that leaked onto PCBs tend to make the PCB rather conductive It might not be due to a large electrolytic either, the end result is a useless PCB.

Just a thought & i hope this isn't the case as the only cure is to cut away the area impregnated with electolyte

cant be since if leacky electros will 100% result to some voltage drop since its class A and there constand current draw
__________________
SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
  Reply With Quote
Old 16th April 2010, 06:53 AM   #6
diyAudio Member
 
east electronics's Avatar
 
Join Date: Jan 2006
Location: Athens GREECE
Quote:
Originally Posted by eyoung View Post
cut the power to the outputs and see what the driver boards are pulling. I use a 1r resistor in place of the fuse and read voltage drop across so you don't have current meandering through your leads and meter...

Regards, Elwood
thanks but no need to do that ...( its a lot of work also to isolate first stage and out ) it is obvious from voltage drop on 0.33 resistors that some ammount of power is consumed in a first stage .....
__________________
SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
  Reply With Quote
Old 16th April 2010, 06:54 AM   #7
diyAudio Member
 
east electronics's Avatar
 
Join Date: Jan 2006
Location: Athens GREECE
Quote:
Originally Posted by Zen Mod View Post
ask Magnum ;

they were pretty helpful and kind in past
yes i can also do that thanks ZEN but i actually cannot find any info on the internet regarding this company can you point me something ....
__________________
SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
  Reply With Quote
Old 16th April 2010, 07:55 AM   #8
Zen Mod is offline Zen Mod  Serbia
diyAudio Member
 
Zen Mod's Avatar
 
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
I'll try to find envelope with addy
__________________
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 16th April 2010, 08:36 AM   #9
diyAudio Member
 
east electronics's Avatar
 
Join Date: Jan 2006
Location: Athens GREECE
kind of you ...thank you very much zen
__________________
SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
  Reply With Quote
Old 12th May 2010, 11:19 AM   #10
diyAudio Member
 
east electronics's Avatar
 
Join Date: Jan 2006
Location: Athens GREECE
any news on that one ????

managed to solve the problem ( located in regulators and generally everything is boiling inside )

it would be very nice to have some info on the original bias ( now is set to about 700 ma ) and that is enough to rise temperature of 65 degrees c on tha main heat sink

other than that the amplifier is nothing that surprized me so much of class A i was expecting it be something magic but is not
__________________
SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
  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
Class A Mosfet Amplifier and PS Kinnja Solid State 102 7th April 2014 06:47 PM
Collection of Class B topologies <100mA Idle and Sound closest by Class A tiefbassuebertr Solid State 37 27th July 2012 08:04 AM
Mosfet reliability in Class D amplifiers Pierre Class D 164 17th January 2011 09:41 AM
help!ab class mosfet amp mbonus Solid State 31 19th September 2005 04:14 PM
class B and AB idle current demons_wing Solid State 6 6th October 2004 05:14 AM


New To Site? Need Help?

All times are GMT. The time now is 01:27 PM.


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