How To Adjust Bias Current? - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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
Thread Tools Search this Thread
Old 14th July 2008, 12:40 PM   #1
Bushulo is offline Bushulo  India
diyAudio Member
Bushulo's Avatar
Join Date: Aug 2006
Location: Goa
Default How To Adjust Bias Current?

Im working on Quasi Nmos500W amp almost finished just waiting to fire up. I need your help to adjust Bias. I think amplifier settings depend on different technique. To set VR1 and VR2.Multimeter I own Fluke 80 Series, I need some clarification, how to adjust Bias current. Can someone please put in detail the procedure? Selection of Multimeter range? How to place multimeter Red probe and Black probe on the circuit point while setting Bias current.

Power amp under development

  Reply With Quote
Old 14th July 2008, 02:11 PM   #2
Corax is offline Corax  Germany
diyAudio Member
Join Date: Feb 2005
Location: south of lower saxon
That's quite simple:

First leave the amp turned off for now:
Turn the idle current pot to the position where the base of the transistor (which is connected to the idle current pot) is connected to the most positive voltage level (closer to the collector potential).
Then hook up your test leads (red/black or +/-, polarity actually doesn't really matter, the relative dc voltage drop is to know), at one of the legs of the output FET pairs, right between the FET and the low-R (0.47 ohm resistors). This way you'll measure the voltage across the two in series connected low-R resistors. In between the output to the speaker.
If a total idle current of 100mA is desired and 3 FET pairs (legs) are provided - resulting in a ~33mA current through each leg - the voltage you should finally measure is approximately 31mV dc.
Now turn on the amp and - if no smoke comes out of it - let it run for a few minutes to heat up slightly.
Now adjust the idle current pot to the aforementioned voltage level (desired idle current). Let the amp run at least 15 to 30 minutes and observe the idle current during that time. In case of any rapid rise turn the amp off immediately and check for any issues that might have lead to this behaviour (i.e. output FETs are not firmly connected to the heatsink, etc.). After 30 minutes of additional warm up, with an appropiate idle current applied, it will be usually necessary to re-adjust the idle current (voltage across the low-R resistors) to the desired voltage drop. Since MOSFETs have a positive temperature coefficient it's more likely that the idle current has dropped a little bit after 30 minutes.
Now you can turn your amp off and remove the testleads/testclips and your amp should be ready to rumble
  Reply With Quote


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
How do I adjust amp bias? fotzepolitic Solid State 31 22nd May 2017 09:55 AM
BGW 500D Bias Adjust ??? Loud & Clear Solid State 36 11th September 2009 08:57 PM
Krell KSA-250 bias adjust willcycle Solid State 0 14th September 2007 09:39 PM
How To adjust Adcom 565 Bias HiFiMonk Solid State 1 25th November 2005 12:09 AM
Cannot adjust bias current, Hafler 220 mmerig Solid State 5 31st March 2005 02:28 AM

New To Site? Need Help?

All times are GMT. The time now is 08:53 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2