Biasing EF output class B amps... - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 25th May 2012, 02:43 PM   #1
diyAudio Member
 
Join Date: Apr 2012
Location: Maine
Default Biasing EF output class B amps...

I am in the process of learning about different amp topologies and designs, and so far, the only thing I have built is a Pass A40. With class A, biasing seems simple in many ways, just give it a lot of bias and call it good. In trying to learn class B, the exact bias point matters, and how to get there is eluding me.

I understand the bias is supposed to be set high enough to eliminate crossover distortion, putting the transistors at the point of conduction, but without going over and drawing too much current. The only thing I came up with is looking at the emitter resistors, but I don't know what to look for exactly.

Without having any knowledge of specs (except supply voltages and general topology), is there an easy way to set the bias to the correct level with just a DMM and o-scope? Any help would be appreciated.
  Reply With Quote
Old 25th May 2012, 03:13 PM   #2
Banned
 
Join Date: Jan 2005
Class B amplifiers have no bias, in place of the bias tempco transistor, they have a string of 2-3 1N4148's.
  Reply With Quote
Old 25th May 2012, 03:37 PM   #3
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
ClassAB EF output stage is optimally biased when Vre ~26mV.
The difficulty is that the re includes the internal transistor resistance as well as the external Re.
As the external Re is reduced, the proportion of Vre across the external resistor reduces to maintain that optimal bias condition. This is best done by monitoring the crossover distortion and setting the output bias to suit. That method is not available to most builders.

Settle for a Vre~18mV for Re=0r1 and rising to ~24mV for Re=0r47. Self listed the intermediate values of Re and the total Vre across the series pair. Self also defined his ClassB the same as what everyone else labels optimal ClassAB.
__________________
regards Andrew T.
  Reply With Quote
Old 25th May 2012, 03:45 PM   #4
Bonsai is offline Bonsai  Taiwan
diyAudio Member
 
Bonsai's Avatar
 
Join Date: Jul 2003
Location: Taipei, Taiwan
I just stick with 26mV across the emitter resistor. This results in a little overbias, but it's quite ok.
__________________
bonsai
Amplifier Design and Construction for MUSIC! http://hifisonix.com/
  Reply With Quote
Old 25th May 2012, 05:46 PM   #5
diyAudio Member
 
Join Date: Apr 2012
Location: Maine
Thanks for the help, everyone. I have an old Sony STR-V6 receiver that I was fixing, and when I first got it, the power supply thermal fuse kept blowing after a while, but when I replaced the fuse it sounded ok. I figured the caps were leaky/old and the bias would be off. I was only seeing ~10mv or so across each .47ohm Re, and it didn't seem right according to what I'd heard. There is indeed a set of 4 diodes in series, and an adjust pot per channel. I've replaced all the major electrolytics, and an open resistor, and now I need to adjust it. Is it possible to see the crossover notch on an o-scope, or will the resolution be too low on a digital scope such as a Rigol 1052, etc?

As soon as I get a feel for understanding how all this works, I'm going to try building a class B amp, but haven't decided what project would be a good one yet.
  Reply With Quote
Old 25th May 2012, 06:14 PM   #6
Banned
 
Join Date: Jan 2005
The crossover distortion is easiest seen with a low signal level like a volt or less on the output with a frequency of 10-20kHz. Sometimes its not a right off flat, but rather a little kink in the waveform around the zero crossings, so watch for these straightening out.

Adjust until the flat parts around the zero crossings just about disappear, check the voltage across the emitter resistors, then let the amp sit turned on for ~15 minutes to let things stabilize and settle down, then recheck the bias and readjust if needed, then let it sit for another ~15 minutes, recheck bias again and your done.

I'd make a tutorial on doing bias adjustment on an amplifier, but after all flack i've gotten from ppl that my circuits, measurements and other stuff are flawed, i'm not going to as i will likely be doing this wrong as well.
  Reply With Quote
Old 25th May 2012, 06:46 PM   #7
diyAudio Member
 
Join Date: Jun 2008
Location: HaiPhong
Quote:
Originally Posted by darkskeptic View Post
Thanks for the help, everyone. I have an old Sony STR-V6 receiver that I was fixing, and when I first got it, the power supply thermal fuse kept blowing after a while, but when I replaced the fuse it sounded ok. I figured the caps were leaky/old and the bias would be off. I was only seeing ~10mv or so across each .47ohm Re, and it didn't seem right according to what I'd heard. There is indeed a set of 4 diodes in series, and an adjust pot per channel. I've replaced all the major electrolytics, and an open resistor, and now I need to adjust it. Is it possible to see the crossover notch on an o-scope, or will the resolution be too low on a digital scope such as a Rigol 1052, etc?

As soon as I get a feel for understanding how all this works, I'm going to try building a class B amp, but haven't decided what project would be a good one yet.
should follow datasheet of power trans. search detail of it and adj correctly value current cut off of power transistor.
  Reply With Quote
Old 25th May 2012, 09:48 PM   #8
diyAudio Member
 
Join Date: Apr 2012
Location: Maine
Sorry to ask so many questions, but I think this will be the last one on this thread. Well, I put the amp on the scope, and fiddled with the bias pot till the crossover disappears and the waveform looked normal at 10khz, which was only around 10mV Re or so... Going beyond that to 26mv would more than double the dissipation, such is class AB right? As this amp was built to be class B, is there any reason (performance or otherwise) other than heat that I shouldn't put it at 26mV and call it good? The heatsinks just barely feel warm after 30 min at idle, and are of substantial size, being at the tail end of the 70's receiver power wars.
  Reply With Quote
Old 25th May 2012, 10:36 PM   #9
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
Sounds like you can definitely try giving a value up to 26 mV a shot. You're still in underbiased territory now, and if the amp can handle more without getting too warm, you can obtain even better distortion performance. But don't overdo it, since SOA on those old power transistors may not be that luxurious and they die more easily when hot.
  Reply With Quote
Old 26th May 2012, 10:24 AM   #10
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Try 15mVre, then 20mVre and finally 25mVre.
Do they measure any different? Do they sound any different?
Can the amp survive the higher temperatures?
__________________
regards Andrew T.
  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
Kulish Corrector for Class-A EF O/P Stage linuxguru Solid State 98 19th June 2013 07:18 PM
low-biasing or high biasing in Class-AB amps Workhorse Solid State 124 18th October 2005 04:45 AM


New To Site? Need Help?

All times are GMT. The time now is 06:00 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