Input resistor problem(3886) please help - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

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 23rd January 2005, 11:45 PM   #1
avenger is offline avenger  United States
diyAudio Member
 
Join Date: Nov 2003
Location: Boston
Default Input resistor problem(3886) please help

Hi Guys,
I have built a non-inv amp (LM3886), the issue is that I forgot to place a resistor from the non inverting input to ground. It worked but was really noise and the output was at a 35V potential(same as power supply Voltage). No mater what I did I couldn't figure out what was wrong specially with the very loud hum. Then I placed a 22k res. there and all is good!! no dc at all on the output.

what is the purpose of this resistor?
Now its very quiet but get a bit hot for my taste even at rest, I have a 3inch x4inch aluminum plate a a heatsink. Would this be normal?

Please reply with any thoughts at all.
Thank you very much in advance for yoru time.
God bless.
  Reply With Quote
Old 23rd January 2005, 11:56 PM   #2
diyAudio Member
 
Tube_Dude's Avatar
 
Join Date: Mar 2002
Location: Aveiro-Portugal
Default Re: Input resistor problem(3886) please help

Quote:
Originally posted by avenger
Then I placed a 22k res. there and all is good!! no dc at all on the output.

what is the purpose of this resistor?

Hi

The purpose of the resistor , is to provide a DC path, for bias the base of the transistor , of the differential pair, that is inside the ship.

Regards
__________________
Jorge
  Reply With Quote
Old 24th January 2005, 12:10 AM   #3
avenger is offline avenger  United States
diyAudio Member
 
Join Date: Nov 2003
Location: Boston
so,
if this resistor is not there what would happen? Would we get the supplied voltage at the output like I got?


God bless.
  Reply With Quote
Old 24th January 2005, 12:16 AM   #4
diyAudio Member
 
Tube_Dude's Avatar
 
Join Date: Mar 2002
Location: Aveiro-Portugal
Quote:
Originally posted by avenger
so,
if this resistor is not there what would happen? Would we get the supplied voltage at the output like I got?


God bless.
Yes!....As one of the bases of the input pair is not biased ,only one "half" of the amp is working and the output will be at the voltage , of one of the rails.

God bless you to.
__________________
Jorge
  Reply With Quote
Old 24th January 2005, 12:24 AM   #5
avenger is offline avenger  United States
diyAudio Member
 
Join Date: Nov 2003
Location: Boston
ok,
that makes perfect sense.
As far as the temperature goes...should it get hot at idle? Mine gets a little bit warm just sitting there.

Muito obrigado e continuacao de uma boa noite. Acho que e a' volta de uma da manha ai em portugal se nao me engano?

God bless.
  Reply With Quote
Old 24th January 2005, 12:32 AM   #6
diyAudio Member
 
Tube_Dude's Avatar
 
Join Date: Mar 2002
Location: Aveiro-Portugal
Quote:
Originally posted by avenger

As far as the temperature goes...should it get hot at idle? Mine gets a little bit warm just sitting there.
About the temperature issue...I have no experience with the LM 3886 .
Have you inspected with a scope , if there is no oscilation , at the output?
Have you used a Zobel?

Quote:
Muito obrigado e continuacao de uma boa noite. Acho que e a' volta de uma da manha ai em portugal se nao me engano?
De nada! Escreves bem Português...pois são ,uma e meia da manhã...


Abraço
__________________
Jorge
  Reply With Quote
Old 24th January 2005, 12:42 AM   #7
Sherman is offline Sherman  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Chicago area
Quote:
Originally posted by avenger
ok,
that makes perfect sense.
As far as the temperature goes...should it get hot at idle? Mine gets a little bit warm just sitting there.

Muito obrigado e continuacao de uma boa noite. Acho que e a' volta de uma da manha ai em portugal se nao me engano?

God bless.

No experience with the 3886 but every 3875 I've built got hot even at low volumes during the first 10 hours or so. For whatever reason, after that "break in time" they all cooled off and operated at a much lower temp. Can't explain why but since it has happened over and over again I guess it is real.

Maybe something similar with the 3886?
__________________
--Sherman
  Reply With Quote
Old 24th January 2005, 12:51 AM   #8
avenger is offline avenger  United States
diyAudio Member
 
Join Date: Nov 2003
Location: Boston
wow, great,
at least mine is not the first to do that.

You had mentioned the oscilation, I will take a look with the scope, but what should it look like? what should I look out for?

Actually I think its oscilating because just now it was making a scrachy noise with the audio and it was a lot different than before and the temp. started to increase alot..I had to shut it off for a few minutes.

Thank you in advance.


Sim, eu sou portugues e vivo nos EUA. sou natural de Ponta Delgada, ilha de Sao Miguel nos Acores.

Continuacao de uma boa noite.
  Reply With Quote
Old 24th January 2005, 01:25 AM   #9
diyAudio Member
 
Tube_Dude's Avatar
 
Join Date: Mar 2002
Location: Aveiro-Portugal
Quote:
Originally posted by avenger


You had mentioned the oscilation, I will take a look with the scope, but what should it look like? what should I look out for?
You must look for a sinusoid at the output ,without any input signal.

Quote:
Continuacao de uma boa noite
Para ti também.
__________________
Jorge
  Reply With Quote
Old 24th January 2005, 03:35 AM   #10
Banned
 
Join Date: Mar 2003
Location: Lisbon, Portugal
How thick is that "aluminium plate" you use as a heatsink?
It it straight, no fins???
  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
Resistor value in 3875 and 3886 haziz Chip Amps 3 29th April 2007 11:40 PM
3886 problem (high DC offset) phresh Chip Amps 5 26th January 2007 02:54 PM
problem with lm 3886 application circuit gothmog42 Chip Amps 19 3rd October 2006 06:06 PM
diff input 3886? Williams Audio Chip Amps 7 9th August 2006 07:05 PM
Resistor-Input vs. Choke-Input vs. Cap.Multip. Blitz Pass Labs 13 26th September 2003 04:59 AM


New To Site? Need Help?

All times are GMT. The time now is 07:41 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