High DC offset with no source (LM4780) - 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 2nd February 2010, 02:32 PM   #1
diyAudio Member
 
Join Date: Nov 2009
Default High DC offset with no source (LM4780)

I run a paralleled Lm4780 config (audiosector) which dispalys an odd behaviour. With no source connected I measure ~2-3V DC on both channels, however if I short the inputs (or connect an preamp) the DC offset is ~16mV on both channels. This is with the preamp off, with the preamp on, the only addition is some minor fluctuations when playing music. I`m using the amplifier and it works great, I was just confused why it would do this with no source? I use 0.1% resistors.

I haven't had speakers connected when I had the 2-3V DC, so no worries
  Reply With Quote
Old 2nd February 2010, 03:03 PM   #2
diyAudio Member
 
Join Date: May 2008
TomTen : do you have a resistor going from the chips input to ground? The input of the chip needs a load. Typically 20K or so. This must be between the chip and any coupling caps.
  Reply With Quote
Old 2nd February 2010, 03:07 PM   #3
diyAudio Member
 
Join Date: Nov 2009
Yes, it is a 22k resistor. This is the schematic: http://www.audiosector.com/lm4780%20amp.pdf
  Reply With Quote
Old 2nd February 2010, 03:12 PM   #4
diyAudio Member
 
Join Date: May 2008
Hmm. I don't see any resistor going from your + input to ground. the - inputs on that when used in stereo are for feedback.

R4 and R0 should be 20-50K. if the channels are isolated.

Last edited by thetube0a3; 2nd February 2010 at 03:15 PM.
  Reply With Quote
Old 2nd February 2010, 03:19 PM   #5
diyAudio Member
 
Join Date: Nov 2009
Do you mean the ci cap? If so, no.

Does it matter that the chips are paralleled?

Well, I only have R4 (22k) installed, not R0 beacause the chips are paralleled and the jumper in the schematic implies that they share R4 I Suppose?

Last edited by Tomten; 2nd February 2010 at 03:22 PM.
  Reply With Quote
Old 2nd February 2010, 03:22 PM   #6
diyAudio Member
 
Join Date: May 2008
If they are paralleled and R4 is in place I'm out of idea's. Tho if you used the Cz caps in the feedback loop it should get rid of all or close to all of your dc offset.
  Reply With Quote
Old 2nd February 2010, 03:25 PM   #7
diyAudio Member
 
Join Date: Nov 2009
Ok, its just strange, I mean 16mv sounds just right (and quite good) and 2-3V doesnt

Last edited by Tomten; 2nd February 2010 at 03:44 PM.
  Reply With Quote
Old 2nd February 2010, 03:31 PM   #8
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
That schematic shows a DC coupled amplifier.

The output offset is ~ gain * ( input offset current * input resistance) + residual output offset of the amplifier.

For your arrangement the input resistance changes from Rin to Rsource when you couple up the pre-amp. That CHANGES the output offset.

If you cannot understand why you should not do that then you should not be building a DC coupled amplifier.
__________________
regards Andrew T.
  Reply With Quote
Old 2nd February 2010, 03:53 PM   #9
diyAudio Member
 
Join Date: May 2008
AndrewT : wouldn't using Cz in the feedback loop fix the offset problem? Or are the input caps still required?

I'll be building another lm3886 amp soon and was going to omit the input caps but keep the feedback ones.
  Reply With Quote
Old 2nd February 2010, 03:59 PM   #10
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
I do not recommend a mixed AC and DC coupled amplifier.
Either DC coupled with all the protections necessary to protect your speakers or AC coupled.
__________________
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
High Dc Offset Issues nic6paul Class D 2 3rd March 2009 02:49 PM
Question about DC offset from source. theAnonymous1 Solid State 2 12th November 2006 08:07 PM
BrianT LM4780 DC offset SEIJI Chip Amps 8 21st April 2005 02:23 AM
36 V DC offset when source connected revell Chip Amps 12 27th February 2005 08:29 PM
High DC-Offset - Why? rampage101 Chip Amps 1 30th April 2004 09:16 AM


New To Site? Need Help?

All times are GMT. The time now is 01:05 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