Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Chip Amps Amplifiers based on integrated circuits

High DC offset with no source (LM4780)
High DC offset with no source (LM4780)
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, 03:10 PM   #11
pacificblue is offline pacificblue  Germany
diyAudio Member
 
Join Date: May 2008
The schematic has a strange layout, which makes it a bit confusing. Cz is not part of the feedback loop.

If you mean to use Ci as per the datasheet, you should have a look into your sources. Most sources have an AC coupled output and in that case you can skip the blocking cap Cin and use only Ci. Since you have to adapt the corner frequencies of both input filters and all sources tend to have different filters, it is still better to use Cin and Ci. If you use DC coupled sources, you should not skip Cin, when you use Ci.
__________________
If you've always done it like that, then it's probably wrong. (Henry Ford)
  Reply With Quote
Old 2nd February 2010, 03:24 PM   #12
thetube0a3 is offline thetube0a3  United States
diyAudio Member
 
Join Date: May 2008
Ah your 100% right. I was thinking it was a cap inbetween R3 and R5. My mistake.
  Reply With Quote
Old 2nd February 2010, 04:22 PM   #13
cbdb is offline cbdb  Canada
diyAudio Member
 
Join Date: Oct 2008
Location: Vancouver
Quote:
Originally Posted by AndrewT View Post
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.
Close.
The output offset is ~ gain * ( V+ - V-) + residual output offset of the amplifier.
Which is gain * (Zin+ * Iin+) - (Zin- *Iin-) where the I's are input bias current of the OPAMP and the Zins are the OPAMP input impedances .
So if the input Zs(which is the opamp and they are) and I's are matched at + and - in of the OPAMP you have no offset from this contribution.
The other factor is the resistances between grnd and the +/- inputs. The OPAMP input bias current * this resistance will give you a voltage drop, so if these resistances arent matched they will also cause an output offset. (FET inputs almost eliminate this problem)

In your cct. this last resistance changes from R7+R4 to R7+(R4//Rsource) Makes a diff, but I cant see it causing that much offset. Are you sure R4 is working (soldered properly, not defective)

Last edited by cbdb; 2nd February 2010 at 04:30 PM.
  Reply With Quote
Old 2nd February 2010, 04:30 PM   #14
thetube0a3 is offline thetube0a3  United States
diyAudio Member
 
Join Date: May 2008
"Ah your 100% right. I was thinking it was a cap inbetween R3 and R5. My mistake."

Er I meant inbetween R3 and ground and R5 and ground.
  Reply With Quote
Old 2nd February 2010, 04:36 PM   #15
Peter Daniel is offline Peter Daniel  Canada
diyAudio Member
 
Peter Daniel's Avatar
 
Join Date: Jul 2002
Location: Toronto, Canada
High DC offset with no source (LM4780) Send a message via AIM to Peter Daniel
Quote:
Originally Posted by Tomten View Post
Yes, it is a 22k resistor. This is the schematic: http://www.audiosector.com/lm4780%20amp.pdf
Did you install the jumper: AudioSector-chip amp kits, dacs, chassis
__________________
To order kits contact me through forum mail.
  Reply With Quote
Old 2nd February 2010, 05:50 PM   #16
Tomten is offline Tomten
diyAudio Member
 
Join Date: Nov 2009
I had my setup on the floor, just to see that everything worked and therefore I didnt have any ground connections from amplifier PCB:s, which I guess then makes R4 useless, so when I shorted the inputs the R4 got in the way and provided a load for the chips and therefore shrunk the DC offset to 16mV. So when I install it in a case I wont have this problem.

I didnt test my theory, but this seems plausibel?

Yes, the jumper is installed.
  Reply With Quote
Old 9th February 2010, 08:40 AM   #17
Tomten is offline Tomten
diyAudio Member
 
Join Date: Nov 2009
I have now installed the amplifier in a case and the problem persists, however, now only in one channel. With no source and without shorting the inputs I have 3.5V on one channel and 200mV on the other. If I short the inputs I get the same as before, which is 16mV (the same on both channels).

I`ve checked the input to ground resistor which is fine on both channels. I have cheched all the solders and I cant see any problems. I have a star ground with both amplifier PCB:s ground connections and the mains safety ground connected to it.
  Reply With Quote
Old 9th February 2010, 09:23 AM   #18
peranders is offline peranders  Sweden
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
High DC offset with no source (LM4780)
Tomten, the design has a certain properties and the main one is unbalanced impedances between the + input and the - input this together with rather high input bias currents.

If you draw (2 x input bias current x R4 x the gain) plus (input offset x gain) = can be quite much => a low impedance signal source (down to DC) is needed.

The current = 0.4-2 uA (from two chips)
Offset = 1-10 mV

If you have varying DC resistances you will get varying output offsets. Mr. Daniel has chosen to keep it simple and by that goes "simple" DC performance if you are unlucky. Tomten, if you check the AN-1192 and the datasheet for LM4780 you'll see that the engineers at National see the basic design differently.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Group buy: DCT03 DC trap for big toroidal transformers. Sign up for interest HERE. 55 pcb's in order. 45 paid.

Last edited by peranders; 9th February 2010 at 09:26 AM.
  Reply With Quote
Old 9th February 2010, 09:44 AM   #19
Tomten is offline Tomten
diyAudio Member
 
Join Date: Nov 2009
peranders, I did your calculations and I see that the DC offset can be quite high but shouldnt the DC offset be a more reasonable value anyway, more like the other channel? Should it differ that much?

I have checked nationals LM4780 pdf and I can see that they use input capacitor and feedback capacitor, and I understand that this will affect DC offset (the feedback cap in this case), but isnt 3.5V a little of the scale even without them?
  Reply With Quote
Old 9th February 2010, 10:42 AM   #20
peranders is offline peranders  Sweden
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
High DC offset with no source (LM4780)
Try to measure the DC voltages across all resistors, then you can see if something is wrong/missing. Are you 100% sure that you have correct resistor values?
__________________
/Per-Anders (my first name) or P-A as my friends call me
Group buy: DCT03 DC trap for big toroidal transformers. Sign up for interest HERE. 55 pcb's in order. 45 paid.
  Reply With Quote

Reply


High DC offset with no source (LM4780)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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
High Dc Offset Issues nic6paul Class D 2 3rd March 2009 01:49 PM
Question about DC offset from source. theAnonymous1 Solid State 2 12th November 2006 07:07 PM
BrianT LM4780 DC offset SEIJI Chip Amps 8 21st April 2005 01:23 AM
36 V DC offset when source connected revell Chip Amps 12 27th February 2005 07:29 PM
High DC-Offset - Why? rampage101 Chip Amps 1 30th April 2004 08:16 AM


New To Site? Need Help?

All times are GMT. The time now is 10:31 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki