BrianT LM4780 DC offset - 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 19th April 2005, 04:41 AM   #1
SEIJI is offline SEIJI  Philippines
diyAudio Member
 
Join Date: Sep 2004
Location: Singapore
Default BrianT LM4780 DC offset

Hi all,

I had just completed 6 BrianT LM4780 KITS set, I did a test Bench Test for all the 6 commpleted module with individual large heatsink by connecting a toroidal transformer with 2 secondary 0-18 VAC (5A) to the module each at a time, the DC offset for the 6 module read 150mVDC +/- 10mVDC with the signal cable open, with the signal cable shorted to ground, all 6 module DC offset read at 15mVDC +/- 1mVDC........

When connected a high efficientcy speaker driver (100db/w/m) to each individual module, they is no audioble noise at all...........and when I touch the signal cable, they is some noise on the speaker driver which show the amp are able to amplifier signal.

Is the above mention DC offset symptom normal????????

Any input from you would be much appreciated..........

Many Thanks ...............
  Reply With Quote
Old 19th April 2005, 06:43 AM   #2
maxw is offline maxw  United Kingdom
diyAudio Member
 
maxw's Avatar
 
Join Date: Jun 2004
Location: London
Default Re: BrianT LM4780 DC offset

Quote:
Originally posted by SEIJI
150mVDC +/- 10mVDC with the signal cable open
I am pretty sure thats normal and you should not measure the offset with with open signal anyway.

Quote:
Originally posted by SEIJI
with the signal cable shorted to ground, all 6 module DC offset read at 15mVDC +/- 1mVDC........
Is the above mention DC offset symptom normal????????
15mV is pretty good!! alot of people have upto 75mV and are not concerned at all
If it bothers you, you can add Ci caps whioh is mentioned here along with plenty of other info about DC-offset:
DC offset problem
  Reply With Quote
Old 19th April 2005, 06:56 AM   #3
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Your values are normal. Worst case if you read the datasheet is 300 mV (10 mV x gain). The changes when your have different DC resistance's, this is also normal becasue you have input bias currents which are some.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Super Regulator SSR03 Group buy
  Reply With Quote
Old 19th April 2005, 11:50 PM   #4
diyAudio Member
 
Russ White's Avatar
 
Join Date: Jan 2005
Location: Nashville, TN, USA
Send a message via Yahoo to Russ White
Your offset sounds close to what I get on my LM4780s which do not have input caps or the "Ci" capacitors as per the datasheet. If you add "Ci" which should be at least 47uf you will get 0 offset and I did not notic any ill effects. In fact my amps ran cooler after the mod. I think it helped tame some oscillation.
  Reply With Quote
Old 20th April 2005, 12:43 AM   #5
SEIJI is offline SEIJI  Philippines
diyAudio Member
 
Join Date: Sep 2004
Location: Singapore
Thank you so much for all the kind reply, the chasis will be fabricate in 5 days time to accomodate the 6 module, is the CI cap need's to be place right at the input of the PCB board or it can be place at the input connector (in my case the neutrik XLR jack) which is about 6-8 inch away form the PCB board?

As the LM4780 will be connected to the output of electronic crossover which already has a output 22uF cap, do I still need the CI cap in this case?

Thanks agian for your kind reply...........
  Reply With Quote
Old 20th April 2005, 12:48 AM   #6
diyAudio Member
 
Russ White's Avatar
 
Join Date: Jan 2005
Location: Nashville, TN, USA
Send a message via Yahoo to Russ White
The input cap and the "Ci" caps are two different caps. The input cap can be a lower value like 2.2uf or 4.7uf Ci is the capacitor that goes between the negative feedback resistor to ground and ground. Look at the schematic for the parallel LM4780 on national's site as it will be more clear than I am being.

As for the input caps I put them right on the input jacks, but it is not important. You could also mount them to the PCB, or even inline.
  Reply With Quote
Old 20th April 2005, 12:50 AM   #7
diyAudio Member
 
Russ White's Avatar
 
Join Date: Jan 2005
Location: Nashville, TN, USA
Send a message via Yahoo to Russ White
If your crossover has output caps you do not need input caps. But if you plan to use the amp with other sources it is a good idea to use them.
  Reply With Quote
Old 20th April 2005, 04:09 PM   #8
diyAudio Member
 
Join Date: Apr 2005
Location: In the Wild, Wild West
As noted in the datasheet, the Ci cap is there so that the DC gain is 1. Without the cap your DC gain is equal to your AC gain so if you have DC present on the input it will be gained up at the output. The input capacitor is there to block any DC from a source. It is not needed if your source is already about GND. The purpose of both of these caps is to keep DC out and from being gained up. That's all they are there for.

-SL
  Reply With Quote
Old 21st April 2005, 01:23 AM   #9
SEIJI is offline SEIJI  Philippines
diyAudio Member
 
Join Date: Sep 2004
Location: Singapore
Thank you so much for all the kind reply, I will be building the module up with a chasis soon, and see whats the result of DC offset after conneted to the electronic crossover without the input and Ci cap and I will post some picture here........

Thanks agian.........
  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
DC Offset JannaCassandra Chip Amps 3 26th July 2008 03:20 PM
Op amp Dc offset legarem Solid State 12 7th February 2008 04:48 PM
sonic differences between inverting/non-inverting LM4780 VS both non-inverting LM4780 jarthel Chip Amps 5 19th July 2007 12:37 PM
2,5v dc offset!! nicoch58 Class D 2 4th March 2007 11:02 PM
GC offset jan.didden Chip Amps 4 3rd July 2004 08:10 AM


New To Site? Need Help?

All times are GMT. The time now is 12:00 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