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

Solid State Talk all about solid state amplification.

need help with opamp DC servo
need help with opamp DC servo
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 21st February 2019, 02:32 PM   #1
snowcatmaiden is offline snowcatmaiden  Hong Kong
diyAudio Member
 
Join Date: Mar 2009
Default need help with opamp DC servo

Hello everyone

I have a question about opamp dc servo with power amp.
Here is the circuit I usually use.

need help with opamp DC servo-dcservo_qa-jpg

I have tried several opamp models.
This works well with opa604 and opa132.
But not with opa1641 and ada4522-1. (High DC at output)
[I cannot remember if I tried opa134 too. If I did, it was not working too.]

The blue words are the voltages when it is not working using ADA4522-1. [under +-23V supply]

I have read some post in the forum and understand that this circuit needs a JFET input opamp.
while opa604 and opa132 are JFET opamp and ada4522-1 may not be so, opa1641 is JFET opamp but still not work properly with this circuit.

Are there any more criteria besides JFET input?
Do I need to change anything if I use 10K in all R1 R2 R3 R5? (as preamp)

Thank you.
Attached Images
File Type: jpg DCSERVO_QA.JPG (184.3 KB, 466 views)

Last edited by snowcatmaiden; 21st February 2019 at 03:00 PM. Reason: change picture
  Reply With Quote
Old 21st February 2019, 02:52 PM   #2
rayma is online now rayma  United States
diyAudio Member
 
Join Date: Apr 2011
Quote:
Originally Posted by snowcatmaiden View Post
Do I need to change anything if I use 10K in all R1 R2 R3 R5? (as preamp).
First remove the connection between R3 and C1.
The lower end of C1 is not grounded.
  Reply With Quote
Old 21st February 2019, 02:53 PM   #3
snowcatmaiden is offline snowcatmaiden  Hong Kong
diyAudio Member
 
Join Date: Mar 2009
Quote:
Originally Posted by rayma View Post
First remove the connection between R3 and C1.
sorry i have drawn wrongly
I change it now

Last edited by snowcatmaiden; 21st February 2019 at 03:00 PM.
  Reply With Quote
Old 22nd February 2019, 01:57 AM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Because you are using 2 megohm resistors then the op amp needs to be more than that input impedance.
__________________
PCBCAD51 download or CDROM 2019 version out now http://www.murtonpikesystems.co.uk
  Reply With Quote
Old 22nd February 2019, 03:27 AM   #5
snowcatmaiden is offline snowcatmaiden  Hong Kong
diyAudio Member
 
Join Date: Mar 2009
Quote:
Originally Posted by nigelwright7557 View Post
Because you are using 2 megohm resistors then the op amp needs to be more than that input impedance.
Thank you for you reply!
Ada4522-1 input resistance for differential mode is just 30KR so it doesn’t work.

What should be the cap value if I use 1mohm?
And what are the differences with different values?

I have checked opa1641 input impedance is 10^13ohm.
Should be high enough?

Last edited by snowcatmaiden; 22nd February 2019 at 03:30 AM.
  Reply With Quote
Old 22nd February 2019, 03:47 AM   #6
Thatguyoverthere is offline Thatguyoverthere  United States
diyAudio Member
 
Join Date: Oct 2014
I suggest reading the lme49600 datasheet. There is a circuit included that has an lme49710, lme49600, and lme49720. The lme49720 functions as a DC servo, and the datasheet explains the capacitor values and resistor choices. It is most important to understand why values were chosen and not to just choose random values.
  Reply With Quote
Old 22nd February 2019, 03:53 AM   #7
nigelwright7557 is offline nigelwright7557  United Kingdom
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Quote:
Originally Posted by snowcatmaiden View Post
Thank you for you reply!
Ada4522-1 input resistance for differential mode is just 30KR so it doesn’t work.

What should be the cap value if I use 1mohm?
And what are the differences with different values?

I have checked opa1641 input impedance is 10^13ohm.
Should be high enough?
If halving resistance to 1meg then capacitor value should be doubled.

10^13 is excellent.
__________________
PCBCAD51 download or CDROM 2019 version out now http://www.murtonpikesystems.co.uk
  Reply With Quote
Old 22nd February 2019, 04:09 AM   #8
snowcatmaiden is offline snowcatmaiden  Hong Kong
diyAudio Member
 
Join Date: Mar 2009
Quote:
Originally Posted by nigelwright7557 View Post
If halving resistance to 1meg then capacitor value should be doubled.

10^13 is excellent.
But opa1641 cannot work.
This is why I cannot understand.
  Reply With Quote
Old 22nd February 2019, 04:14 AM   #9
snowcatmaiden is offline snowcatmaiden  Hong Kong
diyAudio Member
 
Join Date: Mar 2009
Quote:
Originally Posted by Thatguyoverthere View Post
I suggest reading the lme49600 datasheet. There is a circuit included that has an lme49710, lme49600, and lme49720. The lme49720 functions as a DC servo, and the datasheet explains the capacitor values and resistor choices. It is most important to understand why values were chosen and not to just choose random values.
But I cannot understand why some opamp work, but some cannot.
  Reply With Quote
Old 22nd February 2019, 04:33 AM   #10
nigelwright7557 is offline nigelwright7557  United Kingdom
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Quote:
Originally Posted by snowcatmaiden View Post
But I cannot understand why some opamp work, but some cannot.
If the input impedance is too low they wont work.
Maybe a different pin out ?

Some op amps do odd things if the inputs go near the rails, some op amps invert the output.

You need to put the op amp in and check voltages with a meter and a scope in high impedance mode (x100) to see what is going on.
__________________
PCBCAD51 download or CDROM 2019 version out now http://www.murtonpikesystems.co.uk
  Reply With Quote

Reply


need help with opamp DC servoHide 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
opamp for DC servo lucpes Solid State 11 27th December 2018 05:13 AM
Mr. D. Self : Why JFET OPamp Servo? erikovsky Solid State 23 19th September 2018 03:10 AM
Servo on non-inverting opamp ? 00940 Analog Line Level 11 26th June 2013 05:07 PM
Adcom gfa-585 dc servo opamp georgehifi Solid State 16 1st November 2003 12:50 AM
Schematic for Servo opamp. sonnya Solid State 17 26th July 2003 03:38 PM


New To Site? Need Help?

All times are GMT. The time now is 10:17 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki