DC Servo loop tunning - 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 30th September 2008, 02:44 PM   #1
Ted205 is offline Ted205  United Kingdom
diyAudio Member
 
Ted205's Avatar
 
Join Date: Jun 2008
Default DC Servo loop tunning

Hi guys

I've taken this layout from the lm3886 datasheet.

schematic

I want to use in on an opamp with a gain of 2.

I've been playing around with it and though it does work, its very slow. If I reduce R2, to say 2.2kohms, it becomes alot quicker.

However i'm not entirely sure how it works so i dont want to do anything thats going to alter things with out me even realising as after all its a fairly important part of my system (blocking input DC).

Will R2 at 2.2k be ok, or maybe even lower ?



cheers

ted
  Reply With Quote
Old 30th September 2008, 03:05 PM   #2
diyAudio Member
 
Join Date: May 2008
The DC servo counteracts all signals that are below its operating frequency by feeding them back to the inverting input. It is a low-pass filter. If you make it faster, it will start to counteract at higher frequencies. Make it too fast and it cancels out your music signal below its corner frequency.

If you change R2, change R3 likewise. Changing them by a factor of 1000 is way too much.
  Reply With Quote
Old 30th September 2008, 03:27 PM   #3
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
The servo has possibly too low adjustment window and it's too slow also. The opamp runs always at the gain of one!

Each input of the opamp must always have the same impedance. Equal resistors and equal capacitors.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Super Regulator SSR03 Group buy
  Reply With Quote
Old 30th September 2008, 03:58 PM   #4
Ted205 is offline Ted205  United Kingdom
diyAudio Member
 
Ted205's Avatar
 
Join Date: Jun 2008
i've found an error with my schematic. R1 should be 10k. As it should be 10 times the value of Rf (1kohms)
  Reply With Quote
Old 1st October 2008, 11:16 AM   #5
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
now split R1 into two resistors and form a T filter with the resistor junction going via a cap to audio ground. Try 5k1 for both sides of the T filter and 4.7uF or 6.8uF to ground.

Now go and read the servo thread, particularly Tom's contribution.
__________________
regards Andrew T.
  Reply With Quote
Old 1st October 2008, 01:54 PM   #6
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Andrew and everybody else, the opamp itself isn't too slow. The servo is too slow for my taste. I think around 1 Hz is good.

The gain of the integrator is +1 in closed loop and DC in is which means that 1 mV offset will result in 1 mV at the output. Since the integrator is non-inverting it's essential it's important to have identical resistor and cap values seen from the servo opamp's inputs.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Super Regulator SSR03 Group buy
  Reply With Quote
Old 1st October 2008, 01:58 PM   #7
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Quote:
Originally posted by AndrewT
now split R1 into two resistors and form a T filter with the resistor junction going via a cap to audio ground. Try 5k1 for both sides of the T filter and 4.7uF or 6.8uF to ground.

Now go and read the servo thread, particularly Tom's contribution.
This T-link will create a small instability depening of how close it's pole is to the servo's pole. This small resonance will also create a small hump in the frequency response. It's very obvious if you run a simulation which I recommend.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Super Regulator SSR03 Group buy
  Reply With Quote
Old 1st October 2008, 03:13 PM   #8
Ted205 is offline Ted205  United Kingdom
diyAudio Member
 
Ted205's Avatar
 
Join Date: Jun 2008
i found the post andrew as hinting at -> DC Servo question...

Very helpful indeed. Has this been written up formally anywhere?

as it needs to be

they only thing i noticed is the diodes are not included in the above link. What if anything are these for ?
  Reply With Quote
Old 1st October 2008, 04:20 PM   #9
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
These diodes are for faster servo recovery when it's overdriven.
__________________
/Per-Anders (my first name) or P-A as my friends call me
Super Regulator SSR03 Group buy
  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
Pioneer DV-717 tunning bra Digital Source 9 18th April 2006 07:09 PM
Frequency tunning. perpetual Subwoofers 2 22nd June 2005 03:22 AM
sub tunning perpetual Subwoofers 3 20th June 2005 11:07 AM
hum,ground loop,earth loop problem with your answers please frank2395 Pass Labs 2 17th May 2005 06:46 PM
What happens with different port tunning andy2 Multi-Way 15 28th December 2004 06:09 PM


New To Site? Need Help?

All times are GMT. The time now is 02:55 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