Capacitor over feedback Resistor - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 7th January 2012, 05:51 AM   #11
diyAudio Member
 
Join Date: Sep 2009
Location: Pretoria
Quote:
Originally Posted by jcx View Post
I suggest you not mess with the amp with your level of knowledge and just get an external XO
Thanks for the help! duh
  Reply With Quote
Old 7th January 2012, 03:57 PM   #12
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
learning involves paying attention to answers, comments, taking in new knowledge, reformulating your question

the more info about the system, the amp, what your "need" is the better the advice can be

don't just repeat what you've previously concieved of until someone on the web encourages you to do it in the face of other good advice that it has problems
  Reply With Quote
Old 7th January 2012, 04:18 PM   #13
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Hi,

Its easy to roll-off the amplifiers response at the input without affecting stability.
This will have the same response as modifying the feedback loop.

To modify the feedback loop without stability issues you increase the gain at
low frequencies, allowing it to return to normal higher up, this will maintain
the gain margin required for stability. If you reduce gain the compensation
needs to be modified (increased) for the lower loop gain.

rgds, sreten.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow
  Reply With Quote
Old 7th January 2012, 05:52 PM   #14
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
If your goal is to rolloff the frequency responce of the amp and limit it to sub-woofer frequencies, this should be done with a filter before the input, either passive or active. By adjusting the Miller cap value or the value of the feedback resistor (changing the feedback factor), you change the frequecy at which voltage gain = 1, and the phase shift of the output compared to the input is 180 degrees and so positive feedback with gain takes place. If the compensation does not match correctly the amp will be either under damped (unstable ) or over damped. Neither situation is optimal. Placing a resistor in series with the Miller cap creates a zero in the transfer which may help with phase margin about the unity gain frequency. Look up pole/zero compensation. Generally you can determine where the gain crossover frequency is and place the zero to be ~10 times higher. fz= 1/(2pi*R*C)

Using the compensation cap to set the audio signal bandwidth limit is barking up the wrong tree.
__________________
All the trouble I've ever been in started out as fun......
  Reply With Quote
Old 7th January 2012, 09:18 PM   #15
sreten is offline sreten  United Kingdom
diyAudio Member
 
Join Date: Nov 2003
Location: Brighton UK
Quote:
Originally Posted by CBS240 View Post
If your goal is to rolloff the frequency responce of the amp and limit it to sub-woofer frequencies, this should be done with a filter before the input, either passive or active. By adjusting the Miller cap value or the value of the feedback resistor (changing the feedback factor), you change the frequecy at which voltage gain = 1, and the phase shift of the output compared to the input is 180 degrees and so positive feedback with gain takes place. If the compensation does not match correctly the amp will be either under damped (unstable ) or over damped. Neither situation is optimal. Placing a resistor in series with the Miller cap creates a zero in the transfer which may help with phase margin about the unity gain frequency. Look up pole/zero compensation. Generally you can determine where the gain crossover frequency is and place the zero to be ~10 times higher. fz= 1/(2pi*R*C)

Using the compensation cap to set the audio signal bandwidth limit is barking up the wrong tree.
Hi,

Not exactly accurate, your mixing up unity gain with fixed gain, and loop gain.

Miller degeneration is this case will have no useful effect. Its simple, the more
feedback you apply to reduce gain the more you need to nobble the open loop
gain with compensation to ensure stability.

It is also very simple regarding the feedback loop. Increasing gain at low
frequencies has no stability issues. With a feedback loop the compensation
has no bearing whatsoever on the effective audio response. To create a
bass boost and effectively a stepped roll-off put a parallel RC in series
with the higher value of the two feedback resistors.

e.g. say the resistors are 20K and 1K for a gain of x20. Adding a 47K with
a cap in parallel in series with the 20K will give a stepped bass response
with no stability issues, as the high frequency gain is the same.

For the above low gain will be x67 rolling off depending on the
cap to x20. The above will give around 10dB low bass boost.
(And is far more useful for compensating driver roll-offs.)

Its far simpler to simply roll-off the treble at the input.
(with a series resistor and parallel capacitor).

rgds, sreten.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

Last edited by sreten; 7th January 2012 at 09:26 PM.
  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
Capacitor in parallel with negative feedback resistor Tyimo Tubes / Valves 4 16th April 2011 09:33 PM
Feedback Resistor Values Puffin Chip Amps 11 13th March 2008 07:00 PM
feedback resistor vlljpior Tubes / Valves 3 25th November 2004 04:53 AM
how to calculate feedback resistor Lico Tubes / Valves 7 9th June 2004 06:41 AM
feedback resistor.......? tbla Chip Amps 0 29th August 2003 04:27 PM


New To Site? Need Help?

All times are GMT. The time now is 05:41 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