Capacitor over feedback Resistor

diy didi · 05:12 AM

Recently built a subwoofer amp. Placed a 33p capacitor over the 39k feedback res. Now at a certain amplitude, the amp starts to oscillate. Guess there is just one-to-many phase shifts happening.
I want to roll off the amp a bit since it is a sub amp. I saw somewhere, that a resistor in series with this cap before paralleling with feedback resistor, is used to mess up the Q. Any ideas on how to size this series resistor????

ticknpop · 6th January 2012, 05:32 AM

try 10 % of the feedback resistor as a start

Wavebourn · 6th January 2012, 05:42 AM

You need to adjust your amp's compensation: it is not unity gain stable.

If it uses dominant pole compensation just increase a bit the value of the cap there.

jcx · 05:47 AM

yes it is a very bad idea - increases feedback factor - likely the amp is not "unity gain stable " - as are most cheap op amps but few power amps

even if it did "work" it wouldn't give the right transfer function - with the usual non-inverting amp there would be a Zero in the response that would pass high frequencies with a constant attenuation = to your amp's gain (ie it would be "unity gain" for high frequencies instead of rolling them off)

roll off the signal input with a active filter circuit - XO needs to be coordinated with the rest of your system anyway - active analog or digital XO is the way most systems do this today

PMA · 6th January 2012, 05:49 AM

The task might be more complex, like compensation of input capacitance (between +input and virtual ground), or HF phase shifts. Then one would usually find a small FB cap, like 3pF, might help, and unity gain stability is not required.

diy didi · 6th January 2012, 04:54 PM

Quote:

Originally Posted by ticknpop

try 10 % of the feedback resistor as a start

Will give it a try!

diy didi · 6th January 2012, 04:56 PM

The amp is discrete, no opamps.

diy didi · 6th January 2012, 04:58 PM

Quote:

Originally Posted by Wavebourn

You need to adjust your amp's compensation: it is not unity gain stable.

If it uses dominant pole compensation just increase a bit the value of the cap there.

What exactly is dominant pole compensation?

jcx · 6th January 2012, 05:03 PM

I suggest you not mess with the amp with your level of knowledge and just get an external XO

DF96 · 6th January 2012, 05:31 PM

In a typical amp, dominant pole compensation is done by the Miller capacitor on the VAS stage. The aim is to ensure that the loop gain does not enclose the critical point at (1,0) in the complex plane.

6th January 2012, 05:09 AM	#1
diy didi diyAudio Member Join Date: Sep 2009 Location: Pretoria	Capacitor over feedback Resistor Recently built a subwoofer amp. Placed a 33p capacitor over the 39k feedback res. Now at a certain amplitude, the amp starts to oscillate. Guess there is just one-to-many phase shifts happening. I want to roll off the amp a bit since it is a sub amp. I saw somewhere, that a resistor in series with this cap before paralleling with feedback resistor, is used to mess up the Q. Any ideas on how to size this series resistor???? Last edited by diy didi; 6th January 2012 at 05:12 AM.

6th January 2012, 05:42 AM	#3
Wavebourn diyAudio Member Join Date: Sep 2006 Location: Pleasant Hill, CA	You need to adjust your amp's compensation: it is not unity gain stable. If it uses dominant pole compensation just increase a bit the value of the cap there. __________________ If I disappear suddenly, that means I finally created a time machine and pushed wrong button that brought me to Stalin's Russia. In any experiment any result is the result. Even if it is negative.

6th January 2012, 05:43 AM	#4
jcx diyAudio Member Join Date: Feb 2003 Location: ..	yes it is a very bad idea - increases feedback factor - likely the amp is not "unity gain stable " - as are most cheap op amps but few power amps even if it did "work" it wouldn't give the right transfer function - with the usual non-inverting amp there would be a Zero in the response that would pass high frequencies with a constant attenuation = to your amp's gain (ie it would be "unity gain" for high frequencies instead of rolling them off) roll off the signal input with a active filter circuit - XO needs to be coordinated with the rest of your system anyway - active analog or digital XO is the way most systems do this today Last edited by jcx; 6th January 2012 at 05:47 AM.

6th January 2012, 05:49 AM	#5
PMA diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	The task might be more complex, like compensation of input capacitance (between +input and virtual ground), or HF phase shifts. Then one would usually find a small FB cap, like 3pF, might help, and unity gain stability is not required. __________________ Pavel Macura http://www.linkedin.com/pub/pavel-macura/4/783/637 http://web.telecom.cz/macura/audiopage.html

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6th January 2012, 05:32 AM	#2
ticknpop diyAudio Member Join Date: May 2005 Location: toronto	try 10 % of the feedback resistor as a start

6th January 2012, 04:56 PM	#7
diy didi diyAudio Member Join Date: Sep 2009 Location: Pretoria	The amp is discrete, no opamps.

6th January 2012, 05:03 PM	#9
jcx diyAudio Member Join Date: Feb 2003 Location: ..	I suggest you not mess with the amp with your level of knowledge and just get an external XO

6th January 2012, 05:31 PM	#10
DF96 diyAudio Member Join Date: May 2007	In a typical amp, dominant pole compensation is done by the Miller capacitor on the VAS stage. The aim is to ensure that the loop gain does not enclose the critical point at (1,0) in the complex plane.

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