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Old 6th January 2012, 05:09 AM   #1
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Default 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.
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Old 6th January 2012, 05:32 AM   #2
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try 10 % of the feedback resistor as a start
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Old 6th January 2012, 05:42 AM   #3
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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.
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Old 6th January 2012, 05:43 AM   #4
jcx is offline jcx  United States
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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.
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Old 6th January 2012, 05:49 AM   #5
PMA is offline PMA  Europe
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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.
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Old 6th January 2012, 04:54 PM   #6
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Quote:
Originally Posted by ticknpop View Post
try 10 % of the feedback resistor as a start
Will give it a try!
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Old 6th January 2012, 04:56 PM   #7
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The amp is discrete, no opamps.
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Old 6th January 2012, 04:58 PM   #8
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Quote:
Originally Posted by Wavebourn View Post
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?
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Old 6th January 2012, 05:03 PM   #9
jcx is offline jcx  United States
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I suggest you not mess with the amp with your level of knowledge and just get an external XO
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Old 6th January 2012, 05:31 PM   #10
DF96 is offline DF96  England
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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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