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Old 17th July 2010, 02:54 AM   #1
cbj591 is offline cbj591  United States
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Default NFB Clarification

Good evening all,

I wanted to get a clarification on how to tell if I have positive or negative feedback.

I have a 500k pot in my feedback loop. when I increase the resistance, the volume increases. That is positive feedback correct? If so, can I swap the two wires coming off the preamp tube going to the PI caps to correct this?

Also, thanks to everyone for helping me learn the do's and dont's of component placement. My hum issues are pretty well gone.

Special thanks to Azazello, if I wouldn't have added the choke to the power supply that he suggested, I would still have a nasty low level hum from the simple/dirty power supply.

Thanks again
Brian
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Old 17th July 2010, 02:58 AM   #2
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Quote:
Originally Posted by cbj591 View Post
I have a 500k pot in my feedback loop. when I increase the resistance, the volume increases. That is positive feedback correct?
No.
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Old 17th July 2010, 03:31 AM   #3
cbj591 is offline cbj591  United States
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By no, I can assume that it is negative feedback and everything is connected correctly.

Thanks again
Brian
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Old 17th July 2010, 04:03 AM   #4
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Yes.
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Old 17th July 2010, 04:09 AM   #5
cbj591 is offline cbj591  United States
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Thanks so much Wavebourn, I just wanted to make sure I didnt phrase my question in a way that seemed like I was asking or implying the wrong thing.

BTW - Nice site you have.

Regards

Last edited by cbj591; 17th July 2010 at 04:16 AM.
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Old 17th July 2010, 04:28 AM   #6
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Thanks.

In case of positive feedback you would turn your amp into sound oscillator.
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Old 17th July 2010, 05:10 AM   #7
cbj591 is offline cbj591  United States
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Quote:
Originally Posted by Wavebourn View Post
Thanks.

In case of positive feedback you would turn your amp into sound oscillator.
A sound oscillator would be a nasty buzzing/pulsing sound coming out of the speakers? If it is, then i'm in good shape. Just a little distortion on the top end.

Sorry for all the questions, I'm trying to absorb all the info I can

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Old 17th July 2010, 07:18 AM   #8
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Don't be sorry.

If amplification factor through the loop of positive feedback is a little bit greater than 1 on some frequency the amp starts oscillating. Think of it as if signal from output coming back to input is amplified, this amplified signal comes again to the input, is more amplified, and so on...
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Old 17th July 2010, 02:47 PM   #9
jrenkin is offline jrenkin  United States
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Quote:
Originally Posted by cbj591 View Post

I have a 500k pot in my feedback loop. when I increase the resistance, the volume increases.
Just to state in clearer terms than "yes" or "no"
when you increase resistance you are reducing the feedback signal, so if the volume increase with reduced feedback signal, it must be negative feedback.
Obvious to knowledgeable folk live Wavebourn , but I always have to think it through literally
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Old 17th July 2010, 03:17 PM   #10
DF96 is offline DF96  England
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This assumes that the pot is in series with the loop, not paralleled to ground - this would have the opposite effect!

Changing volume via a feedback loop can lead to oscillation at low volume, because this is when the loop gain is at its highest. Is this just a pot during tuning, to be replaced by a fixed resistor once you are happy with the design?
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