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Old 5th October 2016, 04:23 PM   #1
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Default Quick Question on Grid NFB

I couldn't find any threads with the info I needed, so I was hoping I could get a quick answer from you guys. When applying NFB to the grid (like in the included picture), do I need to consider the value of the volume pot into my NFB calculations, or am I ok just using the series and shunt NFB resistors and ignoring the pot value?
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Old 5th October 2016, 04:41 PM   #2
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Quote:
Originally Posted by Nailbunny7 View Post
I couldn't find any threads with the info I needed, so I was hoping I could get a quick answer from you guys. When applying NFB to the grid (like in the included picture), do I need to consider the value of the volume pot into my NFB calculations, or am I ok just using the series and shunt NFB resistors and ignoring the pot value?
The Zout of the pot is part of the feedback network (as is the Zout of the driving stage). Your feedback would vary with volume setting in this scheme.
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Old 5th October 2016, 04:45 PM   #3
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Quote:
Originally Posted by SpreadSpectrum View Post
The Zout of the pot is part of the feedback network (as is the Zout of the driving stage). Your feedback would vary with volume setting in this scheme.
That's what I thought. Thank you.
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Old 5th October 2016, 04:50 PM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by Nailbunny7 View Post
When applying NFB to the grid (like in the included picture), do I need to consider the value
of the volume pot into my NFB calculations.
The pot does form a "tee" network with the grid nfb resistance. For this to have minimal effect on the
action of the pot, size the two parts so the pot's value is the same or less than the grid nfb resistor.
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Old 5th October 2016, 05:00 PM   #5
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Quote:
Originally Posted by rayma View Post
The pot does form a "tee" network with the grid nfb resistance. For this to have minimal effect on the
action of the pot, size the two parts so the pot's value is the same or less than the grid nfb resistor.
There is a lot of tension in this proposed circuit. You want the series feedback resistor to be low to minimize noise but you want the series resistance high so that you can use a reasonable value pot.

Also, the feedback will fight the volume control. At full volume, you will have maximum feedback, so minimum gain from the stage. At low volume you will have minimal feedback, so more gain, higher distortion, higher noise, etc.

Personally, I'd buffer or move the pot to another place.
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Old 5th October 2016, 05:13 PM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by SpreadSpectrum View Post
At full volume, you will have maximum feedback, so minimum gain from the stage.
At low volume you will have minimal feedback, so more gain, higher distortion, higher noise, etc.
The amount of nfb is the same at both low and high volume settings. At low volumes, the pot
reverts to an attenuator, since the grid resistor doesn't load the pot significantly. The nfb does
increase at mid-settings.
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Old 5th October 2016, 05:21 PM   #7
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Quote:
Originally Posted by rayma View Post
The amount of nfb is the same at both low and high volume settings. At low volumes, the pot
reverts to an attenuator, since the grid resistor doesn't load the pot significantly. The nfb does
increase at mid-settings.
Yes, you are correct. I didn't think that all the way through.
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Old 5th October 2016, 05:25 PM   #8
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Thanks for the information
I have one last question:
When calculating the value of the NFB resistors, is the series resistor value added to the value of the pot or parallel to the value of the pot?
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Old 5th October 2016, 07:15 PM   #9
rumors is offline rumors  Italy
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imho works best with cap
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Old 5th October 2016, 07:23 PM   #10
DF96 is offline DF96  England
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Quote:
Originally Posted by Nailbunny7
I have one last question:
When calculating the value of the NFB resistors, is the series resistor value added to the value of the pot or parallel to the value of the pot?
The series resistor is added to the source impedance of the pot, which varies from zero (min and max) to one quarter of the pot value (mid-position).
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