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Quick Question on Grid NFB

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Nailbunny7

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Joined 2014
#1
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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#2
Nailbunny7 said:
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?
Click to expand...

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.
 
#4
Nailbunny7 said:
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.
Click to expand...

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.
 
#5
rayma said:
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.
Click to expand...

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.
 
#6
SpreadSpectrum said:
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.
Click to expand...

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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