Variable NFB

[Alf]

I read about this variable NFB in an earlier thread but I am wondering is it only changing your nfb resistor into a potmeter or is there more to it ?

Who can explain it with an example ?


Thanks beforehand, Alf

[eschertron]

No explanation, but a comment (or two!)

Turning the fixed resistor into a pot allows you to adjust the amount of NFB. That's all there is to it, at first blush. It can get more complicated. I guess it depends on what your design goal is??

In the NFB loop I made adjustable, I had a small value resistor in series with the pot so I could not turn the NFB so high (low resistance) that I caused the amp to become unstable. Simply an 'idiot proofing' measure. The pot is available on the front panel so I can adjust while I play. The minimum value resistance allows the signal to feed back enough to significantly reduce the overall volume of the amp, but not all the way to zero.

I also have a small value cap in parallel with the pot. this allows high frequencies to be fed back all the time, reducing the overall high freq and hiss content of the amp. Purely for my satisfaction.

[Chuck H]

...is it only changing your nfb resistor into a potmeter...

That's it in a nutshell. Usually you won't want more than stock NFB so a comparably large pot is wired as a variable resistor and in series with the stock resistor. My opinion on it is that most amps have a happy place at a certain NFB level both clean and overdriven. I've tried variable NFB. But once I located that sweet spot I almost never turned the knob without it ending up back at that spot. So I'll suggest not mounting it on the front panel. That way, if you decide to remove it, there won't be an unused hole on your faceplate.

[DRH1958]

I have had a different experience with this. I re-built a Hammond AO-43 amp for guitar. Their stock overall vol. control was varying the NFB. It had a 10K pot in series with a 3.8k resistor so as to not go to cutoff. I removed both and put in a 50k pot. The amp sounds killer. The pot acts as master vol and it also adds power amp grind. It sort of acts like the vol on your guitar when you have a dirty sound dialed in. The first part of it when turning down just cleans up the signal, then reduces volume. I can dial in preamp overdrive and also power amp overdrive by varying the NFB. It's really a cool effect. I can actually dial back the preamp vol for a little cleaner and dime the "master" vol and get mostly power amp grind. This way I can choose which part of the amp I get my overdrive from, pre, power or both. It does involve the PI, so this part of the circuit is involved too. I know this used to be an organ amp and you might not want to do this to an expensive amp, just some food for thought. I would put this amp up against any 18 watt amp out there.
As an aside, this is a great amp to mod/rebuild because it has built in reverb circuitry. I reproduced the reverb control that used to be up by the keyboard with a 50k linear pot and a fixed resistor.

[Alf]

Thanks all for your help , I'll give your suggestions a try.

Alf