question about negitive feedback

[fred_farkel]

hi everybody, Ive been reading again and came across an interesting article, just wish I could remember where! The article said that by properly adjusting the negitive feedback, one could expect nice rich harmonics and note bloom from the amp. Could this be true? Thanks for any help.

[rockstah]

hi everybody, Ive been reading again and came across an interesting article, just wish I could remember where! The article said that by properly adjusting the negitive feedback, one could expect nice rich harmonics and note bloom from the amp. Could this be true? Thanks for any help.

it seems it would be true... more or less since neg feedback is injected back into the ciruit so to speak at 190 degrees out of phase which in turn plays a roll in canceling or letting freqs/ocilations through. it is part of your tone for sure.
the more neg feedback the more smoothing / fatter-
the less neg feedback the more highs and gainier feeling.

Mark

[David Root]

You can put a linear pot in the feedback loop and vary the NFB till you find your own sweet spot. Depending on the original amount of NFB, pot value would vary. For example, if the NFB resistor is 100K coming off the 8 or 4 ohm tap of the OT, you could use 250, 500, or even 1M. If it's 22K coming off the 16 ohm tap, you don't want a real large value pot as this circuit has been designed for a lot of feedback and might not react too well to a greatly reduced amount of NFB.

This was first done I believe about 25 years ago and is called a soul control, because as you reduce the NFB (increase the resistance) the tone gets tubier, funkier, bluesier and the amp will get a bit louder too!

[unklmickey]

a presence control basically shunts the HF content of the NFB.

cheap and dirty way to create a "soul control" would be to short the cap in the presence circuit.


cheers,

unk


EDIT:

...since neg feedback is injected back into the ciruit so to speak at 190 degrees out of phase...

i think you made a typo.............that's actually 180 degrees, Mark.

but since you're in Texas, where they always give 110%, an extra 10 degrees would be OK.

[fred_farkel]

Question from a beginner...does negitive feed back have anything to do with note bloom and harmonics from an amp AND would it be worth the effort to install an adjustable resistor/pot like I've seen posted else where?
Thanks

[drz400]

Question from a beginner...does negitive feed back have anything to do with note bloom and harmonics from an amp AND would it be worth the effort to install an adjustable resistor/pot like I've seen posted else where?
Thanks

Sure nothing wrong with it
Like on a Marshall whichs typical 47k~100K you get a center detent pot from mouser 100K linear run you 47K to that pot and use it as a variable resistor so you can go from 47K to 147K I like around 68k.
I find it is an mood control, 47K it is sweeter and tame, 150K and it is very angry
You can also have a switch that changes which speaker tap it goes to.

[doctord02]

Question from a beginner...does negitive feed back have anything to do with note bloom and harmonics from an amp AND would it be worth the effort to install an adjustable resistor/pot like I've seen posted else where?
Thanks

I dont think this is really gonna do what you seem to think it will... I've never found adjusting my presence control had any effect on note "bloom", and a presence control is a variable negative feedback , just with a limited range. It *might* effect the high-end shimmer or edge of whatever harmonic content is already there, but it wont add any that you didnt have before, in my experience.

[drz400]

Question from a beginner...does negitive feed back have anything to do with note bloom and harmonics from an amp AND would it be worth the effort to install an adjustable resistor/pot like I've seen posted else where?
Thanks

I dont think this is really gonna do what you seem to think it will... I've never found adjusting my presence control had any effect on note "bloom", and a presence control is a variable negative feedback , just with a limited range. It *might* effect the high-end shimmer or edge of whatever harmonic content is already there, but it wont add any that you didnt have before, in my experience.

I agree, you will set it to one place, but for the effort and cost it can help dial in your tone. It will cost you a pot

[Normster]

The presence control isn't really a "variable feedback" device, it merely bleeds off highs through the presence cap. The amount of NFB that goes to ground is relatively constant regardless of the presence pot position.

The best info I've read about NFB is on Aiken's site. I highly recommend reading his stuff and then doing some experimentation. My own experience (w/Dumble circuits) is that too much NFB can make an amp sound constipated and boxy. As you decrease the amount of NFB injected into the PI, the amp begins to open up and sounds richer at BOTH ends of the frequency spectrum. Too little NFB and the amp will become farty and raspy at high volumes.

[Noel Grassy]

Thanks for that Normster. You succinctly described the prescence control & NFB so well that I'll shaddup right about now.... I'm off to Aikin's tech discussions to pay a little more retenshum.