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About Negative Feedback

iaresee

Moderator
Moderator
Imagine a Tesla with a built in Axe-Fx (or should I sa an Axe-Fx with a built in Tesla :p).
There was a VW Beetle with a Fender badge on it that had an amp sim and a 1/4" jack on the dash board you could plug your guitar in to...
 

Bman

Veteran
I was unproductive at work today and scoured through this thread trying to understand Negative Feedback. And I think the 'aha' moment may have hit me not longer after Chris' 'aha' moment post a page or two back. I'd stumbled on using the parameter as a result of trying to tame some nuances that occur when I bump up an amp's input trim. I love the added gain that comes with the boost even with just a little boost. It sounds livelier to my ears than boosting with a pedal block. But the low end suffers, which is how I discovered the Neg. Feedback parameter. Boosting from it's default (1.5 I believe) on a Marshall amp block to between 2 or 3 smooths out the bad and feels like it takes another layer of blanket off the sound. It just sounds more pleasing on its own. But I did notice that when playing in a mix it cut less. Now after reading this, the way I understand it is this is because the EQ spectrum is widened out as you increase it. As Cliff sort of said: It becomes addictive and there is a trade-off. I liken it to a scooped EQ tone. Sounds awesome on its own but doesn't cut. It reminds me of when I was young and had a Crate 1x12 amp. I got a Heavy Metal Pedal and the thing sounded like all my hero's stacks.....until I added volume.

Anywayt, the purpose of my post is to get some 'feedback' as to whether I'm correct about my conclusions. I suspect so, but need a sanity check. Then I can move on to solving the mix issue via EQ or Filter.
 

yeky83

Veteran
But I did notice that when playing in a mix it cut less. Now after reading this, the way I understand it is this is because the EQ spectrum is widened out as you increase it. As Cliff sort of said: It becomes addictive and there is a trade-off. I liken it to a scooped EQ tone. Sounds awesome on its own but doesn't cut.
Sounds like you have it backwards...? The part where Cliff says "addictive," he's referring to less negative feedback where the power amp response follows the speaker impedance curve with its scooped mids. If you increase the negative feedback, the speaker impedance has less of an effect on the response of the power amp, so the mids flatten out.
 
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Bman

Veteran
Sounds like you have it backwards...? The part where Cliff says "addictive," he's referring to less negative feedback where the power amp response follows the speaker impedance curve with its scooped mids. If you increase the negative feedback, the speaker impedance has less of an effect on the response of the power amp, so the mids flatten out.
Ok. I think I may have presented it backwards but you've confirmed what I was trying to convey. Increasing it smooths out or broadens the frequency spectrum which in my case may have an unwanted result of smoothing out the boosted mids. More pleasing to the ears on it's own, but doesn't cut as well. Thanks, Yeky!
 

Joe Bfstplk

Inspired
This forum channel's discussion reminds me of Ampage and the Weber VST boards, and even the old dial-up BBS that Ted ran back in the day. :)
 

Handyman

Regular
Random factoid - the patent for negative feedback amplifiers was initially rejected as a perpetual motion machine in the US. It took years and years to get this sorted out with the USPTO.

The patent went through without drama in Europe.

Maybe there's a lesson here, but I'm not sure what it is.
 

Rex

Legend!
From Merriam-Webster:

factoid
noun
fac·toid | \ ˈfak-ˌtȯid \
Definition of factoid


1: an invented fact believed to be true because it appears in print
2: a briefly stated and usually trivial fact



Not sure which definition to go with here. :)
 

Joe Bfstplk

Inspired
I don't get it, the negative feedback is parasitic on the output, not additive...that's backwards from perpetual motion
The US Patent Office does strange things sometimes. Sometimes they check things very closely, other times it seems they are oblivious, and allow people to patent 'prior art'. Mesa has a fair number of things patented that really ought to have been rejected as prior art....
 
Great article and very informative. Thank you for explaining these concepts. If we have presence and resonance to control high and low frequencies respectively, then what's the point of bass, mid and treble controls on an amp? Thank you
 

shatteredsquare

Forum Addict
If we have presence and resonance to control high and low frequencies respectively, then what's the point of bass, mid and treble controls on an amp?
Bass/mid/treble is part of the tone stack, presence and resonance is part of the power section, a way to tailor high and low response (filtering) at a different point in the chain...when there's so much distortion and cross modulation happening, it makes a big difference to be able to have lots of top end at the front, distort the hell out of it, then filter it back down at the end with something like a negative feedback circuit. If you filtered out the top end at the front, you wouldn't be able to get the desired distortion characteristics, and if you can't filter it out at the end, you face get's ripped off.
 
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shatteredsquare

Forum Addict
Thank you for your response. So does that mean that bass, mid and treble control the preamp and presence and resonance control the power amp?
yep. unless there is no negative feedback in the amp and there is still a presence knob. like a mesa dual rectifier in 'modern' mode, there is no negative feedback in that mode, so the presence knob becomes part of the preamp, just another high filter. in 'vintage' or 'raw' mode the presence knob controls power amp negative feedback.
 
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