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  #1  
Old 01.19.10, 3:29 PM
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Please educate me: Negative Feedback loop (NFB) (Pro Jr amp)

so what's the story with the NFB thing?
what does it do for an amp, such as the Pro Jr, and why is it there?
what are the advantages of using it?

give me the details, i would like to learn about this.

i read about some people disconnecting the NFB wire in their Pro Jr...what effect would this have overall? what is the reason for doing this?

thanks for the info.!
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Old 01.19.10, 5:33 PM
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I thought that was the whole concept behind the Presence control, and one of Leo Fender's contributions to the world of guitar, but I might be wrong.
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Old 01.19.10, 5:53 PM
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Quote:
Originally Posted by thebard42 View Post
I thought that was the whole concept behind the Presence control, and one of Leo Fender's contributions to the world of guitar, but I might be wrong.
Pretty much....negative feedback is the Presence control and is used to reduce distortion. People disconnect the NFB line to get more volume and dirt out of their amp: Per Wiki

Electronic amplifiers
Main article: negative feedback amplifier
The negative feedback amplifier was invented by Harold Stephen Black at Bell Laboratories in 1927, and patented by him in 1934. Fundamentally, all electronic devices (e.g. vacuum tubes, bipolar transistors, MOS transistors) exhibit some nonlinear behavior. Negative feedback corrects this by trading unused gain for higher linearity (lower distortion). An amplifier with too large an open-loop gain, possibly in a specific frequency range, will additionally produce too large a feedback signal in that same range. This feedback signal, when subtracted from the original input, will act to reduce the original input, also by "too large" an amount. This "too small" input will be amplified again by the "too large" open-loop gain, creating a signal that is "just right". The net result is a flattening of the amplifier's gain over all frequencies (desensitising). Though much more accurate, amplifiers with negative feedback can become unstable if not designed correctly, causing them to oscillate. Harry Nyquist of Bell Laboratories managed to work out a theory about how to make this behaviour stable.

Negative feedback is used in this way in many types of amplification systems to stabilize and improve their operating characteristics (see e.g., operational amplifiers).
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Old 01.19.10, 6:45 PM
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Does this mean my Bassman will break up earlier if I disconnect the presence control? I would definitely want that.
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Old 01.19.10, 7:44 PM
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Originally Posted by asatbluesboy View Post
Does this mean my Bassman will break up earlier if I disconnect the presence control? I would definitely want that.
Not necessarily....It might just be nastier dimed. I've never done it. I would be careful doing it. Messing with NFB circuits in an existing circuit can cause oscillation. The amp builders/experts over at ampage.com could probably tell you more. There's a continuing discussion about it there. When I build an amp from recycled parts I always toss the original circuits. Old Hammonds and tube hifi amps always have NFB to keep them clean and flat. Guitar amps color your sound.
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Old 01.19.10, 7:59 PM
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btw, i did google this at one point, but a lot of the descriptions i found go over my head. i ain't no electronics engineer. i was hoping for simplified versions by asking here, which so far is coming about, and i thank you all who responded so far.

so thats interesting, it associates with the Presence...i assume that's the same Presence for which some amps have pots for, allowing you to adjust the upper mids in the amp signal?

so, by killing the NFB, i guess that essentially leaves your Presence (or lack thereof) always on or off, for lack of better terms, right? so by doing this, would that mean that the amp's signal output will always have a brighter tonality, or will it have the least amount of that upper mid cut (have a flatter signal)?

i'm probably thinking too far into this, yet alone down the wrong path.

so olddawg, basically, it allows you to "go further" with the amp then. you get more volume, more dirt...so technically, my Pro Jr would go to 13 or 14.

honestly, i'm trying to see if there are reasons why i wouldn't want to do this. one thing is that i still have the stock speaker, and perhaps the extra dirt will piss it off. yea yea, i may upgrade one day. thats another chapter. i'm still trying to give it a chance.

meanwhile, i'll wait for more input about the NFB. i understand it may not be a casual affair. reading olddawg's latest post, maybe thats a reason for me to drop the notion?
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Old 01.19.10, 9:32 PM
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Complex topic, and I don't know how clearly I can describe it.

A negative feedback loop is designed to keep the bias network in check. All tubes need to be biased, which is a steady voltage fed to the tubes. This bias voltage is controlled by the input voltage. When the input voltage rises, more of the bias voltage is released, and when it falls, less is released.

The bias voltage comes from a source, usually a single source, with resistors controlling how much each element gets. Each bias source has a capacitor filter which keeps the bias voltage steady, and that way, if there's a sudden large draw from the source, the cap will keep the stream steady.

The funny thing is, that the subsequent stages of an amplifier are the opposite polarity of the last. When the input is at its most positive voltage, the output is at its most negative voltage. The next stage therefore, is driven the opposite. Because of this, when one stage is driven hard, it causes the next stage to output even more.

This is a runaway signal now. The output will just keep going harder and harder until it rings or oscillates. You need some way to keep this from happening. The answer is negative feedback. This takes some of the output and feeds it back to the input. This counteraction keeps the bias level, and increases the accuracy of the output.

More negative feedback will give you a flatter, more accurate sound. Think Blackface Fenders. Vox amps, on the other hand, have no negative feedback loop. Driven hard, they are quite bright and nasal. This is probably due to the ringing of the feedback.

I can't guarantee that this is totally accurate, but it's at least an idea of what a negative feedback loop is.
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Old 01.20.10, 12:24 PM
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The NFB has nothing to do with the bias in poweramp for example but only in other circuits; it is very rarely used in this way in guitar amps

The way it works is a difficult thing to understand; just remeber that it evens out the frequency range, adds volume and makes the amp more smooth, lesser mid heavy.

Oscillation as mentioned in another post is only really possible by inverting the two sides of the push - pull source. It'll change the negative feedback loop into a positive feedback loop which causes feedback and oscillation.

A knob as called the presence control is to accentuate the feedback and make the amp more / less feedback into itself; it'll change the frequency of the poweramp but also causes other breakup of this poweramp.

Disconnecting it would result in a more raw, pure sound of the amp with more overdrive...... such like old tweed amps
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Old 01.20.10, 1:01 PM
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So, does this mean that with the Presence dimed, there will be more poweramp distortion? I'm assuming that removing the Presence control is the same as diming it.

On my Orange OR50, turning the presence control to 8-10 indeed DOES increase poweramp distortion. I don't know if that is just a design unique to the OR50 or if it's running on the same principles as the negative feedback loop.
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Old 01.20.10, 1:04 PM
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By the way New001, how are you liking the Pro Junior? I know you had to go through hell with guitar center to get it.
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Old 01.20.10, 1:57 PM
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cosmiq, thanks for the follow up. i'm on amp #3, and *fingers crossed* it's doing pretty well. this may be a winner. i gigged it a few times, and i've used it regularly at rehearsals. i'm liking it a lot. toying around with speaker replacement right now, but i can honestly say i'm not hellbent to throw out the stock speaker right now. i guess i'm still evaluating it, considering the amp is still fairly new. part of it is cost, and i like to spend only when i know its a dire necessity, or good investment. right now, things are slow in gig land, so im a bit hesitant to make a bold move.

this NFB stuff is as complicated as i assumed it'd be...haha. not even sure i'd do it, just getting informed for now.
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Old 01.20.10, 3:49 PM
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Well, if I was mostly happy with the sound, I'd just leave the speaker alone. Plus, if it's fairly new, it still has alot of breaking in to do.

You'd be surprised how different a speaker sounds after being broken in.
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Old 01.20.10, 9:35 PM
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Quote:
Originally Posted by berarduur View Post
The NFB has nothing to do with the bias in poweramp for example but only in other circuits; it is very rarely used in this way in guitar amps

The way it works is a difficult thing to understand; just remeber that it evens out the frequency range, adds volume and makes the amp more smooth, lesser mid heavy.

Oscillation as mentioned in another post is only really possible by inverting the two sides of the push - pull source. It'll change the negative feedback loop into a positive feedback loop which causes feedback and oscillation.

A knob as called the presence control is to accentuate the feedback and make the amp more / less feedback into itself; it'll change the frequency of the poweramp but also causes other breakup of this poweramp.

Disconnecting it would result in a more raw, pure sound of the amp with more overdrive...... such like old tweed amps
I've never actually known much about it on musical amplifiers, only HF radio type amps. Let me ask this, do class A amps use an NFB in the power section? And what keeps a basic two or three stage preamp (without a feedback loop) from motorboating or oscillating?
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Old 01.21.10, 3:08 AM
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I'm still working on wrapping my head around the electronics here (leprechauns, right?), I did want to point out that "presence" has no set definition in the world of amp controls. What it does depends on the company and amplifier.
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Old 01.21.10, 3:14 AM
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You could always add nfb on a switch or feed it in with a pot, get some sticky labels and label it something overly dramatic like 'spice'.

An easy enough job for a tech (ie don't get ripped off) and you won't be changing your amp from how it sounds now.

Note nfb also reduces the hum from the amp at idle so switching it out will make it a bit more noisy - if that bothers you.
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