NFB question

[daz]

As i understand NFB, the presence control basically sends whatever frequency the presence cap passes to ground. So with the typical .1uf you are passing everything below a certain treble frequency, lest say everything below 1kHz for the sake of argument. So if i turn the presence up i assume that the NFB, or the signal being fed back is all HF content, all frequencies above 1k. If this is correct, then only a small part of the signal is being fed back and none of the mids and lows. So heres the question.... if NFB is supposed to tighten things up and do all those things to your tone that is always mentioned, does turning the presence up high cause those benefits to be lessened since only a fraction of the signal is being fed back? Do the benefits only show up in the highs when the presence is up? I ask this because when i turn the presence all the way down and compensate for the los of brightness by turning the treble up, the tone seems tighter and punchier. With the presence up it seems looser and lees punchy.

[Jazz P Bass]

Without getting into the theory, I was taught that with the Presence control at zero or off, you get the full benefit of the negative feedback. (ie: the NFB is trying to correct the signal)
I never really did understand just why turning the Presense control to 10 makes the high frequencies so pronounced.
Your explanation does make sense of it.

[nickb]

As i understand NFB, the presence control basically sends whatever frequency the presence cap passes to ground. So with the typical .1uf you are passing everything below a certain treble frequency, lest say everything below 1kHz for the sake of argument. So if i turn the presence up i assume that the NFB, or the signal being fed back is all HF content, all frequencies above 1k. If this is correct, then only a small part of the signal is being fed back and none of the mids and lows.

The presence shunts HF to ground so only LF gets fed back. Thus, gain is increased for HF.

So heres the question.... if NFB is supposed to tighten things up and do all those things to your tone that is always mentioned, does turning the presence up high cause those benefits to be lessened since only a fraction of the signal is being fed back? Do the benefits only show up in the highs when the presence is up? I ask this because when i turn the presence all the way down and compensate for the los of brightness by turning the treble up, the tone seems tighter and punchier. With the presence up it seems looser and lees punchy.

Since the feedback is reduced for HF the benefits only apply to LF.

[daz]

The presence shunts HF to ground so only LF gets fed back. Thus, gain is increased for HF.



Since the feedback is reduced for HF the benefits only apply to LF.

Hmm....you lost me there. If HF is shunted to ground and LF is fed back, why is HF gain increased?

[nickb]

'cos it's negative feedback. Whatever is fed back is subtracted from the signal. So, if it's LF that's fed back that is subtracted from the signal and so the output is reduced.

[daz]

'cos it's negative feedback. Whatever is fed back is subtracted from the signal. So, if it's LF that's fed back that is subtracted from the signal and so the output is reduced.

I've read that before but as with many things i forgot. Thanks. One thing i think I'm finding is that NFB makes the tone sorta weak sounding, while no NFB really livens it up and cuts and punches like Ali. But no NFB also becomes too bright and harsh and dynamics become lost. But i find that if i use very very little NFB like 16 ohm tap and around 300k (i use a 250k pot and a 33k resistor in series) i get the dynamics and it still has a lot of that punch but doesn't get too harsh and bright. Seems to give me a balance of the best of both with very little of the negative aspects of either. I can see how more NFB would be good in a very cutting amp, but mine isn't so it benefits from less NFB. The presence still works too, tho obviously not as effective.

[Chuck H]

I think you've got this one. NFB reduces gain, BUT, also linearizes and reduces distortions. A little NFB in certain guitar amps (for certain types of tones) can help keep the bottom end tight. So... When the presence cap shunts HF out of the NFB circuit, those HF frequencies don't get fed back, get it? You certainly seem to have a handle on how much NFB suits your amp and style of playing. That pretty huge for someone who wasn't even certain how it all worked in the opening post!!! But I do have one piece of advice

Do seek your personal sound through some tweaking, BUT, also try to hear the musical possibilities in every tone you hear. Chasing tonal panecea is an impossible quest. You NEVER catch it. The very best guitar players can play through any turd amp you plug them into. Granted the very best guitar players often have good amps. But not always. Some really great tracks have been recorded on some pretty bunk gear. The best guitarists can find something musical about almost any guitar/amp/setting combo. I'm a hopless obsessive. That's why I'm warning against it. Frank Zappa said it best... "Shut up and play yer' guitar." Which is almost right. Because nothing beats a good player with good tone. But good players spend their time PLAYING!!! Not tweaking amps! Coming from me it's a classic case of the pot calling the kettle black. But I'm sure you see where I'm going. Try to rock on as much as possible without obsessing about your tone. Only then can you find what is good about the tone you are using.

[daz]

Thats true for some Chuck, but it can be much different for others. Take me.....i played bar gigs for 25 years and then one day it stopped. I got old, i had a issue with my fret hand thats killed my playing. But i still enjoy picking an electric up today. But i do it for very different reasons. And i no longer have any desire to play and get better really. I play in part because i AM old and i don't have a family and very little to keep me occupied. So playing is a release from boredom. BUT, chasing amp tone is fun too ! Thats the main reason i do it....as a hobby. It's not constructive in any way whatsoever other than to give me something i enjoy doing that occupies my time and is one more of very few reasons i have to wake up in the morning. So while Frank's album by that name may make sense for some, for me it would kill a good 1/2 of my motivation for breathing as far as musical things go. I don't expect to find tonal nirvana, but i do expect to get closer because i always do given enough time. And the great thing about it is that if during my tweaking i happen to find things that make the tone a lot more enjoyable i then will want to play more. So theres even a reward that is like the cherry on top.

So thats my motivation. I enjoy playing, what little i can anymore, and i enjoy tweaking just as much. If it were only for the possible reward and i didn't enjoy the hunt, i wouldn't bother. Tomorrow after work i plan to experiment with gain staging in the initial stages and see if with the higher voltage that gain works better in certain areas than others compared to before. I know with the much higher voltage theres going to be some things that now will yield much better results done differently than before. Should be fun !

[kleuck]

"Presence" and "resonance" are indeed reduction of the nfb, for the hf or lf, using these controls makes your amp close to or completely without nfb for these frequencies.