Fuzz experience

[Xplorer]

thanks.

may i ask you some little lesson for the zéros, about the negative feedback please ?
since it comes up so often about amps and fuzz, i realise i don't even understand it very well.

is it about some gain coming out of a transistor, reinjected into the transistor "input", while being negative ? ( and there's a positive feedback then i guess ) ..
sort of an echoplex reinjected signal but with a delay there, and not about saturation .. ?

[Tek465b]

thanks.

may i ask you some little lesson for the zéros, about the negative feedback please ?
since it comes up so often about amps and fuzz, i realise i don't even understand it very well.

is it about some gain coming out of a transistor, reinjected into the transistor "input", while being negative ? ( and there's a positive feedback then i guess ) ..
sort of an echoplex reinjected signal but with a delay there, and not about saturation .. ?

Negative feedback is about the same signal and not some gain that is 180degree out of phase or the opposite of the input signal. Like a mirror copy re-injected to the input or one of its input reducing its amplitude.
The stage or overall gain will be affected by the feedback ratio, it will also reduce the output impedance in most case and reduce the interaction with a speaker for an amplifier.
Positive feedback is used mostly for oscillator, signal in phase re-injected to the input(like a mic too close to the output speaker squealing).
Positive feedback can also be used to raise the input impedance of a circuit if you use it in small amount and avoid oscillation..

When you take a wah and a fuzz. The nfb on the fuzz input control the gain. the wah take that signal at its input of Q2B(because of the way the circuit is made) and the output of Q2E is in phase with its input then going to the input of Q1B, wich is an inverting stage and invert it 180 degree.. so now its back in phase to the fuzz, or positive feedback(or just cancelling the nfb in our case). you get oscillation/squealing thats how the gain get soo high with these when you roll the wah pot toward the .22u...

The echoplex i dont know much about these effect, but probably just a much larger phase delay..

[Xplorer]

Negative feedback is about the same signal and not gain 180degree out of phase or the opposite of the input signal. Like a mirror copy re-injected to the input or one of its input reducing its amplitude.

thanks ! this, i think i understand a bit, this is what i imagined. it's like the signal amplified by the transistor gives enough volume to be divided in two signals of equal volume at least as the initial one ?
then if one portion is going back injected into this same process , it means that the sum of it will again be reinjected, and again and again , right ? infinite, with a certain blending amount.

but i don't see how it reduces the amplitude. anyway, the sum of this signal + einjected signal then goes to Q2 and ... etc , right ?

for the positive feedback and the phases i'm sorry , i didn't get it yet.

what makes a signal a negative or a positive feedback ?

[Tek465b]

Negative feedback is about the same signal and not gain 180degree out of phase or the opposite of the input signal. Like a mirror copy re-injected to the input or one of its input reducing its amplitude.

thanks ! this, i think i understand a bit, this is what i imagined. it's like the signal amplified by the transistor gives enough volume to be divided in two signals of equal volume at least as the initial one ?
then if one portion is going back injected into this same process , it means that the sum of it will again be reinjected, and again and again , right ? infinite, with a certain blending amount.

but i don't see how it reduces the amplitude. anyway, the sum of this signal + einjected signal then goes to Q2 and ... etc , right ?

for the positive feedback and the phases i'm sorry , i didn't get it yet.

what makes a signal a negative or a positive feedback ?

The input resistor(or output impedance of the previous circuit and some other variable) and the feedback resistor set the feedback ratio for Q1, that times the stage gain will give the true gain (well i over simplify there). When we add the input resistor we raise the feedback ratio, more feedback less gain for Q1, less fuzz and less oscillation/squeal.
Its like a resistor divider, input on one side, negative feedback on the other side and the Q1B fuzz input in the middle so the fuzz see less input signal so less output signal = reduced gain.

When a signal is positive its the same signal just maby a different amplitude.
when its negative it mean its the invert/opposite of it. The positive portion of the waveform is now the negative one, and the negative portion is now positive. 180degree , just flip it

edit:
Q2E signal is just used as a low impedance output for the nfb(yes like a buffer for the nfb)+ its also used for the bias as we know. And allow control of the gain for both stage with the same control pot..
So for the original fuzz it is more like local nfb, and the Dave fuzz taking nfb from Q3C its global nfb affecting the gain of all the stage and Q2 has this gain/fuzz setting.
So like he said.. many possibilities.

Can't wait to try it.

Dave could explain it better am sure. am more into ham radio.. vco, mixers, filter, pll, dc-dc converter.. that kind of stuff where we dont really use nfb much if at all.

[Xplorer]

Mh well, i get some bits but i think i'm not a zero, i'm rather a minus 10

[Tek465b]

Mh well, i get some bits but i think i'm not a zero, i'm rather a minus 10

There is a good youtube channel from alan w2aew. its nothing about guitar but has some very good videos:
https://www.youtube.com/user/w2aew
https://www.youtube.com/watch?v=zXh5gMc6kyU
https://www.youtube.com/watch?v=K03Rom3Cs28

He do alot better job than me at explaining stuff .

[daveweyer]

Xplorer,
Maybe think about it like this; if you take two signals of the same size and combine them out of phase with each other, you get zero. One signal is positive, its amplitude rises at the zero crossing point; the other is negative, its amplitude falls at the zero crossing point. The sum of both is zero, as long as their amplitude is the same size and current is equal.

The out-of-phase signal in an FF unit is returned to the input to subtract a certain amount of signal from whatever is coming in, controlling the gain of the input stage. Since the out-of-phase signal also contains the distortion from the circuit, that distortion is also subtracted from the input signal, hence cleaning up the signal at the first stage.

For purposes here, there is no delay in the signal fed back to the input; therefore the gain of the circuit is the ratio of the feedback resistor to the input resistor. (There is actually a tiny delay, maybe a microsecond or so caused by the switching time of the transistors and the various capacitances in the circuit which create a phase shift network; as long as there is no gain at the {very high} frequency where the combined phase shifts equal 180 deg. the circuit will not become an oscillator) An oscillator happens when the feedback phase is the same as the input phase.
In this respect, an echoplex is a sort of an oscillator with a long delay.

The important point is that the negative feedback signal subtracts from the input signal, and also subtracts the distortion component of the circuit from the input signal, giving a sum of a partly or wholly undistorted signal, according to the amount of the negative feedback signal--the more negative feedback, the less distortion.

Impedance is another topic.

[Xplorer]

Tek, no offense , and thanks for trying but i understand it very well now thanks to Dave
will Watch your youtube videos though, thanks.
thanks a lot Dave, i think i now see how it's important for an amp , and for the vox for example, which removes completely the negative feedback, according to brian may.

the negative feedback signal subtracts from the input signal, and also subtracts the distortion component of the circuit from the input signal, giving a sum of a partly or wholly undistorted signal

one thing though, just to be sure : where does the distortion come from then ? from after the first stage ? the output of the first stage meaning the "input signal", different from the input signal right at the input jack ?

[Tek465b]

Tek, no offense , and thanks for trying but i understand it very well now thanks to Dave
will Watch your youtube videos though, thanks.
thanks a lot Dave, i think i now see how it's important for an amp , and for the vox for example, which removes completely the negative feedback, according to brian may.

the negative feedback signal subtracts from the input signal, and also subtracts the distortion component of the circuit from the input signal, giving a sum of a partly or wholly undistorted signal

one thing though, just to be sure : where does the distortion come from then ? from after the first stage ? the output of the first stage meaning the "input signal", different from the input signal right at the input jack ?

I know am not a good teacher, don't worry about that.
Am going to leave that to dave haha He has more experience than me.

[Tenderfoot]

I can't believe that I understood some of this! COOL!

[daveweyer]

The distortion comes from Q1 and Q2. The distortion in that signal is subtracted from the undistorted input signal by the negative feedback signal, so when the input signal minus the distortion component goes through Q1 and Q2, they add the distortion component back in, leaving you with positive distortion added to negative distortion, all other things being equal, the sum of a positive and a negative (equals), which is zero, in this case zero distortion.
Sometimes they call it pre-distorting the wave (the input signal), because that's what really happens. The input signal is pre-distorted by the negative feedback so when the circuit distorts it again in the opposing phase, the two distortion components cancel each other. The minimum gain of a circuit like this is 1. The distortion never goes to zero, but the best circuits of this type might be able to get it down to less than 1%.
Getting rid of all distortion was never the goal of the FF obviously, and the circuits we're working with here will always have probably 3 to 5% with the negative feedback at max, just a little grunge.

[Xplorer]

thanks for the explanations

i had a bit of time so i evolved in the build. probably finished tomorrow

http://i.imgur.com/3dsGZAk.jpg

[shakti]

Anxiously waiting for your impressions!

[Xplorer]

Me too !!!

i have a bug, and i hate to debug such circuit. No sound at all. i have no idea what bad connection or stuff i did.

i have one footswitch for the clean / hot fuzz , and one little switch to true bypass the entire circuit. i debugged this one, so now i have either the dry signal straight into the amp, or no sound.

i have no idea of where to start , i usualy have some extreme care right from the beginning, to avoid such debug episode, and have it working right from the first try, because i'm zero and blind at debugging stuffs.

[Tek465b]

Me too !!!

i have a bug, and i hate to debug such circuit. No sound at all. i have no idea what bad connection or stuff i did.

i have one footswitch for the clean / hot fuzz , and one little switch to true bypass the entire circuit. i debugged this one, so now i have either the dry signal straight into the amp, or no sound.

i have no idea of where to start , i usualy have some extreme care right from the beginning, to avoid such debug episode, and have it working right from the first try, because i'm zero and blind at debugging stuffs.

What are the voltage of the transistor? do it bias?
check the input/output wiring.
show picture of your circuit/wiring maby dave or me can spot something wrong..
I love finding electric bug haha. i have something magic i think. am pretty sure i could blind my eye and randomly touch a part of the circuit and it will be the faulty component right away. well am joking but i impress myself at how good am at finding bug/faulty component when i have them on hand.