I know there are a few threads about building the ESR Graphic Fuzz with an adapter for the power supply, but none really covered my questions. At least I didn't found anything like that.
I guess it should work when you just put in a input and output cap and use the original schematic, but with your single or isolated power supply poles connected to where the battery clip leads would go. Is that correct?
And is it now also possible to use it with "normal" (v- is gnd) pedals, that run before or after it, but have a different or isolated power supply? And what would happen, if it is used with a daisy chain together with other pedals?
(https://i.postimg.cc/cgysgdMr/Graphic-Fuzz.jpg) (https://postimg.cc/cgysgdMr)
Thanks for answers!
QuoteI guess it should work when you just put in a input and output cap and use the original schematic, but with your single or isolated power supply poles connected to where the battery clip leads would go. Is that correct?
it would work, but only if you don't daisy chain other pedals. you don't need the in/out caps since the power supply will balance around 0, creating +/-4,5v, that's what the resistors do!
if you plug it in to a daisy chain, it will go all weird and sputerry, or not even work at all, since the -4.5 resistor will be ignored due to a ground connection on both sides, and the 741 not liking a bias of -Vcc
if you want it to work with a daisy chain or non isolated power supply, you are better off redesigning this from the ground up
cheers, Iain
That pedal is designed to connect directly to the guitar pickup. The gain is strongly dependent on the guitar pickup impedance.
If the signal source is a low output impedance pedal you are going to get a lot of gain and a very different character tone. If the signal source is an high-ish impedance pedal the gain will be lower but dependent on the preceding pedal's output impedance (again a different tone to the pickup). Also the ESR Graphic Fuzz presents close to zero AC impedance load to the preceding pedal so it will mess with the behaviour of the previous pedal.
I short it's pot luck connecting it to anything other than a guitar pickup.
From the output side there's no problems connecting it to other pedals.
(PSU side, you can't daisy chain 9V with other pedals. I think you get that one already.)
741 DC offset with a lot of gain could result into crackling output pot, couldn't it.??
Quote741 DC offset with a lot of gain could result into crackling output pot, couldn't it.??
Not to mention driving into a short when the output pot is on min.
That circuit has some crazy things going on.
Maybe search through the archives to find a post on the ideas/motivations of the pedal.
Thanks guys! That's basically what I assumed.
Quote from: Rob Strand on December 02, 2020, 05:48:45 PM
That pedal is designed to connect directly to the guitar pickup. The gain is strongly dependent on the guitar pickup impedance.
Yeah that's what I noticed when I switched to my normally a little darker pickup mode, where the bridge and the neck are paralleled. It has some tighter fuzz sound according to only having about half the impedance now.
Also think that it's nearly only a "right after your guitar" kind of pedal, just like a fuzz face.
Quote741 DC offset with a lot of gain could result into crackling output pot, couldn't it.??
Had that crackling sound. So I decided to put in some caps and now it's fine.
Quoteif you want it to work with a daisy chain or non isolated power supply, you are better off redesigning this from the ground up
There is a version where the LM741 sees V+ and GND on its power pins and is biased with 4.5V. But I just like the original version better as it also doesn't oscillate that much when you use your power adapter.
Take into account what Rob said about pot min setting and place a 470R - 1k resistor between lug 1 and GND.. :icon_wink:
Quote from: antonis on December 03, 2020, 07:32:21 AM
Take into account what Rob said about pot min setting and place a 470R - 1k resistor between lug 1 and GND.. :icon_wink:
Alright, I will try that. But I don't really like the idea of not being able to cut the volume completely, also because the Graphic fuzz is already a extremely loud pedal. Wouldn't it be a better place to stick a resistor in before the volume pot?
Quote from: pokus on December 03, 2020, 08:00:59 AM
Wouldn't it be a better place to stick a resistor in before the volume pot?
Wire pot with lug3 to op-amp output, 2 to circuit output and lug1 to GND..
(no resistor needed..)
Funny thing is, that I did that "accidentally" on my breadboard build as it's the most common technique. You're right this would make the resistor unneeded.
(edit) When I set up the original setting for the volume pot it is now extremely loud from almost the beginning of the pots rotation. I had to switch amps to make it an endurable volume. But as there's only the relatively small LM741s output impedance working against that resistance to gnd now, I hadn't guessed anything much different. Seems a bit brighter and fuzzier, but that can be the different amp.
Another thing occurred to me. As my V- isn't gnd, I can't switch from on to off, when the battery is used, with a simple stereo input jack.
Any way to do this without a extra switch for it?
I thought I read that the original pedals footswitch was just for on/off and not bypass. That now makes sense to me.
Quote from: pokus on December 03, 2020, 10:41:28 AM
As my V- isn't gnd, I can't switch from on to off, when the battery is used, with a simple stereo input jack.
Any way to do this without a extra switch for it?
I'm afraid, no.. :icon_cry:
Quote from: antonis on December 03, 2020, 01:59:14 PM
I'm afraid, no.. :icon_cry:
Damn ;D
Quote(edit) When I set up the original setting for the volume pot it is now extremely loud from almost the beginning of the pots rotation. I had to switch amps to make it an endurable volume. But as there's only the relatively small LM741s output impedance working against that resistance to gnd now, I hadn't guessed anything much different. Seems a bit brighter and fuzzier, but that can be the different amp.
Put in a 500R resistor before the volume pot and now I can dial in a suitable volume at the amp I used in the first place. Compared to my previous setup of the vol pot (lug1 to gnd, 2 to effect out, 3 to output cap), the original one (except the added 500R) is definitely a game changer. Much more treble and bite than before. I simulated this with LTSpice but for both settings it shows me the exact same frequency response just different in volume. But not that audible change I can hear. That's what I really don't get.
Quote. I simulated this with LTSpice but for both settings it shows me the exact same frequency response just different in volume. But not that audible change I can hear. That's what I really don't get.
The volume control is only passive so to first order approximation you wouldn't expect it to do much.
You need to add some of the surrounding real world components.
To simulate the circuit better,
- Model the impedance of the guitar:
https://ironstone-guitar-pickups.co.uk/guitar-impedance-matching-lcr/
- When you simulate the pedal drives the amp input and the cable capacitance.
Say 470k to 1M load and 300pF to 1nF load. You need to know what capacitance matches your set-up
- The opamp is clipping so the frequency response might not show the effect hear by the ears.
What you hear could be changes in harmonics caused the loading on the opamp.
You would need to do a transient analysis with a sine input with the circuit overloading.
Look at the FFT output before and after the changes to see the different harmonics.
Since the opamp models aren't great it still might not match watch you actually hear.
Quote from: Rob Strand on December 03, 2020, 06:10:08 PM
- Model the impedance of the guitar:
https://ironstone-guitar-pickups.co.uk/guitar-impedance-matching-lcr/
- When you simulate the pedal drives the amp input and the cable capacitance.
Say 470k to 1M load and 300pF to 1nF load. You need to know what capacitance matches your set-up
- The opamp is clipping so the frequency response might not show the effect hear by the ears.
What you hear could be changes in harmonics caused the loading on the opamp.
You would need to do a transient analysis with a sine input with the circuit overloading.
Look at the FFT output before and after the changes to see the different harmonics.
Since the opamp models aren't great it still might not show-up with you actually hear.
Did all of that. Still the same in frequency response and the waves look similar at the transient analysis. Also the harmonic content stays the same at FFT.
Wouldn't say it isn't the harmonic content that causes that audible change, but it feels more like a great amount of treble is much more present now. With my first setting it seemed like there was a little too much treble roll-off and it sounded a bit dull.
As I have the two options now, it's not that bad that my simulation turned out so weak. But it kind of bothers me not to know how a simple different pot wiring could lead to that almost drastic change in sound.
(https://i.postimg.cc/9zxQ87Pb/graphicfuzz-FFT2k.png) (https://postimg.cc/9zxQ87Pb)
You maybe have to take into account different impedances seen by op-amp output..
(https://i.imgur.com/K2XYo0z.jpg)
QuoteDid all of that. Still the same in frequency response and the waves look similar at the transient analysis. Also the harmonic content stays the same at FFT.
Wouldn't say it isn't the harmonic content that causes that audible change, but it feels more like a great amount of treble is much more present now. With my first setting it seemed like there was a little too much treble roll-off and it sounded a bit dull.
As I have the two options now, it's not that bad that my simulation turned out so weak. But it kind of bothers me not to know how a simple different pot wiring could lead to that almost drastic change in sound.
It is a bit annoying the opamp models aren't good enough to resolve "the sound of opamps". Even though I use spice a lot, it's not something I expect to work.
The circuit has that 220ohm and 100nF cap on the output as well. No doubt that effects the tone. Even more surprising perhaps is with the network present the pot load has an audible effect.
Is your pot 10k in the real circuit?
Quote from: Rob Strand on December 04, 2020, 04:49:22 PM
The circuit has that 220ohm and 100nF cap on the output as well. No doubt that effects the tone. Even more surprising perhaps is with the network present the pot load has an audible effect.
BTW, it brings to mind a Zobel network (Bucherot cell) designed for some speaker of 180R/5mH, or so.. ::)
QuoteBTW, it brings to mind a Zobel network (Bucherot cell) designed for some speaker of 180R/5mH, or so.. ::)
It looks like that but I suspect the intent is to load down the opamp, especially at high frequencies. The 220R helps prevent opamp oscillations due to capacitve loading.
Quote from: Rob Strand on December 05, 2020, 07:41:34 AM
I suspect the intent is to load down the opamp
Aron Nelson & Jack Orman could also verify it with the purpose of 1k1 Insanity Box resistor.. :icon_biggrin:
Quote from: Rob Strand on December 04, 2020, 04:49:22 PM
Is your pot 10k in the real circuit?
Yes, it's a 10k lin pot. Maybe I will change it to a log one for being able to dial in lower volumes.
Doesn't mean loading down the op amp results in less gain? Maybe what I hear is the low frequencies now have less gain than when the op amp only sees the 100n + 220R network and a relatively high impedance in parallel. Although I would expect the low frequencies to distort less instead of being less present.
But if thats true the volume pot should have a great influence on the bass content. I would try that, but unfortunately I haven't found a way to do that yet without busting my ears.
You may consider 100nF + 220R in parallel with wiper-lug1 resistance..
For wiper at 50%, at 1k6Hz effective wiper-lug1 resistance is about 980R (and NOT 5k) where at 160Hz is about 3k4.. (more close to 5k)..
It is clear for very high frequencies actual Pot setting should be considered negligible.. :icon_wink:
(almost zero output volume variation..)
QuoteYes, it's a 10k lin pot. Maybe I will change it to a log one for being able to dial in lower volumes.
A log pot would help.
QuoteDoesn't mean loading down the op amp results in less gain? Maybe what I hear is the low frequencies now have less gain than when the op amp only sees the 100n + 220R network and a relatively high impedance in parallel. Although I would expect the low frequencies to distort less instead of being less present.
But if thats true the volume pot should have a great influence on the bass content. I would try that, but unfortunately I haven't found a way to do that yet without busting my ears.
When we are talking small effects it can be quite hard to narrow down the cause. Thing we normally consider have no effect may have a small effect. In isolation those effects can be difficult to pickup and it's only when you do detail A/B testing that is becomes clear there is a subtle effect.
Opamps aren't like transistor stages you *really* have to load them down to have an effect. Something like the 100n+220R network where the load is 100's of ohms to k's of ohms we might expect something.
The other effect you might be experiencing is the level pot setting in the original and modified circuits is difficult to set to the same *equivalent* position. it's actually harder than you think.
If we wind back a bit to more tangible engineering causes. If you are using a tube amp the output impedance of the pedal interacts with the overloading preamp tube. Keep in mind that pedal can push out about +/-4V. As far as making the modded volume control work like it did before, perhaps try adding a 10k between the pot wiper and the output jack. That makes the output impedance more like the original.
Another thing you can do is add place 2.2k to 10k from the *opamp* output to ground. A switch to switch the resistor in would be ideal here so you can easily do AB testing. If you can't hear a difference then resistive loading isn't the cause.
A far more repeatable approach would be to put say 22k between the level pot wiper and the output jack, similar to the previous mod, then simply add a small capacitance across the output jack and ground to take out the fizz. Start at say 100pF and increase the cap until you get to tone you want.
Quote from: Rob Strand on December 07, 2020, 04:41:23 PM
Another thing you can do is add place 2.2k to 10k from the *opamp* output to ground. A switch to switch the resistor in would be ideal here so you can easily do AB testing. If you can't hear a difference then resistive loading isn't the cause.
I put in a 470R right after the op-amp with the volume pot in the lug3 to output cap(not the effect out) setting. Now I have that brighter sound the original pot wiring was creating, but I'm also able to control the volume of it a lot better. Making that resistor value higher, the sound comes closer to what I experienced without it and reaches no-difference at about 10k.
So the op amp loading seems to have a huge effect in this circuit. The volume pot in the Graphic Fuzz therefore shouldn't be (intended or not) just controlling the volume. Maybe this is also very op-amp dependent. Would the addable offset-null-control do anything here?
I still don't know which exact sound I like better. The original one has a lot of bite and sounds like that early raw vintage fuzzes, at least at quite low volume regions. But also kind of harsh and not that great when multiple strings are in action. The different volume pot wiring definitely smooths things up a bit and brings back bass content. Maybe with the setting of the resistor after the op-amp you can have/dial in best of both worlds at a reasonable volume.
QuoteSo the op amp loading seems to have a huge effect in this circuit.
That must be it. You did well persevering with that one. Certainly not clear what was going on.
If the sound match is with 470R, you must have had the output Level pot set quite low.
Looking at original circuit, for the Level control to work it relies entirely on loading down the opamp.
Trying to make more sense out of it, if you look at some older Fairchild and National Semiconductor datasheet, they have current limit info and an internal circuit,
https://www.alldatasheet.com/view.jsp?Searchword=LM741MH
The current limit is typically 25mA (10mA to 35mA), so with 470 ohms, 25mA*470 = 11V. That's more than the 4.5V swing. So it's not even current limit that's changing the sound. Nonetheless it's not far off so it's likely the internal drive currents are getting to the point where they are running out of steam and are just on the edge with the heavy load. In other words the outright loading is changing the sound.
Thank you guys for leading me through this.
I think i will stick with the "wrong" volume pot wiring as the tone can be better altered elsewhere in the circuit.
Overall the Graphic Fuzz stays a very weird but also great sounding pedal to me.
It still keeps getting weird ;D
I boxed this thing up (veroboard). On my breadboard the oscillation was really subtle, shining through a bit but mainly coloring the tone.
Now it's really wildly oscillating whenever it's on. Really don't think that all of this is coming from the wires.
My voltage readings of the 741 in the box (on breadboard) are:
0V (0V) at the non-inverting input, as it is tied to gnd.
0.7mV (1.4mV) at inverting pin
50mV (12mV) at output pin
The other readings are the same. About +4.7V on +pin and -4.7V on -pin. Also -4.7V on both null pins.
Tried the null control for the 741 by connecting pin1 and 5 with a 10k pot to V-. The best result, in regards to the oscillation, I'm getting is when pin1 is tied directly to V- through that pot. But it's still very present. The voltage on the output pin than drops to about 35mV, on the inverting pin it rises to 6.7mV.
It also seems like it's a lower oscillating frequency then. When pin5 is connected to V- it's a higher frequency.
Tried this with different chips, but the sound on breadboard always is subtle and in the box it's very present. It's hard to tell, but apart from that I don't think there's any difference in the sound.
Anything that could be done to fix this? Or any idea why this occurs?
QuoteAnything that could be done to fix this? Or any idea why this occurs?
Perhaps the place to start would be to increase the 2x100nF caps across +/-4.5V to say 100uF - maybe add the 100uFs across the 100nFs temporarily.
The cause could be the 100n + 220ohm (and any load resistors) on the output is feeding into the ground. It's a low impedance and is the most likely cause of trouble, especially with the relatively light 2x100nF supply caps. The precise ground wiring and the wiring of the 100nF input cap could tip it over the edge.
Some other things to try would be:
- Add a small resistor say 1k between the opamp - input and the 100nF cap. You could also try 1k between the input jack and the 100nF cap. Resistor values may need to be tuned to find a balance between no oscillation and affecting the sound.
- another possibility is to add a 220 ohm *after* the 470 ohm load you added and before the volume pot.
- and another is to add a small cap across the 1M pot, perhaps 22pF to 100pF ; many need to juggle values for sound vs oscillation trade-off.
The 100uF caps somehow cut my tone completely. 200nFs in parallel with the 100nFs really lowered the oscillation a bit, but also the frequency changed there and the overall sound was a lot duller. (on my breadboard the 100uF caps added oscillation ??? )
That's why I focused on the feedback cap across the filter pot. It's really a sound vs. oscillation trade-off like you mentioned. I found it best with a small 10pF cap, which cuts the oscillation almost completely, but keeps the nature of those high frequencies and the filter control. Keeping pin1 tied to V- also helps.
As it's already on veroboard I haven't managed to try the other options with some added resistors yet.
I left out the 470 ohm resistor, so what the 741's output sees is just the 100n+220R network and the 10k pot (wired as 10k path to gnd).
Quote from: pokus on December 30, 2020, 08:12:54 AM
The 100uF caps somehow cut my tone completely.
:icon_eek: :icon_eek:
Did you place them as Rob suggested..??
(across +/- 4.5V)
Quote from: antonis on December 30, 2020, 08:30:13 AM
Quote from: pokus on December 30, 2020, 08:12:54 AM
The 100uF caps somehow cut my tone completely.
:icon_eek: :icon_eek:
:icon_eek: :icon_eek: Indeed.
QuoteThe 100uF caps somehow cut my tone completely. 200nFs in parallel with the 100nFs really lowered the oscillation a bit, but also the frequency changed there and the overall sound was a lot duller. (on my breadboard the 100uF caps added oscillation ??? )
Those results show we are dealing with a crazy beast. It's like it relies on the ground flapping about relative to the power rails. Very hard to understand and even harder to fix without affecting the tone. Normal thinking probably isn't going to get us to a solution.
So maybe we need to try a sightly different cap scheme. Try putting a single cap across the power rails of the opamp, perhaps 10n, 100n, 10u, 100u.
QuoteAs it's already on veroboard I haven't managed to try the other options with some added resistors yet.
We might have to try all the options (or combinations of option) since it's not looking easy to fix.
Rob, I think we're confused a bit..
It seems to me that OP refers on 100nF output cap (the one in series with 220R) when you point on supply rails decoupling.. :icon_wink:
QuoteIt seems to me that OP refers on 100nF output cap (the one in series with 220R) when you point on supply rails decoupling..
I didn't think of that.
Pokus's post has "100nFs" with an 's' so maybe he did put the 100uF's across the two 100n's on the power rails (for clarity the 100n's across the 2x10k's).
Maybe he can confirm.
I put one 100uF from V-(-4.5V) to ground, in parallel with the 100n and another one from V+(+4.5V) to ground, also in parallel with the already existing 100nF.
It also felt weird to me that it completely cut my tone. No difference if there's one(from V+ or V- to gnd) or two( from V+ to gnd and V- to gnd). Greatly smaller values, like the 200n I mentioned, are only cutting the highs. So maybe that's why the 100uF cuts the bass, too (although I got no plan why it does so here ??? ). With 1u there's still a little bass content left.
The opamp output still sees the 220R + 100n. I did put an LED with a resistor across V- and V+, but that was after I had experienced the tone cut - at least I think so. As with the small feedback cap it sounds and behaves like my breadboard build, I don't think there's anything wrong there. Apart from being that weird.
Just to make things clear.. :icon_wink:
Placing a single cap between + & - bipolar supply rails instead of one cap between each rail and GND is often desirable to prevent each of two caps injectingrail noise to ground..
Try to delete both +/- 4.5 to GND caps and put a single cap between supply rails (with GND left uncoupled..)
P.S.
More a more "sensible" gain circuit, connect IN to 1M pot "open" lug and cut the connection between 100nF cap and pot wiper..
(of course, for max gain setting, signal will still "see" zero impedance so Rob's suggestion for a 1k or so input resistor is always advisable..)
I've been trying to decipher the Graphic Fuzz in order to come up with some more ideas stabilizing the circuit.
So, up front, I don't understand why the bass is cut with the big caps. It could be some non-linear effect but it's hard to visualize at the moment.
So here's what I've come up with so far.
Some circuit bending trying to understand what it's doing, linear case only,
(https://i.postimg.cc/mhVRNKHt/Graphic-Fuzz-Decipher-V1-0-2021-01-01.png) (https://postimg.cc/mhVRNKHt)
From that we can see how the load affects the response. I guess that something :icon_mrgreen: It does not explain the bass cut with big caps.
(https://i.postimg.cc/gxLWpJBZ/Graphic-Fuzz-Response-and-Load-2021-01-01.png) (https://postimg.cc/gxLWpJBZ)
So as far as stabilizing the circuit is concerned. The small feedback cap across the 1MEG still holds. However, perhaps there's a better scheme. Perhaps adding a 470R or 1k in series with the 100n cap at the input of the opamp will help stability. Maybe need to experiment. The way I see it is the 100n is adding lag to the feedback. By putting a resistor in series with the cap the phase shift at high frequencies is removed and that should help stability.
Given this is a rough model and totally I'm ignoring opamp common mode effects, I wouldn't be surprised if the resistor made things worse. Nonetheless we have to start somewhere since we aren't dealing with a normal circuit!
EDIT:
The "circuit bending" part seems OK superficially but it might not be correct. The reason is i've tried to keep things at a high level bt representing to opamp supply path as a voltage source. In reality it probably acts as a current source which changes the required "circuit bending".
From what I can see that pedal was a real pedal,
(https://1.bp.blogspot.com/-2UFnNbzJpDA/X0nLrxA_qgI/AAAAAAAAGLo/OrIf2eYH4MoWtKpICruBUvlEjByQOVJxwCLcBGAsYHQ/s730/Screen%2BShot%2B2020-08-29%2Bat%2B1.27.07%2Bpm.png)
I can see three 10k's on the PCB.
My first guess is extra 10k is simply in series with 1M pot. However, in reply #4 of this thread there is an extra 10k between the opamp+220R+100n and the 10k pot,
https://www.diystompboxes.com/smfforum/index.php?topic=83300.0
(Knowing what we know now and the fact there is a PCB pic, PRR's mod in reply #7 are probably changing the intent of the pedal.)
Schematic credited to Jerms (2009_03_21),
(https://i.postimg.cc/KK600qcc/ESR-Graphic-Fuzz2.jpg) (https://postimg.cc/KK600qcc)
Confirmed correct by retracing PCB.
A few details,
- IC = 741CP.
- 1M pot is called Filter, probably A taper, clock-wise = high resistance
- 10k pot is called Boost, probably A taper
- all caps disc ceramics
- Input socket is a square plastic type with a separate set of contacts to switch the battery 0V ("-4.5V")
Perhaps we can trace it from this bottom shot,
(http://4.bp.blogspot.com/-i5lDSHm3KuE/Tpcg1Ev-0oI/AAAAAAAAAI4/QFdthw_j9Ko/s1600/IMG_0551.JPG)
EDIT: I retraced the board and confirmed the jerms schematic is correct.
EDIT: The difference between this correct schematic and the one at the top of the thread is the 10k between the opamp output + 220R+100n and the output level pot (Boost pot).
Here's a few more points:
As far as fixing the oscillation I'm pretty sure adding resistor in series with the 100nF input cap to ground is the way to go. There is some engineering sense behind this instead of magic and mystery.
There is a minimum value for the resistor which reliably fends off oscillations. The minimum is around 47R to 100R but you need to check both 470ohm loads and 10k loads at the output. A higher value around 470R seems more reliable and will prevent oscillation independent of the load. With very low values you might find the oscillation stops but if you change the load resistance the oscillation might kick-off again. That means you need to increase the value of the series resistor.
The peaks in the response (see plots a couple of posts back) are affected by the load resistance. The resistor in series with the 100nF cap also affects the peaks. You may need to delicately adjust the value to find a balance between oscillation and tone.
So here's the problem: I'm fairly certain the presence of the peak itself is an indication the circuit is close to oscillation. If you are using those peaks to EQ your tone then you are treading a very fine edge between oscillation and getting the sound you want. There seems no way out of this. The peaks are around 3kHz so that means they will be quite noticeable. A better design is perhaps to setup the circuit to have no tendency to oscillate then add a post EQ to get the EQ profile you like.
As far as my "circuit bending" stuff goes. I tried a few other ideas and they give similar types of behaviour. Even if the analysis so far doesn't quite represent reality I do have a feeling the closeness to oscillation vs EQ is coming into play.
I appreciate your determination, all.
I get annoyed with circuits like this. It worked once, for someone, somewhere, with some chip they had in a drawer. And then they published it and tens/hundreds/thousands of other people have wasted hours trying to get the bl00dy thing working ever since. That's a badly designed circuit in my view, and I lose patience with that.
What you're doing is the basic design work that the designer of this circuit should have done originally. It's quite possible, given the surfeit of talent on this forum, that something good will come out of it. But it won't give any credit to the original designer, and it still p1sses me off that you have to bother.
Sorry, I'll stop moaning now... :-X
QuoteI get annoyed with circuits like this. It worked once, for someone, somewhere, with some chip they had in a drawer. And then they published it and tens/hundreds/thousands of other people have wasted hours trying to get the bl00dy thing working ever since. That's a badly designed circuit in my view, and I lose patience with that.
Agreed, the one day it worked circuits are a total pain, especially when they have design problems. (One of my favourite oscillation cases on the forum was the oscillating SHO. The lead inductance to the CRACKLE pot causes the MOSFET to oscillate with some weird symptoms - solution is posted somewhere on the forum.
http://www.luciferstrip.com/fuzz/sho-schematic.jpg )
FWIW, Earth Sound Research weren't known for their engineering. A large section of the business was copying Peavey amps, right down to the PCB in some cases.
After pondering the resistor in series with the input cap, it also occurred to me the guitar cable capacitance at the input jack could set the things off oscillating in the same way the 100n input caps does. Maybe a resistor from IC pin 2 + 100n input cap to the input jack is wise as well. 100R might do it perhaps upto 1k. 1K is better but it might change the sound so the 100R could be a compromise. I mentioned this somewhere above but now it seems like it would be a wise addition.
QuotePlacing a single cap between + & - bipolar supply rails instead of one cap between each rail and GND is often desirable to prevent each of two caps injectingrail noise to ground..
Try to delete both +/- 4.5 to GND caps and put a single cap between supply rails (with GND left uncoupled..)
I put a 100uF cap from V- to V+. All that changed was a reduction of some hissing noise, not affecting the oscillation. That's also great, as the circuit was a bit noisy when rolling back the guitars volume and increasing the boost/volume pot on the pedal.
QuoteI can see three 10k's on the PCB.
My first guess is extra 10k is simply in series with 1M pot. However, in reply #4 of this thread there is an extra 10k between the opamp+220R+100n and the 10k pot
That makes perfect sense. The volume pot therefore shouldn't be affecting the sound in the original circuit, other than what I've experienced with the original volume pot wiring right behind the opamp. Don't know why this is missing at the schematic I posted at the beginning.
QuoteAs far as fixing the oscillation I'm pretty sure adding resistor in series with the 100nF input cap to ground is the way to go. There is some engineering sense behind this instead of magic and mystery.
Just to be clear, you suggest a small value resistor from the 100n cap at the input to gnd, just like the network on the opamps output?
It should be very easy to try this on my breadboard build, but there's almost no oscillation (whyever?!), so it could be hard to hear an effect.
As Rob said, normal thinking maybe won't get us to a solution, I made my breadboard build oscillate again with the 100uF from V+ to gnd (again: whyever?!) ;D
Now I connected the 100n input cap to a 470 ohm resistor and then to gnd. Oscillation is gone! It stays unhearable until about 100R, then comes in gradually (I'm not stacked with every value). There might be a small effect on the frequency response, but really nothing I could define exactly. I didn't change the load, but with the 10k in the real original circuit it should always be 10k minimum.
Great work and a big thanks for your effort on this one.
While we're at it. When my guitars volume is down completely, there's a lot going on for an undefinable time. Crackles and that kind of stuff. Not super loud, but you can hear it. That's on both, the breadboard and the boxed one.
QuoteThat makes perfect sense. The volume pot therefore shouldn't be affecting the sound in the original circuit, other than what I've experienced with the original volume pot wiring right behind the opamp. Don't know why this is missing at the schematic I posted at the beginning.
When the pcb pics were put up a number of people had a stab at tracing it. There seems to be three schematics on the web but only the jerms schematic is correct.
QuoteNow I connected the 100n input cap to a 470 ohm resistor and then to gnd.
Yep, you got it. Just like the output circuit on the output.
QuoteOscillation is gone! It stays unhearable until about 100R, then comes in gradually (I'm not stacked with every value). There might be a small effect on the frequency response, but really nothing I could define exactly. I didn't change the load, but with the 10k in the real original circuit it should always be 10k minimum.
Great work and a big thanks for your effort on this one.
That's good news. Finally something that makes sense since adding that resistor is the right thing to do from a stability perspective.
I look at that circuit now and think how did they get to production without finding oscillation problems. Also your breadboard didn't oscillate. The circuit must be hanging on by a thread.
QuoteWhile we're at it. When my guitars volume is down completely, there's a lot going on for an undefinable time. Crackles and that kind of stuff. Not super loud, but you can hear it. That's on both, the breadboard and the boxed one.
The way the circuit is at the moment the input terminals are DC coupled to the input socket, that can cause some crackles. So the way to fix that is to add a cap between current circuit input (the point where pin 2, the cap and the pot meet) and the input socket. As to the value you might need a low value like 1uF or 2uF, listen for any bass cut. It would also be wise to add say 100ohm to 220 ohm in series with that cap as well (to fend off obscure oscillation issues involving the input cable.)
I actually also own an original of this. Bought in a store when they were current. As I was in high school at the time, it is most likely that it was selected because it was cheap as hell (as compared to, say, a Big Muff), so it's likely it was a pedal primarily "designed" to get out the door quickly. I couldn't have afforded anything decent at the time, so I wouldn't expect much more.
I made a version off the commonly accepted schematic a few years back (for reasons I'll get to momentarily). It also behaved differently whenever it was in the mood. I had to keep pulling it out of the box to change slightly to keep it from screaming bloody murder whenever it felt like it. My recollection is that I redid the power supply to be modern-friendly and then ran a small cap across that feedback loop to keep the screams down. I didn't want to tame it any further because I figured that was just part of its "charm." Works great for that very non-muffy extreme sort of ragged fuzz that is mostly uncontrollable. That can be a good thing at times, but I think if you try to tame it too far, you are just making a new pedal, not "fixing" the Graphic Fuzz.
The reason I made my own version is because I tried my original one day and it simply didn't want to make a sound. I could find nothing actually wrong. It just made no sound. (using my 18watt clone). At some point I plugged it into a modern SS amp and it worked. ??? (I think that was the sequence) after some further investigation, I found that there was significant DC on the output and for some reason that amp didn't like that. Probably biasing it badly in some way. The SS amp seemed to filter that out. Input cap or something. My re-made version did not have that problem. (I'm sort of surprised nobody in various modern threads about this pedal has ever encountered that issue.).
Anyway, I am just posting this here for the record, FWIW, and to voice the opinion that as maddening as this pedal can be, I think that's where we should leave it. If you want a BETTER pedal using a 741 to make fuzz, just make one. Some things should just be left in their wild state.
> significant DC on the output and for some reason that amp didn't like that.
Most tube amp input stages are normally biased AT ground, and assuming no DC from the source, they can omit input caps. Many transistor amp inputs are biased off-ground, thus "must" have an input cap, which incidentally protects from most external DC abuse also. (Many exceptions in both camps.)
Thanks for the background.
QuoteI made a version off the commonly accepted schematic a few years back (for reasons I'll get to momentarily). It also behaved differently whenever it was in the mood. I had to keep pulling it out of the box to change slightly to keep it from screaming bloody murder whenever it felt like it. My recollection is that I redid the power supply to be modern-friendly and then ran a small cap across that feedback loop to keep the screams down.
I'm quite confident the jerms circuit I posted in reply #35 is correct. However, I'm also quite confident the design has some inherent stability issues.
QuoteI didn't want to tame it any further because I figured that was just part of its "charm." Works great for that very non-muffy extreme sort of ragged fuzz that is mostly uncontrollable. That can be a good thing at times, but I think if you try to tame it too far, you are just making a new pedal, not "fixing" the Graphic Fuzz.
Very much the case. It's easy to fix it by changing it to something conventional. The trick is to fix it without changing the character. Adding the resistor in series with the cap is the correct fix from technical perspective and it shouldn't significantly change the character of the pedal.
This thread was a tricky thread.
I updated that schematic btw. I forgot the 10K on the output.
(https://i.postimg.cc/Hcgy7k0L/Fuzz.jpg) (https://postimg.cc/Hcgy7k0L)
When I worked there, they did oscillate a lot. Tony (the other tech in customer repair) made a slight change to the circuit. I can't remember what that was. Maybe increasing the two 0.1 caps in the power supply to 1uf (although I doubt they would increase cost with a cap change) or increase the two 10K that split the power supply, I just don't know for sure.
If I could remember his last name (Italian) maybe I'll find him. Wow, 43 years ago.
I had one, gutted out the box and made a keyboard sustain pedal with it or something. Stuff long gone.
QuoteWhen I worked there, they did oscillate a lot.
Interesting to know.