ESR Graphic Fuzz Power Supply

[pokus]

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?



Thanks for answers!

[iainpunk]

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?

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

[Rob Strand]

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.)

[antonis]

741 DC offset with a lot of gain could result into crackling output pot, couldn't it.??

[Rob Strand]

741 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.

[pokus]

Thanks guys! That's basically what I assumed.

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.

741 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.

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

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.

[antonis]

Take into account what Rob said about pot min setting and place a 470R - 1k resistor between lug 1 and GND.. 

[pokus]

Take into account what Rob said about pot min setting and place a 470R - 1k resistor between lug 1 and GND.. 

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?

[antonis]

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..)

[pokus]

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.

[antonis]

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.. 

[pokus]

I'm afraid, no.. 

Damn 

(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.

[Rob Strand]

. 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.

[pokus]

- 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.

[antonis]

You maybe have to take into account different impedances seen by op-amp output..

[Rob Strand]

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.

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?

[antonis]

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.. 

[Rob Strand]

BTW, 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.

[antonis]

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.. 

[pokus]

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.