ESR Graphic Fuzz  [schematic]

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Re: ESR graphic fuzz traced & layout

Postby rocket88 » 20 Apr 2013, 22:48

just built this like peeps vero to a T, but i get low output. i cant figure out why, anyone have any idea why, and how to fix it?
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Re: ESR graphic fuzz traced & layout

Postby Intripped » 20 Apr 2013, 23:02

...and this is my guess on Jimmy's vero
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Re: ESR graphic fuzz traced & layout

Postby Intripped » 21 Apr 2013, 15:39

^ doesn't work
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Re: ESR graphic fuzz traced & layout

Postby rocket88 » 21 Apr 2013, 17:23

No one with experience has any idea what I should do or look for? I'm going nuts cause it sounds perfect, just have have my volume drop by about 1/2.
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Re: ESR graphic fuzz traced & layout

Postby deltafred » 21 Apr 2013, 19:33

rocket88 wrote:No one with experience has any idea what I should do or look for? ....

ok I will have a shot at this.

I see there are 2 quite different schematics shown, which one is nearer the truth?

My guess is that if you are not getting much audible output it is probably oscillating out of the audio range. A scope would tell a lot here.

About schematic 1
If you look at the formulae for the gain of an inverting amplifier it is the input resistor divided by the feedback resistor. Well there is no input resistor so that is a variable dependent upon what impedance of the driving device is (guitar output resistance combined with tone control impedance and pickup impedance).

The lower the input resistor and the higher the feedback resistor (feedback pot position) the higher the gain, so you could try adding a resistor in series with the input.

The resistor and capacitor on the output act as a zobel network.

About schematic 2
This is configured more of an integrator due to the capacitor/s being in the feedback loop, but still lacks any input resistance so will no doubt have unpredictable behaviour.


The thing is once you start operating outside the design envelope of a device then you are in virtually uncharted territory.

The open loop gain of the opamp will no doubt play a big part in how it behaves.
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Re: ESR graphic fuzz traced & layout

Postby deltafred » 21 Apr 2013, 19:38

deltafred wrote: the formulae for the gain of an inverting amplifier it is the input resistor divided by the feedback resistor.

Wrong, feedback resistor / input resistor. :oops:

(The edit button has disappeared again.)
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Re: ESR graphic fuzz traced & layout

Postby rocket88 » 21 Apr 2013, 19:57

ok I will have a shot at this.

I see there are 2 quite different schematics shown, which one is nearer the truth?

My guess is that if you are not getting much audible output it is probably oscillating out of the audio range. A scope would tell a lot here.

About schematic 1
If you look at the formulae for the gain of an inverting amplifier it is the input resistor divided by the feedback resistor. Well there is no input resistor so that is a variable dependent upon what impedance of the driving device is (guitar output resistance combined with tone control impedance and pickup impedance).

The lower the input resistor and the higher the feedback resistor (feedback pot position) the higher the gain, so you could try adding a resistor in series with the input.

The resistor and capacitor on the output act as a zobel network.

About schematic 2
This is configured more of an integrator due to the capacitor/s being in the feedback loop, but still lacks any input resistance so will no doubt have unpredictable behaviour.


The thing is once you start operating outside the design envelope of a device then you are in virtually uncharted territory.

The open loop gain of the opamp will no doubt play a big part in how it behaves.


thanks delta, i'll look into it. i used the verolayout posted by peeps. i'm wondering if there is a problem with the pots. if i increase the volume pot from 10k to something like 100k would that allow more output? i know this is a very odd circuit, so i'm really lost. the sound is perfect, and spot on. if i lower my volume on the guitar, the feedback almost quacks, when the volume on the guitar is high it sounds like it total earth engulfing fuzz, but the volume is like 75-80% of the clean volume. it makes no sense to me, and no one who has built it has said if this is normal.
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Re: ESR graphic fuzz traced & layout

Postby Nocentelli » 21 Apr 2013, 20:10

Is there anything infront of the circuit or is it direct guitar->input? Also, a different value volume pot is very unlikely to change the output level.
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Re: ESR graphic fuzz traced & layout

Postby rocket88 » 21 Apr 2013, 20:35

Nocentelli wrote:Is there anything infront of the circuit or is it direct guitar->input? Also, a different value volume pot is very unlikely to change the output level.


nope, just guitar->ESR->amp.
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Re: ESR graphic fuzz traced & layout

Postby deltafred » 21 Apr 2013, 20:56

First have you checked the battery voltage.

Increasing the value of the volume pot will increase the output level, dependent upon amp input impedance.

The 741 will swing to within about 2 volts of the supply rails so with a good battery you should get between 4v to 5v output swing. A 10k pot in series with a 10k resistor drops this to 2 - 2.5v, still plenty of (peak to peak) signal. Increasing the pot value (and feeding into a high input impedance) you should be approaching 4 to 5 volts.

Does turning the frequency pot clockwise make it oscillate as on the youtube vid?

I am still inclined to think that it is oscillating at a frequency only audible to dogs and bats, this will limit the audible output. I would try a 10k variable resistor in series with the input and see if that does anything.

I listened to the youtube linked earlier, there are some good sounds on there (and some weird ones that I would not want to use) I am half tempted to breadboard it.
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Re: ESR graphic fuzz traced & layout

Postby rocket88 » 22 Apr 2013, 07:11

deltafred wrote:First have you checked the battery voltage.

Increasing the value of the volume pot will increase the output level, dependent upon amp input impedance.

The 741 will swing to within about 2 volts of the supply rails so with a good battery you should get between 4v to 5v output swing. A 10k pot in series with a 10k resistor drops this to 2 - 2.5v, still plenty of (peak to peak) signal. Increasing the pot value (and feeding into a high input impedance) you should be approaching 4 to 5 volts.

Does turning the frequency pot clockwise make it oscillate as on the youtube vid?

I am still inclined to think that it is oscillating at a frequency only audible to dogs and bats, this will limit the audible output. I would try a 10k variable resistor in series with the input and see if that does anything.

I listened to the youtube linked earlier, there are some good sounds on there (and some weird ones that I would not want to use) I am half tempted to breadboard it.


yep, does everything in the video perfectly, just like i said, low volume output. i'll check the battery and see if a brand new battery will make a difference. it oscillates perfectly, and cuts out when turned fully to the right. how should i check the output swing and input swing to be sure the current is flowing correctly.
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Re: ESR graphic fuzz traced & layout

Postby deltafred » 22 Apr 2013, 09:05

rocket88 wrote:yep, does everything in the video perfectly, just like i said, low volume output. i'll check the battery and see if a brand new battery will make a difference. it oscillates perfectly, and cuts out when turned fully to the right. how should i check the output swing and input swing to be sure the current is flowing correctly.

Try a 100k volume pot, that should almost double the output voltage. If that is not enough then an output buffer with gain would be the answer.

To check the output swing you really need a scope.

The input voltage is whatever your guitar puts out.
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Re: ESR graphic fuzz traced & layout

Postby Intripped » 22 Apr 2013, 16:54

just to clarify about the 2nd schematic and the veroboard layout at the top of this page:

they are my try to R.E. Jimmy's modified circuit
this modified circuit is interesting mostly because it allows you to share a common power supply (daisy chain) with other pedals

anyway
both the (2nd) schem and the veroboard layout - which are consistent to each other - are wrong: i have this circuit on breadboard and it doesn't work.
so the questions still stand: how exactly does Jimmy's mod work? where is the error in my hypothetical schem/layout?
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Re: ESR graphic fuzz traced & layout

Postby rocket88 » 25 Apr 2013, 05:17

deltafred wrote:
rocket88 wrote:yep, does everything in the video perfectly, just like i said, low volume output. i'll check the battery and see if a brand new battery will make a difference. it oscillates perfectly, and cuts out when turned fully to the right. how should i check the output swing and input swing to be sure the current is flowing correctly.

Try a 100k volume pot, that should almost double the output voltage. If that is not enough then an output buffer with gain would be the answer.

To check the output swing you really need a scope.

The input voltage is whatever your guitar puts out.


Thanks man. I changed the pot to a 100k log and the volume is what it should be. It sounds unreal, and really love this pedal. Btw, I'm thinking i should add an LED, but not sure where to connect it. I tried to this wiring, but the LED wont turn on if attached to +9V, -9V, or ground. Any ideas? Its an ultra bright 5mm blue LED.
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Re: ESR graphic fuzz traced & layout

Postby induction » 07 May 2013, 20:03

Forumite insonicbloom has a cool electronics blog with schems and veros for all manner of interesting things, including a floating power supply using a 555 chip. It seems like this would be a simple way to make this effect daisy chainable. Granted, it will more than double the parts count, but that's not saying much with this circuit.

Maybe it's not as clever as whatever Jimmy Behan did, but it should do the trick, I think. If I'm wrong or have overlooked something, please set me straight.
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Re: ESR graphic fuzz traced & layout

Postby agoldoor » 07 May 2013, 22:31

I'm pretty sure if you emailed Jimmy he'd tell you what he did. He is very generous with his knowledge.
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Re: ESR graphic fuzz traced & layout

Postby induction » 08 May 2013, 21:37

I finally bothered to look up 'comparator' on Wikipedia, and now I understand how this circuit works, more or less. It's surprisingly simple, so I'll offer my 2 cent analysis. The open loop gain on an op-amp is theoretically infinite, so if the voltage on pin 2 goes above the voltage on pin 3, the output saturates at the lowest voltage the chip can output. When pin 2 goes below pin 3, the output saturates at the highest voltage the chip can output. Voila: square waves.

To work correctly, the DC voltage on pins 2 and 3 must be the same. The original schematic accomplishes this by tying both pins to ground. Pin 3 connects to both ground and the 1/2-rail voltage via a divider on the battery (which implies that the battery gives +/-4.5V, so adapters won't work unless the adapter is isolated and doesn't share ground with anything else in the chain). Pin 2 is held at ground by DC coupling the input. In principle, an input cap would not change the operation of the circuit, but a pulldown resistor might be necessary on pin 2 (I'm not sure).

But there are other ways to skin this cat. Pins 2 and 3 have to have the same RMS voltage, but it doesn't have to be ground. So we can tie the negative battery terminal and pin 4 to ground (or use a normal adapter instead, daisy-chained, even), and break the ground connection to pin 3 (which means that pin 3 is now at 1/2-rail voltage). Now we add an input cap and supply a 1/2-rail DC offset to pin 2 via a second voltage divider (or a maybe a 2 Meg resistor to the existing divider). Then you'll just need an output cap, since the RMS value of the op-amp output won't be 0V anymore.

ESR_Graphic_Fuzz_schem_my_mods.png


This is simpler than the floating power supply I suggested above (only 4 extra components), but seems like it should work just as well. I haven't breadboarded it yet, though, so a grain of salt or two might be in order. I have no idea if this is the approach Jimmy Behan used or not, but it should allow for standard power supplies and input jack switching.

Or, sure, you could ask Jimmy what he did. His solution might be better.
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Re: ESR graphic fuzz traced & layout

Postby Nocentelli » 11 May 2013, 00:44

induction wrote:But there are other ways to skin this cat.


This works fine on the breadboard, sounds very much like the +/-4.5v version. The oscillation seems maybe higher, but i can't swear i'm using the same 741 (if that would make a difference). To my untrained eye, it looks very much like the version posted by PRR in the DIYSB thread (http://www.diystompboxes.com/smfforum/i ... #msg903540) but with 1uF in/out caps and the 2M2 between pin 2 and 3. Removing this resistor makes no noticeable difference.
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Re: ESR graphic fuzz traced & layout

Postby Goodrat » 12 May 2013, 00:50

Hey guys, I worked at ESR as one of the two technicians there in the customer service repair. It was back in 1978 and my first electronics job. Sometimes we would just play guitar all day.
What I remember about the Graphic Fuzz box is they oscillated like crazy. The other Tech, Tony, made some improvements.
As far as the schematic, I remember there were two 10K resistors as a divider with the center going to ground to make the +/- supply and whatever caps on that.
The IC was a 741.
A lot of amp designs were ripped of from Peavey and they got in trouble eventually for that..

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Re: ESR graphic fuzz traced & layout

Postby induction » 14 May 2013, 16:53

FWIW, the reversed Level control at the output has a drastic effect on the oscillation/cocked wah frequency. It's very interactive with the Frequency pot, and in fact doesn't really function much like a volume control, except at its lowest settings. It's actually easier to control the resonant frequency with the Level control than with the Frequency control, which is more useful for adjusting the distortion characteristics. My approach: use the Frequency control to dial in the right amount of existential despair, and use the Level control to fine tune the oscillation/cocked wah sound.

If I was going to label the knobs on this thing, the knob now called Level would be called "Frequency", and the knob now called Frequency would be called "Travis Bickle".

If you want a Volume control that actually, you know, controls the volume, just add a traditional volume control after the Level control.
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