freestompboxes.org

Any idea how Jimmy Behan (http://www.jimmybehanfx.net/custom/eart ... aphic-fuzz) has got this to work on a standard shared DC adaptor power supply? I looks to me (a total amateur with virtually no knowledge of electronic theory, and very little experience of using opamps) like the original graphic fuzz circuit has a voltage divider (10k/10k) that produces a dual supply of positive and negative 4.5volts from the battery relative to "vref" which is tied to the sleeve ground.

Could one change it to a standard divider with battery/power supply negative tied to ground, 10k/10k to battery/PS positive, and feed the 4.5v centre vref to the original ground points? I have an idea that the opamp's inverting input and output would not need a cap in series with the vref, but the non-inverting input would now need a cap. I also suspect that one would need to add input and output caps and maybe keep the volume pot connection to ground, but I hasten to repeat I have very little understanding of how this circuit works so please correct my misconceptions if you can.

Correction - I think you would need a cap from the output 10k to the new verf, as this part looks to be how the filter pot works as a high pass filter that goes all the way up to total cut-off

^ That seems like a way to do it. I think all the info about Jimmy's is there at DIystomp.
(Well at least very close to what he did at least)

Thanks John, but I've read that thread a number of times, and didn't pick up any info about how to wire it for standard power, just the references to the resistor + cap (680k + 47p) in parallel with the filter pot to delay the onset of the oscillation. I'll go back and read through it again.

Most of the talk is about the filter knob but check Paul's (PRR)
first post which is very close to your scheme, and my question as the initial post.

i've studied a bit the pic of Jimmy's graphic fuzz
the only thing i cannot understand is the jumper indicated by the arrow: it connects pin2 of the IC to....??? the only components in the row (yes, seems to me a veroboard) are the vertical 220 Ohm(?) resistor, and the 100n(?) cap connected to pin6.

if so, the schematic could be like the one attached

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?

...and this is my guess on Jimmy's vero

^ doesn't work

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.

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.

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.

(The edit button has disappeared again.)

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.

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.

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.

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.

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.

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.

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?

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.