Harmonic jerkulator - positive feedback loop?

[Squiggles333]

Hi everyone, first post. Quite new to the DIY pedal community. I've been working my way through a list of simple builds on the bread board trying to learn as I go. Threw together Escobedo's harmonic jerkulator and am blown away with how good the gritty sound is. I started adding things in, just to experiment and see what happened. I came across something bizarre and wonderful that I'm hoping you clever people could help me with.

I think I've created a positive feedback loop (I'm not sure if that's correct). In the original schem there's a 22uf cap going from Emitters of Q1 and 2 to ground. I added a 100k pot in front of the cap and when I turned it up some interesting things happened.

There's an intense squeel or feedback that changes pitch depending on the resistance dialed in. It's not your usual background hiss or breadboard interference, it's a loud sinewave. This plays all the time when at rest and makes the mod somewhat unusable.

When I start to play, depending on the settings, the squeel transforms into one of various amazing modulation type effects alongside my guitar signal, or in place of it entirely.

By rolling the tone controls all the way down on my guitar I got a variety of vastly different textures and effects that also changed depending on the pickup config (strat with a 5 way).

Lowering the volume on the guitar or the input volume to the circuit also varied the effects. The effect seems to be hyper sensitive to the input impedance.

Majority of the different effects being created were synth like, their sounds and textures responding well to my dynamics and position on the neck. At certain points of tweaking I was able to get strong octaves and even some ring modulation. Various of the dialed in settings still had a clean guitar signal in with them too. These sounds aren't subtle, they're like something out of a purple pedal with space or robot in the name.

I have no idea what is going on electrically other than a variable filter between the cap and the vr. I know there's something particular about the transistor pairing that may be contributing to the wild effect. I'm hoping to understand what's creating these effects (with my limited understanding thus far) and if possible I'm seeking to tame this so it can be a usable mod to my build. It would be ideal if the constant high pitch squeel could be eliminated whilst not playing, but with the warped modulation effects still existing as I play.

Does anyone have a scoobie what's going on here?

[antonis]

Hi & Welcome..

Could you post a schematic of the pot placement..??
(i mean, is series as a variable resistor, pot's body between transistors, pot lug 3 to Q1/Q2 junction and wiper to cap, etc..??)

[iainpunk]

yeah, i have noticed the same thing 2-ish years ago when bread boarding and tinkering around with a harmonic percolator (i probably did the same thing, but with the pot going in between the capacitor and ground, not that that changes anything)
i ended up eliminating the squeal sound with a gate circuit, but that was more complex than the pedal is to begin with, and i abandoned that idea. but to answer the question, yes, its a positive feedback loop!

cheers, Iain

[antonis]

So, you guys have turned a cascode amp (CE+CB) into a squealing beast..
Exactly what particular configuration is supposed to prevent..

[iainpunk]

its not a cascode, its two distinct common emitter gain stages where the interaction is negated by the capacitor in the middle. a cascode is a common emitter boosted by a common base gain stage, with as much interaction as possible to negate oscillations.

cheers, Iain

[antonis]

Quite right, Iain..
(upper BJT Base cap isn't grounded - my bad.. )

[Squiggles333]

Hi & Welcome..

Could you post a schematic of the pot placement..??
(i mean, is series as a variable resistor, pot's body between transistors, pot lug 3 to Q1/Q2 junction and wiper to cap, etc..??)

Hey, thanks for the welcome! Here's a schem. The emitters of Q1 and 2 connect to ground via lug 1 in of a VR and lug 2 out to the 22uf cap which is connected to ground.

[Squiggles333]

yeah, i have noticed the same thing 2-ish years ago when bread boarding and tinkering around with a harmonic percolator (i probably did the same thing, but with the pot going in between the capacitor and ground, not that that changes anything)
i ended up eliminating the squeal sound with a gate circuit, but that was more complex than the pedal is to begin with, and i abandoned that idea. but to answer the question, yes, its a positive feedback loop!

cheers, Iain

I hadn't thought aout a noise gate! A separate build maybe. I've been trying all sorts of random things with it but can't seem to diminish the power of the squeel at all.

[antonis]

As Iain told you, it could be quite difficult to get rid of squealing..
You can try 100 - 150 pF Miller caps across R2 & R4 with doubtful result..
A more "elegant" solution should be signal inversion between Q1 Emitter and VR3/Q2 Emitter..
(via a phase shifter BJT CE amp of gain 1X , perhaps..)

[iainpunk]

i didn't use a normal noise gate, but a transistor that acts like a switch parallel to the potentiometer,
in the mean time in found the drawings, ill redo them in Falstad because they were drawn really sloppy by a drunk hand.
i remember it working, but also changing the sound of the oscillations a bit...

i also changed a lot of the harmonic percolator to suit my taste, and used the Bazz Fuss biasing technique.

As Iain told you, it could be quite difficult to get rid of squealing..
You can try 100 - 150 pF Miller caps across R2 & R4 with doubtful result..
A more "elegant" solution should be signal inversion between Q1 Emitter and VR3/Q2 Emitter..
(via a phase shifter BJT CE amp of gain 1X , perhaps..)

but he wants oscillations, but only while he's playing, not when he's quiet!
that's where my gate-ish schematic comes in to play!

ill update and add the schematic of the thing redrawn later this evening

cheers, Iain

[iainpunk]

this is what i drew, the values of components are not 100% correct, since i designed the gate thing by interpretation, not by hard rules, the falstad simulation implies that it works tho.

the fuzz part is changed because wanted to change it for the sake of change!

cheers, Iain

[Squiggles333]

As Iain told you, it could be quite difficult to get rid of squealing..
You can try 100 - 150 pF Miller caps across R2 & R4 with doubtful result..
A more "elegant" solution should be signal inversion between Q1 Emitter and VR3/Q2 Emitter..
(via a phase shifter BJT CE amp of gain 1X , perhaps..)

Thanks for the suggestions. I'll try the Miller caps tonight as I understand what this is at least, need to do a lot of homework to understand your second suggestion. Someone on another site mentioned my transistors might be working as some kind of switch and that got me thinking a out what else I could maybe turn on by means of a transistor switch. This rabbit hole never ends its great 

[Squiggles333]

this is what i drew, the values of components are not 100% correct, since i designed the gate thing by interpretation, not by hard rules, the falstad simulation implies that it works tho.

the fuzz part is changed because wanted to change it for the sake of change!

cheers, Iain

Wow this is awesome! Thank you so much for taking the time to share this. I don't understand or recognise the right hand side of the schematic but the left is familiar enough. I'll need to try this on the breadboard. What transistors is it using?

Also I'm wondering, "the bazz fuss biasing technique" is that using the diodes to bias c to b? Still trying to get my head around biasing.

This happy accident has taken this thing to a whole other place, I think I might finish the jerkulator build I was doing without it and start a new build focused around these crazy oscillations, I'll try your gate section then too.

Thank you so much for your help Iain.

[antonis]

this is what i drew, the values of components are not 100% correct,

1k input impedance of the gate thing..??

[antonis]

need to do a lot of homework to understand your second suggestion.

Nothing special..
(something like below):



The point is to inverse the signal from Q2 Emitter to Q1 one hence turn positive feedback to negative one..
In your original circuit, incoming signal is inversed (180^o out of phase) on Q1 Collector and stays inverted out of Q2 Emitter so it enters inverted into Q1 Emitter - it's positive feedback..

P.S.
I can't ensure you it will work - it's just a rough guess..

[iainpunk]

fist of all 2 problems/oversights with my drawing, sorry for that, i had a long day yesterday
1) my bread board at the time had a build in input and output buffer, so a low impedance was no problem.
2) the connections going to the fuzz and gate circuits should switch place, the fuzz directly to the input and the gate after the first 1k resistor

Wow this is awesome! Thank you so much for taking the time to share this. I don't understand or recognise the right hand side of the schematic but the left is familiar enough. I'll need to try this on the breadboard. What transistors is it using?

its a system that turns off the transistor when the signal is above a certain treshold, and on when its under, basically turning to a near-short circuit and deadening the oscillations
i used BC547 and BC557 transistors, but the specific ones are not critical, the diodes are tho, mix and match them to work and gate at the right point, the right most diode was probably an red LED, the others were probably 1N4148's

Also I'm wondering, "the bazz fuss biasing technique" is that using the diodes to bias c to b? Still trying to get my head around biasing.

yes its the diodes to bias from C to B, it gives quite high gain and a bit of a harsher sound.

This happy accident has taken this thing to a whole other place, I think I might finish the jerkulator build I was doing without it and start a new build focused around these crazy oscillations, I'll try your gate section then too.

i hope you get it to work! the values in the schematic were not the same as i used on my breadboard, but close

Thank you so much for your help Iain.

no problem, i like helping out.

1k input impedance of the gate thing..??

in the mean time in found the drawings, ill redo them in Falstad because they were drawn really sloppy by a drunk hand.

i was drunk, i guess its to lower the signal going in to the gate thing so it triggers at the right volume level, also, my bread board at the time had a build in input and output buffer, so low impedance wasn't a problem.

cheers, Iain

PS, i think you are better off just building a noise gate with effects loop since its a cool and useful utility to have in general!

[iainpunk]

I can't ensure you it will work - it's just a rough guess..

i can ensure you it won't
where would the DC go from both transistors??

[antonis]

So let's delete 2 X 100nF caps and 680k resistor..

P.S.
The idea was to show OP implementation of phase inverter..

[pinkjimiphoton]

Hi everyone, first post. Quite new to the DIY pedal community. I've been working my way through a list of simple builds on the bread board trying to learn as I go. Threw together Escobedo's harmonic jerkulator and am blown

away with how good the gritty sound is. I started adding things in, just to experiment and see what happened. I came across something bizarre and wonderful that I'm hoping you clever people could help me with.

I think I've created a positive feedback loop (I'm not sure if that's correct). In the original schem there's a 22uf cap going from Emitters of Q1 and 2 to ground. I added a 100k pot in front of the cap and when I turned it up some interesting things happened.

There's an intense squeel or feedback that changes pitch depending on the resistance dialed in. It's not your usual background hiss or breadboard interference, it's a loud sinewave. This plays all the time when at rest and makes the mod somewhat unusable.

When I start to play, depending on the settings, the squeel transforms into one of various amazing modulation type effects alongside my guitar signal, or in place of it entirely.

By rolling the tone controls all the way down on my guitar I got a variety of vastly different textures and effects that also changed depending on the pickup config (strat with a 5 way).

Lowering the volume on the guitar or the input volume to the circuit also varied the effects. The effect seems to be hyper sensitive to the input impedance.

Majority of the different effects being created were synth like, their sounds and textures responding well to my dynamics and position on the neck. At certain points of tweaking I was able to get strong octaves and even some ring modulation. Various of the dialed in settings still had a clean guitar signal in with them too. These sounds aren't subtle, they're like something out of a purple pedal with space or robot in the name.

I have no idea what is going on electrically other than a variable filter between the cap and the vr. I know there's something particular about the transistor pairing that may be contributing to the wild effect. I'm hoping to understand what's creating these effects (with my limited understanding thus far) and if possible I'm seeking to tame this so it can be a usable mod to my build. It would be ideal if the constant high pitch squeel could be eliminated whilst not playing, but with the warped modulation effects still existing as I play.

Does anyone have a scoobie what's going on here?

you gotta feedback loop happening, and its causing ring modulation. sounds fun

[Squiggles333]

need to do a lot of homework to understand your second suggestion.

Nothing special..
(something like below):



The point is to inverse the signal from Q2 Emitter to Q1 one hence turn positive feedback to negative one..
In your original circuit, incoming signal is inversed (180^o out of phase) on Q1 Collector and stays inverted out of Q2 Emitter so it enters inverted into Q1 Emitter - it's positive feedback..

P.S.
I can't ensure you it will work - it's just a rough guess..

No guarantee needed thanks very much. So is the idea of this to have the feedback in phase with the original singal? What does this mean for the sound? I still need to read more about signal phasing!