I need help fixing impedance and tone issues with a harmonic jerkulator

[Squiggles333]

Hello wise wizards. I'm on my 4th iteration of Tim Escobedo's harmonic jerkulator and I'm having some frustrating issues after finding various sounds / set ups that I like. I belive my tonal issues are impedance related. I think I understand impedance and the effect it plays on AC signal.

However, I'm struggling to understand how to calculate the impedance at different stages and understand how it is affecting my tone. I would like to perfect every element of this circuits design so that I can add it to my board and use it at every setting. It sounds amazing whilst the output volume is set to max, or vr3=0k resistance, but this volume is so much above the bypass that I have to reduce the volume via vr3. This also causes the tone to become muddy and splatty. I believe it would also benefit from some kind of low pass or band pass filter at the end to remove high end mess.

The "harmonics" (pre gain) vr1 and "body" (bazz fuss tone control) vr2 work fine whilst the output vr3 is at max volume, or minimum resistance. They further exaggerate the impedance and splatter problem when vr3 is increased however.

In and ideal scenario I would also add a vr before the output blending how much of the signal goes to a pair of hard clipping diodes. I managed this with a 500k pot and 2 1n34a diodes to ground but the impedance related issues of tone and splatter where greatly exasperated. In an attempt to simplify the circuit and remove possibility for complications I omitted this section, but would like to keep it if it's possible to simply fix the impedance issues.

There are also some strange tremolo like pulses on the tails of some chords or notes, I can also sometimes hear radio frequencies through it (although it is still exposed on the breadboard) I tried adding a smoothing cap to Q1 and Q2, it helped with Q1 but worsened the radio problem when added to Q2.

I have a multimeter but am.nof experienced with using it yet.

I would greatly appreciate any guidance with getting this circuit pedal board ready!

[iainpunk]

if you want to fix impedance issues, buffers are your friend, this buffer has an output impedance of about 3.4k, but you can take it lower with lower value emitter resistors, but it draws more current.



cheers, Iain

[Squiggles333]

if you want to fix impedance issues, buffers are your friend, this buffer has an output impedance of about 3.4k, but you can take it lower with lower value emitter resistors, but it draws more current.



cheers, Iain

Thanks for the reply Iain, would I need a buffer at the input or the output of my circuit then? I don't really understand how a buffer helps or what even the problem with my impedance is

[iainpunk]

you had problems when the output volume is turned down, suggesting that the impedance problem is on the output, hence the drawing of an extra transistor buffer onto the 2nd gain stage, where the output is.

what a buffer does best is uncoupling impedance's, making sure that a given network of impedance doesn't affect another preceding network (if they are loading each other, this is called coupled loading). in this case, it makes sure that whatever comes after the volume control has no effect on the 2nd gain stage.

There are also some strange tremolo like pulses on the tails of some chords or notes,

this is normal, especially with a gated fuzz, but all fuzzes have it, some more extreme than others.

I can also sometimes hear radio frequencies through it (although it is still exposed on the breadboard) I tried adding a smoothing cap to Q1 and Q2, it helped with Q1 but worsened the radio problem when added to Q2.

radio problems can just be problems with the guitar, i have a bass that if the rear and middle pickup are selected and the tone is on 0, it pick up an Israeli radio station. exposed breadboard can be extremely sensitive to radio as well, i found that having stuck it on a metal base with double sided foam tape, decreases the problem a lot, foam tape is thicker and decreases capacitance between the breadboard and the base, i used a cookie tin, and drilled holes for jacks, pots and switches to be attached. remember to ground the metal!

at this point, it would be helpful for us to see some voltages,
attach the COM lead to ground and use the other lead to measure the voltages denoted in this diagram:


i wish you success

cheers, Iain

[antonis]

this buffer has an output impedance of about 3.4k, but you can take it lower with lower value emitter resistors

This buffer has an output impedance of about 6k8 // [ r_e + Rs/(h_FE + 1) ] , where r_e = 0.025/Collector current, Rs = CE amp output impedance.. (22k, ignoring 100k bias resistor and Collector output resistance..)

About 160R, roughly estimated..

[iainpunk]

this buffer has an output impedance of about 3.4k, but you can take it lower with lower value emitter resistors

This buffer has an output impedance of about 6k8 // [ r_e + Rs/(h_FE + 1) ] , where r_e = 0.025/Collector current, Rs = CE amp output impedance.. (22k, ignoring 100k bias resistor and Collector output resistance..)

About 160R, roughly estimated..

that's the impedance when sourcing current, when the stage is sinking current, the impedance is 6.8k, this averages out to 3.5k .
cheers, Iain

[duck_arse]

There are also some strange tremolo like pulses on the tails of some chords or notes,

this is normal, especially with a gated fuzz, but all fuzzes have it, some more extreme than others.

not sure I'd call it normal. I see you have only 22uF cap on emitters to ground. if you aren't using a tantalum cap here, either try one, or up that value to at least 47uF, see if that doesn't reduce the tremolo. that sound might be some of the famed octave down, but unformed.

edit - t for least.

[iainpunk]

i don't know if it would be octave down, but internal oscillations could be the source, i know that if the cap in between the emitters is to small, it's prone to oscillations, but they are generally at high frequencies, above 300Hz in my experience.
my best guess is that its somewhat gated/mis-biased that gives odd oscillations and tremolo as well when the note dies out, just like a mis-biased fuzz face, the trails can go sputerey and switch on/off when the guitar's output volume gets lower towards the end of the sustain.

cheers, Iain

[Squiggles333]

you had problems when the output volume is turned down, suggesting that the impedance problem is on the output, hence the drawing of an extra transistor buffer onto the 2nd gain stage, where the output is.

what a buffer does best is uncoupling impedance's, making sure that a given network of impedance doesn't affect another preceding network (if they are loading each other, this is called coupled loading). in this case, it makes sure that whatever comes after the volume control has no effect on the 2nd gain stage.

There are also some strange tremolo like pulses on the tails of some chords or notes,

this is normal, especially with a gated fuzz, but all fuzzes have it, some more extreme than others.

I can also sometimes hear radio frequencies through it (although it is still exposed on the breadboard) I tried adding a smoothing cap to Q1 and Q2, it helped with Q1 but worsened the radio problem when added to Q2.

radio problems can just be problems with the guitar, i have a bass that if the rear and middle pickup are selected and the tone is on 0, it pick up an Israeli radio station. exposed breadboard can be extremely sensitive to radio as well, i found that having stuck it on a metal base with double sided foam tape, decreases the problem a lot, foam tape is thicker and decreases capacitance between the breadboard and the base, i used a cookie tin, and drilled holes for jacks, pots and switches to be attached. remember to ground the metal!

at this point, it would be helpful for us to see some voltages,
attach the COM lead to ground and use the other lead to measure the voltages denoted in this diagram:


i wish you success

cheers, Iain

Thank you kindly sir. I didn't catch at first glance that your schem was the output of my circuit with the buffer added in. Your explanation of loading and decoupling impedance is really helpful thank you.

I'll need to try your suggestion for the breadboard noise too. A biscuit tin full of wires sounds much more tidy than occupying half of the dinner table, my partner thanks you.

When I can later I'll get the mm and take some voltages, thank you for the guidance.

[Squiggles333]

There are also some strange tremolo like pulses on the tails of some chords or notes,

this is normal, especially with a gated fuzz, but all fuzzes have it, some more extreme than others.

not sure I'd call it normal. I see you have only 22uF cap on emitters to ground. if you aren't using a tantalum cap here, either try one, or up that value to at leas 47uF, see if that doesn't reduce the tremolo. that sound might be some of the famed octave down, but unformed.

Thanks for the suggestion. I only have electrolytic caps that high but I'll experiment with increasing the 22uf. My first guess was a phase cancellation issue but its an uninformed one. A

[antonis]

that's the impedance when sourcing current, when the stage is sinking current, the impedance is 6.8k, this averages out to 3.5k .

That only stands for great singals amplitude and low loads value..
(the main reason for biasing Emitter higher than Vcc/2..)