Harmonic Percolator w/ 3-band EQ ~FALLING APART~

[Entarteter Lärm]

Hey everyone.  I'm a recent sign up, though I probably should have joined a long time ago after all the times I've referenced these forums.  First off I want to thank everyone for contributing to DIY and making it possible for broke folks like me to make some cool shit and play cool music.



My question is about this pedal I built a few weeks ago that was working perfectly up until recently.  It's basically just two circuits that I've smashed together and very slightly modified as you can see from the image.  It's just Tim Escobedo's Harmonic Jerkulator that I added a simple active 3-band EQ I found on some website with a generic name I can't remember like "electroniccircuits.com" or something to that effect.  The only modifications I made to it were to change the 2 resistors acting as a voltage divider off the voltage source in the Jerkulator portion to get a better response from the external pots (I made the EQ pots internal trimpots) based off the advice I stumbled across on this forum, and pin 4 of the op-amp was originally to a negative voltage and pin 3 was to ground.  I added a simple voltage divider and used VREF as a virtual ground in the EQ portion of the circuit.  The IC could probably be any op-amp.  I originally ran it on an ne5532, then switched to an lm358.  Both work fine, but this sort of segues into my problems:

After a fair amount of troubleshooting and minor modifications, I arrived at the circuit I've shown here.  It sounded great.  @#$%ing MEAN distortion.  However, after awhile, the pedal basically just quit on me.  I can play it for a few minutes, then it won't put out any sound other than a very small crackle.  It will play again after it's "rested" for awhile, but again start sputtering after a couple of minutes of play time.  I've tried switching out the battery, the transistors, my cords, amps, and the IC.  I had some success with switching out the IC.  First I changed from one ne5532 to a fresh one.  It played fine for a few minutes, then back to the crackle.  Then I changed it to an lm358 just for the hell of it.  Same result.

So it seems like the issue is around the IC and I have a few theories, but I'm actually not super well versed in circuit building, so they might be ridiculous theories.  Could the percolator portion of the circuit be hitting the IC to hard and overheating it?  Once when I swapped out the IC, I could have sworn it felt a little warm, but I might have imagined it.  Should I throw a buffer in there?  I'm not even entirely sure what those do.  Should I swap the positions of the percolator and EQ portions of the circuit?  Does it have something to do with the virtual ground I made?

PLEASE HELP.  This is the first pedal I've ever committed to enough to move through the disappointment of the first ten times I housed the circuit and it didn't work, put hours upon hours of troubleshooting into, then finally got it working for a time.  And my housing looks awesome!  Sorry if I'm being dramatic, but I love this damn pedal and felt I had found a unique tone to make some music that didn't immediately sound like a knock off.

[radio]

What did you do with the other half of the IC?

[Mark Hammer]

First, welcome.  C'mon in, the water's fine.

Second, whenever something requires time to go wrong, that usually points to one or both of two things: temperature, and one or more electrolytic capacitors.

The NE5532  can draw a lot of current (bad for LFOs), but the LM358 is considered to have a low current draw (good for LFOs).  So, unless there is a distinct difference between how long it takes to fade down to crackle-and=sputter, I would say it's less likely (though not entirely impossible) for it to be the IC itself.

How do the two transistors feel?  Do they get warm?

[Entarteter Lärm]

What did you do with the other half of the IC?

Literally nothing.  I didn't even ground them.

First, welcome.  C'mon in, the water's fine.

Second, whenever something requires time to go wrong, that usually points to one or both of two things: temperature, and one or more electrolytic capacitors.

The NE5532  can draw a lot of current (bad for LFOs), but the LM358 is considered to have a low current draw (good for LFOs).  So, unless there is a distinct difference between how long it takes to fade down to crackle-and=sputter, I would say it's less likely (though not entirely impossible) for it to be the IC itself.

How do the two transistors feel?  Do they get warm?

Haha thanks =]

No, I just played until failure and the transistors didn't feel warm.  I tried swapping out the IC to see if it would start playing.  Popped in an ne5532.  It didn't start playing again.

[bluebunny]

What did you do with the other half of the IC?

Literally nothing.  I didn't even ground them.

This will upset the unused opamp.  It's probably oscillating off into orbit...

Connect the output pin to the inverting input, and then connect the non-inverting input to V_REF.

Not sure if that will fix all your problems, but at least the opamp will be more relaxed. 

[Rob Strand]

The design has got problems.

1) The percolator is feeding directly into the tone control circuit.
     The input impedance of the tone control is low impedance and
     it will load down the percolator.   Add a buffer between the two.

2) Vref circuit (2x100k) has no cap to ground add a 10uF to 47uF cap to ground.

3) R8 should connect to the opamp -input.  Basically R8 is on the wrong side of C6.
     It might work but it's an error.

[Entarteter Lärm]

Thank you all for taking time to respond!

The design has got problems.

1) The percolator is feeding directly into the tone control circuit.
     The input impedance of the tone control is low impedance and
     it will load down the percolator.   Add a buffer between the two.

2) Vref circuit (2x100k) has no cap to ground add a 10uF to 47uF cap to ground.

3) R8 should connect to the opamp -input.  Basically R8 is on the wrong side of C6.
     It might work but it's an error.

I might be asking for too much, but I'd like to learn a little bit if I can so that I can reference these lessons if I have similar problems in the future.

1) What effect does high/low impedance have on a circuit and why is the tone low impedance?

2) Where am I connecting it?  Next to the voltage source, VREF, or ground (the other end of the voltage divider)?  What effect is the lack of a capacitor having?

3) What purpose does R8 even serve?  Why is its value so low?

[ElectricDruid]

Welcome Entarteter Lärm!

This is what I'd try for starters:

Like Rob said, add a decent sized electrolytic cap on the Vref supply, since there isn't one. I agree, 10-47uF. That fixes (2).

Use the other half of the op-amp as a buffer between the jerkulator and the tone circuit. That fixes the impedance problem (1). I'm not convinced that's what's causing your problem, but you've got half an op-amp going spare, so you might as well. It will improve the circuit.

For the buffer, I'd put it in between the two caps 0.1uF and C1/4.7uF. Signal from 0.1 cap goes to positive input, op-amp output is fed back to negative input and to the 4.7uF cap. Since with caps on the input and output the op-amp will have no DC bias, we need to fix that, so add a 1M resistor from the positive input down to your Vref supply.

(3) I haven't checked, but Rob knows what he's doing, so if R8 is going to the wrong place, try changing that too.

[Rob Strand]

I might be asking for too much, but I'd like to learn a little bit if I can so that I can reference these lessons if I have similar problems in the future.

1) What effect does high/low impedance have on a circuit and why is the tone low impedance?

2) Where am I connecting it?  Next to the voltage source, VREF, or ground (the other end of the voltage divider)?  What effect is the lack of a capacitor having?

3) What purpose does R8 even serve?  Why is its value so low?

Very good questions.

1) This is more about the circuit working as expected.   By itself the two circuit will behaviour as they are expected however
when you put the two together the combined behaviours isn't the same as what would by putting the two together.  When
you put a low impedance on the output of a high impedance circuit the signal level drops.    We call that loading down
the circuit.  The way a tone control circuit like that works is say boosts at certain frequencies.  However at those same frequencies the impedance of the tone control circuit drops so if the circuit feeding the tone control isn't low impedance there is a loading
effect.   The loading effect works against the boost it would normally have.  So in the end the tone control works differently to
what is expected.   Note also even when the tone control are flat there is a loading effect.   That will be less of a loading effect
than when the controls are boosted and it may or may not be acceptable.

2) So in general it is good practice to ground the Vref because it prevent a number of bad effects.
- Noise from the power supply getting into the audio.   
- Noise from the Vref resistors getting into the audio.
- In some circuits, no cap on Vref causes different parts of the circuit to interfere with each other.
- The circuit behaves differently to what is expected.

In the case if this circuit you might get away with it largely because of the third point that there's not other circuit to interfere with.  Nothetheless the other points stand and in fact the non-bypassed (ie. no cap) on Vref means the C5 and R8 might not be so effective.

3) The reason R8 and C5 are present are to prevent oscillation.    However, this specific circuit isn't really required for a TL072/TL082 etc opamp.  More suitable would be a small valued capacitor (maybe 47pF to 100pF) between the opamp output and the opamp - input.    The R8 and C5 circuit originates back to the history of the three band tone control circuit.   The original circuit used a different type of opamp (an uncompensated opamp) and the R8 and C5 parts help that opamp stay stable.

So the circuit will probably work as is but it's not following good practices.   And when you have problems you should stick to good practices.

That also includes the point bluebunny made about the unused opamp.

[Rob Strand]

You might try adjusting the 1M resistor and see if you can get a good sound.

If you suspect the tone control take a step back wire the output socket to the percolator stage to see if it's working OK.
Sort of like C6 on this schematic.  Don't worry about the diodes.

https://www.fredric.co.uk/misc/HarmonicPercolator.sch.png

[Keppy]

I don't think anyone's mentioned yet that C1 has no consistent DC voltage on the left side, or that it's redundant with the cap next to it. C1 should be removed, replaced with a jumper (unless sticking a buffer in between as suggested by ElectricDruid). Without proper DC conditions, C1 might have gone bad.

[Entarteter Lärm]

Again, I want to thank everyone for helping me out.  I'm super grateful to have some experienced people giving me directions since thus far I've been teaching myself how to do this as best I can with varied results.

So just to clarify before I start de-soldering and re-soldering a bunch of stuff--

Welcome Entarteter Lärm!

This is what I'd try for starters:

Like Rob said, add a decent sized electrolytic cap on the Vref supply, since there isn't one. I agree, 10-47uF. That fixes (2).

Use the other half of the op-amp as a buffer between the jerkulator and the tone circuit. That fixes the impedance problem (1). I'm not convinced that's what's causing your problem, but you've got half an op-amp going spare, so you might as well. It will improve the circuit.

For the buffer, I'd put it in between the two caps 0.1uF and C1/4.7uF. Signal from 0.1 cap goes to positive input, op-amp output is fed back to negative input and to the 4.7uF cap. Since with caps on the input and output the op-amp will have no DC bias, we need to fix that, so add a 1M resistor from the positive input down to your Vref supply.

(3) I haven't checked, but Rob knows what he's doing, so if R8 is going to the wrong place, try changing that too.

So to start, in the first image in the section I've labeled A) is this the setup you're referring to?  I ask because out of all the schematics I've perused on the interwebs, I've never seen a buffer or an amplifier set up in this way.  Granted I'm pretty inexperienced, but still, is this accurate?

I might be asking for too much, but I'd like to learn a little bit if I can so that I can reference these lessons if I have similar problems in the future.

1) What effect does high/low impedance have on a circuit and why is the tone low impedance?

2) Where am I connecting it?  Next to the voltage source, VREF, or ground (the other end of the voltage divider)?  What effect is the lack of a capacitor having?

3) What purpose does R8 even serve?  Why is its value so low?

Very good questions.

1) This is more about the circuit working as expected.   By itself the two circuit will behaviour as they are expected however
when you put the two together the combined behaviours isn't the same as what would by putting the two together.  When
you put a low impedance on the output of a high impedance circuit the signal level drops.    We call that loading down
the circuit.  The way a tone control circuit like that works is say boosts at certain frequencies.  However at those same frequencies the impedance of the tone control circuit drops so if the circuit feeding the tone control isn't low impedance there is a loading
effect.   The loading effect works against the boost it would normally have.  So in the end the tone control works differently to
what is expected.   Note also even when the tone control are flat there is a loading effect.   That will be less of a loading effect
than when the controls are boosted and it may or may not be acceptable.

2) So in general it is good practice to ground the Vref because it prevent a number of bad effects.
- Noise from the power supply getting into the audio.   
- Noise from the Vref resistors getting into the audio.
- In some circuits, no cap on Vref causes different parts of the circuit to interfere with each other.
- The circuit behaves differently to what is expected.

In the case if this circuit you might get away with it largely because of the third point that there's not other circuit to interfere with.  Nothetheless the other points stand and in fact the non-bypassed (ie. no cap) on Vref means the C5 and R8 might not be so effective.

3) The reason R8 and C5 are present are to prevent oscillation.    However, this specific circuit isn't really required for a TL072/TL082 etc opamp.  More suitable would be a small valued capacitor (maybe 47pF to 100pF) between the opamp output and the opamp - input.    The R8 and C5 circuit originates back to the history of the three band tone control circuit.   The original circuit used a different type of opamp (an uncompensated opamp) and the R8 and C5 parts help that opamp stay stable.

So the circuit will probably work as is but it's not following good practices.   And when you have problems you should stick to good practices.

That also includes the point bluebunny made about the unused opamp.

Thank you for explaining all that.  I think I'm following.  Also, to be clear, is the way I set up the capacitor in section B) the way you were referring to when you said to add it to the VREF section?

Also, do you mean positioning the 10-47pf capacitor like in the red square of the 2nd image?  So on the tone half of the IC and not the buffer half, correct?  And would that take out the need for C5 and R8?  Would I still connect all that circuitry to VREF?

You might try adjusting the 1M resistor and see if you can get a good sound.

If you suspect the tone control take a step back wire the output socket to the percolator stage to see if it's working OK.
Sort of like C6 on this schematic.  Don't worry about the diodes.

https://www.fredric.co.uk/misc/HarmonicPercolator.sch.png

The 1M in the blue box?

I don't think anyone's mentioned yet that C1 has no consistent DC voltage on the left side, or that it's redundant with the cap next to it. C1 should be removed, replaced with a jumper (unless sticking a buffer in between as suggested by ElectricDruid). Without proper DC conditions, C1 might have gone bad.

I think I'll replace C1 with a fresh cap even if everyone else's suggestions work.  I've probably overheated it way too many times while soldering, and I salvaged it from a previous circuit as is, so it's probably beat to shit.

[Entarteter Lärm]

Sorry I'm asking so many questions, y'all.  I'm trying to soak up every drop of information I can.

[Rob Strand]

Also, to be clear, is the way I set up the capacitor in section B) the way you were referring to when you said to add it to the VREF section?

Also, do you mean positioning the 10-47pf capacitor like in the red square of the 2nd image?  So on the tone half of the IC and not the buffer half, correct?  And would that take out the need for C5 and R8?  Would I still connect all that circuitry to VREF?

Not quite there.

Where R2, R5 and R7 join that stays connected to pin 1 of the opamp.

Now the cap connects to pin 1 like you have it but the left side goes to pin 2.

I'd say 47pF to 100pF would do it.   You can removed C5 and R8 or leave them in.

At this point though I think it would be best to get the "section A" working with the
tone control.

The 1M in the blue box?

Yes that one.

If you have a multimeter the DC voltages on each pin of the transistor would be useful.

[Entarteter Lärm]

Not quite there.

Where R2, R5 and R7 join that stays connected to pin 1 of the opamp.

Now the cap connects to pin 1 like you have it but the left side goes to pin 2.

I'd say 47pF to 100pF would do it.   You can removed C5 and R8 or leave them in.

Like this, then

If you have a multimeter the DC voltages on each pin of the transistor would be useful.

With signal voltage going through it, or at rest?

[Keppy]

I think I'll replace C1 with a fresh cap even if everyone else's suggestions work.  I've probably overheated it way too many times while soldering, and I salvaged it from a previous circuit as is, so it's probably beat to shit.

That's a good idea, but if C1 is really in series with a smaller cap as the schematic shows, then you don't even need to replace it with another cap. Just use a jumper.

The equation for caps in series is (1/c1) + (1/c2) = (1/cTotal)
(1/.1) + (1/4.7) = 10.2
1/10.2 = .098

In other words, the larger cap doesn't matter. It's affect on the total capacitance doesn't even move it out of the range of tolerance of using the smaller cap alone.

[Rob Strand]

Like this, then

Yes.

With signal voltage going through it, or at rest?

At rest.   Normally we measure DC voltages with no signal.  (There are a few exceptions like when the DC voltage depends on the signal, for example when we test a rectifier circuit.)

[Entarteter Lärm]

UPDATE!


It's working!  I swapped back to an lm358, added a buffer in between the .1uf and C1, replaced C1 with a fresh cap, and after 15 solid minutes of playing, no sputtering!  On the buffer subject--

Use the other half of the op-amp as a buffer between the jerkulator and the tone circuit. That fixes the impedance problem (1). I'm not convinced that's what's causing your problem, but you've got half an op-amp going spare, so you might as well. It will improve the circuit.

For the buffer, I'd put it in between the two caps 0.1uF and C1/4.7uF. Signal from 0.1 cap goes to positive input, op-amp output is fed back to negative input and to the 4.7uF cap. Since with caps on the input and output the op-amp will have no DC bias, we need to fix that, so add a 1M resistor from the positive input down to your Vref supply.

I'm a bit embarrassed that I said earlier I had never seen a buffer set up like that before, because after searching around a bit, 9 out of 10 buffer schematics used that set up, so my inexperience is incredibly apparent. 

I haven't yet put an electrolytic on the VREF supply, though that's probably good practice, so I should do that next.

A bit of a caveat: without warning, it started working without fail before I added in all the new circuitry, so there might be a chance that it will still fail later on down the line, at which point I'll probably turn to y'all for help again.

But until then, I want to thank everyone for their help in getting this thing working and teaching me a few new tricks along the way.  I will record and post a demo video later on for everyone to take a gander at my (our) work!   


P.S.

If you have a multimeter the DC voltages on each pin of the transistor would be useful.

Also Rob, if you still wanted to know for some reason, the voltages on the transistors are:

2n3904
Pin 1 - 4.75V
Pin 2 - 5.35V
Pin 3 - 5.35V

2n3906
Pin 1 - 4.75V
Pin 2 - 4.02V
Pin 3 - 3.66V

[Rob Strand]

It's working!  I swapped back to an lm358, added a buffer in between the .1uf and C1, replaced C1 with a fresh cap, and after 15 solid minutes of playing, no sputtering!  On the buffer subject-

Cool.

Also Rob, if you still wanted to know for some reason, the voltages on the transistors are:

The idea was those voltage would show any biasing issues in the percolator stage.  That could cause sputtering.
Obviously they are OK and the problem was elsewhere.
I'm sure those voltages will help someone one day.