some major debugging help needed for new built (BIG effect)

[birt]

ok. so i just built my wah pedal (pictures coming very soon)
this is the schematic i used: http://users.pandora.be/vandijckheffen/bert/whackywah.jpg
and this is the lay-out: http://users.pandora.be/vandijckheffen/bert/perflay-out.pdf

so i did a first quick test... in bypass there was lfo ticking and lots of bleedtrough from the synth part (lower right in the schematic above the lfo). That part is also extremely loud when switched on. so this needs to be tamed a bit. altough i had lfo ticking the lfo indicator doesn't show the pulses, but i can control the brightness with the depth pot.
i didn't hear any wah sound yet because i immediatly pulled the 9v plug out when i saw smoke and a little flame coming from the dual gang wah pot (lower left in the schem), it stinks now.

i guess i'll have to solve this problem before i can move on to the next :p

[birt]

ok here are the pics:

[Mark Hammer]

You know, there are world-class performers who have their choice of piano all over the world because they could simply never bring their preferred piano with them on tour.  In your case, if you ever got famous, you'd have to arrange to have several of these built at different places and leave them there for touring purposes as well.  Only in your case it wouldn't be because the item could not FIT on the airplane.  It would be because there is no way that anyone inairport security would permit you to bring it on the airplane.   That thing looks positively dangerous.

LFO ticking:  One of the other forum members (whom I shall not name in order to keep his e-mail manageable) was gracious enough to send me some photocopies of several select issues of Stompboxology, the Boscorelli newsletter that was out for a while.  One of the things that Boscorelli advocates, and uses religiously, is a slight modification tot he traditional 2-opamp LFO intended to reduce audible ticking.

Some backtracking, first.  The standard 2-opamp LFO found on so many modulation devices is essentially a comparator and integrator.  A reference voltage is fed to the input of one opamp, and when a different voltage exceeds that refeence, the output of that op-amp swings high instantaneously.  THAT square wave is now fed to the next opamp integrator stage, where a cap in the feedback loop smoothes out the square wave so that it is transformed into a triangle (slow rise, slow fall).  That triangle wave output is what provides the voltage going back to the first opamp which the reference is compared to.

This circuit tends to produce ticking because when the square wave is produced, that sharp instantaneous rise to the full voltage-swing of the op-amp creates a sudden current draw from that op-amp.  The "tick" you hear is the result of available current being momentarily sucked away from other stages.

There are several approaches to fixing that.  One is to use low-current op-amps for the LFO so that whatever current draw there is will not be excessive.  That's why you often see TL022 op-amps in LFO circuits.  They may not be great for audio signals, but they will produce a decent LFO control signal with little current required, and little current momentarily pulled away from other stages or parts of the circuit.  A second strategy is to decouple the LFO by providing the op-amp its own little reservoir of stored current - sort of like "petty cash" for short emergencies.  This is seen in the form of a small resistor from the main V+ line to the V+ pin on the chip, and an electrolytic cap (say, 10-47uf) from the chip's V+ pin to ground.

The third approach - the one that brings us back to Boscorelli - is to modify the LFO circuit slightly so that it does not produce a square wave output, but rather produces a trapezoidal primary waveform, with a slight bit of lag on the rise and fall.  The top is still flat, and the circuit still produces the same old lovable triangle output, but the "raw material" for the square-to-triangle conversion has juuuuuuuust enough lag added to the rise time that the current draw doesn't happen all at once, thereby eliminating the "tick" and replacing it with a "ttttttttiiiiiiiiiiiiiiiiiiiccccccckkkkkkkk" that happens slow enough that you can't hear it.

An example of it is shown here, in the Boss BF-2 flanger: http://www.freeinfosociety.com/electronics/schematics/audio/pictures/bossbf2.gif. 
Look at the LFO built around IC5.  Normally, pin 6 would go directly to the junction of the voltage divider formed by R40/41.  What is different, though, is that the 100k resistor now links the divider and pin 6 and a .01uf cap goes from the output of that op-amp back to pin 6.  Contrast the BF-2 schematic with the LFO found in the CE-2 chorus, and you'll see where the differences lie. http://www.freeinfosociety.com/electronics/schematics/audio/bossce2.pdf

[gez]

Nice one Mark, that's a cool little trick!

PS  Couldn't get links to work (but description was good enough)

Edit:  Got it now (pasted address in and it works)

[birt]

the weird thing is that even in bypass mode (in and output wires are shielded, only the short in and outwires from board to switching relays aren't) the synth part is very audible and changes when i turn the frequency knobs.

and that burning pot bothers me



oh and mark, i take the dangerous part as a compliment

[birt]

bump for pot burns

[Mark Hammer]

oh and mark, i take the dangerous part as a compliment

That's how it was meant, brother.

[slacker]

Nice to see you actually built that thing, very cool 

The noise from the synth part might be getting on to the ground wires. I had that problem with my uglyface and unfortunately I couldn't solve it.
Burning pots normally means you've got  9 volts getting on to the pot some how.  check out this recent thread about it http://www.diystompboxes.com/smfforum/index.php?topic=50154.0

[birt]

Nice to see you actually built that thing, very cool 

The noise from the synth part might be getting on to the ground wires. I had that problem with my uglyface and unfortunately I couldn't solve it.
Burning pots normally means you've got  9 volts getting on to the pot some how.  check out this recent thread about it http://www.diystompboxes.com/smfforum/index.php?topic=50154.0

well it does have 9 volts on it as you can see in the schematic. i use the led/ldr setup where the led brightness is controlled by a pot as seen on geofex.. i just need a fix for the problem.. maybe a small resistor in series with the pot to have a minimal resistance?? (i think the problem is only on the B pot)

the synth part is just way to loud. i think the interference might go away if i could tame it down a bit...

[gaussmarkov]

There are several approaches to fixing that.  One is to use low-current op-amps for the LFO so that whatever current draw there is will not be excessive.  That's why you often see TL022 op-amps in LFO circuits.  They may not be great for audio signals, but they will produce a decent LFO control signal with little current required, and little current momentarily pulled away from other stages or parts of the circuit.  A second strategy is to decouple the LFO by providing the op-amp its own little reservoir of stored current - sort of like "petty cash" for short emergencies.  This is seen in the form of a small resistor from the main V+ line to the V+ pin on the chip, and an electrolytic cap (say, 10-47uf) from the chip's V+ pin to ground.

hi mark!  thanks for the Boscorelli trick.  and finding it in the Boss BF-2.  Mr. B's copyrights remain intact.

how different is it to put a decoupling cap, say .1uF, from the V- pin of the chip to V+?  i have been using this approach on R.G.s stuttering pedal in this thread.  i picked it up from the femtoverb schem by davetv and the layout by metalguy and a related thread.

cheers, gm

[slacker]

maybe a small resistor in series with the pot to have a minimal resistance?? (i think the problem is only on the B pot)

Sorry I hadn't looked at the whole schematic before. I think you'll need a current limiting resistor in series with the pot otherwise when you turn the pot down the current gets too much for it. The other way to do it would be to use smaller pots and wire them up as voltage dividers something like this.

that way you avoid the problem of burning them out.

[birt]

well one of the wah post is wired as a voltage divider but i think it's the other one (where resistanse goes down till the middel and then up again) that is problematic... i guess i should wire a resistor (330ohm or something?) in series with the middel lug.

[$uperpuma]

MOTHER OF GOD. THAT is quite an undertaking...