Parapedal Help!!!

[A.J.]

Hi Everyone -

I put a Parapedal together using the layout on Geofex (the top one of the two presented) and I can quite seem to get it working.....

I get sound - ok with just U1, clean, seems to work...but when U2 is popped in it starts to screech - when the pedal is not depressed is sound kind of wah-ish (good sign!), but as soon as the pot turns it swells quickly - sound like a siren - pitch varies as the pedal moves.  In fact - it reminds me a lot of a Feedback Loop, like a TSA.  Very loud.

Checked all of the traces, parts, joints, changed the pot (2 gang 10K lin), etc, all the usual stuff...

Voltages are:

U1
1  3.43
2  3.30
3  3.28
4  7.75
5  3.28
6  3.31
7  3.31
8  0

U2
1  3.31
2  3.31
3  3.27
4  7.75
5  3.27
6  3.31
7  3.31
8  0

Battery measured 8.46V - tried both 1458 and 4558 IC - these were with the 4558

Does anything look amiss - any help or advice would be greatly appreciated....

A.J.

[sfr]

I don't remember my voltages now (I got's 'em written down somewhere) but I will say I breadboarded a version of this, and basically had the same problem - the thing screeches.  I have it wired to two pots right now while I'm trying to figure it out, but it's basically a tuneable oscillator at this point.

So yeah, just seconding this.    Which layout of the two on the page did follow?

[A.J.]

Sounds about the same - I used the top layout with all of the offboard wiring together in a row.....

Can't figure it out at all...

A.J

[R.G.]

It's oscillating. (Well, duh, R.G.!)

I ran back through the layout and schemo. The circuit simulates correctly, which just indicates that there is some chance it works in the real world, not a certainty. I also ran back through the layouts.

There is a bug in the top layout but not the bottom one. The cap designated "C20" is incorrectly connected across ground and -V. It should be connected across R13 as the schematic correctly shows.

I believe that this high impedance on the reference voltage may be causing the oscillation. The voltages you show don't indicate any thing wrong on the components/pinout/connection side.

The bottom layout is correct with the schematic.

If that's the bug - sorry. Try pulling "C20" loose and connect its (-) lead to the bias voltage. I think that will help.

[Paul Marossy]

Hmm... I haven't had any problems with mine. Must be because I used the lower PCB layout. 

[R.G.]

Just based on what I found tracing, I'd bet that's true.

High impedance in the reference voltage will definitely make this prone to oscillation.

The simulator shows that up as well.

[slacker]

Aren't the pin 4 and 8 voltages wrong? I know it's a positive ground pedal but shouldn't pin 8 be 0 volts and pin 4 be –9volts?

[R.G.]

The voltages are all inverted, but I took that as measurement - put the red/+ lead on ground, then everything reads as shown. Also, the pin 4 voltages show 7.75V rather than 8.4, which I took to be battery sag and the series diode drop.

[slacker]

thats makes sense. I guessed that might be the case, just thought it was worth mentioning.
Do you know if there's any particular reason why it's postive ground, seems a bit of an odd thing to do with op amps.

[R.G.]

It was worth mentioning.

As to why it's positive ground, I have no idea. Most opamp circuits simply don't care much whether you do positive or negative ground. In this circuit, I believe that there would be very little needed to make it negative ground.

[A.J.]

Thanks R.G.et al - I checked against the schem but must have missed that - I'll try tongiht to see if it fixes it.

Looking forward to getting this one going - gutted a Dunlop Rotovibe (worst pedal I've ever owned) that's been sitting in my closet for about 10 years without being played - thought I ought to at least put the shell to good use....

A.J.

[R.G.]

I updated the layouts at GEO for the Parapedal to fix the C6 bug. I also added a negative ground version of the same circuit. I think I got it right, but checking PCBs nearly always needs another set of eyes.

[A.J.]

Just a quick update:

Fixed that cap and it worked right off - thanks RG!

Had a couple of holes in the shell so I added a 100K pot in series with a 47K resistor in place of the 100K resistor at U1 to give a bit of a volume control and added a 10K pot in series with the 10K resistor to control the resonance - oh - and also a switch to vary the input cap.

Nice clean sounding wah, with a bit of a volume swell - sounds really good with a Foxx Tone Machine in front of it.

Thanks again,

A.J.

[Rocket Roll]

...but when U2 is popped in it starts to screech - when the pedal is not depressed is sound kind of wah-ish (good sign!), but as soon as the pot turns it swells quickly - sound like a siren - pitch varies as the pedal moves.

Had the same problem, but with U1: oscillating in both (true) bypass and active state. Voltages were pretty much in the same ballpark as A.J.s, only difference was that the U1's pin 1 was cca 8V. I tried removing pulldown resistors, running shielded wire on input and output, etc - nothing worked. I've used the new, negative ground circuit (the second, "Modern" PCB on http://www.geofex.com/FX_images/newparapedal.pdf).

Then I replaced TL072's with NE5532's and the oscillation was gone. Bad opamp? Different input impedance between 072's and 5532's? Something else? I don't know. However, I'd love to have a scope so that I could check for the oscillation beyond audible range, just in case.

[Rocket Roll]

Finaly debugged it: don't add pulldown resistors, U1 will self-oscilate on pin 1! Here's how it sounds: http://www.box.net/shared/1ncajcu4ej