Buffered output Wah oscillates - but not everywhere

Started by Lino22, October 14, 2022, 07:29:52 AM

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Lino22

Antonis the pop is caused by oscillation, this is not the leaked DC pop.
My setup is guitar/or nothing > wah > amp.
When the core started to glow and people started yelling, he promptly ran out the door and up a nearby hill.

antonis

#21
Quote from: Lino22 on October 19, 2022, 09:41:08 AM
Antonis the pop is caused by oscillation, this is not the leaked DC pop.

If so, it leads in oscillating buffer..

I'd try a BJT based buffer (don't worry about buffer's input impedance for the moment) of low output impedance..
(about 5mA working current should be OK..)

P.S.
I still can't see any obvious reason for using a buffer in the output of a Wah pedal but it should be "academically" interesting to find the cause of your issue..
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

Lino22

Yes, BJT was gonna be my next move, i will report back.
When the core started to glow and people started yelling, he promptly ran out the door and up a nearby hill.

GibsonGM

I'd like to see the pop on a scope.  Or, can you make an audio clip of it?
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anotherjim

The buffer is so the wah works with low impedance loads such as FF, but a 10k-15k in series with the output to the jack tip works well too if you don't mind a little level drop.


Lino22

#25
Quote from: GibsonGM on October 19, 2022, 10:21:44 AM
I'd like to see the pop on a scope.  Or, can you make an audio clip of it?

Good call, here you are. 1.33MHz, but amplitude modulated by something i can hear.





When the core started to glow and people started yelling, he promptly ran out the door and up a nearby hill.

antonis

Probably I'm wrong but it seems to me like oscillation due to positive feedback via Gate to Source capacitance..

Could you try a 1k or so resistor in series with JFET Gate..??
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

anotherjim


That's what I was seeing. Where did you try adding a cap?

Rob Strand

QuoteThat's what I was seeing. Where did you try adding a cap?
Also, look at the waveform when you have the added caps in place.

QuoteCould you try a 1k or so resistor in series with JFET Gate..??
And a 1k in series with the output lead.   Perhaps see if pulling the output jack,
which will remove any load capacitance, affects things.

I suspect the oscillations are there *before* be buffer.   However, you might
find placing a x1 CRO probe can stop the oscillation, so you need to do measurements
before the buffer with a x10 CRO probe.   A way around that might be to look
for oscillation on the output of Q2's emitter.
Send:     . .- .-. - .... / - --- / --. --- .-. -
According to the water analogy of electricity, transistor leakage is caused by holes.

Phend

Mechanical fix:
Put in a stop so the pedal won't go down so far.
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GibsonGM

Quote from: Phend on October 19, 2022, 06:51:16 PM
Mechanical fix:
Put in a stop so the pedal won't go down so far.

Or mess w/the R across the wah inductor to adjust the Q a little (trim pot?).  I'd like to HEAR the problem.  Every wah I've ever messed with needed something done with the "33k" across the inductor, to be honest.  Adjusts honkiness.
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MXR Dist +, TS9/808, Easyvibe, Big Muff Pi, Blues Breaker, Guv'nor.  MOSFace, MOS Boost,  BJT boosts - LPB-2, buffers, Phuncgnosis, FF, Orange Sunshine & others, Bazz Fuss, Tonemender, Little Gem, Orange Squeezer, Ruby Tuby, filters, octaves, trems...

Lino22

#31
Quote from: Phend on October 19, 2022, 06:51:16 PM
Mechanical fix:
Put in a stop so the pedal won't go down so far.

... or maybe when the fellow starts quacking i'll say don't do it.  :D

Thank you guys, i am pretty sure the solution is at the door. I will have time to deal with it tomorrow, and report back.
When the core started to glow and people started yelling, he promptly ran out the door and up a nearby hill.

Lino22

Quote from: anotherjim on October 19, 2022, 04:51:47 PM
That's what I was seeing. Where did you try adding a cap?

I tried 47p between the wiper and ground as well as across the 10k emitter resistor.
When the core started to glow and people started yelling, he promptly ran out the door and up a nearby hill.

Lino22

#33
Quote from: antonis on October 19, 2022, 04:28:42 PM
Probably I'm wrong but it seems to me like oscillation due to positive feedback via Gate to Source capacitance..

Could you try a 1k or so resistor in series with JFET Gate..??

Yes, this may be it, i will definitely try it. Thank you. Or as Rob says one serial 1k at the output - it creates a low pass filter with the cable if i am not mistaken. And my coil cable audibly makes it less loud.
When the core started to glow and people started yelling, he promptly ran out the door and up a nearby hill.

Lino22

When the core started to glow and people started yelling, he promptly ran out the door and up a nearby hill.