Buffered output Wah oscillates - but not everywhere

[Lino22]

I added a standard J-FET buffer to a standard VOX wah, and sent it to my friend. It works perfectly at my home, but when he plays it in his home, it oscillates in the heal position. The buffer doesn't have a "volume" resistor (or trimpot) between its output and ground (after the output cap) - can that be the cause of the oscillations?

We are talking about guitar > battery powered wah > amp setup.

[GibsonGM]

Not sure, Lino...can you post a schematic of the buffer you used?  Input buffer I assume?

If your amp was set slightly cleaner than his, can't that be a possible explanation?  I've had wahs squeal at various positions because of the level of disto I was using on the amp...

[antonis]

The buffer doesn't have a "volume" resistor (or trimpot) between its output and ground (after the output cap)

Are you sure you're talking about "buffer" (Source follower)..??
And is that buffer placed on Wah IN or OUT..??

[Lino22]

Guys i am sorry i didn't say this clearly in the first post

- i added a True Bypass switching and an output buffer to the standard Vox wah
- i reduced the input serial resistor from 68k to 47k to equal up the wet and dry volume
- the buffer is a J-FET buffer. Some people add a volume trimpot to its output to have more room for setting the output level, i didn't.

Here is the buffer



The fellow said it was oscillating in the heel position on all guitars and amps he has tried, even at home and in his rehearsal room.
I can't make it oscillate at my home.

[GibsonGM]

I've never felt I needed an output buffer on a wah...I've done input buffers, but everyone has different tastes, and it SHOULD work.  I've also done the 'lower input R' trick with no bad results.

Can't see why it should have a sensitive spot with the buffer. At this point, I'd just put the trimmer in and see if that does anything - you never know.  While you're in there, of course have a look around, make sure the switch wiring is correct and all that.    And examine lead dress...be sure board in/out wires don't pass too close to trannies or Q's and so on.    I might even swap in/out wires for shielded, grounded at one end.   

[anotherjim]

I take it the output buffer would prevent it from becoming a seagull simulator when plugged in backwards?

[antonis]

Vox Wah pedal output impedance can range from 630R up to 6k8 (dependent on sweep pot setting) so I don't think an output buffer is essential..

[Lino22]

I wish i could make it oscillate. I will take it to our rehearsal room and try my luck. If there is any progress, i will report back. Thank you guys.

[Rob Strand]

- i added a True Bypass switching and an output buffer to the standard Vox wah

Apparently wah's can oscillate when you add an *output* buffer.   It seems the wah needs
the cable capacitance (on Q1's collector) to roll-off the upper frequencies.

Try adding a small cap say 47pF from the base of Q2 to ground.  (I think anotherjim played with this a few years back).



Alternatively you could try adding a capacitor from Q1 collector to ground, maybe bit higher say 100pF and upto say 470pF.  The cap will preserve the tone of the wah driving a cable (the question is what length cable do you want to emulate.)

[anotherjim]

In my situation, IIRC, the oscillation was ultrasonic but caused audible crackling with the operation of the wah pot. There were many places a damping cap (47p) fixed it. For a built example, the easiest place to try might be the wiper of the wah pot to its grounded lug.

[Rob Strand]

In my situation, IIRC, the oscillation was ultrasonic but caused audible crackling with the operation of the wah pot. There were many places a damping cap (47p) fixed it. For a built example, the easiest place to try might be the wiper of the wah pot to its grounded lug.

It's possible there's more going on with the current problem.

Given the output buffer was added onto a working wah pedal it has to narrow down the possibilities.

If the response is too peaky it can oscillate.   That could happen in the heel position as sometimes the response peak can be high there.    Perhaps the emitter resistor on Q1 is too low or the 33k across the inductor is too high.   
Another change was the 47k to 68k.    I'd have to look more closely to see if that change could upset the balance and make it oscillate.   If we assume it's not those then perhaps something shorted when the 47k was changed?

[Lino22]

This forum really makes me happy. Such nice, clever and helpful people here.

[Lino22]

I finally tried the wah out in our rehearsal room on a few guitars and amps - it wasn't oscillating, but there was a weird "pop" in the sound in the heel position. Like a scratched wah pot but loud. It was the same on all the amps and all guitars and cables, just one cable (output) made it less loud - my ancient coil cable i received with my first guitar some 30 years ago.

[Lino22]

That looks like the additional capacitance of the old cable helps. I will start with the 47p caps.

[Rob Strand]

I finally tried the wah out in our rehearsal room on a few guitars and amps - it wasn't oscillating, but there was a weird "pop" in the sound in the heel position. Like a scratched wah pot but loud. It was the same on all the amps and all guitars and cables, just one cable (output) made it less loud - my ancient coil cable i received with my first guitar some 30 years ago.

Oscillation can cause "unexplained" pops.

That looks like the additional capacitance of the old cable helps. I will start with the 47p caps.

See how you go.   Doesn't hurt to try a few things with weird problems.

[anotherjim]

but there was a weird "pop" in the sound in the heel position

Yeh, I could get that and maybe more as I had the pot hooked up to the breadboard so could spin it further than a treadle mount does. Cap cured it.

[Lino22]

I tried the dumping 47-100p caps as you suggested, but it didn't change a bit. I first made sure it really is the buffer, so i disconnected it and the sound was just fine. Then i put it back nad used my audio probe, and the pop seems to appear on the J-FET output.
It sounds fine on the gate. I replaced the MPF102 with J112 but no change.

[antonis]

Ground 1μF output cap with a 1M or so pull-down resistor..

[Lino22]

I did, i used 100k and it didn't help

[antonis]

What's next effect..??

P.S.
In case of next effect input cap existence (as stands for the vast majoriy of them) I'd try without buffer's output cap..