How can I get rid of distortion in wah circuit?

[josahasa]

Hi!

I've built a McCoy wah-clone from the schematic at fuzz-central. It also incorporates the suggested output buffer. Everything is working, but I can't get the sound not to distort at heel-down position. With bc109b's and bc109c's there is somewhat more bass than with "modern"-type high-gain 5884 etc. transistors, but no matter what transistor I try it with, I can't get it clean in the lower register. I've matched the gains on all of the applicable transistors. It seems as if the lower mids are getting boosted quite a bit, and therefore causing distortion.

Is this typical, and is there a way for getting rid of the unwanted distortion?

Thanks!

-Jonah

[earthtonesaudio]

I don't know if it's typical, but a frequency-dependent gain problem seems to beg for a frequency-dependent attenuation solution.

I would start with decreasing the value of the input cap.

[Quackzed]

do you know if the distortion is coming from the output buffer or the pedal itself? i'd tap an output from before the buffer and see if its still distorted. 

[R.G.]

do you know if the distortion is coming from the output buffer or the pedal itself? i'd tap an output from before the buffer and see if its still distorted. 

Ding! A correct answer! Divide and conquer.

And another one snatches the pebble! 

[josahasa]

Unfortunately it is coming from the pedal itself...

[Quackzed]

check out r.g.'s excellect 'technology of wah pedals' article for a close look at what does what in a wah...
http://www.geofex.com/Article_Folders/wahpedl/wahped.htm
i'd start at the transistor gains and bias first, see if you can't dial out the distortion by adjusting the collector resistors.

[Paul Marossy]

Unfortunately it is coming from the pedal itself...

That can happen in a wah circuit. I think I have at least a couple of wah pedals that have a little distortion being created by the circuit itself. It's not enough to be unacceptable, though.

i'd start at the transistor gains and bias first, see if you can't dial out the distortion by adjusting the collector resistors.

Yeah, that is probably exactly what I would do, too.

[jacobyjd]

I was under the impression that distortion at the peak of resonance was exactly what is desired in a wah (for the most part). I generally find that if there's no distortion present whatsoever, it sounds bland.

Not to question what your preferences are, but are you SURE you want it to be completely distortion-free?

[josahasa]

No no, not distortion-free. It's just that usually in a funky comp one likes to hit the chord at the bass end of the wah and open it up in mid chord for the actual "wah". This phenomenon is somewhat interrupted and disturbed by muddy and loud, peaky distortion at the the heel down position. Actually there's no real (audible) distortion at the treble end, where it actually could be kind of nice. (Think "Shaft")   

Are there any basic guidelines for optimal transistor gain for bc109b ? How about the bias?

Thanks again!

-Jonah

[joegagan]

jacobyd, i agree, some distortion at low-mid peak is what makes a wah cool in my opinion. i have a theory that commercial companies try to engineer this feature out of their wahs so that people don't complain.( factor in the difference between  HBs and single coils, it gets even harder to please the public.)

i call it a 'pop',  that gives it the 'wow'

even with clean tones, this 'pop' in mid sweep adds a lot of character, as in the cool wah part in everlast's 'what it's like'

http://www.youtube.com/watch?v=-vWhdz5svvQ

similar to a wah i modded for a similar pop  - (my elec. guit. track on a new age music project that the artist never finished)

http://www.soundclick.com/bands/page_songInfo.cfm?bandID=669714&songID=

even the snarling dogs mold spore has a pronounced pop (0:32 thru 1:15 or so)

http://www.youtube.com/watch?v=OCMo-ynOFh4

for a smooth, 'popless' wah, try a shin ei, or other transformer based wah.

the internet rumors say that shaft guitar part was played with a maestro boomerang. i never tried one. do they pop?

[joegagan]

oh , jonah, i get what you're saying, you have a pop too far at the bass end. not sure how to alter where the pop happens.