Opamp wah at Geofex - Anyone build it?

[C Bradley]

In the Technology of the Wah article, there's an opamp wah. Very simple circuit. I was thinking of using that circuit with the squarewave oscilator circuit from my own tremolo design (555 timer chip) and controlling the wah with it.

Anyone tried that circuit? It seems like a no-brainer (which is probably why I'm so interested in building it! )

Chris B

[Aharon]

I did not build the Geo op amp wha but I built the Craig Anderton Wha-anti-wha that uses a dual,a couple of caps and a 50K pot.It worked fine.
Aharon

[Paul Marossy]

I wonder if it would sound warmer than a transistor circuit. Maybe the inductor wouldn't have to have magical properties since the dual triode would probably create some more asymmetrical waveforms than transistors.

[C Bradley]

I did not build the Geo op amp wha but I built the Craig Anderton Wha-anti-wha that uses a dual,a couple of caps and a 50K pot.It worked fine.
Aharon

I put the opamp wah together, and it's pretty cool! It sounds like a Boss Auto Wah or similar. I've just got to figure out how I'm going to use it now...

Chris B

[C Bradley]

I'm having problems with a squeal when I turn the wah to the treble side of it's range. Any ideas about what could be causing that? It's practically a carbon copy of RG's schematic, except for the 22k input resistor instead of the 27k RG has, and an LDR or a 100k audio taper pot instead of the 500k pot RG has.

What I want to do is use an LDR/LED setup to control the wah, and have the LED driven by an LFO.

Chris B

[mattv]

Perhaps a resistor in series with the LDR would help.

[Paul Marossy]

You might want to try the 500K pot, too.

[C Bradley]

Perhaps a resistor in series with the LDR would help.

I used a 220 ohm in series with the LDR, but it reduces the treble end of the wah's range (where the squeal occurs). I want the treble, but not the squeal. Maybe I built it too small? Everything is real close together, mounted on a Radio Shack PCB.

I'm going to re-build it on a bigger board with the LFO when I get around to it. Maybe the problem will cure itself. I'm thinking it's parasitic oscillation because of poor layout?

Chris B

[mattv]

I doubt it will fix itself. Lots of filters can oscillate. Surely there are other methods of avoiding this besides my advice, but I don't know of them.

[OliveR_FF]

What opamp did you use to make the Geo Wah? I'm using a TL081 and it doens't seem to be wah-ing

I assumed the capacitors in the diagram were written in microfarads as no unit was specified - other than that i'm pretty sure i've done it correctly

[OliveR_FF]

here's the link to the schematic i'm using:

http://www.geofex.com/Article_Folders/wahpedl/opampwah1.gif

[Eb7+9]

Analogue Oscillator and Filter circuits are just Unstabe/Stable versions of each other ... drop the gain of the circuit and you'll get rid of the squealing - do this either by increasing the input (27k) resistor or dropping the feedback (1Meg) resistor around the op-amp ... engineers call this a Butterworth Bandpass circuit - Craig Anderton used it in his BiFilter Follower ...

[R.G.]

Decouple the power supply to the opamp with 0.01uf and 10uF electro as close to the power pins as you can get them. Be sure you're feeding this from a low impedance source - an emitter follower buffer will work, or the output of another opamp will be even better. Also be sure that you're set up properly for the power supplies - if you're single supply, make sure your bias voltage is clean and your grounding is correct.

Filter circuits in particular are not terribly tolerant of ransom power wiring and often need decoupling at the device. Low drive impedance is going to help as well.

The circuit has worked exactly as shown.

[Eb7+9]

I'm having problems with a squeal when I turn the wah to the treble side of it's range. Any ideas about what could be causing that? It's practically a carbon copy of RG's schematic, except for the 22k input resistor instead of the 27k RG has, and an LDR or a 100k audio taper pot instead of the 500k pot RG has.

creeping pole leading to an oscillator function ?!!



nothing to do with op-amp type, rail decoupling, or pot values either ...

---

simply, an unstable filter ...

http://diy.smallbearelec.com/Projects/ISeeWah/ISeeWah.html

http://diy.smallbearelec.com/Projects/WMouse/WMouse.html

[EBK]

You couldn't wait until the 15-year anniversary of that quoted post before replying?  We were so close.   

[Eb7+9]

my penchant for approximation theory, thought I was plenty close enough already ... 

checking out a bunch of online WAH inventions lately,
Keen's, Hollis, and the SK WAH ... they all exhibit some degree of peculiarities

the SK is quite different in the approach, boostrapping trick producing a wider range Gyrator

think these are earlier versions of Keen's circuit that were published,
simmed with ideal source, and first order PU

looks not that different than one with Steve Daniels published values ...

ideal:



first order PU modeling:




and Hollis'

ideal:



with first order PU modeling:



... what's the source of instability in the Keen/Hollis designs ?!
curious problem in itself

correct solution falls out of basic (and singular) small-signal analysis of Kushner-Plunkett circuit

[thermionix]