True "variable inductor" wah...

[Paul Marossy]

Doesn't sound too bad. That's not a bad circuit you came up with. Good for if you want something that sounds a bit different.

The only commercially produced variable inductor wah ever produced that I know of is the old Kay wah made sometime in the 70s.

[armstrom]

Looks like a great circuit. Where did you find the inductor?

as for your issue with LTSpice, I had the same problem. To get 1M type in 1meg. Kinda strange.

[JKowalski]

I think LTspice reads 1M as one milliohm. That's why you have to type out meg.

That's a really neat idea - I might try that someday! It's alot cheaper/easier then buying or making your own wah pedal with all the mechanical issues and expensive pots.

[gmoon]

Doesn't sound too bad. That's not a bad circuit you came up with. Good for if you want something that sounds a bit different.

The only commercially produced variable inductor wah ever produced that I know of is the old Kay wah made sometime in the 70s.

Thanks. Didn't really expect it to sound like a vox/thomas wah clone, so "different" is good.

I'm aware of the Kay W-1, and this circuit is fairly close. Wouldn't want anyone to think this was completely my idea. But I was unable to find a schematic, so the best I could do was find a blurry photo of the Kay wah (which has a tiny schematic) and figure out the general topology.

And it's just a single-transistor preamp driving a passive LCR fiilter. Couldn't be simpler. I read somewhere that even a passive LCR filter is second-order, so it's more effective than one might think...

Looks like a great circuit. Where did you find the inductor?

as for your issue with LTSpice, I had the same problem. To get 1M type in 1meg. Kinda strange.

Thank you--the inductor is from a Wurlitzer tube organ tone generator. There were several (well, a whole octave's worth) of inductors, of two types: ~70-550mH, and ~115-800mH. I used the first type; I'll certainly try the second.

I've also got some other fixed inductors (from a Lowrey organ) that run the gamut from 70mH up to 1.5H--including two ~500mH. They are more similar to typical wah inductors (in size, inductance and DC resistance.) And other more oddball inductors, too.

Thanks for the LTspice tip. Yep, "meg" works great.

I think LTspice reads 1M as one milliohm. That's why you have to type out meg.

That's a really neat idea - I might try that someday! It's alot cheaper/easier then buying or making your own wah pedal with all the mechanical issues and expensive pots.

Appreciated, thanks. The pedal mechanicals are from an organ "swell" pedal, but you're right--this is simpler than the typical construction, and "from scratch" wouldn't be too difficult. The wooden parts were added because the inductor is tall, and the core needs to "exit" the inductor during the down stroke...

[Joe Kramer]

Hey Gmoon,

Really cool!  Sounds unique.  Thanks for the nice video and schemo.  I recently found some inductors from an old Thomas organ similar to the ones you are using.  Now I have a possible new way to use them! 

By the looks of your circuit, I wonder how much influence the output load has on the depth of the effect.  Have you tried hanging a hi-impedance (Darlington or FET) buffer on the output?

Thanks again for the nice work.

Joe

[gmoon]

Really cool!  Sounds unique.  Thanks for the nice video and schemo.  I recently found some inductors from an old Thomas organ similar to the ones you are using.  Now I have a possible new way to use them! 

By the looks of your circuit, I wonder how much influence the output load has on the depth of the effect.  Have you tried hanging a hi-impedance (Darlington or FET) buffer on the output?

Thanks, Joe, that's a good idea.

Maybe a follower? I might have a JFET or two around here...

[jgarnold]

Kay Wah schematic (cleaned up from the original)...

-Jeff

[Electron Tornado]

Way cool wah! Sounds nice.

[earthtonesaudio]

Check out that 1.5V supply... amazing headroom!   

[PRR]

> Kay Wah schematic

Thanks.

I wuz thinking your plan had low input impedance, but man that KAY really sucks on the guitar. About 5K-10K in. Bass down to half and all the top end loaded away.

> 1.5V supply... amazing headroom!

Yeah.... yet it may be fine. Gain to collector less than 3, and that 2:1 at the input, it may not clip often. When it does, the broadband splatter is then narrow-filtered by the L-C. So you don't hear fuzz, you hear a sweep of individual overtones.

Remember how a radio transmitter makes a perfect sine. You smack a tube into 1,000% distortion and swing an L-C flywheel which rounds it out  near-perfect.

And the L-C system has "passive gain" at its peak. Jeff may know how much. Counting with two rocks I get gain of 30 at the peak..... it will be much less with all the losses I ignored, but gain of 3 at the peak would give a fine signal.

[jgarnold]

>I wuz thinking your plan had low input impedance, but man that KAY really sucks on the guitar.

I was only presenting the original Kay schematic in a more readble format. I didn't change any of the original values. Obviously, the circuit could use a high-impedance FET input buffer (and maybe an output buffer as well).

I like the idea of making a wah pedal with a variable inductor. However, I'm very surprised that its inductance is specified as 3000mH. That's six times the inductance (500mH) found on your standard VOX/Dunlop wah pedal. The inductor doesn't look very large, but I don't know if a 6x increase in inductance necessarily means that the inductor will be six times larger.

Kay wah video... http://www.youtube.com/watch?v=iaS8uzvFckc

-Jeff

[zambo]

That thing makes the Robin Trower sound quite well imho. I really liked the tone on that vid. Cool you made your own tredle too. Hats off to you good sir!

[jasperoosthoek]

Thanks guys for bringing this up. My clone wah scratches a lot.  I should have bought the slightly more expensive pot. This might be scratch free.

Maybe this might be done with an old single coil pickup. Just remove the magnets and mount some ferrite material to the pedal.  

I remember seeing a wah circuit years ago with a variable radio tuning capacitor. I think it was tube based. I cannot find it anymore. Sounds familiar? That might be cool too.

[Skruffyhound]

I would add that I have never before read a post by Nasse where he was anything other than polite and informative, that organ must really have pissed you off mate.

Like the sound of this a lot Doug, thanks for the info.

[Derringer]

That thing makes the Robin Trower sound quite well imho. I really liked the tone on that vid. Cool you made your own tredle too. Hats off to you good sir!

yes ... what I heard in the vid brought to mind a controlled vibe-phase more than a wah

cool sounding indeed

[jgarnold]

Could somebody explain the relationship between the size of the inductor and the size of the capacitor in a wah pedal (as far as sonic qualities are concerned). The original poster is using a 550mH inductor and a .33uf capacitor, while the KAY pedal supposedly uses a 3000mH inductor and a .047uf capacitor. I understand why both circuits work because the ratio of the two sets of components is about the same (5:1). However, what are the advantages of using a larger inductor and a smaller capacitor (or vice-versa)?

3000mH / .047 uf
1500mH / .10 uf
1000mH / .15 uf
750mH / .22 uf
500mH / .33 uf
250mH / .68 uf
100mH / 1.5uf

At what point does scaling the size of the inductor down cease to give a wah pedal its unique sound?

-Jeff

[gmoon]

Thanks, Jeff, for resurrecting this project. I found those Kay wah photos online, too. Although I totally missed the transistor type at the bottom of the schematic, cool! It's a very simple, elegant circuit and now it's fully documented (except for the construction particulars of the inductor).

It's a simple parallel LC (or LCR, even without a separate resistor) band-pass filter. You can find the resonant frequency with: (or use an online calculator, if you prefer.)


For a given inductance, increasing the capacitance will lower the resonant freq. Impedance issues might have more to do with the large-value inductor than anything else (or more likely, it was readily available). Or more inductance = physically larger, and larger was more robust.

And thanks to everyone else for commenting, especially about the sound. I like it too (I like the Kay Wah, too.)

Yeah, this is "scratch free." As noted, the transistor doesn't have much gain, just enough to make up any losses in what's essentially a passive filter block. In fact, you could use the LCR filter by itself, and you'd still hear the effect (attenuated, but there).

Unfortunately, I never went beyond the "proto" stage of the project. Real life, other projects, etc. The usual story. I tried a lot of variations, buffers, etc., but still have ideas. I should have built one as-is, then moved on to other variations...

One idea used negative feedback for a more true active filter. It oscillated like mad, but I recently noticed a Twin-T circuit from a 1973 issue of "101 electronic projects" that uses a single transistor and NFB. They recommended a 2K-5K trim POT for the emitter resistor, adjusted to eliminate oscillation. I'll try that next...

I've also looked into other variable inductor types: variocouplers, variometers, spider coils. I've also checked out material permeability, etc. The geometry itself of a variable inductor would be fun to play with (like a pot taper, essentially).

[jgarnold]

>Although I totally missed the transistor type at the bottom of the schematic, cool!

The only reason that I knew that it was the transistor number is because my Fulltone pedal uses the same transistor. The BC109B and 2SC1815 would probably work just as well and they are much quieter.

>It's a simple parallel LC (or LCR, even without a separate resistor) band-pass filter.

I understand the formula for getting the resonant frequency, but how does a particular inductor/cap combination (with the same resonant frequency) affect the sound. Will a pedal that uses 3000mH/.047uf  sound the same as a 500mH/.33uf version? I could take this to an extreme by using a much larger capacitor and a much smaller inductor, such as .5mH/330uf (although a variable .5mH inductor would probably be too small to control with a foot pedal). I'm trying to understand how the particular values are chosen. There must be a reason why all VOX style wah pedals use a 500mH inductor.

-Jeff

[gmoon]

I suppose there's a realistic range that's partly dictated by the impedance of other surrounding components--much as RC coupling values in a tube amp. There also might be a sweet spot as far as DC resistance and "Q" value for a specific gauge/winding range. Maybe really small inductors (10mH) tend to saturate too readily at audio freq--I don't know, I've never tried one. Small inductors certainly work well with RF. Other factors are at play, too. Larger inductors cost more, and are just plain big.

I think you could easily build a working wah with a typical Vox circuit using an inductor within that range (3H - 100mH). At least one production wah pedal used a value around ~200=250mH (I can't recall which one, but someone else will).

Technically, there's no "strict" electronic difference between two widely different value inductors--assuming all the other factors are equal. But there are many more parameters to an inductor than a resistor or capacitor, not to mention core types and material. And most guitarist don't believe that a polyester and a ceramic cap are equal, even if the values are the same. So I won't speculate on inductor types--iron core, ferrite, etc., and which is "best." 

Personally, I'm not a big believer in inductor "mojo," but I should explain this. I've owned a Thomas Organ wah for ages (pre-Dunlop CryBaby). I used it for years but didn't really like it--until I got just the right amp. A little bit of natural overdrive, a taste of spring reverb and the classic wah sound just JUMPED out. The flaw was never in the wah at all.

That said, I'm not closing the door on the idea that different inductors can change the sound. But no single "sound" works for everyone, anyhoo...

[Paul Marossy]

I think you could easily build a working wah with a typical Vox circuit using an inductor within that range (3H - 100mH). At least one production wah pedal used a value around ~200=250mH (I can't recall which one, but someone else will).

That was the Vox "grey wah" with a 250mH inductor. I understand that the smaller the inductor is, the more the mids are accentuated and the less bass you get. I don't think you get a wah pedal with a 100mH inductor - too small to make it a resonant circuit.