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Hello, everybody! New member here.

Anyone else interested in the variable-inductor thing? When I was researching it, I ran across a thread or two--but not a whole lot.

I've managed to hack one up. Be glad to add more info if there's any interest....

Here's a youtube vid:
http://www.youtube.com/watch?v=lL9ToDSFz0E (http://www.youtube.com/watch?v=lL9ToDSFz0E)

Veeeeerry cool! - I like the woodwork!

SOunds like shit, my cheap computer came with crappy soundcard they say it is bad on every audio forum and it has autowah better than this, and so does many more... looks like finnish pine, fast grow weak wood shitty q wooden construction

Quote from: Nasse on April 03, 2009, 03:28:27 PM
SOunds like sh*t, my cheap computer came with crappy soundcard they say it is bad on every audio forum and it has autowah better than this, and so does many more... looks like finnish pine, fast grow weak wood sh*tty q wooden construction

Nice. Is this a typical reply here?

Quote from: gmoon on April 03, 2009, 03:34:41 PM
Nice. Is this a typical reply here?

No, it is not. Normally, if someone doesn't have anything nice to say, then they don't say anything.

:-[

I'm at work, I can't hear it... but I just took a quick look at your vid, the wood construction seems fine to me.

Quote from: jefe on April 03, 2009, 03:40:27 PM
I'm at work, I can't hear it... but I just took a quick look at your vid, the wood construction seems fine to me.

Thanks. I could post the usual excuses: it's very much a work-in-progress; kind of experimental; something about my chops being rusty, etc., etc., etc.

Buy why bother? I don't have much of an ego about that stuff. I just thought someone else might be interested, and that maybe folks here also discussed projects that were unusual and not "fully formed."

(the unfinished pedal is a combo of douglas fir (base) and rough cedar (range stop in front.) It will get treated to sanding and routing later. No circuitry's been added inside, yet--still protoboarded.)

@ alanlan:  :)

I´m just kiddin, but this is serious fx forum...

Watched the Youtube,  Nice job on the pedal!!!!

Keep up the cool work!

Peace!
Arn C.

Quote from: gmoon on April 03, 2009, 03:49:12 PM
Thanks. I could post the usual excuses: it's very much a work-in-progress; kind of experimental; something about my chops being rusty, etc., etc., etc.

Buy why bother? I don't have much of an ego about that stuff. I just thought someone else might be interested, and that maybe folks here also discussed projects that were unusual and not "fully formed."

(the unfinished pedal is a combo of douglas fir (base) and rough cedar (range stop in front.) It will get treated to sanding and routing later. No circuitry's been added inside, yet--still protoboarded.)

@ alanlan:  :)

I like your attitude. I never make any apologies for my crappy playing. And half-baked ideas get discussed around here all the time... A LOT. If people only presented their completely finished projects here, with all the bugs worked out... there would be a lot less projects to discuss.

I'll give this a listen later when I'm home.

Wow, that sounded real good. Very full, thick throaty and vocal. Seems like it has a lot of "substance" to the tone.

Care to show/explain what is happening inside?

Quote from: Nasse on April 03, 2009, 03:49:50 PM
I´m just kiddin, but this is serious fx forum...

Yes, serious about helping out beginners and experienced folk alike.  Not serious as in "holier than anyone else".

Thanks, Arn C.

Quote from: kurtlives on April 03, 2009, 04:01:30 PM
Wow, that sounded real good. Very full, thick throaty and vocal. Seems like it has a lot of "substance" to the tone.

Care to show/explain what is happening inside?

Sure--I should upload images to the "general" pictures gallery?

The inductor was scavenged from an old Wurlitzer organ, and was used in the tuning circuit. It's a typical rotational-adjustment variable inductor. It has a ferrite core attached to a brass rod. With the end tap removed, the core slides easily in / out of the cardboard tube that the coil's wrapped around. There's an outer ferrite housing, too. The inductor's range is 550mH to 72mH (I have a cheapo LCR meter--thank you Hong Kong); the core moves ~1.35 inches.

The circuit follows the Kay W-1 wah--but I couldn't find a schematic, so I had to enlarge online photos of the schematic that's mounted inside. My version is somewhat different, of course--the Kay wah used a 3H inductor (plus I'm guess many of the component values.)

I worked it up in LTSpice, which at least got me in the ball park.

It's really nothing but a preamp feeding a passive LCR filter. Just a single transistor. Could a more effective active filter (NFB loop) be used? Sure! I just haven't gotten there yet...

The pedal mechanism was also scavenged from an organ. With the core moving in / out of the inductor, a "well" beneath the pedal housing was needed--hence the wooden platform. The inductor was "jumping" on the last part of the youtube vid--I was tilting the pedal for photos, and it came "unstuck." Easy to fix.

So the treadle actually moves the ferrite core in and out of the inductor coils? That's very cunning! :)

Nice sound too. Early on, at least, it had a sort of vibey rather than trad wah sound to it. I liked it.

>but this is serious fx forum...

Yes, we are serious about being nice to one another. Please keep the negativity down.

Quote from: Nasse on April 03, 2009, 03:49:50 PM
I´m just kiddin, but this is serious fx forum...

You missed when I came up with the Riot Zone, then...

Quote from: Andi on April 03, 2009, 05:45:32 PM
So the treadle actually moves the ferrite core in and out of the inductor coils? That's very cunning! :)

Nice sound too. Early on, at least, it had a sort of vibey rather than trad wah sound to it. I liked it.

Grazie. It's not a unique idea, but not very common, either.

Since the treadle motion is really an arc, neither the coil or the core move in a truly linear fashion. So both are fixed to flexible anchors.

(http://www.instructables.com/files/deriv/FBG/5LVC/FT3K4CL5/FBG5LVCFT3K4CL5.LARGE.jpg)

I had a more sophisticated design, but dropped it. This simple approach seems to work so far.

The inductor, unmodified (except it's been removed from the aluminum housing.)

(http://www.instructables.com/files/deriv/FWL/2JLX/FT3K4CL3/FWL2JLXFT3K4CL3.LARGE.jpg)

The modified inductor: the delicate parts re-enforced, length shortened, etc.
(http://www.instructables.com/files/deriv/FZN/OJL9/FT3K4CL4/FZNOJL9FT3K4CL4.LARGE.jpg)

And here's a schematic.

More info:


-- Clearly, a very simple circuit.

-- Substituted the rectifier diode for the emitter bias resistor, and like the change. Even tried an LED, which worked also. Paralleling a resistor with the diode (10K or so) is kinda nice, too.

-- The DC resistance of the inductor is pretty high (185 ohms), which certainly effects the "Q". Might try "rolling my own" inductor...

-- Chose the 2n2222 simply because LTspice had the model. Other BJTs would likely be preferable. Not bad, though.

-- LTspice doesn't like 1M (converts to 1m)--maybe there's another abbrev it will tolerate... Until then, 1000K should be clear enough.

(http://www.instructables.com/files/deriv/FZY/ASTN/FT3K4HHJ/FZYASTNFT3K4HHJ.LARGE.jpg)

I am sorry about my stupid post. I was too tired when tuning my bottle blow organ, needed to drop the tuning. And my speakers are crap too, listened with another set and it sounds nice. Nice report, thank you.

Quote from: Nasse on April 05, 2009, 06:02:43 AM
I am sorry about my stupid post. I was too tired when tuning my bottle blow organ, needed to drop the tuning. And my speakers are crap too, listened with another set and it sounds nice. Nice report, thank you.

Thanks...and it's water under the bridge, my friend. We all have those days...

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.

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.

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.

Quote from: Paul Marossy on April 05, 2009, 10:10:19 AM
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...

Quote from: armstrom on April 05, 2009, 11:14:12 AM
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.

Quote from: JKowalski on April 05, 2009, 12:23:17 PM
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...

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

Quote from: Joe Kramer on April 06, 2009, 03:14:24 PM
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...

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

-Jeff


(http://www.goldenhawk.com/PICS/KayWahSchematic.jpg)


(http://www.goldenhawk.com/PICS/KayWahOriginalSchematic.jpg)

Way cool wah! Sounds nice.

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

> 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.

(http://i55.tinypic.com/119wiz6.jpg)

>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

(http://www.goldenhawk.com/PICS/KayWahInductor.jpg)

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!

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.

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.

Quote from: zambo on September 24, 2010, 03:05:41 PM
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

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

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.)
(http://www.whatcircuits.com/online_tools/LC_image/lc_resonance_formula%202.png)

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).

>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

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...

Quote from: gmoon on September 25, 2010, 07:56:16 PM
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.

Paul, I was thinking the Vox circuit, but with some value changes (R, C). Whatever limiting factors are involved, I think inductance is only a part. 500mH is a sweet spot for that particular circuit, I'm sure. But the stock values in Vox and Crybaby wahs vary by about 200mH anyway. If 250mH worked in the "grey wah," then 100mH would also. Whether it would sound good is another story.

The variable inductor in this simple circuit is about 550mH to 75mH, and it certainly works over the entire range. I won't say that it doesn't begin to attenuate the signal at the higher inductance, but the inductor (and the LC filter) itself might not be to blame.

About the time this came up I was thinkin about an active "Q" circuit for something, perhaps it was op amp wah. Cant remember what circuit it was, perhaps too much for my brains- try to find it some day

Quote from: gmoon on September 25, 2010, 09:24:34 PM
Paul, I was thinking the Vox circuit, but with some value changes (R, C). Whatever limiting factors are involved, I think inductance is only a part. 500mH is a sweet spot for that particular circuit, I'm sure. But the stock values in Vox and Crybaby wahs vary by about 200mH anyway. If 250mH worked in the "grey wah," then 100mH would also. Whether it would sound good is another story.

The variable inductor in this simple circuit is about 550mH to 75mH, and it certainly works over the entire range. I won't say that it doesn't begin to attenuate the signal at the higher inductance, but the inductor (and the LC filter) itself might not be to blame.

Looking at the schematic, which I didn't know was in this thread, I see your point. I was thinking it was more like the CryBaby circuit, where I think it would matter very much if it were only a 100mH inductor. But it might still work fine. Might be an interesting experiment for someone to try...

> 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)?

Impedance.

550mH inductor and a 0.33uf capacitor is 370Hz. 3000mH inductor and a .047uf capacitor is 424Hz, not real different since both are variable.

What is the impedance of 550mH at 370Hz? 0.55H*370Hz*6.28 is 1,278 ohms.

On that other hand,  3H*424Hz*6.28 is 7,988 ohms.

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

When the impedance gets so small that it shorts everything out.

A teeny coil could need a HIGH current driver like a loudspeaker chip. At present prices, a '386 may be cheaper than a high-Inductance coil. So it's just a design-economics decision.

Flexible anchors? Have you considered a sort of "cradle" or hinge of some kind? It would probably be a bit more robust.
Cool video. I'm not sure if the video was synched with the audio, but it was fun to listen to.

Does anyone know where you can get these type of inductors? The kay wah seems like a very nice project that I would like to build. And I like working with germanium trannies ;D.

>And I like working with germanium trannies

The KAY wah pedal uses the 2SC828R, which is a standard silicon transistor. Both the BC109B and 2SC1815GR (very low noise) appear to be suitable replacements.

-Jeff

Nice work, pedal looks and sounds awesome to me.

1.st link, figs.. 6-11... (http://www.diystompboxes.com/smfforum/index.php?topic=87244.0)

I thought what if there is a resistor too and it was made variable too if it is such circuit

Quote from: PRR on September 26, 2010, 03:53:14 PM
When the impedance gets so small that it shorts everything out.

A teeny coil could need a HIGH current driver like a loudspeaker chip. At present prices, a '386 may be cheaper than a high-Inductance coil. So it's just a design-economics decision.

Well put, thanks.

There are other factors that limit using smaller inductors as well. For a parallel RLC circuit the higher the inductance (and the lower the capacitance), the wider the bandwidth (lower the Q), or the L/C ratio. So there's probably a realistic range. Too high a "Q" just doesn't sound vocal, and too low isn't a "wah."

Of course, using a series circuit (notch) in a negative-feedback configuration could address the wider bandwidth issue, since series RLC works opposite of parallel, bandwidth-wise...

Quote from: jasperoosthoek on September 27, 2010, 08:03:41 AM
Does anyone know where you can get these type of inductors? The kay wah seems like a very nice project that I would like to build.

Nope. This was either a custom part at that time, or a long-obsoleted off-the-shelf part. Probably custom, 'cause the movable core is completely detached from the coil. Mechanical variable inductors are going the way of the dinosaurs, at least in the larger inductances--as are variable capacitors, and large transformers in general.

Variable inductors were very common in radios, etc.--variocouplers, variometers, and cool things called "spider coils." Unfortunately, most of these are LARGE (large is often expensive).

Including materials with high magnetic permeability (ferrite, etc.) changed how inductors are made, increasing inductance while reducing size. There is so much potential for reproducing the Kay inductor, or a reasonable replacement (or an equivalent audio circuit with a different inductance range, which mine is) and that's exciting to me.

Obviously, I find inductors and inductor materials fascinating... :-\

Quote from: BadIdeas on September 26, 2010, 07:17:21 PM
Flexible anchors? Have you considered a sort of "cradle" or hinge of some kind? It would probably be a bit more robust.
Cool video. I'm not sure if the video was synched with the audio, but it was fun to listen to.

I designed what I *think* is a better pedal mount, but it's not really important at this stage. Especially if other inductors are made/found.

If you do a little research on the Kay W-1 wah, you'll find that for one of the two I know of (posted on the interwebs), the owner had to repair/replace the mounting plate, which broke after years of flexing. So even the original had mechanical flaws (at best, according to some  :) ).

The vid might be a bit off. I can't edit videos in the native format of my camera. Some of that could be "undamped" resonance, too.

Quote from: bacanador on September 27, 2010, 05:27:37 PM
Nice work, pedal looks and sounds awesome to me.

Thanks for all the encouragement everyone is giving. I'm gonna revive this, and I already have a new jump-off point...

Quote from: Nasse on September 28, 2010, 12:00:51 AM
I thought what if there is a resistor too and it was made variable too if it is such circuit

In an RLC filter, at least, the "R" mostly effects the Q factor and bandwidth. In a Vox/Thomas wah, the resistor works in concert with other components to vary the capacitance (you probably already knew that...)

Thanks! Wah inductors don't have a donut shaped core but a cylinder, right?

Maybe if I buy a cheap inductor I can make it moveable. Maybe use the crybaby circuit or the crybaby without the second feedback transistor and a bigger cap... Then I just as well could build a Kay type wah.

Quote from: jasperoosthoek on September 28, 2010, 01:50:13 PM
Thanks! Wah inductors don't have a donut shaped core but a cylinder, right?

Maybe if I buy a cheap inductor I can make it moveable. Maybe use the crybaby circuit or the crybaby without the second feedback transistor and a bigger cap... Then I just as well could build a Kay type wah.

Meaning they are not toroidal? Yes, in general (although a toroidal one with the correct inductance would probably work in a Vox wah).

You can find coils in transformers and speakers, too. The toughest part is finding a close-fitting, high-permeability core to move in and out. Steel is passable, Ferrite and other materials are better. Air has very low permeabilty.

The original Kay coil (like mine) has the coil encased in ferrite as well. AFAIK, most wah inductors use a ferrite enclosure and core (stationary), too. But their small size works against them in this application...

Quote from: gmoon on September 28, 2010, 09:34:51 PM
Meaning they are not toroidal? Yes, in general (although a toroidal one with the correct inductance would probably work in a Vox wah).

That was the word I needed :). So you mean the ferrite is around the coil instead of in the coil? Never thought about that possibility, but why not.

How about tunable capacitors from old radios? They are very cheap and easy to get. I bought a few old Philips tube radios for 2 euros. They usually contain some nice tubes (EL84, EZ80 etc.) a chunky power transformer that can be used for an AX84 P1 or High Octane and, a tunable capacitor.

I should get one of those cheap LCR meters from Hong Kong so I can easily measure them myself. Anyone bought this one? http://cgi.ebay.com/DM4070-2000uF-3-1-2-Digital-LCR-meter-w-self-discharge-/170505428466?pt=Radio_Control_Parts_Accessories&hash=item27b2ea5df2 (http://cgi.ebay.com/DM4070-2000uF-3-1-2-Digital-LCR-meter-w-self-discharge-/170505428466?pt=Radio_Control_Parts_Accessories&hash=item27b2ea5df2)

Quote from: jasperoosthoek on September 29, 2010, 05:50:23 AM
That was the word I needed :). So you mean the ferrite is around the coil instead of in the coil? Never thought about that possibility, but why not.

Yeah, there's a ferrite sleeve around the coil. The core and sleeve work together to alter the inductance--not that different than typical transformers. Chokes, for instance, are constructed like transformers and have a wraparound core.

Quote from: jasperoosthoek on September 29, 2010, 05:50:23 AM
How about tunable capacitors from old radios? They are very cheap and easy to get. I bought a few old Philips tube radios for 2 euros. They usually contain some nice tubes (EL84, EZ80 etc.) a chunky power transformer that can be used for an AX84 P1 or High Octane and, a tunable capacitor.

I should get one of those cheap LCR meters from Hong Kong so I can easily measure them myself. Anyone bought this one? http://cgi.ebay.com/DM4070-2000uF-3-1-2-Digital-LCR-meter-w-self-discharge-/170505428466?pt=Radio_Control_Parts_Accessories&hash=item27b2ea5df2 (http://cgi.ebay.com/DM4070-2000uF-3-1-2-Digital-LCR-meter-w-self-discharge-/170505428466?pt=Radio_Control_Parts_Accessories&hash=item27b2ea5df2)

Radio inductors are usually in the uH range; good for RF applications. They're mostly air-core, or have a small tuning slug. It's possible their inductance could be lowered with a ferrite sleeve and core...

It is usually the "iron" that's most valuable in old radios and hifi amps. Used tubes are iffy, but the power transformers and output transformers are the two single most expensive parts of a tube amp, so that's a find. Variable capacitors are generally very low capacitance, in the 100's of picoFarads range for the larger ones. Not quite so useful for audio.

I don't have that specific LCR meter, but I've got a similar (~$20) "Hong Kong special."

Quote from: gmoon on September 29, 2010, 08:53:29 AM
Variable capacitors are generally very low capacitance, in the 100's of picoFarads range for the larger ones. Not quite so useful for audio.

Very low indeed but maybe if you would use a suitable Miller feedback arrangement like in the crybaby it could be boosted. I remember that someone built a tube based wah with a variable capacitor (8 years ago). Can't find it anymore and I'm not that sure about it...

Take this one for instance: http://cgi.ebay.com/Triple-3-Gang-500pF-Variable-Radio-Tuning-Capacitor-/150490943445?pt=UK_ConsumerElectronics_SpecialistRadioEquipment_SM&hash=item2309f58fd5 (http://cgi.ebay.com/Triple-3-Gang-500pF-Variable-Radio-Tuning-Capacitor-/150490943445?pt=UK_ConsumerElectronics_SpecialistRadioEquipment_SM&hash=item2309f58fd5)
3x500pF = 1.5uF. That's a very big one... If I take the LC values from the Kay wah the capacitor is 31 times too small. I'd have to run some simulations to see if you could boost the capacitance that much without oscillation and if you wouldn't loose headroom. Maybe I'll look at the crybaby wah for inspiration.

3 X 500pF is 1500pF or 1.5 nF or 0.0015 uF. Pretty small
I have played around with a large air core variable cap in a few circuits, the ends of the range gave at best a barely audible difference. You would have to strengthen the response significantly for it to be interesting.
Then there's the thing that they are enormous and can be expensive.
Good luck to you though, a circuit that could make a variable cap viable would be a great find.

Scruffyhound's right on this one. Still, if anyone wants to create a "true" variable-capacitance wah, who cares if it's half the size of your guitar case? Impractical or not, it would still be cool...

Info on the Wurlitzer inductors:

#1
Stamped on the alum cover: "500805 SECO"
550uH - 70uH range (7.86:1)
195 ohms DC resistance

#2
Stamped on the alum cover: "500807 SECO"
790uH - 110uH range (7.18:1)
260 ohms DC resistance

Each has a paper label on the end, with:
Pasted on the end:
"WURLITZER CORINTH" & "TUNE TO F" (the note "F" is different for each)

I've only used inductor #1 in my project. I'm inclined now to think #2 is the better choice, although the inductance ratio is greater on the first. I wish the DC resistance for both were lower; a design consideration for the future. I'm sure either would work, with fine-tuning of capacitance values.

The core extends out of the inductor when "floored,"--i.e., higher inductance in the "up position" (bass range), lower L in the treble.

I doubt the change in inductance is linear. It's rather difficult to tell.

What I meant was something like this:
(http://img844.imageshack.us/img844/774/variablecapacitor.png)
The opamp basically replicates and inverts the signal on the left side of the capacitor. So an inverted signal amplified 33 times will appear on the other side of the cap. This means that the current into the cap will become 34 times larger which effectively makes the capacitance 34 times larger turning it into a 0.051uF cap. Because the 100k resistor is present in the original circuit it can be conveniently used in the opamp arrangement. ;D

I don't know if it's stable. Maybe a 4.7p or 10p cap across the 3.3M might help to cut frequencies above 5 or 10kHz resp. You need some extra headroom but I don't think that will be a problem. Say a 100mV signal on the left side will be 3.3V on the other side. Nothing that spectacular. Just an idea.

Give it a go, jasperoosthoek.

Seems to me there's a one-word name for that capacitance circuit, but typically, it escapes me...

That word is actually Miller. All classic wahs are based on sending an inverted signal to the other side of the cap thereby multiplying the caps value.

All I do is use a variable cap instead of a variable gain with a pot. And I use bigger coils and smaller caps.

Right now I don't have the time to try that idea. :( I've been receiving packages all week as I'm building a new guitar ;D. Also my Firefly has been finished halfway...

Naw, I'm familiar with the Miller effect, from mucking with tube amps. It's another term.

Maybe I'll recall it. Or maybe I'm mistaken. Anyway, have fun!

Quote from: gmoon on September 30, 2010, 03:59:40 PM
Give it a go, jasperoosthoek.

Seems to me there's a one-word name for that capacitance circuit, but typically, it escapes me...

Gyrator?

Quote from: SISKO on September 30, 2010, 08:31:22 PM
Gyrator?

THAT'S the one... thanks.

(and I was a little off the mark, as it's simulating inductance rather than multiplying capacitance...)

> the wider the bandwidth (lower the Q), or the L/C ratio. So there's probably a realistic range.

(as you know) You adjust this with the R.

In this plan, there's two. The amplifier (transistor) has an output impedance in series with (part of) the cap (and thus the tank). The coil *always* has series resistance. Lower is higher Q. The resistor parallel to the coil also matters,higher is higher Q.

> So there's probably a realistic range.

There's a "practical" range of impedances. The 0.55H 0.33uFd team is nominal 1.3K. For significant Q we want series resistance of a few hundred ohms and shunt resistance of many K. Coils this big are hard to get. A 0.5 milliHenry coil is easier to find. Load can be a few dozen ohms (easy) but source should be under One Ohm (oof!). Even a LM386 will have trouble driving one ohm impedance; 1-transistor stages won't do the job in guitar systems.

> Too high a "Q" just doesn't sound vocal, and too low isn't a "wah."

I don't know wah. In general audio manipulation, Q of 0.7 to 1.3 will give a significant broad boost. Anything higher, in general audio, sounds "ringy" and "peaky". Obviously a wah works in the "ringy zone". But if Q were far over 10, it would peak an overtone then there would be a "gap" until it crosses the next overtone. So the "ideal Q" depends on how dense the overtones are. We could postulate and interpolate. It may be easier to just try. Easier yet to copy Q ratios from existing wahs.

And there's another conflict with reality. Because of coil winding resistance, even with closed iron cores, it is hard to get Q of a small coil over unity at the bottom of the audio band. In the midrange Qs of a few dozen are not too hard... but a variable inductor usually won't be a _closed_ core so Q will be lower. So it may be real difficult to end up at "too high" a Q.

Using Hammond/Wurlie coils is brilliant. They are audio-scale values, and VERY well designed. An organ has a dozen, and the organ-harvesters usually throw away the tone generators.

> I wish the DC resistance for both were lower

The Q was "as high as reasonable" for good oscillator stability. The old guys knew what they were doing. To lower R, you could use larger wire, but the larger build means lower L, so you need more turns to get your L, and that's more R. Q probably scales up with size, but probably very slowly: square-root or even cube-root of linear dimension. If it is ^0.33 then double Q means 8 times longer and 500 times as much costly stuff. We may have better ferrites, though I think most work is for better >5KHz performance (audio is a dead-end).

> How about tunable capacitors from old radios?

Generally 356pFd maximum. Which is about 200K at 2KHz, 2Megs at 200Hz. Your shunt resistance must be much higher, meaning many-Megs. I guess it could be done.

The high impedance does attract electrostatic interference. This same trick, though usually with a bigger (very custom) cap, was used in many sine-wave audio oscillators. The case must be VERY tight, the internal wiring very clean, or the output is contaminated with buzz and fluorescent lamp hash.

Also these tuning caps ARE getting rare, hard to find.

Quote from: gmoon on September 30, 2010, 08:51:08 PM

Quote from: SISKO on September 30, 2010, 08:31:22 PM
Gyrator?

THAT'S the one... thanks.

(and I was a little off the mark, as it's simulating inductance rather than multiplying capacitance...)

Don't take my word for it:
(http://www.geofex.com/article_folders/wahpedl/wah7.gif)
Taken from the wah bible: http://www.geofex.com/article_folders/wahpedl/wahped.htm (http://www.geofex.com/article_folders/wahpedl/wahped.htm)

No, it's not a Gyrator. I don't simulate an inductance with a feedback circuit. I use a true inductor. Of course you can use a gyrator anyway... As I said, It's a Miller effect capacitor with a parallel resistor. What I did is basically the same as R.G.s circuit, albeit with a single opamp. And I've used the Q resistor in the feedback loop. I miss one resistor though which can be easily implemented.

Quote from: PRR on October 01, 2010, 12:18:47 AM
Generally 356pFd maximum. Which is about 200K at 2KHz, 2Megs at 200Hz. Your shunt resistance must be much higher, meaning many-Megs. I guess it could be done.

If you use the bare capacitance it will be quite difficult. I agree.

Quote
The high impedance does attract electrostatic interference. This same trick, though usually with a bigger (very custom) cap, was used in many sine-wave audio oscillators. The case must be VERY tight, the internal wiring very clean, or the output is contaminated with buzz and fluorescent lamp hash.

Maybe the biggest problem. That's one big antenna.

Quote
Also these tuning caps ARE getting rare, hard to find.

Search for "variable capacitor" on ebay. I get 339 hits, more than half of them could work. I only get 34 hits for "variable inductor". There is one 33H unit which might be suitable...

Very rare for someone trying make a living out of it. Quite abundant for the lone tinkerer who decides to build a single unit and very abundant compared to variable inductors. I can also buy them right now from a thrift stores. I probably threw away a couple of them some years ago. I know I shouldn't have but I try not to be a hoarder...

(Evil misunderstood villain voice)
You guys just keep pushing and pushin! :icon_evil: With this circuit I will rule the world!
(/Evil misunderstood villain voice)

I might even try to breadboard it. ;) Not to prove anyone right or wrong but I'm kind of interested myself if you can use it at audio frequencies. I have a 5H hammond choke that I can use as a passive gyrator simulation :icon_mrgreen:.

But I actually feel that if you Millerize the capacitor with a feedback loop you might as well go all the way and replace the passive inductor with a gyrator. Maybe even take a plastic tuning capacitor from an 80s radio.

Quote from: jasperoosthoek on October 01, 2010, 06:25:35 AM
Don't take my word for it:

Oh I didn't. I've had a fair understanding of the Vox/thomas circuit for a while (thanks, R.G.), and looked up the online patents for multiplying Miller capacitance. The "Gyrator" comment was an aside (but one that belongs in the conversation).

Not getting enough encouragement? OK, third time  :) : Work on it. Please. It's an idea worth exploring.

Quote from: jasperoosthoek on October 01, 2010, 06:25:35 AM
Search for "variable capacitor" on ebay. I get 339 hits, more than half of them could work. I only get 34 hits for "variable inductor". There is one 33H unit which might be suitable...

Very rare for someone trying make a living out of it. Quite abundant for the lone tinkerer who decides to build a single unit and very abundant compared to variable inductors. I can also buy them right now from a thrift stores. I probably threw away a couple of them some years ago. I know I shouldn't have but I try not to be a hoarder...

OK, just playing "devil's advocate" here. Another member of the forum has identified a source for the same Wurlitzer inductors I'm using; and they're NOS, untouched. Apparently they have LOTS of them. Wurlitzer was a fairly large company, and must have had "authorized" regional repair centers, etc. Independent service centers bought up the parts, probably in the 1980's (or the authorized centers were already independents). I'll let the other forum member identify the source, if he wants to.

Plus there are WAY more inductors, chokes, transformers (and ferrite objects) currently being produced the world than those old butterfly caps, so a custom variable inductor, while expensive, could be ordered and delivered fairly quickly (if it weren't radically different in design).

Frankly, it's all beside the point, from my POV. I'm exploring an interesting design, not setting up for production. To paraphrase an old saying:

Want to make a small fortune building boutique guitar effects? Start with a large fortune...

Thanks, PRR. As always, very pertinent and useful.

Quote from: PRR on October 01, 2010, 12:18:47 AM
And there's another conflict with reality. Because of coil winding resistance, even with closed iron cores, it is hard to get Q of a small coil over unity at the bottom of the audio band. In the midrange Qs of a few dozen are not too hard... but a variable inductor usually won't be a _closed_ core so Q will be lower. So it may be real difficult to end up at "too high" a Q.

Right, as usual. I've disassembled and tested a few chokes. Removing the interleaved steel core reduces the inductance tremendously. "Interleaved" and "closed" being of equal importance--inserting the core leaves from one side doesn't replicate the flux of the closed core.

Sorta drives home concepts about output transformer construction, and why that "air gap" is needed for SE OTs.

Quote from: PRR on October 01, 2010, 12:18:47 AM
The Q was "as high as reasonable" for good oscillator stability. The old guys knew what they were doing. To lower R, you could use larger wire, but the larger build means lower L, so you need more turns to get your L, and that's more R. Q probably scales up with size, but probably very slowly: square-root or even cube-root of linear dimension. If it is ^0.33 then double Q means 8 times longer and 500 times as much costly stuff. We may have better ferrites, though I think most work is for better >5KHz performance (audio is a dead-end).

Size matters  :). That becomes apparent when examining old school inductors (like the spider coil). Makes perfect sense: dimension vs. area, etc.

Some funky stuff is at least worth thinking about: combinations of interleaved coils (ala spider), ferrite enclosures, and even movable, selectively interleaved closed cores are possible. Maybe Mu metal leaves as well between coils. Alright, these are a little much, compared to known, proven tech  ;)...

Quote from: gmoon on October 01, 2010, 09:22:38 AM
Oh I didn't. I've had a fair understanding of the Vox/thomas circuit for a while (thanks, R.G.), and looked up the online patents for multiplying Miller capacitance. The "Gyrator" comment was an aside (but one that belongs in the conversation).

Not getting enough encouragement? OK, third time  :) : Work on it. Please. It's an idea worth exploring.

I always read too fast. I thought you meant that I designed a gyrator and you needed more convincing. You did know exactly what we were talking about, nice :).

Encouragement is not what I'm after. I thought you guys meant that it would never work. You are right, inductors any plenyful but saying variable caps are hard to obtain is wrong. On ebay you still have a large selection to choose from for a price lower than a good wah inductor.

The interesting thing is that this could both work in a miller and gyrator type circuit. The last one is even better as you don't need an inductor anymore. I have one lying somewhere else. I might even breadboard it after the weekend. :)

You could press the polepieces out of a single coil pickup and replace them with slightly-smaller ferrites, then use that as your variable inductor. 


For that matter, how about a guitar pickup with all its polepieces mounted on a common lever, so you could just drop them all down or raise them up to adjust output? :icon_idea:

Quote from: earthtonesaudio on October 02, 2010, 10:06:04 AM
You could press the polepieces out of a single coil pickup and replace them with slightly-smaller ferrites, then use that as your variable inductor.  
For that matter, how about a guitar pickup with all its polepieces mounted on a common lever, so you could just drop them all down or raise them up to adjust output? :icon_idea:

Good thought. However, the DC resistance of pickups (even single coil) is too high to maintain a decent Q factor. At least in this circuit, at these frequencies.

The Wurli inductor is 190 ohms. Vox wah fixed-inductors range between 15 and 85 ohms. It's possible to find cheapo 1960's MIJ pickups as low as 1.3K DC Res, but the frequency response looks like an ordinary RC filter above 1K. It flattens out considerably @ 500-600 ohms.

But the idea of using a pickup bobbin, one with a slot rather than holes--and a ferrite bar--is inline with what I'm saying about inductor geometry. The shape itself could really have a effect on the linearity of the response, just like a custom-taper POT...

I like that idea!

Goes really off topic, true inductor but this must have been what I was readin

http://www.elektor.com/magazines/2000/december/1-kh-synthetic-inductor.56662.lynkx (http://www.elektor.com/magazines/2000/december/1-kh-synthetic-inductor.56662.lynkx)
http://electronicsprojects.mediadir.in/category/electronics-circuits/page/38/ (http://electronicsprojects.mediadir.in/category/electronics-circuits/page/38/)

Quote from: Nasse on October 03, 2010, 02:28:32 AM
Goes really off topic, true inductor but this must have been what I was readin

The gyrator stuff looks pretty useful to me for filters. AFAIK, they may have already been used in an inductor-less wah, somewhere.

Gyrators do fall short of inductors in some ways, though. They can't store energy in the same fashion, and the damping characteristics are different, so they might not resonate quite like a real inductor. For some applications, that's a good thing. For a wah, maybe not. I'm sure they are worth investigating, and I'd guess that the results would be at least as good as a Twin-T wah...

Seems to me (unsubstantiated, at this point), that a variable-inductor filter interacts to dynamic core movement in a fairly complex fashion.

> Gyrators ... can't store energy in the same fashion

No, they get energy from the battery. So they can be "better", or you can throttle to essentially the same action.

> you might as well go all the way and...

At this level of complex abstraction, you may as well go to the next step. Lots of variable-center bandpass filter designs exist. The simple Moog ladder is low-pass which can be fed-back as a ringy low-pass, not a true bandpass. Two multipliers (may be opto-resistor dividers or OTAs) and a quad opamp makes a lovely 2nd-order filter (same as an L and a C). Switched-capacitor filters can be varied with a digital clock. Both of these offer in addition to bandpass, hi, lo, and notch connections (not a Wah, but maybe of some other use).

But stone-age and bronze-age technology IS fun to play with.

> ebay.. There is one 33H unit

I'll bet the seller can't read milli/micro inductance units.

Some of the 339 variable caps will be 30pFd-10pFd jobs which are great for FM/VHF work but will be hard to use in audio.

Many larger caps are "grounded frame" which spoils simple Miller-multiplication. You can float the frame on plastic and use a plastic knob, but it still tends to be a buzz/radio antenna (and Miller will multiply that too).

No doubt you can get something to work, without any post-1960 technology, and have fun in the process.

Quote from: PRR on October 06, 2010, 01:31:59 AM
Many larger caps are "grounded frame" which spoils simple Miller-multiplication. You can float the frame on plastic and use a plastic knob, but it still tends to be a buzz/radio antenna (and Miller will multiply that too).

My fear also as expressed in a previous post. Indeed my variable cap has a 'grounded frame', at least if you mean that the frame is connected to one side of the cap (the moving side in my case). Mine is currently connected to a breadboard with a 5H hammond choke. I'm eager to start testing this idea. The choke was not designed for audio either but at least it only has a 135 ohms series resistance. So I expect the Q to be very similar or even slightly larger than the 500mH fixed inductors with 15 to 85 ohms series resistance.

I haven't been completely asleep. I rejected a different two-transistor circuit, and moved on. Here's a new version; this one uses an LM324 op amp instead of the BJ transistors. Here's a short and rough vid clip:

http://www.youtube.com/watch?v=wQOg6E0zFrk (http://www.youtube.com/watch?v=wQOg6E0zFrk)

The LCR filter is setup as a notch filter in a negative feedback configuration, which is different than the Kay original. Thanks to the higher impedance op amp, there is WAY more high end definition than with the BJT. Since I only used two of the quad amps, I'm sure a non-inverting input buffer stage could be added for higher input impedance (and an output buffer, too).

The vid sample omits the R3 resistor, so the Q is at the max for this setup. Frankly, it's more vocal with a lower Q factor, but it still sounds cool.

I apologize (again) for the audio quality, it's a crappy camera.
EDIT: yeah, that really is poor quality audio. It's not that distorted.

(http://www.instructables.com/files/deriv/FK6/5HOZ/GFA3YPA9/FK65HOZGFA3YPA9.LARGE.jpg)

Here's the freq response (approx, I had to overlay one graph and resize that by hand, but it's within a couple dB of the sim). Blue is @ 0.07mH, Red is @ 0.57mH.

(http://www.instructables.com/files/deriv/FWL/6578/GFDX0J6U/FWL6578GFDX0J6U.LARGE.jpg)

Mind you, there are a lot of variables I don't know about the inductors, so LTspice sims should be taken with some skepticism. However, I don't think it's too far off.

I'll watch your video tomorrow when I have audio. :)

I haven't slept all the time either. Among other things I breadboarded the variable capacitor wah. Unfortunately it oscillated albeit at audio frequencies from a mid to a high 20k-ish pitch. It least it proves that the miller arrangement worked. I need to simulate it to get some better results.

I have one question about the Kay: the output resistance of the transistor is 330 in parallel to the collector impedance. How can such a small load resistance generate a good Q? I tried to replicate that value in my cap wah but it sounded like a high cutoff with a slight peak maybe. It oscillated at higher resistances loading the cap and inductor. At least the scope traces with a block signal didn't show a decaying sine, which is what you expect. So how does that work in the Kay?

I have to spend some more time on this, maybe I just overlooked something.

I couldn't stand the audio quality on that vid, so I recorded a couple more clips with decent equipment.

http://www.youtube.com/watch?v=MuY76kIDLGk (http://www.youtube.com/watch?v=MuY76kIDLGk) and
http://www.youtube.com/watch?v=KZEcr5WWQWc (http://www.youtube.com/watch?v=KZEcr5WWQWc)

No additional effects. Two mics. Minimal post-processing (just fade in/fade out).

Oh, and R3 is 100K in these clips. The Q was too narrow and harsh in the previous vid...(for my taste, anyway).

There's a few more construction details in the second vid, although the pedal is a little different now.

Quote from: jasperoosthoek on October 18, 2010, 01:52:39 PM
I have one question about the Kay: the output resistance of the transistor is 330 in parallel to the collector impedance. How can such a small load resistance generate a good Q? I tried to replicate that value in my cap wah but it sounded like a high cutoff with a slight peak maybe. It oscillated at higher resistances loading the cap and inductor. At least the scope traces with a block signal didn't show a decaying sine, which is what you expect. So how does that work in the Kay?

I'll let the EEs answer that, but I view the Kay circuit as a boost, followed by the LCR filter. The "Q factor" is the result of a filter with innate resonance. Even in a passive circuit *I think* it's a second-order filter.

I have a good theoretical understanding of these filters, but haven't put it into practice. Indeed by nature LCR or LC filters are second order filters.

I'm very impressed with your variable inductor wah! I must say that it is much easier to get some results compared to the variable capacitor. If the variable inductor is too small you can just use a bigger capacitor. ;D In the variable cap case you would have to go to 100H inductors (!!!) or use a Miller multiplying circuit which works only to a certain extend. Mine started oscillation at high amplification.

Where do find the variable inductor? I have a hard time finding a source for them. ???

Quote from: jasperoosthoek on October 19, 2010, 07:06:25 AM
I'm very impressed with your variable inductor wah! I must say that it is much easier to get some results compared to the variable capacitor. If the variable inductor is too small you can just use a bigger capacitor. ;D In the variable cap case you would have to go to 100H inductors (!!!) or use a Miller multiplying circuit which works only to a certain extend. Mine started oscillation at high amplification.

Where do find the variable inductor? I have a hard time finding a source for them. ???

Thanks! It's improving.

Using really large inductors might lead to selectivity problems, too (Not to mention their size and cost).

I encountered a lot of oscillation trying to implement a NFB version with BJTs. I got one version to work by "tuning" the emitter resistor (a POT), but it was noisy and inferior. Using op amps eliminated the problem.

I'm not a "wiz" with op amps--the last time I used one was probably 15 years ago playing with C. Anderton's compressor designs. It's odd to me, for instance, that my op amp circuit works so well with DC coupling between the stages...

Try searching the web for Wurlitzer organ parts. Someone here indicated they are available (I listed the part number somewhere in the thread). Maybe it was "Morelock's Organ Parts." It's a place to start, anyway.

Hi
Because large value variable inductors are large (ie a pain in the butt), and the resonant frequency depends on LC - I wonder whether a fixed inductor and variable capacitor would be easier/better?  e.g. the old ganged plate contraptions in radios (around C=500 pF).  If that were "Millered" (or otherwise converted) up to a higher capacitance, a small fixed inductor would work well.  It could be a DIY wound inductor, around 1 mH.
cheers

Quote from: brett on October 19, 2010, 08:39:19 PM
Because large value variable inductors are large (ie a pain in the butt), and the resonant frequency depends on LC - I wonder whether a fixed inductor and variable capacitor would be easier/better?  e.g. the old ganged plate contraptions in radios (around C=500 pF).  If that were "Millered" (or otherwise converted) up to a higher capacitance, a small fixed inductor would work well.  It could be a DIY wound inductor, around 1 mH.

I don't think size is really an issue. Most tuning capacitors I've seen are a lot larger than my Wurli inductor. Of course those little handheld AM radios had much smaller variable caps--maybe one would work. Those are somewhere in the 50 to 150pF, I think. Vari caps do have one advantage--they are rotary devices.

With a 1mH inductor, you'd have "Millerize" the cap up to the 40-10uF range for a wah. That's a pretty high capacitance-multiplication factor. Probably better to stick with a standard Vox inductor. Or go with the larger inductance (1-5H) of a small choke.

Would I like to see/hear a vari-cap wah built? Yeah, I would.

Personally, I'll stick with this project for now. It's different, and it works. Never could multitask very well...

Quote from: gmoon on October 19, 2010, 11:08:09 PM
Vari caps do have one advantage--they are rotary devices.

Hmm, I would call that a disadvantage. :) So a wah is normally based on a rotary device and still you need gears to convert a semi-linear motion into a rotary motion...

Quote
With a 1mH inductor, you'd have "Millerize" the cap up to the 40-10uF range for a wah. That's a pretty high capacitance-multiplication factor. Probably better to stick with a standard Vox inductor. Or go with the larger inductance (1-5H) of a small choke.

My thoughts too. I would say pretty high is an understatement, it would be around 40,000. Meaning that if you have a 100mV signal on one side of the cap you need a 4kV signal on the other side... Any multiplication factors above about 30 are impractical and are very sensitive to oscillation.

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Would I like to see/hear a vari-cap wah built? Yeah, I would.

(http://img688.imageshack.us/img688/7331/variablecapacitorwah.png)

(http://img194.imageshack.us/img194/3985/breadboard.jpg)

The circuit doesn't work yet. Any thoughts would be appreciated. I tried various values for the ?? resistor. 1k didn't have a notch and at high values it oscillated at audio frequencies... A haven't put too much effort into it but will probably do some simulations the coming days.

I did some simulations and this circuit seems to work. It is pretty much a crybaby circuit with all the impedances a factor of 10 higher.

(http://img51.imageshack.us/img51/1332/variablecapacitorwah2.png)

Vary the 30pF cap between 10 and 500pF to get some nice looking frequency sweeps. It might need some higher amplification factors in the second opamp stage.

Quote from: gmoon on October 19, 2010, 01:33:40 PM
Try searching the web for Wurlitzer organ parts. Someone here indicated they are available (I listed the part number somewhere in the thread). Maybe it was "Morelock's Organ Parts." It's a place to start, anyway.

I sent a mail to Morelock's Organ Parts if they have the 550mH inductor (500805 SECO). They have lots of them! Price including shipping, handling and insurance is $5. But they ship to US post offices and want me to send them my credit card details by email or fax. Hmm, I rather not by email and I don't have a fax except at work.

At least I have a source for them. I'm going to send them an email if they ship to continental Europe and accept paypal.

Yep, that must be the source he mentioned. Not expensive, either--less than a custom-taper wah POT.

Have you seen this from Ax84? (PDF in link):

http://www.ax84.com/media/ax84_m58.pdf (http://www.ax84.com/media/ax84_m58.pdf)

It's a tube-based, variable-capacitor wah. Uses a 60H choke with a three-gang variable tuning capacitor. They list the availability of the cap as "excellent." I can't find it w/google, but it appears to be three 400pF sections, so a similar tuner with 1200-1500pF would work. "New" ones don't appear to cheap, though.

It's very similar to the approach I'm using--LC notch filter returned as negative feedback (which becomes a bandpass filter on the output). The good news is there's some "prior art" to vindicate your idea. You know it works.

Any luck with the multiplier? I'm having a little trouble following the second schematic, partly 'cause the two batteries aren't in a bipolar setup (but the op amps are the more "ideal" type without power input, so maybe that's why), and the drawing is a little confusing. Which sim are you using? Also, on the first schematic, shouldn't Vb be used as the signal ground throughout? (input and filter).

EDIT: Oh, yeah--rotary vs linear (arc, really) motion: the advantage to rotary is simply that existing petals already do that conversion, so using an old wah shell is a viable option...

> I haven't been completely asleep. ... uses an LM324 op amp

LM324 is not a great audio amp. It distorts badly for loads under 100K unless you add a pull-down resistor (which spoils its low power consumption). The input current is higher than we might like.

TL072 would IMHO be a better pick.

The input impedance of the inverter is 100K (||1Meg) which is on the low side for some guitars. Also the 100K (and the 47K!) inserts noise. I guess you inverted because the second stage inverts; absolute phase is not needed here. Use non-inverting with 1K and 470 NFB resistors. The dummy resistor on the other input pin is usually not needed in audio, and not with JFET TL072.

The 0.01uFd against the ~~90K input is a heavy bass-cut. The 0.02u at the output could be too, depending what comes after.

The simulated inductor should have series resistance; it matters enough that you want to include it. I'd guess hundreds of ohms. If you have an inductor in hand, just put the ohm-meter on it.

If you are going to model the 5H of the pickup, you should add 30pFd per foot of cable, 200pFd-1,000pFd, AND the 250K pot most guitars have. (Sweep this alone before you run it with other circuits; the peak/fall response is quite severe. I would assume some perfect source, also assuming that the pickup-winder designed for desired response.)

Quote from: PRR on October 21, 2010, 12:01:52 PM
LM324 is not a great audio amp. It distorts badly for loads under 100K unless you add a pull-down resistor (which spoils its low power consumption). The input current is higher than we might like.
TL072 would IMHO be a better pick.

It's what I had on hand (other than a couple TLC274s). The TL07x would be my first choice. I'll have to order a few...

Quote from: PRR on October 21, 2010, 12:01:52 PM
The input impedance of the inverter is 100K (||1Meg) which is on the low side for some guitars. Also the 100K (and the 47K!) inserts noise. I guess you inverted because the second stage inverts; absolute phase is not needed here. Use non-inverting with 1K and 470 NFB resistors. The dummy resistor on the other input pin is usually not needed in audio, and not with JFET TL072.

Yeah, I was planning on using one of the additional stages as a noninverting buffer, I think I mentioned that... I'm aware of the noise issue and the input impedance of ~100K (why the 100K is there in the first place). With a non-inverting buffer (stage 1) I'd drop the 2nd inverting gain stage values but use Rf/Rin ratios so I can get 2-3X gain. yes, gain's lower now, but I've discovered it's a little below unity.

The 4.7K resistor between the stages is essential, at least for the LM324. I tried removing or changing the value, and it makes a difference. I still feel weird about DC coupling, I didn't expect this to work at all.

I'd like to keep the overall circuit noninverting, but I'm not stuck on it. I have limited op amp experience, and I always appreciate the extra input (pun intended). Simple is better.

Quote from: PRR on October 21, 2010, 12:01:52 PM
The simulated inductor should have series resistance; it matters enough that you want to include it. I'd guess hundreds of ohms. If you have an inductor in hand, just put the ohm-meter on it.

Entered in the sim parameters for the inductor from the start of the proj...

Quote from: PRR on October 21, 2010, 12:01:52 PM
The 0.01uFd against the ~~90K input is a heavy bass-cut. The 0.02u at the output could be too, depending what comes after.

Intentional. Partly personal taste, partly the specific amp, partly due to the bass-heavy nature of the circuit, which was much worse with the BJTs. Maybe with a higher-impedance input I can increase the cap.

I don't generally care for bass-heavy or high-gain... Certainly not during testing or development. Just looking for a nice (subjectively) linear sound.

Quote from: PRR on October 21, 2010, 12:01:52 PM
If you are going to model the 5H of the pickup, you should add 30pFd per foot of cable, 200pFd-1,000pFd, AND the 250K pot most guitars have. (Sweep this alone before you run it with other circuits; the peak/fall response is quite severe. I would assume some perfect source, also assuming that the pickup-winder designed for desired response.)

Noted, that would be an easy improvement. Regardless, I take LTspice sims with a grain of salt. They make a great starting point, though.

> I think I mentioned that...

Apologies. Between server up/downs, doctor date, brother driving up tonight, I've only skimmed the recent posts.

> Entered in the sim parameters for the inductor

Ah. My sim aint that smart. (I now have two here, and both have remarkable stupidities.)

> I still feel weird about DC coupling, I didn't expect this to work at all.

No, it should work. That's one thing op-amps can do well. FAR bigger systems are totally DC-coupled. A dozen DC-coupled vacuum-tube opamps can get a little hairy, trying to trim #12 and get a result before #1 drifts out of your error band. But in audio huge DC "error" is acceptable.

> The 4.7K resistor between the stages is essential

Yes. Without it the gain is "always infinite". In real-world it comes down to parasitics and clipping.

It's a parallel tank. Impedance is infinite for one freq and zero for faraway freqs. So gain looks like zero/zero which is a funny number. LF gain actually has coil resistance, but 333/zero is another funny number. With the 4.7K in front all the gains are readily calculated (readily but laboriously, so we either plot 3-5 easy points or we ask the computer to do some work).

If you don't mind >= unity gain away from resonance, you can do hi-Z input and resonance in a single opamp. Non-inverting, ~~~2K "-" to ground, LRC from Out to "-".

Yeah, get some TL072. I'm bitter about the LM324 because when it was very new, I needed a buffer FAST for a paying gig. Solder-up, passes signal, leave it running while packing.... some nasty there?? 'Scope says yes. Then I actually read the app-note and discovered the deep-B output stage.

Quote from: PRR on October 21, 2010, 12:01:52 PM
The simulated inductor should have series resistance; it matters enough that you want to include it. I'd guess hundreds of ohms. If you have an inductor in hand, just put the ohm-meter on it.

It's even written on my inductor... Doh! http://www.entertonement.com/clips/pwmlfkvdfx--HomersDoh-MP3 (http://www.entertonement.com/clips/pwmlfkvdfx--HomersDoh-MP3)

---

Quote from: gmoon on October 21, 2010, 09:18:22 AM
Yep, that must be the source he mentioned. Not expensive, either--less than a custom-taper wah POT.

Good news! They accept paypal and will ship to Europe for $8. Giving the current $US/Euro exchange rate they are very cheap indeed. Especially because they replace both the wah pot and the inductor!  ;D

Quote
Have you seen this from Ax84? (PDF in link):

It's a tube-based, variable-capacitor wah. Uses a 60H choke with a three-gang variable tuning capacitor. They list the availability of the cap as "excellent." I can't find it w/google, but it appears to be three 400pF sections, so a similar tuner with 1200-1500pF would work. "New" ones don't appear to cheap, though.

Funny that you mention that one, on page two of this thread I wrote 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. I saw it about 7 or 8 years ago. At the time my understanding of electronics was in it's infancy. Right now I'll be able to tackle that one a lot better.

There are dozens of variable capacitors on ebay, a couple are in that range. :) All taken from old radios etc. For example http://cgi.ebay.nl/Triple-3-Gang-500pF-Variable-Radio-Tuning-Capacitor-/150490943445?pt=UK_ConsumerElectronics_SpecialistRadioEquipment_SM&hash=item2309f58fd5 (http://cgi.ebay.nl/Triple-3-Gang-500pF-Variable-Radio-Tuning-Capacitor-/150490943445?pt=UK_ConsumerElectronics_SpecialistRadioEquipment_SM&hash=item2309f58fd5)

They are not cheap though. Mine came from a tube radio I converted into a guitar amp about 5 years ago. The complete radio was somewhere between 3 or 5 euros...

Quote
It's very similar to the approach I'm using--LC notch filter returned as negative feedback (which becomes a bandpass filter on the output). The good news is there's some "prior art" to vindicate your idea. You know it works.

Only it is not my idea actually. :) That was one reason why I bothered. I knew someone had built it before.

Quote
EDIT: Oh, yeah--rotary vs linear (arc, really) motion: the advantage to rotary is simply that existing petals already do that conversion, so using an old wah shell is a viable option...

That's true of course. But the problem is that they don't fit in wah shells. Especially the chunky 1200pF ones that are about 4 inches long... So indeed rotary is an advantage because it fits existing technology. But some years ago I wanted to build a way using studio faders. The problem was that the only faders with adequate quality were too long. The short ones were crappy.

---

Quote
Any luck with the multiplier? I'm having a little trouble following the second schematic, partly 'cause the two batteries aren't in a bipolar setup (but the op amps are the more "ideal" type without power input, so maybe that's why), and the drawing is a little confusing. Which sim are you using? Also, on the first schematic, shouldn't Vb be used as the signal ground throughout? (input and filter).

I have learned a lot from the simulations. But I lost an hour again because stupid PSpice doesn't recognize 1M resistors (thinks it is 1 ohm). But it does recognize k(ilo), n(ano), p(ico), even f(emto) but no M(ega). :S

Remember that I said earlier on page 5: I have one question about the Kay: the output resistance of the transistor is 330 in parallel to the collector impedance. How can such a small load resistance generate a good Q?
I found the answer, you cannot get a Q with the 330 ohm load resistor. The answer is the very low 0.033uF 'coupling cap' that connects to the LCR circuit in the Kay. The cap is smaller than the filter cap! A suitable coupling cap should have a higher impedance than the circuit it loads. This was no design error but the cap itself has the high impedance that makes it work. If it were much larger the peak would be very broad (the simulation confirms this). Think of the Kay wah as a voltage divider with two inductances Z1 on top and Z2 on the bottom: Z1 is the 330 resistor in series with the 0.033uF cap, Z2 is the parallel LCR.

For good measure I did some simulations on the passive part of the Kay circuit. I didn't include the transistor because I can only use high gain silicon trannies. Still it is good to get a rough idea.
(http://img220.imageshack.us/img220/9118/kayb.png)
The two simulations were done with a 3H and 0.1H inductor.

I always thought that the wah has a flat frequency response left of the peak:
(http://www.geofex.com/article_folders/wahpedl/wah6.gif)
Taken from R.G.'s wah bible http://www.geofex.com/article_folders/wahpedl/wahped.htm (http://www.geofex.com/article_folders/wahpedl/wahped.htm).

Well, this is not what actually happens: The vox/crybaby wah does not have a flat response left of the peak, it actually has a cutoff. It's because of the small size 0.01uF coupling cap. This means that for low frequencies the 0.01 coupling cap cannot load the rest of the circuit leading to a band pass. (The inductor can be seen as a short and the coupling cap needs to load the 1.5k resistor.)

Anyway, I thought that my variable capacitor was too small to get a good wah. But the existing tube design doesn't use Miller amplification as present conventional wahs! It is very similar to your design where you use the bare LCR circuit in a feedback loop. The biggest difference is that they used a tube and you used an opamp. If I adapt the crybaby circuit to 10x higher impedances (caps 10x smaller value, resistors and coils 10x larger value) I only need a 1000pF variable cap. I don't know how big mine is but Millered up bit more might work without becoming unstable. I'll try to build the circuit I posted above with R12 as a 3.3k trimpot. That way I can easily adjust it to the value of the capacitor I have. :D

Nice, I've finished a new design that I tested with an oscilloscope and a block wave generator:
(http://img403.imageshack.us/img403/5408/variablecapacitorwahspi.png)

It produces all the right curves on the oscilloscope! The resonance frequency of the circuit is between 380Hz and 2.8kHz. R16 and C8 form a 20kHz low pass filter necessary to avoid oscillation at 500kHz when the variable cap is at its highest value. They might not be needed on a PCB. I'll test it later today with my amp :).

It's noisy as hell on my breadboard but it works ;D. I have to look at the oscillation problem though but with suitable 20kHz lowpass filters here and there it should work fine. Putting in on a piece of perfboard and in a box should kill most noise.
The variable cap is as long as my wah shell is wide so I cannot just pop it in.

Hello all -

FWIW - This may be off point at this time but many of the upright styles of organs use a 3in square box that appears to contain a variable capacitor -- I have never measured it. It is connected to a pedal on the organ not sure what is does. This might be a source for a pedal ready vari-cap. I will take some pics this weekend and try to post.

By all means, please take pictures! I cannot use my standard wah enclosure with my vari-cap. It fits inside but the gear is in the middle making it just too big.

Lotsa new stuff here; I'll try to keep my response < epic.

Quote from: jasperoosthoek on October 22, 2010, 12:25:19 PM
It's noisy as hell on my breadboard but it works ;D. I have to look at the oscillation problem though but with suitable 20kHz lowpass filters here and there it should work fine. Putting in on a piece of perfboard and in a box should kill most noise.
The variable cap is as long as my wah shell is wide so I cannot just pop it in.

Alright!--cool. I just noticed my project is somewhat noisy with other amps, but sounds great with the other. Yeah, shielding will help.

Good job. Now I can see RG's Vox-to-op amp circuit in your schematic, too.

@PPR

Quote>> I think I mentioned that...

>Apologies. Between server up/downs, doctor date, brother driving up tonight, I've only skimmed the recent posts.

>> Entered in the sim parameters for the inductor

>Ah. My sim aint that smart. (I now have two here, and both have remarkable stupidities.)

No apologies necessary. We've had my 88 yr-old Dad as a house guest the last few days (and a good chunk of my time lately is spent cleaning out his house).

Anyway, I hadn't entered the serial resistance in the sim after all--that was in my earlier BJT sims before, but I forgot. Of course, entering it really mucks up the high-freq response, which I've been dealing with from the beginning...  There's no doubt that op amps are better, but ideally a variable-inductor with < 75 ohms of DC resistance would be my wish--or a vari L in the 5H-.5H range. Hey...kinda like the Kay original...

QuoteIf you don't mind >= unity gain away from resonance, you can do hi-Z input and resonance in a single opamp. Non-inverting, ~~~2K "-" to ground, LRC from Out to "-".

Simmed it, and that does work. Maybe not quite as well as two, but it's pretty cool.

So here's what I've come up with:
--first stage: non-inverting with Rin/Rf of 1K/1K, for a gain of 2, and the high input-impedance expected of a non-inverting stage.
--second stage: as before.

One more addition: between the op amp output and the LC tank I've added a series resistor. It doesn't effect the Q, or the height of the resonant peaks, but it does raise the "floor" of the rejected freqencies. Something up to 2.2K or so; 1K works well. It does, of course, increase the subjective volume, 'cause it's raising all the rejected frequencies by 10 or 20dB. But even with 1K inserted, the high-end peak is 15-18dB or so above the floor (and the low-end even more). This can be adjusted to taste, of course. Makes the effect a little less "reedy" (which again is a taste issue).

Yes, it's inverted overall, but so was the Kay and my first transistor-based models. No biggie.

I'll post a schematic and clip whenever I get a chance. Might be at least a couple days, for the clip, anyway...

I'm near to "boxing" this one, but I'll wait until I order and test with TL072s.

Quote from: jasperoosthoek on October 22, 2010, 02:33:32 PM
By all means, please take pictures! I cannot use my standard wah enclosure with my vari-cap. It fits inside but the gear is in the middle making it just too big.

Thanks to moonbird for bringing this up. The one I'm using is a re-purposed organ pedal. And I've got two more organ "swell" or expression pedals. I could post a couple photographs, too.

Organ expression pedals from my basement junk pile. These are resistive, not capacitive, though. I've never run across one of those...

#1: LDR with bulb/cell beside the can caps.
#2: POT, mech coupled with those strap/friction wheels
#3: multi-layer resistive strips that deform when the pedal is pressed.

1:
(http://lh6.ggpht.com/_Z7b6PJeTtfE/TMW2GFb4DOI/AAAAAAAAABE/Kh9jZSUu42M/s800/_DSC1904.JPG)
(http://lh3.ggpht.com/_Z7b6PJeTtfE/TMW2HI7hGpI/AAAAAAAAABM/JyCi2r7RgVo/s800/_DSC1907.JPG)
(http://lh5.ggpht.com/_Z7b6PJeTtfE/TMW2IHPDp_I/AAAAAAAAABQ/Wd25Swv6taQ/s800/_DSC1910.JPG)

2:
(http://lh4.ggpht.com/_Z7b6PJeTtfE/TMW2I8-7wMI/AAAAAAAAABU/zg8VLCEx5cM/s800/_DSC1911.JPG)
(http://lh3.ggpht.com/_Z7b6PJeTtfE/TMW2Jqvne2I/AAAAAAAAABY/H8le2snaGhA/s800/_DSC1912.JPG)
(http://lh5.ggpht.com/_Z7b6PJeTtfE/TMW2KdvyEtI/AAAAAAAAABc/m-QLydDaAEU/s800/_DSC1913.JPG)

3:
(http://lh5.ggpht.com/_Z7b6PJeTtfE/TMW2LDLLv0I/AAAAAAAAABg/7rn0aLDCzPI/s800/_DSC1915.JPG)
(http://lh6.ggpht.com/_Z7b6PJeTtfE/TMW2L_vtHyI/AAAAAAAAABk/eC8Yloe1XUs/s800/_DSC1916.JPG)

> stupid PSpice doesn't recognize 1M resistors (thinks it is 1 ohm).

Older pSpice uses the no-letter-case parser. "M" and "m" are ambiguous, mega or milli. You MUST write "meg", "Meg", "MEG". "M"/"m" alone should parse as "milli". If you are very frustrated, write "1000K".

Quote from: jasperoosthoek on September 24, 2010, 05:11:34 PM
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.

Eric Barbours big L small variable C wah: http://smg.photobucket.com/albums/v324/caseyseffectpage/Wahs/?action=view&current=svetlanafuzzwah.jpg

Hi there -- exactly!! great photos - these are the type of peddles I am fiddling with. The one I have -- that I CANNOT find (yet) has a variable capacitor that is controlled by the peddle movement. Hope to find it this weekend while cleaning the garage (ugh).