### Author Topic: True "variable inductor" wah...  (Read 46599 times)

#### gmoon

##### Re: True "variable inductor" wah...
« Reply #40 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.

#### Nasse

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##### Re: True "variable inductor" wah...
« Reply #41 on: September 26, 2010, 02:05:07 AM »
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
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#### Paul Marossy

##### Re: True "variable inductor" wah...
« Reply #42 on: September 26, 2010, 10:42:57 AM »
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...

#### PRR

##### Re: True "variable inductor" wah...
« Reply #43 on: September 26, 2010, 03:53:14 PM »
> 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.
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##### Re: True "variable inductor" wah...
« Reply #44 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.
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#### jasperoosthoek

##### Re: True "variable inductor" wah...
« Reply #45 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. And I like working with germanium trannies .
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#### jgarnold

##### Re: True "variable inductor" wah...
« Reply #46 on: September 27, 2010, 11:10:08 AM »

>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

##### Re: True "variable inductor" wah...
« Reply #47 on: September 27, 2010, 05:27:37 PM »
Nice work, pedal looks and sounds awesome to me.

#### puretube

##### Re: True "variable inductor" wah...
« Reply #48 on: September 27, 2010, 06:34:02 PM »

#### Nasse

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##### Re: True "variable inductor" wah...
« Reply #49 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
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#### gmoon

##### Re: True "variable inductor" wah...
« Reply #50 on: September 28, 2010, 10:57:54 AM »
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...

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...

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.

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...

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...)
« Last Edit: September 28, 2010, 11:07:50 AM by gmoon »

#### jasperoosthoek

##### Re: True "variable inductor" wah...
« Reply #51 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.
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#### gmoon

##### Re: True "variable inductor" wah...
« Reply #52 on: September 28, 2010, 09:34:51 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...

#### jasperoosthoek

##### Re: True "variable inductor" wah...
« Reply #53 on: September 29, 2010, 05:50:23 AM »
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
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#### gmoon

##### Re: True "variable inductor" wah...
« Reply #54 on: September 29, 2010, 08:53:29 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.

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

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."
« Last Edit: September 29, 2010, 09:09:09 AM by gmoon »

#### jasperoosthoek

##### Re: True "variable inductor" wah...
« Reply #55 on: September 29, 2010, 10:34:20 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...

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.
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#### Skruffyhound

##### Re: True "variable inductor" wah...
« Reply #56 on: September 29, 2010, 02:59:15 PM »
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.

#### gmoon

##### Re: True "variable inductor" wah...
« Reply #57 on: September 29, 2010, 05:36:23 PM »
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.

#### jasperoosthoek

##### Re: True "variable inductor" wah...
« Reply #58 on: September 30, 2010, 06:03:31 AM »
What I meant was something like this:

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.

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.
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#### gmoon

##### Re: True "variable inductor" wah...
« Reply #59 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...