Wah inductor anatomy

[Mick Bailey]

Has anyone taken wah inductors apart to discover their anatomy?

Some are fairly obvious, but it's the potted/sealed types that interest me, such as the El-Rad or the black Dunlop, which are quite compact and I'm interested in how they achieve the inductance while keeping the resistance low in such a small package. I want to wind a batch and experiments so far have resulted in fairly low inductance/high resistance inductors that are no good. I want to stay away from the Halo/Whipple type if possible, as I can't find a UK supplier of the core sets at a reasonable price and prefer the look of the El-Rad.

It seems to be the bobbin type that's the problem and I wonder if anyone has  managed to come up with a DIY inductor that is as compact as the ones mentioned?

[deadastronaut]

+1 . i'd be interested in this too...

[Paul Marossy]

Some are fairly obvious, but it's the potted/sealed types that interest me, such as the El-Rad or the black Dunlop, which are quite compact and I'm interested in how they achieve the inductance while keeping the resistance low in such a small package.

Less windings with larger gage wire? That would lower the resistance, right? I'm sure the core material must also be a big factor - size, shape, the composition of the core material. I imagine it's mostly finding the right balance between those two things.

I have also wondered if maybe these inductors use a torroidal core. I believe that the current Fasel inductors use one, but I have read that the original Fasels weren't torroidal. I don't think the El-Rads/Dunlops are torroidal, but you never know...

[zombiwoof]

Some are fairly obvious, but it's the potted/sealed types that interest me, such as the El-Rad or the black Dunlop, which are quite compact and I'm interested in how they achieve the inductance while keeping the resistance low in such a small package.

Less windings with larger gage wire? That would lower the resistance, right? I'm sure the core material must also be a big factor - size, shape, the composition of the core material. I imagine it's mostly finding the right balance between those two things.

I have also wondered if maybe these inductors use a torroidal core. I believe that the current Fasel inductors use one, but I have read that the original Fasels weren't torroidal. I don't think the El-Rads/Dunlops are torroidal, but you never know...

In the case of the recent reissue Fasels, the red one is toroidal, the yellow isn't.  Yellow is the vintage sound, red a more modern hi-fi sound, according to the descriptions.

Al

[R.G.]

Inductor Anatomy

1. Every wire has inductance, because it causes a magnetic field to exist circularly around it when current flows. Mother Nature said that any flowing current causes a magnetic field, and so it is. The field has a direction around the axis of the wire. Mother Nature's Rules of electromagnetic induction were discovered independently by Michael Faraday and Joseph Henry in 1831. [... Henry? Yep, that's where it comes from.] If you grasp your right hand on a wire, with your fingers wrapped around the wire and your thumb along the wire, then current in the direction your thumb is pointing makes M-field flow circularly in the direction your fingers are wrapped around the wire. This is called (oddly enough  ) the Right Hand Rule.

2. The inductance can be computed from the physical size and materials of the conductor. However, for the wires we would ever deal with, the variations from physical size of just the wire can be ignored.

3. When you coil a wire, the fields cancel between adjacent wires, and combine along the axis of the coil. The field strength per ampere per length of the wire becomes transformed into the inductance per turn.

4. Inductance per turn depends on the physical setup, notably the area of the wire coil loops.

5. Inductance is proportional to the *square* of the number of turns. Double the turns, 4x the inductance. Half the turns, 1/4 the inductance.

6. Inductance is also proportional to the material the M-field flows in. The property of space which affects inductance and M-field flow is called "permeability". The actual units of permeability are computable, and complex, but useless for effects hackers. Instead, we can just say that a coil has an inductance of whatever it has in a vacuum (and in fact, in most other materials, notably air), but if you stick ferromagnetic materials into the coil, the inductance goes up. Ferromagnetic materials (essentially, iron, cobalt, and nickel) have a relative permeability that can be many times that of free space. Some varieties and mixes of these three materials and a spicing of other stuff can make for permeabilities of hundreds of thousands of times more than free space. The down side of this is that ferromagnetic materials do their trick by having each atom be like a little bar magnet, and aligning with the M-field going through it. When all the bar magnets are aligned, that's all the more it can do, and the material is saturated. Each increase of current/M-field beyond that acts like the material was free space again. This is what "saturation" is.

With that as background, you can now talk about how to make a wah inductor.

The simple way is to get a bunch of wire and wind turns, measuring the resulting inductance til you get to 500mH. The only down side to that is that the number of turns and physical size is HUGE. You'll have more wire in there than would fit in a wah pedal, or bigger turns (like, a meter in diameter) which would also pick up incredible amounts of interference junk noise.

So the clever trick is to use some kind of ferromagnetic core. Iron has permeabilities on the order of 10,000 without even breaking a sweat. So an iron core reduces that coil-too-huge-to-wind down to something as small as a cube 0.5"/1.3mm on a side with a hundred turns, more or less. This is the primary of the Xicon transformer sometimes used. Another alternative is to use a ferrite core, as in most all commercial wah inductors. "Ferrite" is a composition of magnetic iron oxides, cobalt, barium, oxygen, cinnamon and coriander. (I made up that cinnamon and coriander. ) It has a modest relative permeability (maybe 3000 instead of 10,000) but corrects other flaws in iron that I haven't talked about.

Ferrite cores solve the issues of size by high permeability, and noise pickup by being physically formed so the ferrite material wraps around the coil, shielding it from external magnetic fields.

Once you get to here, you know in theory at least how to design a wah inductor. (1) Pick a physical size ferrite core; (2) look up the manufacturers' data on permeability, inductance factor A_l, and (3) compute the needed number of turns. From that, (4) calculate from the physical space inside the core what wire diameter will fit, (5) look up the wire size in a wire table and pick one small enough to get that number of turns in the space available.

When you do this, you will find that (6) it won't fit without insanely small wires (7) the cores which will give you what you want are not available in ones; ( 8 ) cores which you can get have A_l which is too small, and ( 9 ) you can't get either insanely small wire or the right core.

You then start again at (1) and iterate until you find some combination of core you can get, wire you can get, and physical/magnetic properties which give you a working inductor.  This is what the poor schmuck in the basement office back at the inductor company does all day long.

To answer your question directly:

Has anyone taken wah inductors apart to discover their anatomy?

Yes. All of the people selling replacement wah inductors have, or they buy from people who have. I do know one guy who made almost a life's-calling out of finding vintage wah inductors and replicating them. He spent years on it, and is understandably reluctant to tell anyone how it was done, as he feeds his family on that information. There are others who may be more (or less) inclined to share.

I'm interested in how they achieve the inductance while keeping the resistance low in such a small package.

They use special high A_l cores which are also small. These are competing requirements, as you can appreciate. They also have access to any wire, no matter how fine. And they pay that poor schmuck to keep tinkering til he gets it right or gets fired, whichever comes first. The "poor schmuck" probably has at least a master's degree and does magnetic design 8-10 hours a day forever, so he has some experience by now. He's forgotten more than I personally know about magnetics and I used to make a living doing transformer design.

Resistance is ((rho)*l)/A, where rho is resistivity of the material, l is the length, and A is the area. In copper wire, this reduces to a resistance per foot/meter for a given wire size. Zillions of turns = high resistance because length is long. High A_l means not only fewer turns, but probably shorter length per turn because you get the same inductance in a smaller size core.

So - the resulting answer is: the use high A_l cores to get fewer turns, allowing thicker wire and hence low resistance from both shorter total length and thicker wire.

I have also wondered if maybe these inductors use a torroidal core.

Some of them are. Toroidal cores are the ne plus ultra of a magnetic core, as this completely eliminates any air gap in the M-field path, resulting in absolutely highest A_l. Also easiest saturation, but that's not a huge factor in low-signal inductors.

I have read that the original Fasels weren't torroidal.

To the best of my knowledge, this is correct.

This is another of those issues where if it was easy, everyone would already be doing it. 

That's not what you're looking for, but it's the right background information. It's where to dig.

[Govmnt_Lacky]

WOW 

That must of took a while to type out!! 

Now I'm gonna go back and get me some KNOWLEDGE! 

[Paul Marossy]

So the clever trick is to use some kind of ferromagnetic core. Iron has permeabilities on the order of 10,000 without even breaking a sweat. So an iron core reduces that coil-too-huge-to-wind down to something as small as a cube 0.5"/1.3mm on a side with a hundred turns, more or less. This is the primary of the Xicon transformer sometimes used. Another alternative is to use a ferrite core, as in most all commercial wah inductors. "Ferrite" is a composition of magnetic iron oxides, cobalt, barium, oxygen, cinnamon and coriander. (I made up that cinnamon and coriander. ) It has a modest relative permeability (maybe 3000 instead of 10,000) but corrects other flaws in iron that I haven't talked about.

And there are different types of ferrites, right? Different recipes for different purposes. I want cinnamon in my wah inductor, it would make it more spicey!   I have always assumed that wah pedal inductors use a ferro-magnetic core to get it in a small package, but I don't really know for sure.

I have also wondered if maybe these inductors use a torroidal core.

Some of them are. Toroidal cores are the ne plus ultra of a magnetic core, as this completely eliminates any air gap in the M-field path, resulting in absolutely highest A_l. Also easiest saturation, but that's not a huge factor in low-signal inductors.

End result is that it can be in a pretty small package for a given value, right?

I have read that the original Fasels weren't torroidal.

To the best of my knowledge, this is correct.

Interesting...

[deadastronaut]

cool reply rg, i thought there might be a ferrite ring/core or something in there...ive been messing around with 'toroids,  wound bifilar though...not a single wire wrapped...

may have to experiment (be the schmuck in my living room)....covered in coriander too....   cheers guys...

[Ronan]

Has anyone taken wah inductors apart to discover their anatomy?

About 15 or 20 years ago I pulled a colorsound inductor apart, measured the turns and the wire diameter, and attempted to put it back together, but I couldn't quite get all the wire back on the bobbin creating a force fit between the ferro halves and one of the halves cracked when I did up the mounting nut, which destroyed it. I found some ferro cases and bobbins to suit in the pre-internet days, 280 miles away. I think I saw an add in an electronics mag and wrote them a letter, the company owner sent a letter back asking if the ferro was for audio or radio, I said audio, and eventually I bought a set of eight with bobbins, because they only sold in sets of eight. I came across a dead sewing machine motor and put a crank handle on one end and mounted the bobbin on the other to get the 460 turns on the bobbin to build a new inductor for the colorsound wah pedal. The new inductor worked great, no complaints from me nor the other band members, it sounded the same, looked the same, and was the same physical dimensions.

[Mick Bailey]

I guess what I'm after is essentially the inductor as a commodity item, and recognize that wah inductors are never going to have a large enough marketplace to bring the price down to consumer prices like transistors. The fact is though, that in the end it's just a coil of wire on a core and the Mojo factor has elevated this to disproportionate levels. £27 for a red Fasel? Someone's having a laugh. £18 for the Dunlop? - they're still laughing.

The key to an economical DIY design has to be obtaining the right core. Wire is an easy variable to play with, but the shape/section/material of ferrite isn't and there seems to be an endless selection of cores available - it's just hitting on the right part number. I bet nearly everyone is using off-the-shelf cores.







 

[R.G.]

The fact is though, that in the end it's just a coil of wire on a core

That is quite true. This is a great illustration of the details mattering, though.

The key to an economical DIY design has to be obtaining the right core. Wire is an easy variable to play with, but the shape/section/material of ferrite isn't and there seems to be an endless selection of cores available - it's just hitting on the right part number. 

Again, correct. You could start by looking at 1810 potcores, ungapped, with as high an A_l as you can get. Non-proprietary round potcores are sized by their physical dimensions in millimeters. I have a vague memory of a wah inductor I measured being 1810. But that's not the smallest available.

As a side note for 'small ferrite cores', at one time you could get ferrite toroids so small that it was like threading a sewing needle to get wire through. The were used for computer memory. I have a 1K core plane somewhere here...

The ferrite material will be another variable that will affect the result, as ferrite recipe changes the permeability a lot. If they haven't changed the terminology for ferrites in the last 20 years, look first at 3C8 for a general linear BH loop high permeability ferrite.

I bet nearly everyone is using off-the-shelf cores.

Actually, I'd bet a substantial amount that no one uses a core that's not off-the-shelf. Making ferrite cores is not a garage-shop industry. Only the big boys get to play in this one.


 

[asatbluesboy]

Gawd, R.G., if you were immortal it could finally be proved God exists.

Thank you immeasurably.

[R.G.]

Don't get too excited. That's just the simple stuff.

There are several more steps from knowing that to having a recipe for a working wah inductor.

[Mick Bailey]

This looks a good starting point;

http://www.cwsbytemark.com/index.php?main_page=index&cPath=206_231

The table suggests that a wah inductor could be wound with the 'correct' resistance and inductance values quite readily.

[R.G.]

Go for it, Mick!

[Mick Bailey]

Went to place an order, but shipping cost to the UK from this company is insane - $50 minimum on $25 worth of smaller cores. I buy components all the time from the US and I know an air mailed jiffy bag full of parts is inexpensive. Need to source the same components from the UK.

[Ronan]

Small Bear have a kit here. Looks pretty reasonable, might be a good place to start.

[Paul Marossy]

I know this thread is going in a different direction now, but I would sure like to see one these old wah inductors dissected.

[Gus]

Something to add to this thread a little off topic

I have seen a inductor in a guitar that looked to be wound on a steel 1/4-20 bolt.  I looked to be two section, hum bucking wired.

It might be fun to wind some inductors for a wha on a steel bolt.

[gmoon]

I have seen a inductor in a guitar that looked to be wound on a steel 1/4-20 bolt.  I looked to be two section, hum bucking wired.

It might be fun to wind some inductors for a wha on a steel bolt.

A two-section coil with a non-air core is probably acting via mutual inductance, similar to the old-school adjustable variometers. With mutual inductance, the total inductance can be greater than the sum of both coils (or less, if the coils are out of phase).

All this inductor stuff is fascinating...