Kay wah - true variable inductor wah

[mac]

I wonder why this is one of the few, if not the only one, true variable inductor wah-wah ever made.

By just inserting an iron core inside a coil you can make a circuit to change its peak freq easily.
A metal core having 500x more relative permeability than air can change the freq by 22x, ie, from say 100hz to 2200hz. { f = const/sqrt(L) }
The good thing is that no expensive pot that-will-go-scratchy is needed.

So back to my wondering,  I resist to think that is because a wah-wah circuit not used by Jimi Hendrix is not a wah-wah at all.
Maybe there are another reasons ie, the freq changes too fast when you move the core inside the coil? Mechanical parts costs?

mac

[R.G.]

I suspect it's purely practicalities, as you speculate. Fixed inductors are expensive and difficult enough to work with all on their own. Making an inductor variable, and worse yet, repeatably so, is even more expensive and difficult. Then there's the issue of the inductance-per-motion as you again noted. Getting linear, square, or square root inductance per unit of movement would require some precision mechanics, as would getting a suitable range of rocker motion to move the mechanism.

I've used a Kay wah, back in the 90s when I did the reverse engineering on it to post the schematic. They are very touchy in terms of rocker movement, and hard to do anything but full bass-to-treble and back on, or at least this one was for me.

But only the engineer and accountants at whatever "Kay" was at the time can really answer that one.

[Keppy]

I've used a Kay wah, back in the 90s when I did the reverse engineering on it to post the schematic.

That's funny, that you had to reverse engineer it and the original in the video came with a schematic. I bet that type of thing happened a lot more before forums like this one.

[pinkjimiphoton]

weird, i had a kay wah back in the day, it was one of my first pedals back in the mid 70's... i remember it as having a fairly decent sweep as a wah pedal, i liked it better than the guild or crybabys my friends had.

and yep, it had the schematic inside, but i doubt if it was right.

[PRR]

> A metal core having 500x more relative permeability than air can change the freq by 22x

A completely closed iron core can do that or better.

Sliding a slug in a coil, most of the magnetic path is still through air, so the inductance changes only slightly.

Split an E-I core, you can get 99% of mag path in air, or less; but you have to go WAY apart to get the low inductance, the high-inductance range is all scrunched-up in the last tenth-inch (few mm).

There are ways to do some better, because the old standard cheap arc welder used variable inductance to get a non-linear but not horrible scrunch range of output currents. However the welder maker _knows_ the input voltage/frequency and can rely on iron saturation. Guitar is (hopefully) more varied in amplitude/freq than a power line.

Hmmmmm.... a ready-wound trial part may be a solenoid, even found in washing machines. The inductance does rise when the slug is sucked (or pushed) home. A 120V/230V winding will be considerable audio impedance. However the self-resistance may be high for good resonance.

I suspect solid-slug solenoids will have massive eddy-current loss at audio. Many AC solenoids are laminated like transformers. (You can tell from afar: the laminated kind tends to be square slug, not solid bar-stock.)

[mac]

I made a coil today, 800 turns around a thread spool.
And the core is a long ferrite bar from the antenna of an old radio. It has the same diameter as the spool's hole 
Maths says 400 turns to have 500mH with iron core. I doubled that number just to be sure. Wire resistance is around 30 ohms.
I also have a bobin filled with hundreds of turns, and the iron of a hammer that fits tightly in the hole. Two coils to experiment.

I breadboarded the Kay Wah and another circuit I had in mind. Right now I have to move the coil with one hand while playing
I have to build the rocker pedal to "feel it". Doh!

RG, by winding a coil asymetrically one can have a non linear change of inductance while inserting the core. Besides the coil can be of variable diameter, like a "V", and of a proper lenght.

Paul, by adding an external iron "C" that touches both ends of the bobin, the magnetic path is closed when the iron bar is fully in, increasing final inductance.

______________
! __________  !
! !         ! ! external fixed core "C"
! !         ! ! __________
/////////////// __________!  moving core <--->
      coil



mac

[greaser_au]

The old mechanical pushbutton car radios used a rack of slugs moving in & out to set the station. If you have one of these things in pieces*, you may be able to find slugs that will make the mechanical side easy.

david

*don't wreck a good one, vintage radios are beginning to bring good money!

[mac]

I ran a simulation of this circuit, and i noticed that unlike a Cry Baby, the peaks are not of the same height, decreasing as L decreases.
This is because of the inductor wire resistance. Below 100mH the wire resistance is not negible at resonance. To avoid this fact, it is better to use a coil of 1H or more. BUt at the same time wire resistance has to be small, say <50 ohm, what means bigger wire diameter and therefore bigger bobin size.
I think this is the main problem with variable inductor wah... unless you put the coil into liquid N2  





mac