I'm sure you've grasped it all, but from my (quick) understanding the much cherished old VOX wah were analysed under lab conditions & ultimately acted in a way that wasn't expected. It would appear the inductor core (which is made of ferrite not meant to have to hold any permanent magnetic qualities) had become in part magnetized (he'd guessing the DC current the wah circuit has permanently running through the industor may have led up to this). The end result is that when you feed an AC signal into the inductor, the inductor core will saturate faster in one half of the signal waveform than the other...apparently this sounds good in a wah circuit! (I can't say...this is all news to me!)
Therefore he goes on to hypothesize that this could be 'frigged' using a modern bespoke wound inductor, buy adding a second winding and then pumping some DC current through it. This would generate a magnetic field, utlimately turning the ferrite core into a pseudo magnet...you then have the aformentioned non linearity that is sought.
he doesn't give any figures for the amount of turn the second core should have...so you'd be very much into trial & error territory. And without having access to his data (scope traces at set frequencies for the original VOX inductor etc), it'd likely be a futile (& long drawn out) affair!
But by the looks of it, the inductor used in the Wah circuit has a ferrite core, an inductance of 500mH and about 50-70 ohms of DC resistance. I'm not sure how physically big those inductors are, but that surely amounts to a lot of windings (so I'm guessing they use very thin copper wire)
Last edited by HankMcSpank; 25-06-2009 at 12:21 AM.
Thanks Hank. yeah I got the basics, although I understood your explanation better. I was looking for a starting point for the magnatising coil, as you said there are no values given.
The main inductor bit is easy, its about 550 turns of 38AWG wire on a pot core. finished size is about the same as a stack of 3 £1 coins.....
As I said I planned to look into this later (too many projects) but Ive got side tracked when I saw this thread about coil winding..
May have to pick your brains when I get round to it.
Over the weekend I had a bit of a play with my homemade CNC - I made a lot of simple mistakes, like forgetting to zero the tool after a tool change, only to watch the drill bit plunge right down through my material & a little into the table- eeek!), here's what I've been on with (rather than six little coils - one per string - becuase I don't have those parts I need, I had a opo at a larger coil.....
(I'm not sure if you'll notice, the bit marked 'bottom' part has a slight chip at the top left corner, this is because when changed cutting bits ,I had the feed rate too high, the machine went ballistic & gouged into the part a little! But since this is just a protoype, I couldn't be bothered to re-run the cut! The other two parts turned out well though...particularly happy with that middle bit - that's just 1mm between the edge holes & the bobbin edge at each end - remember my leadscrews are just M8 rod, & my drive nuts are badly homemade delrin ones!
The colour scheme(!) wouldn't be of my choosing, but I was limited to those colours because of the thickness of the acrylic (I needed 2mm thickness which I only had as clear acrylic & also 4.5mm which I only had in translucent royal blue!). Here it is as a temporary 'push fit'
those poles sticking up are actually 20mm lengths of Alnico magnets ...they obviously need trimming...they were an *extremely* tight fit. I screwed up with the middle part too! Basically, I read somewhere to get rid of the cutter marks that acrylic can be flame polished along the edge ...apparently, all you need to do is run a butane torch gas flame along the edge - I'm sure it normally works well, but not when the part so small....the acrylic heated the small part up & distorted the acrylic! (shrunk it a fraction...hence a couple of the poles not being bolt upright)
Here it is ready to be wound (I've not glued the top to the middle, because, after the coil is potted, I'd like to see how it looks without the top, hence the large washers clamping the top to the middle for while it's being wound)...
(the square edging makes it look somewhat utlitarian & of "East European circa 1970!" design ...but it's just a prototype so I have expended much effort giving it bevelled/rounded edges!
So then, next I wound 150 turns of 0.235mm wire (which is actually 0.27mm if you include the enamel coating!), this gauge is much thicker than I'm used to & is quite a bugger to 'control' while winding on to the bobbin....
....those two large screws atop are simply holding the top part to the remainder (pending potting - after which, the screws will come out & the magnets will go in)
Last edited by HankMcSpank; 29-06-2009 at 01:41 PM.
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