Colorsound Wah+Fuzz Deep Dive - 7

Posted by Jim Black on 12/25/2015 to Bust Em Out

Our luck has changed. There are many resources for our hand wound pot core inductor for Greg's ColorSound Wah+Fuzz. Making our own inductor is not hard but it isn't easy either. We only need to define a few parameters in order to make a selection. It doesn't look like it will be too expensive either.

I did more research on the markings on our inductor to no avail. Except that the N22 may be a material designation.

We are lucky in that we have a few things to go by to make some guesses as to what we need to duplicate the inductor. The form factor and materials of the inductor were standardized at the time of manufacture. We should be able to get close.

What we know so far about our inductor.

  • Pot Core
  • Soft Ferrite
  • 14mm x 8mm
  • 500mH
  • 40 AWG Wire

The first thing to do is to select a company that makes our type of inductor. A short search on the internet produced a company in Pittsburgh named Magnetics, Inc. They have been around since the 1940s so they were manufacturing the pot cores we need back in the day. We know the wah-wah application requires very low current, voltage and frequency. The wah circuit will not produce high temperatures in the inductor since our current and voltage are so low.

The great folks at Magnectics, Inc. have made it easy for use in their extensive design guides. We need to calculate the number of turns of wire on the bobbin to get the 500mH. We need to consider the wire gauge can handle the current and the number of wire turns has to fit number on the bobbin. The 40AWG can easily handle our 1 to 10mA. The tiny diameter of the wire (0.00314) should allow around 400 turns or more. Remember that 500mH is a big inductance value and is going to take a lot of wire.

I found the material selection guide. Magnetics, Inc recommends material C, V or E for filters. I have to take the size of the pot core (1408) into consideration. The chart gives a value for AL and our part number UE41408UG. The Inductance Factor (AL) is used to calculate the number of turns of wire to get the 500mH needed in the wah-wah.

The equation shows L as the inductance in mili-henries. The AL is the inductance factor we just found and N is the number of turns we need on the pot core bobbin.

Plug in the numbers and we find that we need about 490 turns of wire to get our 500mH.

There are a couple of things to consider before we put this topic to bed. The data sheet for our pot core says that the AL has a tolerance of +/-25%. That is pretty broad range. That means that AL can be any where between 1575 and 2625. Our number of turns to get 500mH can be 436 (+25%) or 563 (-25%). What does that do to the inductance? Turns out it comes out to about +/- 3% for the inductance. Definitely can live with that. If you want to really nail it, take the geometric mean of 1575 and 2625, sqrroot(1575 * 2625) = 2033 AL. Run that through the equation for the number of turns again and you get 495. I think that is what we'll go with.

We haven't really talked about the Q factor of the wah filter. We want a very high Q since our frequency range is so small. Guitars are from 82HZ to about 1200HZ. A high Q in the inductor will give a very small band width of our wah. Small meaning 100HZ. That is 10%. There are a couple of parameters of the inductor that affect the Q. It's not like we can change much in the inductor to affect the Q since it is based on physical properties called "permeability components", it is just a good idea to have a look to see if we are in the ball park. The manufacturer has a specification on the data sheet for the pot core inductor called a loss factor (LF). The thing to remember is the smaller the loss factor is the higher the Q. In our case the LF is less than 6 x 10^(-6) or 0.000006. That is a pretty big Q. We are in good shape.

OK, the theory and engineering are done now we move to the practical building side. I've got to convince a distributor to sell me just a couple of the pot cores and I have to figure out how to wind 495 turns of 40AWG wire on a bobbin by hand. This could be a problem. Check out the solution next time...

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