Wah Inductors and magnetism

[aquaboy1993]

Just to add in a thought here from a guy who has never modded a wah before....  I do have one ready to attack however.

Talking about the magnetism of an inductor.  The little Rat Shack transformers have both primaries and secondaries.  Typically you cut the primaries and only use the secondaries as an inductor.  But is is possible/useful to pass some DC current through the primary?  Wouldn't that create a magnetic state in the core that would then be passed onto the secondaries?

Or am I just thinking out of the bottom of my chair?   

Mark

[RedHouse]

...is possible/useful to pass some DC current through the primary...

That's been talked about here before over the years, nobody really pursued it though. Boldly go dude!

[Philippe]

All was well and good until I swapped out the L6 for a 283 Chevy motor, bad move that. 

That short-block Chevy 283 is a terrific motor...a popular engine conversion on Jaguars, Toyota Landcruisers & Datsun Zs during the late 70s-early 80s.

I'd  imagine that a V8 sort of changed the overall personality of your Zcar.

[ORK]

Just to add in a thought here from a guy who has never modded a wah before....  I do have one ready to attack however.

Talking about the magnetism of an inductor.  The little Rat Shack transformers have both primaries and secondaries.  Typically you cut the primaries and only use the secondaries as an inductor.  But is is possible/useful to pass some DC current through the primary?  Wouldn't that create a magnetic state in the core that would then be passed onto the secondaries?

Or am I just thinking out of the bottom of my chair?   

Mark

Or just short the primary?

[R.G.]

Just to add in a thought here from a guy who has never modded a wah before....  I do have one ready to attack however.

Talking about the magnetism of an inductor.  The little Rat Shack transformers have both primaries and secondaries.  Typically you cut the primaries and only use the secondaries as an inductor.  But is is possible/useful to pass some DC current through the primary?  Wouldn't that create a magnetic state in the core that would then be passed onto the secondaries?

That was one of my first efforts. I tried it, didn't notice it being any better. But transformer iron has a saturation flux density about 5X the saturation density of ferrite, and a different transition into saturation, so it may be that iron cores are not well suite to this line of attack, or need a lot more TLC to get right, whatever "right" is.

Or just short the primary?

You have to drive the secondary with a high impedance current source to avoid doing exactly that. And high impedance current sources are noisy, so it adds extra hiss for you, too. The added magnet biases the core without adding hiss.

[nepalnt21]

i didn't really want to start a new topic, as this thread seems like a pretty decent start to an ultimate wah inductor + magnetism compendium. i just wanted to share my experience with this subject, as it pertains to a breadboarded wah circuit (i pretty much stuck with r.g. keen's tech of wah circuit, switched the input resistor and cap order, for no good reason).

i have three inductors out-of-circuit to play with: a newfangled dunlop yellow "fasel", a plain black cylinder inductor from a standard cgb-95, and a "halo" style from small-bear (eleca is the brand).

i gathered up magnets from various sources, from really weak 16th inch by quarter inch ceramic discs that can barely hold their own weight to a couple one-inch diameter neodymium discs.

the one i settled on is probably a half-inch in diameter, maybe at most 8th inch thick, probably rare-earth magnet from a broken drive motor for a conveyor system node.

here is what i noticed: i really have to play with it to find a sweet spot. if i just willy-nilly stick it, chances are, the inductor will turn into a useless short and the wah pedal will not wah, or it will change nothing about the inductor, and will sound just like it did before it was placed.

not only do i move it around the whole surface of the inductor to find a sweet spot, but i also change the orientation of the magnet.

i have the yellow fasel on the breadboard at the moment (makes the most drastic differences), with the magnet on its spine (not sure if positive or negative), in the middle of the white plastic plate of the inductor.

i changed out a few values (im guessing that if the magnet lowers the inductance, lower inductances mean a more trebly sweep), put in a bigger sweep cap, and i like a lower value on the "q" resistor (parallel to the inductor) as people call it. everything else seems to be stock, i think i messed with the 22k (at the 9v+ to collector of q1 position) value, lowered it i think (all of the resistors are wired as rheostats atm, one leg out of circuit), and the 1k5 "midrange" resistor i tuned till i liked the midrange, havent measured yet.

some positions of the magnet make EXTREME changes to the sweep. before the point where the magnet is making too strong of an effect (i think it pushes the sweep out of audible range, personally) the sweep is VERY trebly. it is completely unusable except for the higher harmonics. sounds more like a chipmunk going "wee wee wee", instead of a wah pedal wah-ing. i tend to like the "ee" sound at the toe part of the range, so i play with it JUST until the point where it shows, and fiddle with the rest until the midrange and bass are balanced with it.

dang, i really want all the sounds, actually. lower sweeps (stacking two series inductors, or raising the value of the sweep cap) are great for muted "waka waka" things, and filtering crunchy tones.

anyway, the halo style inductor sounds better stock than with the magnet, which makes very little difference, even in the position where it DOES finally make a difference. maybe it is the metal brackets, which also make it a pain to work with the magnets.

i really want to make a video to demonstrate these experiments.

hope this thread necromancy will be tolerated!

[Rob Strand]

You have to watchout with those magnets.   Some ferrites can be permanently changed if you put a strong magnet like a rare-earth magnet on them.

The other issue is if you put a magnet on an inductor and take it away it can magnetize the ferrite so even though there's no magnet it is not longer the inductor it was before.  You will need to demagnetize the ferrite using an AC coil, then slowly move the coil out of the field.

some positions of the magnet make EXTREME changes to the sweep. before the point where the magnet is making too strong of an effect (i think it pushes the sweep out of audible range, personally) the sweep is VERY trebly. it is completely unusable except for the higher harmonics. sounds more like a chipmunk going "wee wee wee", instead of a wah pedal wah-ing.

Yes, the magnet makes the inductance smaller.    If you are DIY'ing an inductor you can add taps to the coil.   For each tap you can try tweaking the cap, series resistance and Q resistor.

[nepalnt21]

that's good to know, thanks. luckily i am not particularly attached to any of these three inductors. lotsa fun!

btw, in what way/ why can the ferrite be damaged?

[Rob Strand]

why can the ferrite be damaged?

I've never seen the details explained but I remember some manufactures mentioning exposing some of their ferrites to strong fields would cause irreversible change. I guessed it can somehow screws-up the structure of the ferrite at the magnetic domain level.  From that I suspect other ferrites are probably affected but in a minor way.   Those rare-earth magnets are pretty darn strong they will push most materials well into magnetic saturation.

Unless you have a demagnetizer it's hard to completely remove the magnetization from something.   In the past I first use the magnet I magetized something with in reverse then use a demagnetizer.   Another way is to drive a gradually decreasing AC current into the coils of the device (/inductor) but you need to start with a lot of current and you have to be careful not to fry the coils due to over current!

[nepalnt21]

in case anyone cares:



sorry if the guitar sounds like poo, i was focused on wah sounds rather than making sure my strings were in-tune.

after the video, i tried two inductors in series, with one taking the magnet. sounds nice once i got the sweep cap dialed in.

i swear i had it sounding like a univibe last night... there was a clear volume drop at the heel-down position, so it made this AMAZING swell with a nice subtle wah. i wish i could find that arrangement again, let alone make it repeatable once in an enclosure...

[Rob Strand]

in case anyone cares:

It actually won't play on my PC I'll have to pull-out my unix box.

[Paul Marossy]

I know this is an ancient thread but recently I've been playing around with wah pedal inductors, an oscilloscope and a few different kinds of DIY curve tracers. This morning I had a thought to see how placing a magnet close to or directly onto an inductor would affect its characteristics. My basis of comparison is between the red & yellow Fasels and the old El-Rad inductors found in the Maestro Boomerang. The red & yellow Fasels clearly show some asymmetry on the waveforms, as well as the El-Rad.

Inductor measurements:
Red Fasel 641mH, 15 ohm DC resistance
Yellow Fasel 545mH, 22.9 ohm DC resistance
El-Rad 470mH don't know DC resistance because I didn't want to remove from PCB to check it.
RadioShack inductor is nominal 1 Henry, measured 1.025 Henries with 226 ohm DC resistance
Foot note: The Fasels and RS inductors are constructed in a vey similar fashion.


So I took this old 1H inductor designed to be "clean", pulled from an old RadioShack graphic EQ, and did some experiments. When feeding it a 1-volt 500Hz sine wave, it reproduces it without any noticeable distortion. Then I tried a refrigerator magnet, absolutely no effect on waveform. Magnet on end of small screwdriver, no effect. A fairly strong magnet I've had forever which was pulled from a speaker voice coil and can pick up a 3.5 ounce steel object, well now finally something! But not what I expected. All it appears to do is shift where the resonant frequency occurred. It did not introduce any asymmetrical distortion, "the magic sauce". Interestingly, I could change the amplitude from almost no effect at all to at least doubling it, simply by rotating the magnet and/or slightly shifting it. I haven't done exhaustive testing on a dozen different inductors with ten different types of magnets, but so far I'm leaning towards this is not really doing anything special to the behavior of a small inductor that has a very high resonant frequency.