Wah Inductors and magnetism

[Philippe]

It is? How exactly is that?

Playing around with the Re1, Rq, & Rfb circuit resistors seems to make/have more noticeable impact than obsessing/scrutinizing the oftentime subtle differences between modern-day aftermarket Fasel red/yellow and/or Halo inductors.

It appears (to me) that as long as an inductor falls within a certain prescribed/recommended frequency/impedance range and isn't excessively microphonic, the aforementioned resistance modifications play a far larger role in determining the final sound of the wah circuit.

[Paul Marossy]

Playing around with the Re1, Rq, & Rfb circuit resistors seems to make/have more noticeable impact than obsessing/scrutinizing the oftentime subtle differences between modern-day aftermarket Fasel red/yellow and/or Halo inductors.

It appears (to me) that as long as an inductor falls within a certain prescribed/recommended frequency/impedance range and isn't excessively microphonic, the aforementioned resistance modifications play a far larger role in determining the final sound of the wah circuit.

I can agree with that based on my own personal experience.

[MoltenVoltage]

The core itself was magnetized, and signal ran into saturation earlier on one polarity than the other. Looking at the circuit, I found that it puts a bit of DC current through the core for biasing Q1. The core material was fairly cheap and had some significant remanent flux, and there you are. The core was magnetized. Better ferrites don't have much remanence.

Possibly ignorant question, but can years of use magnetize the core of the inductor and change its properties over time?

[R.G.]

The core itself was magnetized, and signal ran into saturation earlier on one polarity than the other. Looking at the circuit, I found that it puts a bit of DC current through the core for biasing Q1. The core material was fairly cheap and had some significant remanent flux, and there you are. The core was magnetized. Better ferrites don't have much remanence.

Possibly ignorant question, but can years of use magnetize the core of the inductor and change its properties over time?

That was my theory on how it got that way. There is a DC current through it in operation, the bias current for Q1. All by itself, that should cause some detectable magnetic offset; however I tested it out of circuit where the core material should have relaxed from the DC offset down to its remanant field value. What surprised me was that any remanence effect was detectable at all. I was used to working with power conversion style ferrites where remanence is a disaster to be avoided, and where air gaps were common.

For magnetically soft ferrites and electrical iron, the remanence (that is, the remaining static magnetic field when any external field is removed) is quite low. Permanent magnet material is selected for **high** remanence to keep a field. Transformer and inductor cores are selected for low remanence because they can saturate on the first cycle if started on the wrong phase, and because the core magnetizing losses get larger as the area inside the BH loop gets bigger, and high-remanence materials maximize the loop area. Taken together, those mean that an audio inductor should have nearly zero remanence, and any remaining field when the current is turned off should be very nearly zero.

Materials are not perfect,though. Remanence is a parameter of the material.

Air gaps in general reduce remanence by reducing the flux density to which the core is driven.

I do not know the exact material of the "magic" core I tested. It was unmarked, and I didn't have the equipment to do laser spectroscopy (that was in a lab I didn't have access to, and the staff there was carefully counselled to not let random engineers use the toys. No amount of promises of beer helped.  )
However, cheapness and poorer properties went hand in hand and at the time, cheap high-mu ferrite had many imperfections. The best-fitting theory is that the ferrite was picked for highest permeability (mu) and other properties could go hang. That would minimize the amount of both ferrite and turns needed to get to half a henry.

The ferrite cores I had access to in the power lab were high quality, high mu, low remanence. I did do the necessary test, winding up a half henry inductor from new ferrite, then testing it. No second harmonic. I also had access to devices and power levels that would magnetize anything that could be magnetized. I did that to the new wind. No measurable second harmonic. I wound a second magnetizing winding on the newly-wound one and finally got second harmonic by driving the second winding with a constant current. The second winding I eventually abandoned as too complicated. The constant current source needed to not load down the Q of the inductor was as complicated as the rest of the wah circuit.

But people reported good results - or at least SOME results by sticking magnets to the core of a wah inductor, doing the same thing without a second winding.

Kewl.

[Paul Marossy]

However, cheapness and poorer properties went hand in hand and at the time, cheap high-mu ferrite had many imperfections. The best-fitting theory is that the ferrite was picked for highest permeability (mu) and other properties could go hang. That would minimize the amount of both ferrite and turns needed to get to half a henry.

Sound like a good theory to me. That's probably exactly what drove the inductor design - cost. Looking at some of the Italian made Vox wahs that had Fasels in them, cost looks like it was definitely the first consideration. Poor workmanship, really cheap wire, hodge podge of components, etc.

[joegagan]

simple question. would it be possible to magnetically recreate this offset by winding a figure eight core with deliberately mismatched size or type of magnets for each half of the 8?

[RedHouse]

I wonder if the cores are all the same dimensions?

In pickup winding with all else aside, altering the bobbin core shape effects the tone without effecting the meter readings.

5000 turns on a Strat bobbin doesn't sound the same on a humbucker bobbin, even if you move put the alnico magnets into the humbucker bobbin.

Just a thought, ok now, back to your regularly scheduled thread.

[R.G.]

simple question. would it be possible to magnetically recreate this offset by winding a figure eight core with deliberately mismatched size or type of magnets for each half of the 8?

Maybe... 

I wonder if the cores are all the same dimensions?

Nope. All of the follow on cores were seemingly obtained by asking a vendor of wound ferrite cores to give them half a henry, with a Q greater than X at frequency Y and DC resistance below Z. I'd wager a significant amount of money that the specs for the companies making wahs didn't include ferrite type, BH-curve remanence, or even loss curves. IMHO, the offset effect was the purest kind of happy accident.

[mac]

Joe,
I was thinking a few days ago about making the core not a cylinder but like this



If an external magnet altering the shape of the magnetic field has some effect in tone, why not trying a not symetrical core shape? Just for fun.

mac

[Joe Hart]

I tried a magnet on the inductor of my Vox 845 and it seems to add a bit of "growl" to the midrange and to smooth the transition out a tiny bit. I'd say all-in-all it's pretty subtle, but the changes do seem to be there. Very interesting. I will say that my 845 is the best wah I've ever played stock (I like my modded 847 a little better). Too bad it has a lot of SMD components and a lot of extra components that I can't figure out. I'd love to fine tune it (and make it the best wah I've ever played??), but I can't figure out what all the parts are. Oh well. But anyway, that's my take on the magnet thing. It is a very interesting mod (and super easy to do)!
-Joe Hart

[george]

Playing around with the Re1, Rq, & Rfb circuit resistors seems to make/have more noticeable impact than obsessing/scrutinizing the oftentime subtle differences between modern-day aftermarket Fasel red/yellow and/or Halo inductors.

It appears (to me) that as long as an inductor falls within a certain prescribed/recommended frequency/impedance range and isn't excessively microphonic, the aforementioned resistance modifications play a far larger role in determining the final sound of the wah circuit.

I can agree with that based on my own personal experience.

My personal experience not being as extensive as Paul's or Phillipe's, but I would have to disagree ....

I have a GCB-95 wah that I first tried all the mods on, then put a yellow fasel in, still didn't like the sound of it.  Then I put in a red fasel which I liked, but found the mods made it too bassy so I took them out and I now like the wah fine with just the stock resistor values!  So in my experience the difference definitely isn't subtle.   So much so I have a yellow fasel which I just don't need anymore.

Sure the differences may be in resonant frequency and/or impedance but I just haven't been very scientific I suppose.

[joegagan]

i never found a modern fasel red or yellow that i thought sounded worth a crap. i grind them up and eat them for breakfast.

[gmoon]

@george
No one's denying that swapping inductors can change the sound.

What Paul and Phillipe are saying is that the inherent DC resistance alone has a large effect on the Q (bandwidth)-- and unlike the inductance variation, that can be compensated for.

GBlekas chart on page one shows the DC resistance of his collection varies between 14.4 and 106.3 ohms. Sure, there's quite a variation in inductance, too. But we CAN tune the circuit for that difference in resistance (actually, we can tune it for the inductance variation, too).

I've done quite a bit of hacking w/variable inductors in fairly different circuits, as well as running simulations. The Q factor in ALL instances decreases as the DC resistance of the inductor increases. However, it decreases MUCH more prominently at higher frequencies. I.E., an inductor with higher resistance will tend to have "mellower" highs, while being a more effective filter in the lower frequencies. How much the Q decreases (and where) depends on the other components in the circuit (as well as the inductor itself).

Of course, tuning the Q via external resistors isn't quite the same as the inherent DC resistance of the inductor itself. It's just arguable that most of the "magic inductor" effect could be due to various other variables.


RE: 2nd order harmonic

RG and the EEs can slap me down hard on this, but it seems to me that any LCR variable filter is a harmonic-generating machine, including even-order--when you sweep the filter.

A LCR resonant filter, or any thing with hysteresis, has a time component by nature. If there's an inductor (w/reluctance and permeability) there's a time-dependent rebound. So when the filter sweeps, there's a mixture of signals that's probably asymmetrical (with some resultant even-order harmonics--probably).

I hate to resort to the "but I can hear it" argument, but in my variable inductor experiments it's pretty obvious that altering the inductance rapidly has a greater effect than a slow change. I think that's why a resonant wah filter is so much more successful than other alternatives like the Twin-T, etc.

But  that's somewhat different than RG's magnetic offset, which in theory would be generating 2nd order harmonics without the sweep.

Personally, I've heard (and built) enough tube amps to take the whole "Class A" and "even-order distortion magic" thing with a grain of salt. There have certainly been a lot of BAD tube amps out there. An amp with only a 2nd order harmonic is pretty boring. Most amp mods strive to get more odd-order harmonics...

I'm not discounting this completely, but it's right back to "personal preference" again.

[Paul Marossy]

I've done quite a bit of hacking w/variable inductors in fairly different circuits, as well as running simulations. The Q factor in ALL instances decreases as the DC resistance of the inductor increases. However, it decreases MUCH more prominently at higher frequencies. I.E., an inductor with higher resistance will tend to have "mellower" highs, while being a more effective filter in the lower frequencies. How much the Q decreases (and where) depends on the other components in the circuit (as well as the inductor itself).

That's an interesting bit of info there. That could explain a few things, depending on what type of changes you are expecting to hear when you change out an inductor. Any differences I *think* I have heard solely by replacing the inductor were very very subtle. So much so that I had to question if I really heard anything at all.

[Philippe]

i never found a modern fasel red or yellow that i thought sounded worth a crap. i grind them up and eat them for breakfast.

Being mass-produced as a retro-inspired component, some/many probably deviate a bit from the 'ideal numbers' as the inconsistancies & variances of modern-day Fasels have been well documented/noted by various builders.

Prefer a spec'd halo myself...along with the 'dialed-in' resistance values.

[RedHouse]

I'm with George on this one.

I too have tried many inductors over the years, I settled on the RED fasel which does it for me.
(that and replacing the MPS-A18 transistors)

[joegagan]

great points all around. thanks to everyone for their input.

phillipe, i base my statement on modern fasels after eartesting ( tuning each as tested) - i have run these tests on 4 yellows and three reds, they all exhibit the same limp-weiner sound to me. if you want a sweet sounding (  think sonic rainbow ) wah but that is also very boring, use a modern fasel. just my opinion.

that said, what i am looking for in a wah is a resonant circuit that is lively, with a growl-ey bark in the low mids to mids. this is what makes wahs dynamic and fun to me. if a wah just has a nice filtering sweep from high to low, it might as well be a twin T circ. to me, the advantage of the inductor is the ability to create a very dynamic instrument from it.

i have seen 10 years of people giving opinions on which inductor they like best, but i would guess that the majority of them simply swapped inductors ( maybe did the usual 33k mod, Q1 emitter R mod etc) but they did not take the time to tune each inductor with its ideal cap to ground. it is near impossible to say which inductor sounds best without taking the time to tune each one individually. and this makes AB comparing nearly impossible.

[Philippe]

that said, what i am looking for in a wah is a resonant circuit that is lively, with a growl-ey bark in the low mids to mids. this is what makes wahs dynamic and fun to me. if a wah just has a nice filtering sweep from high to low, it might as well be a twin T circ. to me...

Can't argue with that assessment.

A great wah tone is reminscent of barreling down an open road (or highway) with the sound of dual Weber sidedraft carburators running wide open...that throaty growl is unmistakeable.

Sounds like you'd dig the sound of a Macari wah with the scooped mids as recorded/heard on Disraeli Gears/Wheels of Fire.

[gmoon]

I've done quite a bit of hacking w/variable inductors in fairly different circuits, as well as running simulations. The Q factor in ALL instances decreases as the DC resistance of the inductor increases. However, it decreases MUCH more prominently at higher frequencies. I.E., an inductor with higher resistance will tend to have "mellower" highs, while being a more effective filter in the lower frequencies. How much the Q decreases (and where) depends on the other components in the circuit (as well as the inductor itself).

That's an interesting bit of info there. That could explain a few things, depending on what type of changes you are expecting to hear when you change out an inductor. Any differences I *think* I have heard solely by replacing the inductor were very very subtle. So much so that I had to question if I really heard anything at all.

It's a pain to pull a web-friendly graphic from LTspice, but here's a couple pics to illustrate, at two different inductance values. The inductance values and circuit are the same in each plot, only the series resistance changes--15 ohms (green) vs 105 ohms (red). That hifreq peak is under 4dB for the higher resistance value, vs about 18 dB for the lower resistance inductor.

low freq response


high freq response



Yes, this isn't the classic Vox/Thomas/McCoy circuit. But I've simulated and tested a ton of different LCR tank circuits, (following boost, NFB, w/opamps, jfets, bjts) and they all have the tendency to drop off more in the higher frequencies when the DC resistance is higher.  How precipitous it drops depends on a lot of factors, but this just illustrates how isolating and changing one factor alone (not the inductance) can have a pretty large effect.

The change is still there, but somewhat less prominent in the Vox circuit because a 500 mH inductor is on the edge of the sweet spot for a linear response in the audio spectrum. A higher value inductor (1-2 H) would probably be even better, but bigger and more expensive.

I know there's tons of other variables, too--ferrite material, geometry, etc.

Fun stuff!

[RedHouse]

... the sound of dual Weber sidedraft carburators running wide open...that throaty growl is unmistakeable...

Back in the 80's I had a Datsun (Nissan) 260Z with that inline-6 and it had 2 of those Weber dual side draft carbs, I had the hood off for a while and you are so totally right, the sound they make is un-mistakable.

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