Microphonic Wah Inductors?!

[Paul Marossy]

I had an inductor that I pulled from a cheapo wah pedal that I put into a wah pedal I just built. But it picks up every noise from the wah pedal operation to any taps on the enclosure. It will even pick up my voice if I speak into it! Weird...

Anyone ever have this experience?

[davent]

Hi Paul ,

No experience with microphonic inductors but often see people mentioning adding damping to tubes (silicon O rings) or caps (Blue Tack) or dampening an enclosure to reduce ring and microphonics (in the HiFi world). Just speculating but could you try adding some mass to the inductor with something like Blue Tack

dave

[juse]

Is it potted? Do you have a pic?

[Paul Marossy]

Hi Paul ,

No experience with microphonic inductors but often see people mentioning adding damping to tubes (silicon O rings) or caps (Blue Tack) or dampening an enclosure to reduce ring and microphonics (in the HiFi world). Just speculating but could you try adding some mass to the inductor with something like Blue Tack

dave

Hmm... that's an idea to try. Perhaps part of the problem is with the wah shell itself? It's a stamped steel Colorsound wah shell, so it doesn't have a lot of mass to dampen external noises on the enclosure and stuff. 

Is it potted? Do you have a pic?

I don't think it is potted. It does the same thing with a Fasel inductor as well, just not quite as bad. I don't know if those are potted or not.

[R O Tiree]

I had this once with a Fasel inductor - these are not potted, BTW. The ferrite core is actually 2 halves and the coil sits in the middle, then the yellow (red) part of the casing is positioned over the white part during manufacture and then heat is applied to bond the plastic parts together firmly... or not. In the end, I had to send it back for a replacement. The problem was that the casing was cracked, so the coil and the 2 halves of the ferrite core were able to move around slightly - voila! a microphone.

I tried potting it, but that actually changed the characteristics of the inductor beyond belief.

I tried super-gluing the casing - no joy, because even super-glue wasn't strong enough (in tension) to hold the shell together. I didn't try something like smothering it in epoxy - that's something I might try if I ever have this problem again. You'd have to clamp the shell somehow while the epoxy cured and leave it for 24 hours, I guess, to make sure it had really cured.

I also tried an Elaca Halo (? it was a while ago) and, while that was not microphonic, I didn't really like the sound very much.

What you need to do is find some way of clamping the coil and ferrite parts together very firmly, so they cannot move in relation to each other at all, without introducing any more metal (which would probably alter the characteristics of the inductor). A tie-wrap?

Let's face it, this is a microphonic inductor, so whatever you do can't actually make things worse, can it? And if it breaks, or becomes unuseable, then you haven't actually lost out at all, because it's unuseable anyway as is.

[Paul Marossy]

Interesting info about the Fasel inductors. I noticed that it was only microphonic when it was in front of a really high gain distortion.

I found a way to isolate the circuit board a little bit with some rubber feet from an old (I mean old!) CD player that I tore apart. They were the four rubber isolators on the CD tray. They happened to fit perfectly in the holes on the four corners of the perfboard I used. And the screws holding the PCB in place have some rubber insulators between the circuitboard and the screwheads, and the screws are just barely tightened, only just enough to keep them from coming out. I also oiled the moving parts a little bit and that helped as well.

It's still not totally immune to extraneous noises, but it's a LOT better now, even with high gain distortion you really can't hear much of anything anymore. 

[juse]

So you've had the same symptoms with two different inductors on the same board? Maybe it's the board itself? Have you checked for lifted traces underneath the inductor area?

I'll bet it's the inductor itself, though.... just happens to be a coincidence with the Fasel. Have you tried to put the Fasel in another board to see if it is truly microphonic? If you put it in another board & it's okay, I would tend to think it was the other board itself with the problem.

But, if you have access to the windings of the original inductor, you could try wrapping a piece of ribbon soaked in glue around it, or better yet, a larger piece of shrink tubing that will hug it nicely when warmed up. It sounds like a stabilization issue, so a big 'ole squirt of silicone might work too.

[Paul Marossy]

So you've had the same symptoms with two different inductors on the same board? Maybe it's the board itself? Have you checked for lifted traces underneath the inductor area?

I'll bet it's the inductor itself, though.... just happens to be a coincidence with the Fasel. Have you tried to put the Fasel in another board to see if it is truly microphonic? If you put it in another board & it's okay, I would tend to think it was the other board itself with the problem.

But, if you have access to the windings of the original inductor, you could try wrapping a piece of ribbon soaked in glue around it, or better yet, a larger piece of shrink tubing that will hug it nicely when warmed up. It sounds like a stabilization issue, so a big 'ole squirt of silicone might work too.

Yep, two different inductors on the same circuitboard do the same thing, but the Fasel inductor isn't as bad as the no name cheapo I first used. It's built on perfboard, so it can't be a case of a lifted trace. I think it has something to do with the mass of the wah pedal shell, and with it being only a relatively light weight steel casing. With the rubber isolators in place and the circuitboard being held loosely in place by two screws with rubber grommets on them, the problem is vastly reduced.

[Paul Marossy]

I think there is one other factor to consider, although it doesn't seem like much, but maybe it has something to do with this phenomenon?

On the vintage Colorsound circuitboard that I used as a guide to make my own Colorsound clone, I noticed something a bit unusual about the inductor. It was an affair where the two ferrite cups have a machine screw & nut thru them which holds to two halves together. Not unusual, I have seen that before on some types of inductors. What is somewhat unusual on the Colorsound circuitboard is that the screw goes thru the circuitboard, and is also connected to ground on the PCB. I wonder if that helps with it not being microphonic?

[Exactopposite]

i had a yellow fasel that was very microphonic. Since it was new I got a replacement for it that worked fine. I also had a TDK inductor in an old thomas organ wah become microphonic when it hadn't been before. In both of these cases it was definitely the inductor because the problem ended when the inductor was replaced. Since I have encountered it twice myself I'm thinking it's not all that uncommon for a wah inductor to be microphinic.

[Paul Marossy]

i had a yellow fasel that was very microphonic. Since it was new I got a replacement for it that worked fine. I also had a TDK inductor in an old thomas organ wah become microphonic when it hadn't been before. In both of these cases it was definitely the inductor because the problem ended when the inductor was replaced. Since I have encountered it twice myself I'm thinking it's not all that uncommon for a wah inductor to be microphinic.

Huh, interesting. Thanks for the info! 

[Lurco]

is there an air-gap or a plastic insulation between the two halves in those Fasel inductors?

[petemoore]

Y Fasel, microphonic, until now it hadn't occured to me.
  But the description fits exactly, any touching of the case injected case noise into the signal path with the modded V-847 wah, it's since become a wah case with wah parts in it.

[Paul Marossy]

is there an air-gap or a plastic insulation between the two halves in those Fasel inductors?

They are a "cup core" toroidal type. I don't think there is two halves...

[amptramp]

A lot of guitar pickups themselves are microphonic and some players prefer it that way and complain that potted pickups sound "dead", although most are now potted in either epoxy or wax.  I have no problem believing that an inductor can be microphonic.  Changes in the very tiny air gap can make a large difference in the inductance because this is most of the reluctance in the magnetic path (reluctance is the magnetic equivalent of resistance).  The equation covering the effect of an inductor is usually expressed as:

E = L (di/dt)

but is actually:

E = d(Li/dt)

In effect, this is E = L (di/dt) + i (dL/dt) by the rules of differential of a product.  When L starts varying, all bets are off.

[Paul Marossy]

A lot of guitar pickups themselves are microphonic and some players prefer it that way and complain that potted pickups sound "dead", although most are now potted in either epoxy or wax.  I have no problem believing that an inductor can be microphonic.  Changes in the very tiny air gap can make a large difference in the inductance because this is most of the reluctance in the magnetic path (reluctance is the magnetic equivalent of resistance).  The equation covering the effect of an inductor is usually expressed as:

E = L (di/dt)

but is actually:

E = d(Li/dt)

In effect, this is E = L (di/dt) + i (dL/dt) by the rules of differential of a product.  When L starts varying, all bets are off.

Yep, and I have experienced both microphonic pickups and wah inductors. Some of those super cheap pickups made overseas don't have any potting whatsoever on them. Ask me how I know. 

[DaveM]

Apparently a BMP is microphonic at high gain as well.  http://www.youtube.com/watch?v=vkDLQAvrEQQ  (1:12)