Question about relationship of eddy currents & hum/buzz

[jamesmafyew]

I've read a lot of the discussion on this forum about eddy currents, and most of it seems to have to do with frequency response -- that stuff makes sense to me. But I haven't found much about the relationship of eddy current interference and hum/buzz.

I'm curious because I have a custom all-aluminum guitar made by the guy who is doing the Travis Bean 'reissues' -- EGC in Florida. The guitar came with his in-house humbuckers which are based on the Travis Bean HBs (not the early WRH ones). They have the big alnico slugs set into a VERY thick steel baseplate, and epoxy-potted. They are fairly high output but also quite bright, as the Beans were known to be. They're bolted through the back of the neck, and not height-adjustable.

The sound of those pickups is not just a product of the aluminum neck/body, because I retrofitted the thing with vintage Gibson Dirty Fingers pickups. The sound was much more 'traditional' with these (if you could call the DFs that, though still much more 'normal' than the Bean-style pickups). I added nickel-silver covers to the DFs and wax-potted them to maintain the look of the EGC pickups. I mounted the DFs in chrome-plated brass rings.

My question comes because of how differently this guitar responds to EMI than my other guitars. I had these vintage DFs in two other guitars prior to this, and they didn't hum at all, even with enormous gain. When they are out of the guitar, on the bench, they do NOT hum (I am looking at the hum on a scope). As I lower them by hand into the aluminum top of the guitar body, I can see the hum increase to 15mVp-p on the scope. It peaks when the top of the pickup is flush with the top of the aluminum body. The neck position does it more than the bridge position (6mVp-p). Again, both pickups have negligible hum (not perceptible on the scope) when in other guitars or out on the bench.

I have coil taps on both of these, and both hum LESS in the single coil mode (regardless of which coil is selected) than in humbucking mode, when they are in the guitar. Out of the guitar, they only hum in single coil mode.

Now to the EGC pickups. They hum whether in OR out of the guitar. There is NO DIFFERENCE in how much they hum whether in or out of the guitar. Like the DFs-in-guitar, they ALSO hum LESS in single coil mode than in humbucking mode, but again in OR out of the guitar. Except for this fact -- they only hum in a few certain places (my workroom in my house being the main one, ugh). But when they do hum, that hum is a LOT louder than the DFs when in this guitar. But in other places they are dead quiet.

(Fact about my house: it has wiring problems, I know this. Two circuits have the polarity reversed and have open grounds, and one circuit in my workroom has an open ground but a GFI that ties the safety ground to the neutral right there -- meaning that the 3rd prong on that circuit is about 2VAC above true earth. So there is a lot of interference, yes -- but my wood guitars DO NOT HUM even with these same DFs installed.)

So my question is admittedly vague, but here it is: are eddy currents in the aluminum body/neck of the guitar responsible for the DFs' behavior? Are the super-thick steel baseplates responsible for the EGC pickups getting so much more interference in here? Is the effect of the baseplates in those pickups swamping the effect of the aluminum body somehow?

I guess I'm asking for help teasing out several interrelated factors. I like the EGCs enough that I've put them back in the guitar. Note: no difference in their behavior with the brass rings compared to before. So anyway, this is mostly moot from a practical standpoint. But I do want to understand what's happening here.

[jason lollar]

The steel plates add noise- same thing happens in the original wide range fender pickup which is basically what the TB pickup is but the TB has a thicker steel inductance plate, firebirds and thunderbirds with a steel inductance plate, check it out make a firebird with and without the steel plate and see. Its not a huge amount of noise but youll certainly hear it if you listen for it and it could easily be measured on a scope.

[jamesmafyew]

Thanks Jason. I know you did a ton of research on the WRHs for your Regal HB. Does it have the same thing going on, or did you find a way around the steel plate?

Edit: IF you wouldn't mind sharing. Not trying to steal your secrets or anything.

[Mike Sulzer]

As I lower them by hand into the aluminum top of the guitar body, I can see the hum increase to 15mVp-p on the scope.

Is the aluminum guitar body grounded when you do this?

[jamesmafyew]

Is the aluminum guitar body grounded when you do this?

Absolutely. And I tried it both ways, made no difference.

[Mike Sulzer]

Absolutely. And I tried it both ways, made no difference.

OK, that is a very good indication that the problem is magnetic, rather than electric; that is, it results from currents, rather than voltages. So you do have to wonder if the currents the aluminum body induce significant currents in the high permeability steel plate. I am not familiar with these pickups, but I have tried joining the pole pieces in two humbucker coils with thick high permeability material, and the resulting hum is unusually bad. In this geometry, the hum into the two coils adds, not subtracts, and so the fact tht it is a humbucker works against you.

[jason lollar]

I tried everything I could think of including isolating the plate and giving it a seperate ground lead- that sort of design just has some extra noise. Its not that much really but its there. I never noticed it and neither did several builders and repair guys but I had a handful of people mention it - maybe 5 out of 500 noticed it. Once I had 4 or 5 people mention it I went looking for the noise. I had to go out side the shop where the ambient noise level is lower and I had to turn the amp up quite a ways but I compared a wide range without the reflector to one with and there is a level of noise from the inductance plate. I did the same tests with firebirds. Its the plate for sure. However its - well I figure if you stop playing you should roll the volume control off, the level of noise is practically insignificant unless maybe you are recording and you have some extremely quiet passages or something external is adding to the noise level. I only use the plates where its needed to enhance the performance, like on the wide range- why have it in the neck? Makes little sense but if a guy did want an accurate set like vintage they could buy two bridge pickups and you would have a set like they were made in the 70's, both wound the same- interchangeable. The thing I didnt test was if the thickness or size of the plate made any difference- those TB pickups have a whole lotta steel under them and the magnets are enclosed so maybe those are noisier- I have worked on them many times but never used them much- the TB pickups that is.

[jamesmafyew]

The thing I didnt test was if the thickness or size of the plate made any difference- those TB pickups have a whole lotta steel under them and the magnets are enclosed so maybe those are noisier- I have worked on them many times but never used them much- the TB pickups that is.

Seems likely. Though the I think the increase in interference is offset by the unbelievable shielding you get from all-aluminum construction (which is maybe why the single-coil modes are relatively quiet). And maybe that's why they're not affected by the eddy currents in the aluminum body?

Just thinking 'aloud' here...
I should qualify my complaints again -- I don't find the TB-style EGC pickups to be noisy in MOST environments. I know that my house is particularly bad, it just seems weird to me that these pickups seem to selectively magnify noise in ways/places that none of my other pickups/guitars do, and the aluminum body doesn't seem to have any effect on them at all. I have only noticed their noise in my home, and maybe one or two other spots, when playing at high gain. It's nothing a Decimator wouldn't cure. I can't hear it when I play totally clean. The noise from the DFs in the aluminum body was noticeable with high gain but was still mostly acceptable (just a lot worse than the same pickups in a Les Paul). At my rehearsal studio, the EGC pickups are as quiet as any of my other humbuckers. And the single-coil mode on them is the quietest single-coil I own by a long shot, in any environment (including at home).

I suppose the extra noise with the WRHs is not a big deal, because basically no one uses those pickups for uber-gain anyway. But there are plenty of people who use the Beans and similar stuff for monster gain. Yet you don't see too many complaints about them being unacceptably noisy. So I come back to these few specific environments, which makes me wonder about whether certain types of wiring problems cause different levels of magnetic interference....? The noise definitely changes when I face a different direction with the guitar.

Mike, your explanation makes a lot of sense -- that induced currents in the aluminum could cause issues with the Gibson DFs. When I think about interference I am used to thinking in terms of induced voltage and this is different. From what I'm hearing here, I'm guessing that the noise in the EGC pickups is due to the steel plate and not so much the aluminum body. Their construction is totally different from the Gibsons, which have the triple bar-magnet arrangement, vs the alnico slug magnets. So maybe the physical orientation/construction makes the EGC less prone to the eddy current interference from the aluminum body (as opposed to them being enclosed), but then it's got that steel plate with lots of eddies doing something similar.

(If I've got a bunch of equipment with grounded chassis sitting on that GFI circuit, where the 'ground' is actually the neutral and a few volts above true earth, is all that chassis metal the source of my magnetic interference? I've got a huge radiator in the room too, which is at actual earth potential. Yikes. Probably beyond the scope of a thread in the pickups forum.)

[big_teee]

The quietest pickup I've made to date is a bass soapbar humbucker with ceramic magnets, and no steel or other metal plates.
I made one with a steel baseplate, and had a light static buzzing sound.
I did like JL and tried grounding it, even with a separte ground, it still buzzed.
I took the steel plate out and replaced it with sheet plastic, and it was back quiet again.
With or without the plate no noticeable change in tone.
No scope, so I don't really know the cause, but the steel plate picked up the buzz.
T

[kmensik]

1. Conductive magnet in a humbucker makes more buzz than a non conductive one. E.i. alnico bar is more noisy than ceramic.
2. Steel HB baseplate adds even more buzz and sort of shorts the magnetic field, makes the pickup lower output thus propagating the noise even more.
3. Connecting polepieces with a thin strip of steel sheet, even grounded, adds wierd static noise. Better leave poles ungrounded. Had that issue in a plastic EMG style passive HB.

So far had no problems with metal pickguards or metal mounting frames.

I guess it is the combination of magnetic and conductive propperties in one element inside the humbucker that picks the most noise from the space, it is then converted to eddy current in that element and induced directly to the coils.

That is all I can tell from my experience, can not tell what the aluminum body does. I play hi gain and care a lot about noise level in humbuckers.

[jamesmafyew]

2. Steel HB baseplate adds even more buzz and sort of shorts the magnetic field, makes the pickup lower output thus propagating the noise even more.
3. Connecting polepieces with a thin strip of steel sheet, even grounded, adds wierd static noise. Better leave poles ungrounded. Had that issue in a plastic EMG style passive HB.

Jason can probably correct me if I'm wrong, but I'm pretty sure my EGC (TB style, but not necessarily TB clones) pickups have the alnico polepieces ungrounded. They seem to be insulated from the steel baseplate by epoxy. They might have been dipped before the pickup was assembled (hard to tell since the whole thing is potted in epoxy). There are holes in the baseplate for all twelve polepieces, and it looks from the back like the poles are set into those holes. But when I punctured the epoxy potting with my meter probe, I got no continuity between the poles and the steel baseplate. Poles ungrounded, steel baseplate grounded.

I don't doubt what you're saying at all, but I think in my case the main source of noise in these pickups vs. others is eddy currents in the steel baseplate inducing noise in the coil. But I won't replace them at this point because 1) they sound unbelievably good, and 2) other pickups will make nearly as much noise due to the aluminum body.

I've never used noise gates, but this guitar sounds so good (at all gain levels) that I think it's time to invest in one for noisier venues.

[jason lollar]

i dont know i have never seen a travis bean encapsulated in epoxy with the .25 thick steel baseplate- all the ones i worked on had the pickup cover riveted to the steel baseplate and then had silicone partially sealing the bottom. I wonder when they changed from one to the other..... I have seen later 1979 tb pickups without the steel baseplate using a brass hanger for the height screws filled with epoxy- different design though, maybe at some point they went to epoxy before changing the design

[fieldwrangler]

All this is reminding me of the hum problems I have in my sidewinders that use a common permeable core in the bobbins instead of saturated magnets...

[soundguruman]

Yeah Travis did have a lot of single coil hum. I have owned 5 Travis bean guitars...including 1 longhorn Bass.
1 Artist, One Wedge, 2 Standards, and a Longhorn.
The Artist was the best guitar. The Longhorn was a KILLER sounding Bass.

The "sound" is coming from brass nut and metal bridge saddles...should replace both with TUSK, carbon fiber, bone, Delrin, etc...
you will get a less "metallic" sound.
The Travis sounds better with a carved "dished" top. The flat top ones don't sound as good. Example is Travis Artist Professional Model with carved top.
Also, mahogany body or Koa body is much better sounding.

Single coils will never stop humming. No matter what you do.

NO GROUND in your house? Expect noise, no matter what pickups.
There is no solution, except humbucking pickups, and a proper ground for the power.

Travis? he was a genius. The people who worked for him went on to make Kramer...
The early Kramers are very similar to Travis Bean Guitars.
But Travis liked Cocaine. And died young of a heart attack. RIP, sad.

The other guitar I liked, from this era, was a "Moonstone."

[ScottA]

I make Tele bridge pickups with and without a copper/nickel plated low carbon steel baseplate.

The ones with the baseplate buzz a little bit, even when they are properly grounded.

The ones without the plate are pretty much dead quiet.

I use the ferrous plates on the lower wind, "Vintage", models. The juice they add is part of the tone and the buzz is the price you pay for it. On higher wind models they thicken up to the point that I don't feel the plate is required.