Why is only one set of pole pieces in a humbucker adjustable?

[ryan mead]

I suspect it has something to do with common mode rejection.

[Andyh 56]

Looks around,shuffles uneasily on seat, timidly puts up hand.
"Er what's common mode rejection?" Is there anything I can take for it?

[Hugh Robjohns]

I'm no guitar expert, but I imagine it's because the corresponding string is in the nearfield of the adjustable coil, and by adjusting hte pole piece you can adjust the sensitivity of that coil to balance the strings etc.

Ambient hum fields which the humbucker coil is there to reject are, inherently, in the far field and thus affect both equally. Consequently there is nothing to adjust on the humbucking coil...

And for the other question: common mode rejection is the ability of a differential system to reject a signal when it appears equally on both its inputs. This is the basis of the EM and RF immunity afforded by balanced interfaces, and in the case of humbucker coils, is created by the fact that the humbucker coil is wound in opposition to the string coil. Thus a hum field picked up equally by both will result in equal but opposing (ie, common mode) currents and thus a cancellation of the hum field in the output signal.

Hugh

[zenguitar]

Hmm...

Just out of interest I paid a virtual visit to the US Patent Office and read Seth E Lover's original Patent for the Humbucker. He makes no reference there to adjustable pole pieces.

There are actually humbuckers with no adjustable pole pieces, one row of adjustable pole pieces, and two sets of adjustable pole pieces. And even 3 adjustable pole pieces on one coil and three on the other coil (the Fender Humbucker for Telecasters that was also designed by Seth E Lover).

The adjustment is not there for any fancy electronic tuning. It is there to fine tune the string to string balance. Just as raising or lowering the pick-up as a whole will increase or reduce it's output, raising one pole relative to the others will increase the output for that string and lowering it will make it weaker. Generally, there's rarely any need to do that. It's likely to be a hangover from the period when players switched from a wound G string to a plain one. It is similar to the change that Fender made in the heights of strat pick-up pole pieces in the 60's, they reduced the height of the G string pole piece in response to a modification that was done by many players at the time.

Andy

[ryan mead]

Thanks Hugh!

I'm no guitar expert, but I imagine it's because the corresponding string is in the nearfield of the adjustable coil, and by adjusting hte pole piece you can adjust the sensitivity of that coil to balance the strings etc.

Right but then why wouldn't both sets of pole pieces be adjustable? Typically only one is. (On any naked humbucker I've ever seen, one set has slots for adjustment with a screwdriver, whereas the other doesn't. Presumably if I were to pop the top on a covered LP-style humbucker I'd see the same thing, as only one set pokes through the cover.)

Ambient hum fields which the humbucker coil is there to reject are, inherently, in the far field and thus affect both equally. Consequently there is nothing to adjust on the humbucking coil...

D'oh! Of course...

[ryan mead]

Cross-posted there Andy! Still a little baffled but guessing that only one of the two poles for each string would need to be adjustable in order to set the overall volume for that string relative to the other strings.

[ryan mead]

Looks around,shuffles uneasily on seat, timidly puts up hand.
"Er what's common mode rejection?" Is there anything I can take for it?

Take two coils and call me in the morning...

[intense]

The WikiPedia article doesn't really explain how humbuckers, or guitar pickups in general, actually work!

A simple explanation is that each string behaves as a component in the magnetic circuit that imposes increased magnetic reluctance as it vibrates away from its rest position. Reluctance is analogous to resistance in an electrical circuit and the cyclic changes of reluctance cause variations of the magnetic equivalent of 'current', which is the flux density, in sympathy with the string's vibration. This gives rise to the alternating EMF appearing at the terminals of the coils of a humbucker, or the coil of a single-coil pickup, in line with Faraday's law of induction.

In which case, a single screw per string is all that's needed to adjust the rest-state value of the reluctance for each string. The smaller this value, the closer the screw-head to the string, the more difference a given amount of string movement makes, and the greater the resulting EMF (or signal amplitude).

However ... by this explanation, the generator action is symmetrical about the rest position of the string so all electric guitars would emphasise the second harmonic of each string's actual vibration frequency. But, then again, so do acoustic guitars - the 'top' is compressed with each half-cycle of each string's movement, irrespective of the direction in which it moves. Does that mean we're all accusomed to the fact that guitars sound the way they do because of their inherent frequency doubling? ... !

Some have put forward the theory that the string is polarised by the field from the pickup and this makes the 'generator' process skew-symmetric. Considering the small width of the strings in comparison with the amplitudes of their vibration, this seems very unlikely (a bit like the 'wind carries sound' idea when the speed of sound is at least ten times that of the wind).

Chris

[ef37a]

Err?
I was a little perplex by the question as well. The adjustments are indeed just there to balance string levels, what possible purpose would it serve to have two per string?

Be like having two sets of machineheads!(and yes, I know about locking nuts and bridge tuners ta!).

Dave.

[ryan mead]

Err?
I was a little perplex by the question as well.

Well, I'm an idiot then.

[CaptainChoptastic]

Well, I'm an idiot then.

I wouldn't say so - maybe just overthinking it a bit:

In a humbucker, the coils are wired in series (when it's operating in humbucking mode), so the same current that flows in the screw coil will also flow in the slug coil. So any change that is made to the output of the screw coil (by adjusting the screw) will also affect the output of the whole coil pair for that string. So there's no necessity for both coils to have adjustable poles in order to have the desired effect.

Other reasons for pickups typically having one slug coil and one screw coil could be that the manufacturing costs for the slug coil are a bit lower - I'm guessing that screws and their associated holes will require more machining and assembly.

Also, when splitting a humbucker it's often the slug screw that becomes the "active" coil in default wiring schemes (OK, not always the case, but...): this might provide a more single-coil-like tone from the split than the screw coil would, since SCs only ever seem to use slugs (you never asked why SCs don't have adjustable pole-pieces!).

Also, a good proportion of pickups do use two rows of screws or bolts - check out the images on e.g. Dimarzio's site: they use a variety of pole-piece styles, probably because of the specific tone they provide.

Have a look at the pickups on the Suhr site: you can get essentially the same pickup with either a single row of screws and a single row of slugs (SSV/SSH), or with a double row of screws (DSV/DSH). According to Suhr, the former have a bit more bite and snarl, the latter have a smoother, sweeter high end.

Si

[ryan mead]

(you never asked why SCs don't have adjustable pole-pieces!)

OK, why don't they? (I've always assumed they were adjustable if one dug a little deeper.)

[zenguitar]

(you never asked why SCs don't have adjustable pole-pieces!)

OK, why don't they? (I've always assumed they were adjustable if one dug a little deeper.)

In a typical Fender style single coil the pole piece is the magnet. The six magnets are then placed between two fibreboard caps and the enamelled wire is then wrapped right around the magnets between the two fibreboard pieces. Because the enamelled wire is against the magnets, any movement risks breaking the wire, or wearing off the enamel insulation and creating shorts.

Typical humbuckers have a complete bobbin that carries the ferrous pole pieces inside and the coil is wrapped around the outside. The pole pieces are connected to a bar magnet at the bottom of the pick-up.

Of course, there are exceptions. A lot of budget Fender style single coils are built with a bar magnet below and ferrous poles. P90's have a bar magnet below the coil, but can have screw pole pieces. And there are pickups where the coils are horizontal rather than vertical.

Andy

[intense]

It's always seemed to me to be perpetuating poor practice to wind the wire directly onto the magnets in Strat-style pickups, but I guess the cognoscenti of originaldom would have it no other way. On at least one occasion, either in a mag or from the web, I have read someone purporting to be an 'expert' advising that 70s-style pickups can be made to look 60s-style by whacking the B-string magnet down with a hammer and a piece of dowel. Dangerous on at least two counts!

Still, who's worried by a shorted turn or two when some two-pickup guitars have their volume pots wired to progressively short-out the pickups at lower settings. Also, I was surprised to discover, some of the variable humbucker-splitting schemes progressively short one of the coils. It doesn't seem to affect the string vibration ... which supports the belief that guitar pickups work by variable reluctance rather than any kind of 'generator' principle.

Chris