Centre tapping a single coil?

[Steve Conner]

No, if it sums the signal, it will sum the noise too. Can nae break the laws ae physics, Cap'n.

[peskywinnets]

No, if it sums the signal, it will sum the noise too. Can nae break the laws ae physics, Cap'n.

Edit: ok, thanks for clarifying - I'm still struggling here.....if the noise signal always has the same phase as the 'string signal', then how is it possible to have a humbucker (the penny will drop soon, but each Google'd explanation I've read is not clear to pesky's unfeasibly small heed)

[Steve Conner]

Edit: ok, thanks for clarifying - I'm still struggling here.....if the noise signal always has the same phase as the 'string signal', then how is it possible to have a humbucker

Because that statement is not generally true. It's only true for the pickup configuration you're experimenting with, where you have two closely coupled coils all wrapped around the same set of magnets.

In a humbucker, you have two coils each with its own set of magnets. One set of magnets has the opposite polarity, which reverses the string signal in that coil, but doesn't reverse the noise signal. So, one coil picks up (noise + signal), the other picks up (noise - signal), so you can take the difference between them to get 2*signal and 0*noise.

But you can't do that, because all your coils are wrapped around the same magnets.

[peskywinnets]

In a humbucker, one set of magnets has the opposite polarity, which reverses the string signal in that coil, but doesn't reverse the noise signal.

Thanks Steve...I guess if I read that one sentence enough, it'll eventually sink in!

Ok, so bearing in mind my requirements are for a very small humbucker (narrow - left to right) )- I'd therefore rather not have a two bobbin side by side humbucker arrangment, I'm guess I'm now toying with a stacked humbucker - two coils on one bobbin - how is it best to achieve magnetic separation in such an arrangement? (surprisingly, I'm not too bothered how the end result sounds - weak/feeble is fine - what's more important is that I end up with a signal from the pickup that's very EMI resistant!)

[Steve Conner]

Well, I'm sure the pickup gurus will have their own ideas, but what about Fender's Samarium Cobalt Noiseless setup?


It's a triple decker with a couple of slim bar magnets in the middle, and flat coils above and below them. I guess the bottom one is more or less a dummy coil: it picks up the same amount of noise, but considerably less string signal, because its pole pieces are sticking out of the bottom, nowhere near the strings.

I guess you can also think of it as a sidewinder that got mounted 90 degrees out.

[peskywinnets]

I like it - you're my new bestest friend!

So essentially slap the magnet in between the top/bottom coils...the bottom coil only serves so as to retrieve the 'noise signature' so it can then be used to remove the noise aspect from the top coil's string signal.

I guess such an arrangement would work with six 5mm diameter slim Neos (assuming 5mm pole pieces being used) in place of the slim magnetic bar (which I bet are a bugger to source?)

[Joe Gwinn]

a bifilar wound coil - two coils wound in reverse into a differential amplifer (which ultimately flips one of the signal polarities)

A bifilar coil is one wound with the magnet-wire equivalent of zipcord (two parallel but insulated conductors). The effect is that both counductors see exactly the same magnetic environment.

It does not matter what the two coils of the bifilar winding are connected to, so the differential amplifier is not relevant to the definition of bifilar.

Bifilar coil - Wikipedia

a traditional humbucker which has two coils wound in reverse but with the polarity of magnet in one coil reversed (which flips one of the signal polarities)

The key is to arrange the two coils (ignoring the magnets for now) such that external 60 Hz magnetic fields produce equal and opposite hum voltages, and thus cancel: this is called hum bucking.

But one does not wish the music to also cancel, so something must differ: the magnets. If one magnet is reversed compared to the other, the two coils will produce equal and aiding music voltages, and so the music will come through loud and clear.


There is a longer analysis in a forum thread on the effect of unbalanced coils (different turns counts) on humbucking effectiveness.

Here is a start: http://music-electronics-forum.com/t4130/

[Mike Sulzer]

Notice that they have made the pickup deeper than a standard Fender SC. In your application you might have room available to make it even deeper, improving both the isolation between the dummy coil and the strings signal and the strength of the signal. (It is more difficult to design a magnetic shield between the two.)

Rare earth magnets are good to use in this application for two reasons: 1. they are very strong so they can be small; 2. they have very low permeability and so they do not couple the top and bottom coil together any more than empty space does.

I like it - you're my new bestest friend!

So essentially slap the magnet in between the top/bottom coils...the bottom coil only serves so as to retrieve the 'noise signature' so it can then be used to remove the noise aspect from the top coil's string signal.

I guess such an arrangement would work with six 5mm diameter slim Neos (assuming 5mm pole pieces being used) in place of the slim magnetic bar (which I bet are a bugger to source?)

[peskywinnets]

A bifilar coil is one wound with the magnet-wire equivalent of zipcord (two parallel but insulated conductors). The effect is that both counductors see exactly the same magnetic environment.

It does not matter what the two coils of the bifilar winding are connected to, so the differential amplifier is not relevant to the definition of bifilar.

I agree that a bifilar wound coil & a differential amp aren't necessary strict bedfellows, but if the end wire of the first wire is joined to the start wire of the second wire, you've a centre tapped coil....feeding that into a differential amplifier will at least give you a balanced cable run ....thereby cancelling any hum as picked up in the cable runs.

[David Schwab]

It's a triple decker with a couple of slim bar magnets in the middle, and flat coils above and below them. I guess the bottom one is more or less a dummy coil: it picks up the same amount of noise, but considerably less string signal, because its pole pieces are sticking out of the bottom, nowhere near the strings.

A Bill Lawrence design (7227076). I think part of his idea was to get the two coils far from each other, so he used those squat coils.

I guess you can also think of it as a sidewinder that got mounted 90 degrees out.

I bet the SCN would sound even better laying on it's side!

You can do a stack with the magnets or poles passing through both coil, and also with the poles passing though and with the magnets on the bottom like a P-90. I did a stacked Tele pickup like that using a blade and two ceramic magnets on the bottom.

Here's the SCN.

[peskywinnets]

Those diagrams aren't clear to my (untrained eye), & having just swallowed the concept to get humbucking you need two coils with opposing magnet polarities, can you please explain how it's possible to have a stacked humbucker with the poles running throughout the same two coils (is that what you're saying even?! )

One other bit of info (ie if it helps you guys pinpoint the best type of small humbucker for my needs!) - this mini humbucker pickup will feed into a preamplifier - so i don't need heaps of windings....the coils' signal voltage can easily be amplified onboard the guitar.

[Steve Conner]

having just swallowed the concept to get humbucking you need two coils with opposing magnet polarities, can you please explain how it's possible to have a stacked humbucker with the poles running throughout the same two coils (is that what you're saying even?! )

Yeah, how does that work? Is it a case of the non-uniform field that Mike Sulzer was talking about earlier?

As in, the music signal cancels, but because the two coils get different amounts of it, it only partly cancels? I'd have thought putting the magnet on the bottom of the stack would lead to more cancellation, hence less output than the SCN setup.

Thanks for posting the diagrams of the SCN, Dave.

[peskywinnets]

As in, the music signal cancels, but because the two coils get different amounts of it, it only partly cancels? I'd have thought putting the magnet on the bottom of the stack would lead to more cancellation, hence less output than the SCN setup.

Damn, just when I was all settled, that's set me off again...and may be just what I need! The bottom coil gets the full noise reception, but not so much signal....just add the (-ve) bottom coil signal, to the top (+ve) signal . Sure, I'll get less overall signal output, but who cares cos the hum has been just about eliminated - & I can simply 'pump up the volume' of the resulting (reduced)string signal in the preamplifier.

In fact my earlier aborted inner/outer coil coil tap arrangement (shared pole pieces, all wound on the same bobbin...one coil encircling the other coil - not bifilar)....

start wire 1-----> end wire 1 (first 'inner' coil) ..

proceed with the wind encircling the first coil....

start wire 2-----> end wire 2 (second 'outer' coil)

...now join end wire 1 to start wire 2 - this yield a 'centre tap', resulting in...

Start wire 1 (+ve signal)
Ground (end wire 1 connected to start wire 2)
End wire 2 (-ve signal)

Sum the two signals & since the noise will be pretty the same in both coils it cancels out (ie +ve noise plus -ve noise = nada), but the signal is less in the outer coil (cos it's further from the core) & therefore doesn't fully cancel ....amplify the resulting signal - a humbucked output!

I guess it would need a delicate balance of coil winding to ensure sufficient noise is cancelled, but not a lot of signal is.

[Steve Conner]

Sure, but that won't work with the bifilar coil you were using. The reason being that the two coils are mixed together and occupy the same space, so they both see the same magnetic field.

The thing to do is probably to try and keep the coils separate, like in the Bill Lawrence SCN, then you'll get more output.

[David Schwab]

can you please explain how it's possible to have a stacked humbucker with the poles running throughout the same two coils (is that what you're saying even?! )

Yes, that's what I'm saying. If you think of a rod magnet, it has poles on each end, and the middle is neutral. So if the top pole is South, the top coil is South, and the bottom is North. EMG Strat pickups are made the same way.

I did a stack with a blade running between the top and bottom coil and then used two magnets on the bottom like a P-90. That also worked well, but was very bright since the lows were somewhat canceled out.

Here's the Duncan stack, and the Tele stack I made back in '06.