Centre tapping a single coil?

[Kazooman]

The electrical polarity of a center tapped coil is +-+-. (The two parts are wound in the same direction.) You can connect the two ends to a differential (+-) amp, and the center to ground and get full signal, but it will not cancel magnetic hum. If you connect both ends to positive inputs you will cancel the magnetic hum, and most of the signal since the outer coil sees almost as much flux as the inner one.

That's what I was thinking, but put more succinctly. If I understand it correctly, winding the center tapped coil in one direction with one magnet would give you a pickup where (here is where I will put my foot in my mouth with incorrect terminology) the phase of the signal and the phase of the hum in each half of the coil being the same. If you wire it up to have the signals in phase, the hum will be also and vice versa.

[peskywinnets]

The electrical polarity of a center tapped coil is +-+-. (The two parts are wound in the same direction.)

Agreed, but those middle two connections are grounded, so the polarity is....

+ve signal output (start wire)
GND (centre tap -ground wire)
-ve signal output (end wire)

Essentially, by centre tapping, I've created two coils but of opposite polarity....therefore, the theory behind my thinking was correct, but where I failed was the oh so simple 'visualising' how I'd be physically 'winding' this in practise. In my head, I'd visualized a stacked setup, but what I've ended up with is a coil within a coil (doh!) - but nevertheless, I'm still going to end up with....

+ve (start wire)
GND (centre tap -ground wire)
-ve (end wire)

....but because the second half of the coil (ie outer coil) has ended up physically further away from the core, its 'induced' signal will be weaker, therefore the two half coil 'signals' will not be of the same magnitude ...this could perhaps be evened out within the circuit I mate the pickup up with, but it's always best to start with good 'source material' in the first place, so I have my doubts that would be a great solution.

My wife (of all people) came up a reasonable proposal for my small humbucker (as luck would have it, she used to work for a company who rewound failed transformers!) - her idea is for me to wind a pickup feeding two wires onto the bobbin at the same time ....then just reverse the polarity of one the coils into my circuit! Gotta love her! (I'm sure this latest proposal will result in "aah, but what about the capacitance" )

[akimball442]

Out of curiosity, if you wind two wires side by side as you mention in your last post, wouldn't it cause the two out of phase signals to completely cancel each other and result in no sound whatsoever, or result in an extremely weak sound?

[Joe Gwinn]

Out of curiosity, if you wind two wires side by side as you mention in your last post, wouldn't it cause the two out of phase signals to completely cancel each other and result in no sound whatsoever, or result in an extremely weak sound?

It's called a "bifilar winding", and it will cancel if so phased.

[peskywinnets]

Update: The first wind I did (inner coil/outer coil)....didn't work very well ....the outer coil had a much weaker signal than the inner coil - no surprises there! ( the concept was reasonable, the visioning wasn't becuase I'd envisaged a stacked coil, but what I got of course was an inner coil + outer coil)

Having since wound & trialled such a config I can report that a bifilar wound coil works just fine (two wires wound onto the bobbin at the same time....then the end of wire 1 is soldered to start of wire 2 - the 'joint' forming a centre tap if you like) ....you do have to invert one of the two resulting signals electronically...ie you feed both signals into a standard op amp cct - one into the positive leg & one into negative leg, the hum gets cancelled but the signal doesn't. (in essence the opamp performs the function of the magnet polarity being flipped in a traditional humbucker 'second coil' arrangement). So this config fulfills its purpose - a nice simple small humbucker 'wind'.

[Mike Sulzer]

That is not a humbucker. A humbucker cancels time varying magnetic fields (hum and buzz), while keeping the time varying magnetic field from the string. A bifilar wound coil does not do that because each winding senses both types of magnetic field with the same polarity.

Update: The first wind I did (inner coil/outer coil)....didn't work very well ....the outer coil had a much weaker signal than the inner coil - no surprises there! ( the concept was reasonable, the visioning wasn't becuase I'd envisaged a stacked coil, but what I got of course was an inner coil + outer coil)

Having since wound & trialled such a config I can report that a bifilar wound coil works just fine (two wires wound onto the bobbin at the same time....then the end of wire 1 is soldered to start of wire 2 - the 'joint' forming a centre tap if you like) ....you do have to invert one of the two resulting signals electronically...ie you feed both signals into a standard op amp cct - one into the positive leg & one into negative leg, the hum gets cancelled but the signal doesn't. (in essence the opamp performs the function of the magnet polarity being flipped in a traditional humbucker 'second coil' arrangement). So this config fulfills its purpose - a nice simple small humbucker 'wind'.

[earthtonesaudio]

Having since wound & trialled such a config I can report that a bifilar wound coil works just fine (two wires wound onto the bobbin at the same time....then the end of wire 1 is soldered to start of wire 2 - the 'joint' forming a centre tap if you like) ....you do have to invert one of the two resulting signals electronically...ie you feed both signals into a standard op amp cct - one into the positive leg & one into negative leg, the hum gets cancelled but the signal doesn't. (in essence the opamp performs the function of the magnet polarity being flipped in a traditional humbucker 'second coil' arrangement). So this config fulfills its purpose - a nice simple small humbucker 'wind'.

You have two oppositely-wound coils, then invert one with your amplifier, which should give 1 - (-1) = +2 ...but since there's no magnetic inversion going on, this is true for both signal and noise currents, and no common mode rejection occurs.

Is it possible your pickup is simply less sensitive to hum due to the lower impedance/lower number of turns? I'm not sure how you'd measure it to find out though.

[Jag]

+1

Running an amp clean with no significant sourece of EMF nearby would be pretty quiet as well.

[peskywinnets]

You have two oppositely-wound coils, then invert one with your amplifier, which should give 1 - (-1) = +2 ...but since there's no magnetic inversion going on, this is true for both signal and noise currents, and no common mode rejection occurs.

Is it possible your pickup is simply less sensitive to hum due to the lower impedance/lower number of turns? I'm not sure how you'd measure it to find out though.

Indeed it is possible....& I've now sacked my wife.

You have two oppositely-wound coils, then invert one with your amplifier, which should give 1 - (-1) = +2 ...but since there's no magnetic inversion going on, this is true for both signal and noise currents, and no common mode rejection occurs.

I'm inclined to agree with you (since you know what you're talking about whereas I'm just poking randomly in the dark!), but I'm reading that EMI phase within the pickup is not sensitive to magnetic polarity, just the direction of the windings.

http://www.blueshawk.info/detailed_pickup_stuff.htm

extract...

"The polarity of the noise signal is independent of the magnets -- it depends only on the direction of the coil winding - so the noise signal across the two coils is cancelling, or out-of- phase."

if that's the case, then a bifilar wound pickup fed into a differential amplifier ought to be humbucking?

[Steve Conner]

if that's the case, then a bifilar wound pickup fed into a differential amplifier ought to be humbucking?

No, any configuration of coils wound round a single magnet that's humbucking is also signal-bucking to some extent.

Humbuckers have reversed magnetic polarity in one coil, allowing the hum voltages to cancel while the signal voltages add.

Mike Sulzer makes a rather refined point about the use of a non-uniform magnetic field. If the field due to the pickup magnets is non-uniform, but the hum field is uniform, then you can have complete humbucking and only partial signal-bucking. A regular humbucker is of course an extreme case of this: the field is very non-uniform because it's in opposite directions in the two coils!

But for a bifilar coil I'd expect both hum and signal cancellation to be pretty perfect for one polarity and non-existent for the other. The two strands are right next to each other, so the hum and signal voltages induced in both should be identical in magnitude and phase, no matter whether the fields are uniform or not.

[peskywinnets]

Humbuckers have reversed magnetic polarity in one coil, allowing the hum voltages to cancel while the signal voltages add.

i'm not arguing (genuinely)...I just need to bed this one away in my head.

I'm struggling to see the difference between...

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

vs

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)

I guess the root of my line of questioning here is - when two coils are wound in reverse (bifilar style) - is the 'noise signal' aspect 'in phase' or 'out of phase' wrt what each coil receives?

[Steve Conner]

The difference is that the differential amp can't tell the difference between the hum voltage and the signal voltage, so if you wire it to cancel one, it must also cancel the other. So by flipping polarity of one coil in your bifilar pickup, you can choose between no humbucking, or no output at all.

In other words, if a pickup wasn't humbucking to begin with, a differential amp can't make it so. And if the pickup was humbucking to begin with, a differential amp isn't needed: the coils and magnetic circuit have already "told the difference" between signal and hum for you.

Flipping polarity in this situation gives you a choice of full output without hum, or lots of hum and no signal.

To recap, the diff amp doesn't do anything that you couldn't do by connecting the coils in series or parallel with appropriate phasing. (Except for buffering cable capacitance and increasing the output level, etc.)

[peskywinnets]

Apologies I wasn't clear, my bifilar wound coil is centre tapped (confusing the issue a little). I essentially join the end of wire 1 to the start of wire 2.

Therefore, what I get (signal wise) from the output of my bifilar wound, centre tapped coil is two signals....

+ve signal .....ground centre tap ..... -ve signal

so, two signals - but with their respective polarities reversed.

Therefore, if the noise induced into each of those coils is in phase (& I don't know), feeding the two signals into a differential amplifier would add the signals but cancel the noise.

So it comes back to...

with two coils wound in opposite directions...is the noise induced signal aspect in phase or out of phase?

[Steve Conner]

The induced noise voltage has the same phase as the induced signal voltage in both coils. You can't cancel one without cancelling the other, no matter how you connect them up.

[peskywinnets]

The induced noise voltage has the same phase as the induced signal voltage in both coils. You can't cancel one without cancelling the other, no matter how you connect them up.

Therefore a bifilar wound coil (two wires fed onto the bobbin consecutively) arranged as such...

start wire 1 -------- end wire 1
start wire 2 -------- end wire 2

but with 'start wire 2' soldered to 'end wire 1', therefore forms a centre tapped coil, yielding this output...

+ve signal
ground
-ve signal


feeding these 180 degrees out of phase signals into a differential amplifier will sum the (out of phase) signals but cancel the (in phase) noise ....*if* the noise as you say is "in phase" with such an arrangement! (& it's this last bit I'm not clear about!)