Get your tuts out for the lads....(re stacked humbuckers)

[peskywinnets]

Thinner wire has higher resistance, so it's going to alter the tone of the top coil more.

Not sure I'm understanding that? The aim is for a dummy bottom coil with very little 'string signal' content, but an equal dose of noise content (wrt to the top coil). So why would the using thinner wire (higher DC resistance) on the lower dummy coil, affect the tone of the top (signal) coil?

[David Schwab]

So why would the using thinner wire (higher DC resistance) on the lower dummy coil, affect the tone of the top (signal) coil?

Because you have basically an inductor in series with the pickup coil. Mike or Joe would have more technical answers, but you are changing the inductance and impedance of the pickup.

You can try it out yourself. Take a coil that is not sensing the strings and wire it in series with your pickup. The tone is going to change, and not for the better. It it were that easy than all single coil guitars would have had dummy coils years ago, or at least would be stacked humbuckers.

By using a low resistance bottom coil you are minimizing the load on the string sensing coil.

Active dummy coil arrangements solve that problem.

Here's a few patents to read up on for stacked humbuckers and dummy coils:

DiMarzio:

4442749

5811710

Duncan:

4524667

Beller (Duncan)

7166793

Kinman:

5668520
7189916
5834999
6103966
7022909

Stich (Bill Lawrence)

7227076 (fender scn)

And lastly Chiliachki/Suhr:

20050204905

[peskywinnets]

Because you have basically an inductor in series with the pickup coil. Mike or Joe would have more technical answers, but you are changing the inductance and impedance of the pickup.

I'm fine with inductance, DC resistance blah blah - it's worth underlining that in the arrangement I am proposing, the two coils will not be in series. Each coil will be essentially independent & feed into it's own leg of a differential amplifier. It's in the differential amplifier that they will be summed & I'm hoping I can get the balance right (either by coil design or diff amp gain) to have the negative noise cancel out the positive noise - I'm hoping that no signal from the bottom coil (or at least as little as possible if I can get decent enough separation), will be present to affect the tone of the top coil to any degree.

PS Thanks for the links - I'll give them a go, but in the main patents make me glaze over, because I find they're often worded ambiguously enough not to give the game away but still leaving the patent owner an angle for a claim if anyone uses something approaching his idea!

[David Schwab]

Each coil will be essentially independent & feed into it's own leg of a differential amplifier.

If you are doing an active setup, than you don't have to worry about that.

[Mike Sulzer]

Yes, once you are willing to use an amplifier, there are several ways tht will work fine. If you can use identical coils, then a series connection is OK since the the two inductors are not loaded by the cable capacitance. You can load the series combination with a smaller cap if that gives you the sound that you want.

If you do not have identical coils, then it is convenient to use separate amplifiers so that you can adjust the relative gains more easily when you subtract them (or add if the polarity reversal is handled by reversing the coils leads on one or winding one in the opposite direction).

[peskywinnets]

Well, it won't win any beauty contests (& the first attempt had to be thrown away!) a couple of windings on the upper coil edge worked loose and had to be untidily superglued down again!, but at least I can move on to phase two - playing with it in an amp cct!



About 350 turns of 0.15mm wire on the upper coil (44 ohms) and a bit less on the smaller lower coil (305 turns/39 Ohms)...the pole pieces are only 3mm diameter (a tad under 1/16"").three wires will connect up to the pcb on the far side +ve sig, centre tap (which will be sitting at 3V, as this means I can do away with dc decoupling caps at the inputs into my differential amp), -ve noise.

It's just @7.5mm high (5/16") -& that's including the pcb - so it'll slide under my guitar's strings near the bridge allowing me to have a dabble.

[David Schwab]

About 350 turns of 0.15mm wire on the upper coil (44 ohms) and a bit less on the smaller lower coil (305 turns/39 Ohms)...

What gauge wire did you use?

[peskywinnets]

What gauge wire did you use?

38SWG (0.15mm)...which is exactly between 34 AWG & 35AWG.


Ok, so to my first test...... I placed a neo at the top of the top coil directly under the strings (no pole piece just a neo). In the bottom (dummy) coil just an air core (ie no pole piece or magnet). I figured in this setup, there'd not be a lot of magnetic 'field' to bleed down into the lower coil. I'm feeding the +ve (signal) & -ve (noise) into a differential amp - I expect everyone says this about pickups they've made - but it sounds crystal clear - very bell like in timbre.

It bucks hum/noise very well.....see the scope trace below....

the green trace is the top (signal) coil) the yellow trace is the lower (dummy) coil. The red trace is a math trace (the summed signal)



The problem with this scenario is neo I'm using (very small) works very well on the open strings, but as you fret past the 12th fret the string gets closer to the neo & it then chokes the string ringing.

So, tomorrow I will dabble with a more traditional arrangment with this pickup. A magnet at the bottom of the top signal coil & a metal polie piece above it. I will still just leave an air gap in the lower coil.

And so ...off to bed.

[David Schwab]

38SWG (0.15mm)...which is exactly between 34 AWG & 35AWG.

For such small coils, why not use much thinner wire, like 44AWG?

but it sounds crystal clear - very bell like in timbre.

I would hope so with such a low impedance pickup!

[peskywinnets]

For such small coils, why not use much thinner wire, like 44AWG?

What does that win me here?

If you mean use the same amount of turns but thinner wire, I guess that would win me a physically narrower pickup - if you mean using more turns but with thinner, that'd win me more output (at the expense of top end), but I don't need more output as I'm preamp'ing them?

[StarryNight]

I'd be curious to know if you shorted out the bottom coil is there a noticeable increase in noise from the top bobbin? I would think with such a low impedence coil there would be very little hum to begin with. Not to dismiss the value of your work, but rather just to hear the difference the bottom coil is making.

[peskywinnets]

I'd be curious to know if you shorted out the bottom coil is there a noticeable increase in noise from the top bobbin? I would think with such a low impedence coil there would be very little hum to begin with. Not to dismiss the value of your work, but rather just to hear the difference the bottom coil is making.

Well, there'd be a noise/hum floor approaching the green trace (vs the red trace) - whether you consider the 'differenced' signal a significant win ...I'm not sure (I never carried out an audible test on the noise floor - it was late!)

I'm doing some more tests over the next few evenings (on slightly different coil configs) & will report back.

Re dismissing the value of my work - I'm just experimenting/earning really, so there's no worries in that respect .....so feel free to doubt away!

[Mike Sulzer]

Low impedance is effective in reducing the hum from electric fields. A noise canceling coil is for reducing hum from magnetic fields. Low impedance will not help; the ratio between the signal and the hum is not affected with magnetic hum. When you increase the voltage with a transformer or preamp you just get back the same hum.

I'd be curious to know if you shorted out the bottom coil is there a noticeable increase in noise from the top bobbin? I would think with such a low impedence coil there would be very little hum to begin with. Not to dismiss the value of your work, but rather just to hear the difference the bottom coil is making.

[peskywinnets]

Low impedance is effective in reducing the hum from electric fields. A noise canceling coil is for reducing hum from magnetic fields. Low impedance will not help; the ratio between the signal and the hum is not affected with magnetic hum. When you increase the voltage with a transformer or preamp you just get back the same hum.

I'm not locking into what you're saying here? My main push is to cancel magnetic hum. The dummy coil on the bottom should pick up the magnetic hum - & even if the coils aren't an exact match in spec, I should be able to tweak the gain for each signal into the diff amp so that hum is minimized - or what am I missing? (& incidentally, the only reason I'm going low impedance, is because I'm pushed for guitar 'pickup' space - less windings = smaller pickup - also, since I've onboard electronics already, I can readily use a differential amp to get the levels up - in other words, a low impedance pickup is not the identified 'solution to cure hum, it just so happens that the real estate necessitates going the low impedance path)

[David Schwab]

if you mean using more turns but with thinner, that'd win me more output (at the expense of top end), but I don't need more output as I'm preamp'ing them?

If you have to boost the signal that much, you are also going to be boosting any noise along with it.

You can wind more than 350 turns of wire and still retain your high end.

I'm also not sure why you insist on having to be able to slide the pickup under the strings.