Stacked pickups - I'm missing something

[chad h]

A friend asked me to repair a SD Tele "hot stack" bridge pickup, and it's dawning on me that I don't really understand how it works.

We have two coils. SD says the pickup is hum-cancelling, so I presume the two coils are wound in opposite directions. But the coils are wound around a single bar magnet. In other words, the magnet has the same orientation relative to both coils. So, if the coils are wound in opposite directions, won't ALL the signals generated in the coils be out of phase?

Maybe I just don't understand magnets as well as I should.

[copperheadroads]

Who winds humbuckers in opposite direction ? & why ?
wire it you don't need to wind it opposite .

[big_teee]

If both coils wound in the same Direction?
It would be like a regular humbucker, either tie both the finish leads, or tie both the start leads togheter.
If coils are wound in different directions, then tie one start to one finish.
T

[chad h]

The wiring isn't the problem. My question is understanding how it works. Running the hum signal in different directions around two coils will mean that hum signal is cancelled. But if the magnetic field around both cores is the same, then surely the signals from string vibrations will also cancel.

It would be different if the second coil was an air coil. But in these pickups, both coils are magnetized, and are magnetized the same way.

[Joe Gwinn]

A friend asked me to repair a SD Tele "hot stack" bridge pickup, and it's dawning on me that I don't really understand how it works.

We have two coils. SD says the pickup is hum-cancelling, so I presume the two coils are wound in opposite directions. But the coils are wound around a single bar magnet. In other words, the magnet has the same orientation relative to both coils. So, if the coils are wound in opposite directions, won't ALL the signals generated in the coils be out of phase?

The coil closer to the strings gets the bigger signal from the strings, while both coils get the same hum signal (because the source is remote). When the coils are wired in series opposing, the hum signal fully cancels, while the string signal is only reduced a bit.

Maybe I just don't understand magnets as well as I should.

The magnets provide a bias field that energizes the strings, but have no effect on hum fields.

[Mike Sulzer]

It would be different if the second coil was an air coil. But in these pickups, both coils are magnetized, and are magnetized the same way.

Coils are not magnetized; parts of the strings are. If you made the lower coil air core, you might get a bit less signal there, but you could affect the hum cancellation. Most magnets (not neo) have enough permeability to redirect magnetic fields, and so tend to increase the hum field strength through the coil. If you did that in only one coil, you would get different magnitudes of hum voltage. Some stacked pickups use different cores, but compensate, perhaps with different numbers of turns.

[chad h]

The magnets provide a bias field that energizes the strings, but have no effect on hum fields.

Yeah, I understand that bit. The part I don't get is why the flux in the magnetic field around the coils doesn't create signals that cancel as well.

The coil closer to the strings gets the bigger signal from the strings, while both coils get the same hum signal (because the source is remote). When the coils are wired in series opposing, the hum signal fully cancels, while the string signal is only reduced a bit.

This is what I didn't appreciate. Thanks. Although, it surprises me. That extra 1/4 of an inch distance makes that much difference? I guess it's all a matter of degree. If one of these coils is wound hot (Seymour Duncan winds one of these to 10.7k, and the bobbin is around the same size and shape of a humbucker bobbin), and the second coil cancels "a bit" of the signal, output could still be reasonable if "a bit" was quite a large fraction.

[ScottA]

The magnets provide a bias field that energizes the strings, but have no effect on hum fields.

As Mike pointed out, AlNiCos have relative permeability > 1 so they do increase the inductance of the coil and therefore the efficiency with which hum signal is generated. A coil with AlNiCo 5 in the core will generate more hum than an air coil, A2 (being higher permeability than A5) even more so.

Coils are not magnetized; parts of the strings are. If you made the lower coil air core, you might get a bit less signal there, but you could affect the hum cancellation. Most magnets (not neo) have enough permeability to redirect magnetic fields, and so tend to increase the hum field strength through the coil. If you did that in only one coil, you would get different magnitudes of hum voltage. Some stacked pickups use different cores, but compensate, perhaps with different numbers of turns.

The key to good hum cancelling is matching the inductance of the anti-polarity hum gathering "antennas".

[ScottA]

Yeah, I understand that bit. The part I don't get is why the flux in the magnetic field around the coils doesn't create signals that cancel as well.

This is what I didn't appreciate. Thanks. Although, it surprises me. That extra 1/4 of an inch distance makes that much difference? I guess it's all a matter of degree. If one of these coils is wound hot (Seymour Duncan winds one of these to 10.7k, and the bobbin is around the same size and shape of a humbucker bobbin), and the second coil cancels "a bit" of the signal, output could still be reasonable if "a bit" was quite a large fraction.

Basically, it's a very lossy design. It's physically impossible to magnetically isolate those coils from each other. That's the big reason why stacked hum cancelling pickups don't really sound as good as conventional pickups.

[chad h]

Basically, it's a very lossy design. It's physically impossible to magnetically isolate those coils from each other. That's the big reason why stacked hum cancelling pickups don't really sound as good as conventional pickups.

I wonder what the nature of the loss is? The top coil produces a signal. The bottom coil is just like the top one, but is shifted by about 1/4 of an inch, and is hooked up in the reverse direction. If that 1/4 of an inch shift simply attenuates the signal by some factor that is independent of frequency, then the loss wouldn't really affect the frequency response of the pickup. But it seems at least plausible that some important filtering happens here.

[big_teee]

The pickup sounds IMO as good or better split, as long as you're using the top coil.
Except they do not hum cancel when only using the top coil.
The bottom coil seems to be more like a dummy coil.
The Stack is inherently bright, and the only way I got them IMO exceptable, was to use a lot of turns with 44awg.
I prefer other types of pickups like Scott said.
T

[chad h]

The pickup sounds IMO as good or better split, as long as you're using the top coil.
Except they do not hum cancel when only using the top coil.
The bottom coil seems to be more like a dummy coil.
The Stack is inherently bright, and the only way I got them IMO exceptable, was to use a lot of turns with 44awg.
I prefer other types of pickups like Scott said.
T

So, the SD website says that the split coil has DCR around 10.7k (and the full humbucker 21.4k). I'm guessing I need around 9000 turns of 44awg on a bobbin to get there. Like you said: lots of turns with 44awg!

Chad

[big_teee]

So, the SD website says that the split coil has DCR around 10.7k (and the full humbucker 21.4k). I'm guessing I need around 9000 turns of 44awg on a bobbin to get there. Like you said: lots of turns with 44awg!
Chad

I think your're close.
9,00-9500 turns per bobbin.
T
**BTW here's a thread where a stack was a project on the B/H forum.
You might want to woller through some of that.
The later posts where I made a stack using humbucker bobbins is similar to what you are doing.
http://music-electronics-forum.com/t32472/
check out post #77.

[copperheadroads]

I wound one a couple weeks back with 1/4" height to wind on with 44 gauge wire ,It was full @ 8400 turns x2 was pretty much 20.0k

[Joe Gwinn]

The key to good hum cancelling is matching the inductance of the anti-polarity hum gathering "antennas".

It's actually the area-turns product, multiplied by the effective permeability, that governs the voltage generated by a given hum field. It's a bit of math to see if this is proportional to the self-inductance of the coil.