Author Topic: Alembic Pickups  (Read 600 times)

David Houck

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Alembic Pickups
« on: May 25, 2005, 08:22:42 AM »
Here are some excerpts taken from posts made by Mica, Bob Novy, and David Fung regarding Alembic pickups.
All the non-Series Alembic pickups are stacked humcancelling pickups, we don't make any humbucking pickups. Even the ones that look like single-coil guitars pickups are stacked humcancelling pickups.  I associate humbucking pickups as designs with two magnetic core coils, and a flipping magnetic domain (the bucking part). Our design is much more simple with one magnetic core coil and one non-magentic core coil stacked and no bucking involved. I suppose in the world outside of Alembic the terms may be interchangable, though I feel the distinction is important because I don't believe the tone from our pickups is what most people would associate with humbucking pickups.
The pickups are single coil in a Series I/II system and require the dummy humcancelling pickup to eliminate the hum that plagues the single coil design.
An AXY is electrically and tonally identical to an MXY. The only difference is that the AXY is in a larger plastic housing; the guts are identical (at some point in history, they shrunk the housing as much as possible to create the MXY look).  
A FatBoy uses the same housing as an AXY (you can't tell by looking at them, at least from the top), and is the same humcancelling design and (probably?) the same number of windings as an AXY/MXY - except that it is wound on a magnet which is larger, along the direction of the strings. This results in a wider aperture, meaning that the pickup is listening to a longer section of the string, roughly twice as much.
First, start with a single coil pickup. You make that by winding a coil around a magnet and disrupting the magnetic field with something like a vibrating ferrous string. The magnetic interference has the effect of generating a electrical signal in the coil, which you can amplify to reproduce an analog of the strings motion. You can tweak the sound of the pickup by varying the strength of the magnet, the shape of the magnetic field (this is the aperture), and the type and amount of wire on the coil.  
The problem with single coil pickups is that in addition to picking up the motion of the string, they also will pick up radiated electromagnetic (EM) fields, which you hear as hum and buzz.  
Humbucking and humcancelling pickups cleverly address this problem by using a matched set of pickup coils. If you reverse the polarity of a single-coil pickup magnet, you'll reverse the polarity of the output signal. You can also reverse the polarity of the signal by reversing the direction the coil is wound in. If you flip the magnet AND wind the coil in reverse, the pickup should sense string motion exactly like the regular pickup does.  
But picking up an EM/hum field doesn't depend on the magnet, just the coil. If you hook up a regular pickup and a reverse pickup, what will happen is that the magnetically-sensed sound will add normally, but the EM-induced buzz (which isn't dependent on magnetic polarity) will add up out of phase and will be cancelled out like magic. This is how the two coils in a Gibson-style humbucker are wired, or the two halves of a P-bass pickup. In modern Strats, the middle pickup is reverse-wound/reverse polarity which is why the in-between tones don't hum.  
In something like a Gibson humbucker, the two coils in each pickup have magnets and hear the string in slightly different places. This means that the audio outputs from the coils are slightly different and that small difference causes some additional out-of-phase cancellation of the audio signal. That's one reason why most humbuckers are less bright sounding than single coils.  
One way to reduce the amount of high-end cancellation is to stack the two coils one above the other. It works the same as side-by-side, but both coils are pretty much seeing the same string motion now. The problem with this design is that one pickup is farther away from the string, so you still don't get a perfect signal match (the closer pickup will have hotter attack for instance, so the summed signal won't be a perfect match). This is how stacked Strat pickups work.  
If you're willing to dedicate one coil to hum cancelling only, then you can perfectly capture the single coil sound by leaving the magnet out of one of the two coils entirely. The coil with the magnet hears the string, and the air coil only hears EM which it will cancel out of the other signal. This is what the Series bass is doing. The pickups are true single coils; the hum canceller (shared by both pickups) is a coil with no magnet which only serves to remove the hum when summed with the pickups. The EM/hum radiation is basically going to read identically anywhere on the instrument, so, with proper matching, you should be able to perfectly cancel the hum with minimal effect on the single coil sound.
[With the FatBoy,] there's a pickup on top and a humcancelling coil beneath [and thus] there's some compromise vs the Series setup.  [Since] it's an air coil humcanceller, then the problem is that the proximity of the air coil to the magnets of the pickup will cause some weaker signal to be induced again mismatching the outputs. That's why the Series humcanceller is physically located away from the pickup magnets.  
A couple of additional observations. First, Leo Fender was pretty clever in making the split-coil P-pickup. By making it that way, you can build a one pickup bass that doesn't hum and doesn't have high-frequency cancellation (only one coil reads each pair of strings but together then cancel the hum). Second, if you can pry the magnets out of the middle pickup of your Strat and do a little rewiring, you'll have the purest, humfree neck and bridge pickup tones you've ever heard. I'm surprised that you don't see more Gibson-style humbuckers with a live coil and an air coil, but I guess that doesn't sound at all like a traditional humbucker.  
Finally, this is a great example of Alembic's no-compromise design. There's lots of ways to get pretty good results, and only one way to do it that's ideal, and that's what you see on the Series. I know of only a few other basses that have this system - the original Paul Reed Smith basses had an extra hum-cancelling coil on the back, and there was a short run of Fender Elite Jazz basses that had this in the 80's. Alembic was first on this (certainly the first significant player) and never stopped since the 70's.
And Mica again:
[The middle two adjusting pots on the back plate of a Series instrument are] hum-balance pots. The hum is adjusted to null for each pickup, because even if they have the exact number of turns, each pickup is slightly different and needs a different amount of influence from the humcancelling coil.
(Message edited by davehouck on March 20, 2006)
(Message edited by davehouck on March 21, 2006)