FAQ - Pickup Magnet or Pole Piece Grounding...
If you are getting noise or popping when you touch your pickup magnets or pole pieces then:
Ground the Magnets or Pole Pieces. The best way to ground the parts is to apply conductive copper foil with conductive adhesive against the bottom of the pickups and run a new wire to ground. Note: this ground wire must be connected to the output jack or cavity ground and not to the pickup common.
With most conductive adhesives the conductivity will vary with pressure and often decreases with time. To ensure the foil stays pressed against the poles, try inserting some foam under in the cavity below the pickup. The pressure of the foam against the bottom of the pickup will keep the conductivity of the glue low and help mechanically stabilize the pickup.
If you solder the wire to the copper strip after you press it onto the magnets then make sure you do not overheat the magnets. Alnico magnets (used in most higher quality pickups) are made by cooling the raw work piece with a magnetic field applied as it cools from 900 to 600 degrees C. Normally the lowest critical temperature for demagnetizing Alnico magnets is around 500 degrees C at which point an irreversible phase change of the magnetic material occurs. While it would be difficult, it is possible to reach this temperature with a very hot soldering iron.
If you incorrectly connect the foil ground and pickup pole pieces to the pickup common instead of ground and your strings are grounded, then when the strings come in physical contact with the pole pieces you will get an extremely loud pop. The pop is created because the pickup common is set at about 1/2 the battery voltage and touching the strings to the pole pieces just shorted that voltage out. The pop will be present at all volume control levels. If you have this problem simply remove the connection to the pickup common and reconnect the wire to ground.
Why is this recommended?
The direction the coils are wound will effect the noise you hear when you touch the pole pieces with your fingers. You may find one pickup is noisy but not the other pickup or only one coil of a hum bucking pickup might be noisy. If your pickups do not have a bobbin around the pole pieces the problems will be larger.
The drawing below shows the 4 possible configurations. The top 2 drawings
have floating (not grounded) magnets, the bottom 2 have the parts grounded.
The drawings with the hot terminal wire is inward, or closest to the
magnetic pole, are on the left. The alternate is the Common terminal
wire inward, on the right.
When pickups are wound with the magnetic wire directly against the magnets or pole pieces, i.e. no thick bobbin, then there will be strong capacitance coupling in between the magnetic parts and the wire. If the pickup hot terminal is wired closest to the pole pieces (left 2 drawings), then noise introduced onto the magnetic parts when you touch it will be injected into the preamp (top, left) unless you ground the pole piece (bottom, left). If the pickup's common connection is wired closest to the magnetic parts (right 2 drawings), the common will shield the rest of the coil from most of the noise. The problem or signal is really being generated by the pickup and will be amplified by the preamp but it will be especially noticeable in High Z-Mode.
Why can't I just swap the hot/common wires?
If both pickups have the same symptoms then try just swapping the hot and common wires on both pickups.
When 2 coils are used in a hum cancelling setup (this can be 2 separate pickups in a Jazz Bass or dual coils in a hum bucking pickup) then the 2 coils are wound in opposite directions to each other and the magnets are reversed. This arrangement is called "noise-cancelling"; the signals that come from noise subtract or cancel each other out. But the signal related to string movement is added together. If you simple reverse the hot and common wire on one coil, the pickups will no longer be in noise cancel noise - the coils in the pickups would no longer be arranged in the so called "noise-cancelling" configuration but will now be in the "out-of-phase" configuration. This is a "thin" sound as low frequencies will be canceled more efficiently than high frequencies. While this is often used for guitars, it is rarely used for the bass.
But there are 2 different ways to reverse the wire direction in the coils. The pickup builder could turn the winding machine the opposite direction when winding the coil OR the hot and common wires for 1 pickup could be reversed. This might appear to be the same thing but it really isn't for the following reason; reversing the winding machine will allow the pickup common to always be located close to the pole pieces - the preferred orientation for a low noise level. If the wires were simply reversed, the hot will be closest to the pole pieces for 1 of the coils and you'd have the noise problem described in the first paragraph.
Why does the pickup designer not reverse the winding direction?
The problems created were not understood or considered significant.
Can this effect be measured?
I took the following oscilloscope pictures to show the effect.
Pickup Common Inward - human not touching the magnet
Pickup Common Inward - human touching the magnet (Top, Right Drawing)
So the noise increases but not by much!
Next put the hot inward toward the magnets
Pickup Hot Inward - human not touching the magnet
Pickup Hot Inward - human touching the magnet (Top, Left Drawing)
Pickup Hot Inward - human touching the Grounded magnet (Bottom, Left Drawing)
Are there any other advantages to grounding the pole pieces?
When you first touch the pole pieces you inject a voltage on the pole piece which is equal to the static level on your body. This can be very noticeable and measurable. This effect will be bigger when the pickup hot is wound inward but can be large in both cases if the wire is close to the magnets. If your strings are grounded by a bridge ground wire this problem will be smaller in practice as your body will discharge each time you touch the strings.
Pickup Hot Inward - Grounded magnet, at touch (Bottom, Left Drawing)
Pickup Hot Inward - floated magnet, at touch (Top, LEft Drawing)
The easy solution is to always ground the magnetic parts if you have pickups with out bobbins to separate the winding for exposed pole pieces.
Does this happen with a passive bass?
Yes, the effects are real (Note: No bass preamp was used to take the scope pictures) but the size of the effect will be smaller with a passive bass just like the size of the effect is present but smaller in Mid Z-Mode compared with High Z-Mode.
Are there any other disadvantages to grounding the magnet parts?
Not in my opinion if you have grounded strings but it will not be required by most pickup designs.