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.
Some winding equipment will not wind in the opposite direction.
Some builders wind 2 coils at the same time to make them as identical
as is possible.
Can this effect be measured?
I took the following oscilloscope pictures to show the effect.
The setup:
Nordstrand NJ5 pickup
Tektronix TDS 3012B scope
Tektronix ADA 400 A Low Noise Differential Preamp (single ended)
3 kHz filter enabled - cap coupled - 10 x gain
1 X scope probe
* combined loading effects of scope probe + preamp are 1 Meg ohm +
80 pF
** NOTES:
1)
No Audere preamp is being used just a oscilloscope
2) The Nordstrand pickups are tested in some configurations
where I invert the wire connections from the recommended wiring.
I selected
this type of pickup because I know the design does not use a bobbin.
Cary Nordstrand makes very nice high quality pickups!
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
*The single inverts because the scope is triggered on line
Pickup Hot Inward - human touching the magnet (Top, Left Drawing)
*More noise especially higher frequency noise in this case
Pickup Hot Inward - human touching the Grounded magnet (Bottom, Left
Drawing)
* same as when not touching it now
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)
* I suspect most of the spike is caused by physical pickup movement
of the pickup.
Pickup Hot Inward - floated magnet, at touch (Top, LEft Drawing)
* Note the scope scale size has been changed 20:1 so this spike is
about 40 times bigger.
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.
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