Fig. 1 The most popular bass pickup
shapes in historical order from older (top) to
newer designs. Fig. 2 A single-coil pickup
with magnets, bobbin, and final wiring.So far in the short history of this column,
we’ve discussed several ways to manipulate
bass tone, yet carefully avoided naming
or identifying the origin of this electric
current—the pickup. A pickup converts
the mechanical movement of our strings
into an electric signal that we can modify
and amplify. But how do we translate string
movement into an electrical signal? Several
ways to do this come to mind:
Piezo. In 1880, French brothers Jacques
and Pierre Curie discovered the piezoelectric
effect. Putting mechanical pressure on certain
types of non-conductive crystals leads to
a directly proportional, microscopic voltage.
Optical. Direct a source of light towards
a moving string and use a photodiode—a
semiconductor that converts light to an
electric current—to sense these alternating
moments of shade and light. The changes
in the sensor give us a direct electric picture
of what the string does. Ron Hoag displayed
the first pickup of this kind at the
1969 summer NAMM show in Chicago.
Electromagnetic. A permanent magnet
emits a magnetic flux. Moving a ferrous
material in this field alters the magnetic
flux, inducing a voltage in a wire that’s
wound around the magnet. While piezo
pickups and optical sensors work with all
string materials, the electromagnetic pickup
is limited to ferrous ones. No problem here,
as we typically use steel strings.
Technically, these pickups are all called
transducers, as they convert one form of
energy (mechanical movement) into another
(electric current or voltage). I can’t think of
any more ways to convert a string’s physical
movement into voltage. (If you know
of other ways, either contact me or head
directly to your patent office. Don’t mention
microphones, as they are simply an electromagnetic
transducer that uses more parts.
And we won’t count condenser mics either.)
Let’s look more closely at these three
types of converters, starting with the one we
all think we know best, the electromagnetic
pickup. Our bass pickups come in several
shapes and sizes. You can see the most popular
profiles in Fig. 1. The Precision and
Jazz pickups started as single-coils, while
the Music Man and soapbar pickups initially
came as humbuckers. Today, with the
huge replacement parts industry, you can’t
judge the number of coils simply by shape:
There are humbucking P and J pickups, as
well as single-coil soapbars. Another thing
you can’t see—at least as long as the battery
cable is hidden—is whether a pickup is
active or passive.
Due to their construction, humbuckers
and single-coils can be classed as different
types of pickups and both are known for
certain tones—although the lines between
these sounds are not that strict. Since
the market offers thousands of magnetic
pickups, there must be something else that
sets them apart. The basic construction
is very simple and their only parts are a
permanent magnet and a coil surrounding
said magnet. Add something that holds
everything in place and voilà—you have a
pickup (Fig. 2).
So what else can differentiate pickup
models? Really it comes down to the material,
number, arrangement, shape, and size
of the permanent magnet(s) and the material,
diameter, and number of turns of the
wire that forms the coil.
Do these variables justify the existence
of thousands of pickup models? While preparing
to write this month’s column, I dug
much deeper into this subject than I’d initially
intended, and ended up re-reading a
few hundred pages on electrodynamics and
magnetism. This is an extremely interesting
area of physics, but did it help me better
understand pickups? Yes, but not as one
would think.
The magnetic pickup is surrounded by
a cult—a land of mystical promises and
nebulous PR. Add the word vintage and it
gets even stranger—and way more expensive.
We deal with voltage and current all
the time, from the 12V power in cars to the
110V or 220V outlets in our homes. We’ve
all dealt with electric shocks, so we know
its there, but we rarely see or feel the power
or reactions of magnetic forces. The physics
can be extremely complex, and those who
really understand these forces rarely end up
winding pickups. Hence, marketing guys
can tell us almost anything.
There are no tricks for making the
perfect pickup. The Holy Grail doesn’t
exist and every advantage has its drawbacks.
Does it help to swap pickups? Yes,
sometimes. But why begin with the most
expensive component on your instrument?
It makes more sense to start with the surrounding
elements and investigate pot load,
cable quality, and circuitry.
You might want to re-read “Maxwell’s
Silver Hammer of the Gods”—the March
2012 PG column by Gregg Stock about passive
pickups. As bassists, simply ignore the
word “guitar”—the physics behind bass pickups
is the same—and try to understand some
of the interdependent factors Stock describes.
In the end, you can just stay with what
you have, or fall for the advertising, or
start to experiment and educate yourself.
If you’re interested in the latter, stay tuned
because we’ll be looking at a few guidelines
in upcoming columns.
Heiko Hoepfinger is a German
physicist and long-time bassist, classical
guitarist, and motorcycle enthusiast. His
work on fuel cells for the European orbital
glider Hermes got him deeply into modern
materials and physical acoustics, and
led him to form BassLab (basslab.de)—a
manufacturer of monocoque guitars and basses. You can
reach him at
chefchen@basslab.de..