Invented in the 1930s, the magnetic
pickup still rules the market, and we
explored this technology in last month’s
column [“Choosing Magnetic Pickups,”
September 2012]. Now let’s look at two
other types of pickups—piezo and optical.
The piezo transducer has mainly found its
niche in acoustic instruments, while optical
pickups still fight for visibility. Let’s do
a quick rundown on these two methods of
sensing string vibration and then compare
all three technologies.
Piezo pickups. Piezo materials are
non-conductive crystals that emit a voltage
when a mechanical force is applied. The
output signal is directly proportional to the
changes of the outer pressure over a wide
frequency range, and this makes piezo crystals
a good mechanical sensor.
Piezos have a very high impedance and
the output signal is pretty weak. Putting
any potentiometer load on them would
weaken their signal even more. So for starters,
we need to put them through a buffering
preamp, which involves a battery.
Most often used as undersaddle transducers,
piezos work with all sorts of strings,
even non-magnetic ones. Almost all acoustic
basses use them as a one-pieced strip
under the bridge. Since they are highly
dependant on pressure, it can be tricky
to get a constant volume from all strings.
More advanced systems use separate piezos
for each string, and sometimes these individual
pickups are also separately buffered
to offer more control over each string’s signal.
Such multi-pickup designs are ideal for
triggering MIDI controllers.
Piezo systems are popular because they are
inexpensive, although the preamp adds to the
cost. These pickups are less prone to feedback
and easier to handle than microphones.
Most piezos sound trebly and pretty
harsh, especially compared to their magnetic
buddies. Many players claim piezos
lack bass, but this impression is typically
a result of a piezo pickup’s increased highs
masking the low frequencies. Some newer,
more sophisticated piezo models can sound
natural and warm.
Optical pickups. The least obtrusive
way of sensing a string’s vibration is to track
it visually. We do this by pointing a source
of light towards a moving string and using a
photodiode—a semiconductor that converts
light into an electrical current—to sense
the alternating moments of shade and light.
Using infrared light makes an optical sensor
less prone to interference from surrounding
light sources.
This sounds futuristic, but the first
pickup of this kind was actually shown in
1969 at the Chicago summer NAMM and
patented in 1973 by Ron Hoag. Definitely
a very promising idea. But, like many
worthy ideas and designs in the music business,
it never really took off. Ron has now
retired, but he did several prototypes over
the years—including the mid-’70s instrument
shown in Fig. 1 and Fig. 2—and
even built one prototype equipped with a
MIDI converter.
Fig. 1. Equipped with an optical pickup, this
mid-’70s prototype bass was designed and
built by Ron Hoag.
Fig. 2. The bridge and optical pickup assembly
in Hoag’s light-sensing prototype bass.
Photos courtesy of opticalguitars.comCurrently, the only optical pickups on
the market are made by Lightwave (lightwave-
systems.com) and these are used in
their own line of basses.
Optical pickups have a frequency range
that far exceeds our audio band and they
also boast an extraordinary tracking speed.
And as with piezos, optical pickups work
with all sorts of strings—unlike magnetic
pickups, you’re not limited to ferromagnetic
materials. Of course, creating a light source
and amplifying the signal requires electrical
energy, so there’s a certain amount of circuitry
involved with an optical pickup.
Given today’s pickup technology, I think
it’s fair to say that optical systems are the
most sonically transparent.
Magnetic, piezo, and optical: How
do they compare? There’s one critical
relationship we haven’t yet covered: the
interaction between string and pickup.
The pull of a magnetic pickup on a metal
string is something that really influences
tone. Not only does it suck up vibrational
energy from the string, resulting in less
sustain, it also transposes upper harmonics
to higher frequencies. When the magnetic
pull gets too strong, it can sound like a
chorus effect or simply out of tune, (our
friends with the flimsy strings call this
Stratitis). Unfortunately, the magnet pull
affects the string, whether or not the
pickup is in use.
A piezo mechanically vibrates along with
the string, but the dampening within the
crystal is so weak, it’s not worth discussing.
This is even more true for the optical pickup,
which doesn’t even contact the string.
Compared to the hum problems of magnetic
pickups, both the piezo and especially
the optical pickup operate with very low
background noise.
It might appear as if piezo and optical
technology would be superior to magnetic
pickups, but sometimes imperfection is the
right choice. While most metal-head bassists
wouldn’t even consider anything but
magnetics, those looking for a more upright
or atmospheric tone should have piezo and
optical pickups on their list. Combining
strong fundamentals from a magnetic pickup
with the airy high frequencies of a piezo
or optical pickup can be very addictive. By
combining technology, you can expand the
tonal range of your bass and thus enhance
your musical expression.
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..