If you are experiencing problems with your bass pickups, such as hiss, hum,
shielding and other problems, you may wish to do some
investigation and look at the text below.
First, of all, you need to determine, what type of pickups you have.
There are basically three types: Passive, Active, and Active-Boosted. The
latter is really a High impedance passive pickup with an active pre-amplifier and
tone controls to modify the sound of the bass.
Passive Pickups (or rather--sensors) have no battery or external power to the
sensor. They are essentially a wire coil on magnet structure. The
common resistance of these sensors is 4000 ohms to 8000 ohms. If the
sensor is disconnected from
the system, you can measure the resistance with a simple VOM (Volt-Ohm-Meter).
If you measure the resistance, and appears to be infinite, you most likely have
an open (a break in the magnet wire), and this quite difficult to repair.
Most sensors have a wax or epoxy coating on the entire wire coil, to prevent
micro-noise from wire vibrating inside the coil.
Active Sensors are a Passive Sensor with a built in amplifier. The
sensors are often low impedance types, but not always. Active Sensors require a battery to operate
the amplifier, and usually have three wires or
four like the ones below. Below, the red is the hot (positive) the black is the
ground, and white is the signal wire, which usually goes to the potentiometer or
signal control board.
The photo above is just one example of many types of
for bass guitars. The ones above are an active Jazz Bass set made by EMG.
Active sensors have a small built-in amplifier inside the pickup housing. They
also require a battery (usually 9 volt, or sometimes 18volt) to operate.
You cannot measure the sensor resistance with active type pickups because, the
output side is really an operational amplifier. So all you can do is
connect them up correctly, and try them out. If it does not work, it may
be the sensor (and amplifier inside it) or you may have some other problem like a bad
If you have one sensor, like a soap bar type, and you have problems, your
solutions will not be too complicated.
If you have two or more sensors, you have the added problem of correct
phasing, which is corrected by changing the wiring of one of the sensors.
Another common problem is no signal, which usually results in just hiss. This is usually the
sign of an open in the circuit, which could be a break in the magnet wire (not
likely), or a bad ground (most likely). Poor soldering methods can lead to
cold joints and poor electrical connection will result, leading effectively to
an open circuit. If the solder
joint does not flow onto the part or tab connection well, but instead looks like
balls of solder, and lots of flux paste residue, and carbon residue, you likely
have a bad connection which will need to be re-soldered.
Another issue is hum. This can be caused by electrically exposed wiring
in the bass itself. (My 1974 Rickenbacher 4001 bass had no shielding at
all!) The wiring compartment should be shielded with copper
foil or CRT(Cathod-Ray-Tube paint--a resistive paint) paint, and then grounded
to the system ground in the compartment. If your hum only occurs
when your hands are off of the strings, then you likely do not have a grounded
bridge. Bridges must be grounded. Sometimes the hum comes from a
poorly shielded control cavity. Sometimes you can install a brass or copper plate
under the sensor, and ground it to the system. Be careful to not short the
sensor to the brass plate. You can use electrical tape between the sensor
and the plate to prevent this from happening. Avoid using aluminum or
aluminum foil as it is hard to electrical bond to the material. Also,
aluminum oxidizes and increases resistance especially at joints.
Humbucking sensors should not hum, this is the whole point of making them. But if the wiring to them is open to
electrical signals, you may be getting a noise from lights and nearby radio
stations (which always gives a professional impression in most gigs). If
the wiring from these pickups is not shielded, the wiring should be twisted,
which causes the hum and noise to cancel when it hits the wires.
P-Bass split sensors are meant to be wired out of phase, so that the noise signals
which fall onto the G & D coil, also fall out of phase onto the A & E coil, and
thus cancel each other out.
Jazz bass sensors are meant to be fully humbucking, but only when the volume
is equal on both pickups. This is the inherent downside of the Jazz Bass,
since most bass players like a little more of the bridge signal for clarity
(e.g. Marcus Miller). So if you turn up the volume on both pickups on your
Jazz bass, you should not get any hum. Now, here is where you might want
to consider installing split coil Jazz Bass sensors. DiMarzio and
Bartolini are the only manufactures that I am aware of which make sensors like
these. The advantage here is that each pickup is really two out-of-phase
parts wired in series or parallel. So with just one sensor on, you can have humbucking enabled.
The above sensor on the right are a split coil design that can be wired to be
hum canceling. However, since I have it coupled with a single coil Jazz
on the right,
I have them wired as a single-coil type. If you wire the P-bass sensor, on
a bass like this, as hum-canceling, in connection with a single coil Jazz bass
sensor, the result will be that two of the four strings will be out of phase
(two strings will sound like they have no bass).
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