FAQ - Noise and/or Hum...
The Audere preamp has a very low noise level which allows it to capture
the true sound of your playing.
If there is noise in the pickup signal
from the external environment then the preamp will capture the noise also.
Most instruments use multiple methods to prevent external noise from entering the output signal
of the bass like shielding the electronics cavity.
However one point of entry for external
noise is often over looked - the pickup cavities.
This source of noise problem is fairly easy to identify in it's most common
form. When you are touching either the controls or strings on your
bass the noise is removed. If you remove your hand from the strings
and controls the noise reappears. The noise is often described as a
hum or buzz. This is a very old and common problem with all types of guitars and not just basses
- watch a professional guitar player with a Strat which is easily the most
common type of six string ever recorded - they will keep a hand on the strings
when they are not playing.
If you do not want to be always touching the strings on your bass
the situation can be easily improved.
Place a ground shield under and around the pickups
The more complete your ground shielding job the more noise you will
remove.
Check out this example of a very complete shielding job that
was done by one of our customers - Reed Ricker.
The best way to understand why shielding the pickup is
important is to understand the path the electrical noise takes to
enter the system and how installing a grounded shield stops it.
The drawing below illustrates the basic problem
The left side of the drawing shows the player is connected to
the power system in a room by capacitive coupling.
In other words, your body has a voltage imposed on it by
the local AC line power system. In the U.S. this will be a 60 cycle
signal and higher harmonics - in other countries the base frequency
can be 50 or 60 cycles. Often the problem will be bigger in some other countries
because the voltage on the power line is often higher and the grounding
standards can be different or less well enforced.
How much noise you collect from the environment you are playing in
will be controlled by a lot of variables which might surprise you such
as how close are you to a power line in the wall you can
not see, what types of shoes are you wearing, are there any light
dimmers around or other objects which control the power level
by switch on/off the line voltage very fast etc.
The right side of the previous drawing shows the player
is also capacitively coupled to the pickup windings.
Most bass guitars are manufactured of wood.
The wood body will not decrease the coupling but more likely increase
it - often denser woods would provide better coupling. Thinner backs
would have increased noise coupling. Bigger pickup coils would have
more coupling. If the pickup wire exits on the inside as opposed to
the outside the coupling might be better or worse. There are a lot of
variables which you will have limited control over.
This noise injection problem is fixable
When a grounded shield is inserted in-between the player
and the pickup then the noise on the player's body is no longer coupled
into the pickup. The noise is now coupled to the shield.
A current related to the noise on the player
will now flow from the shield to the output jack then to the power
amp and eventually back to the grounding rod driven
into the ground. The shield will stay at 0 volts.
The pickup is now capacitively coupled to the shield at a constant 0 volts so no noise is produced.
The better you shield the back and sides of the pickups
the more the noise will be reduced. The best commonly available shielding
material is copper foil which is significantly better than conductive
paint, however, shielding a larger percentage of the area is often
more important than using a better conducting material.
If a shield is installed but not connected
to ground this will often increase the noise as opposed to decrease
it so be sure to connect the shield to ground.
Does this change the tone of the pickup?
Yes, slightly in High Z-Mode. If you look at the top illustration
you will see the pickups are now capacitively coupled to the ground.
A very complete shielding job, as shown above, will add approximately
90 pF of additional capacitance to the pickups. This additional capacitance
will slightly lower the frequency of the response peak in the High
Z-Mode. This capacitance can be compensated out by 95+% of our customers,
if desired, but it is rare because most people choose a High Z-Mode
C loading larger than this. The pickup's tone in the other Z-Modes
will not be changed.
Is this important with hum cancelling pickups?
Yes, ideally you would shield them. Hum cancelling pickups do a good
job of rejecting the lower frequency
noise, but as the frequency increases the 2 coils will not look electrically
close enough to each other to fully cancel the higher frequency noise
so you will still get a lot of this higher frequency noise so it
will sound more like a static noise source.
Does this happen with a passive bass?
Yes, the effects are based on simple physics (note: No bass preamp
was used to take the scope pictures) but the size of the effect will
be smaller with a passive bass. The magnitude of the effect is reduced
by the increased loading effect from the combined
passive bass electrical parts: pot, caps etc. And a passive bass also
loses more of the high frequencies when driving a cable, this includes
high frequency noise.
Supporting Measurements/Data
If you are in a typical room then
your body will not be at 0 volts - it will have several volts
at the power line frequency impressed onto it, assuming
you are not touching a grounded object. Below
is an oscilloscope picture of the noise on my body in my lab as detected
by an oscilloscope with a 1 Meg Ohm probe.
My body is swinging more than 11 volts peak.
To understand how this body voltage affects a pickup we will take
a single coil pickup and connect it to a scope input. For this example,
a Nordstrand NJ5 was used in the testing and
is connected to the scope with a Tektronix
ADA400A preamp and a 1x scope probe (this combined load will be about 1M + 200 pF).
The scope display below shows the noise introduced into the pickup
when my body is about 1 inch away from it.
The scale of the scope is now 1 mV per division
so the noise is now about 4 mV peak to peak.
The noise is broader in bandwidth because
capacitive coupling works better at higher frequencies so even
though the voltage on my body is
strongest at the line frequency the change in coupling efficiency
with frequency causes more of the high frequency signal to be transferred
to the pickup.
Next I reached out with my hand and touched ground (like when you
are touching grounded strings).
You can clearly see the reduction in noise which is cut
by 50%.
Next I added a shield in-between the pickup and my body.
At this point - it does not make any visible difference when I touch
ground or not.
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