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.
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 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.
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.
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.
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.
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 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).
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.