Understanding pickup noise

[megafiddle]

This is a practical discussion of the problems.

There are three types of noise that affect electric instruments: magnetic, electrostatic, and electromagnetic (RF). The sources of these types of interference are different in type and different in their prevention.

MAGNETIC

Imagine a powerful bar magnet on a table top and a magnetic compass some distance away. If you turn the magnet very slowly, the compass needle will follow. Now imagine turning the magnet 60 times per second. If the compass needle can keep up, it will rotate 60 times per second. To be technically accurate, an electromagnetic wave is present. But it is only necessary to look at the magnetic component to understand and deal with the problems of magnetic noise.

This type of of noise is characterized by a 60 Hz deep sound, like an organ pedal. J bass and Strat users are likely familiar with "single coil hum". The pickups on these instruments consist of a single coil of wire. This same coil that senses the string vibration, also senses any external magnetic field variations. These external fields can typically come from motors and power transformers. The power transformer in the instrument amp is often a source of this noise. You can not shield aganst this noise. Anything that prevents the external magnetic fields from reaching the pickups, will also prevent the magnetic influence of the string's vibration from reaching the pickup. There are two ways to eliminate or reduce single coil hum: remove the noise source, or reorient the instrument. There is usually a position of the instrument where the noise falls to zero (or very low). Also, when the noise can be eliminated by
reorienting the instrument, it is an indication of magnetic noise.

A common solution to this type of noise is to use a humbucking pickup. These pickups reject external magnetic fields, while still sensing the string vibration. But you change the sound of the pickup with the double coils.

ELECTROSTATIC

Imagine a pair of plates with an insulator between them; +100V on one plate and -100V on the other plate. If you were to spin those plates at 60 times per second, you would have an electrical field that varied at 60 times per second. And like with the rotating magnet, you would also have an electromagnetic wave. But the local electrical field is much more significant in this case, just as the local magnetic field was more significant in magnetic noise. Note that "electrostatic" here does not refer to static electricity.

This type of noise is more of a 60 Hz buzz.
Shielding can be very effective against this type of noise. It is not just the pickups that are sensitive to this noise though. Wiring is also sensitive. Shielding can be as simple as layer of gronuded foil between the circuit (pickups and wiring) and the noise source. A grounded foil on the opposite side of the circuit can also be effective if the circuit elements are close to the foil. It reduces the electrical field gradient in the vacinity of the circuit.

Grounding is important here. It provides the "zero volts" that the shielding presents to the circuit. If the shield were not grounded, it would simply rise to the voltage level of the external field and act itself as a noise source.

The most common source of the electrostatic fields that the instrument sees is your own body. You pick up these voltages from the electrical fields in the room. That is why touching the strings can silence the noise. If the strings are grounded, you also become grounded, and are no longer a source of noise.

RF (electromagnetic)

60 Hz radio waves are not the problem here; they are simply too weak. This type of interference is from radio transmission or RF buzz from things like fluorescent lights or neon signs. The frequencies are very high here, and so involve some type of "detection" to convert them to audio frequencies. This detection occurs somewhere in the amp, or instrument preamp. Faraday shielding is normally used to shield against this. The entire circuit is surrounded with a grounded foil.

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[megafiddle]

Electrostatic noise is common in instruments with no shielding or poor shielding. The sound is more of a buzz,
higher in frequency than 60 Hz. The reason is that the 60 Hz power line frequency is rarely a pure sine wave.
It contains harmonics. The noise enters the instrument circuit by capacitive coupling, and the capacitance between
the noise source and circuit affects how different frequencies pass through. The higher frequencies see a lower
impedance (an easier path). So that is what you hear mainly, the higher frequncy harmonics, ie, a buzz.
There are two things that contribute to this type of noise. One is inadequate shielding. The other is a noise source
in close proximity to the instrument circuit. That noise source is typically yourself. You pick up electrical fields.
So there are also two ways to deal with problem:
1) Eliminate the noise source. You can do this by keeping yourself grounded. You generally do this by remaining in
contact with a grounded part of the circuit, usually the strings or bridge. If you are willing to do this, it can be a
temporary solution, like for getting through a recording session.
2) Shield the circuitry. You don't need to necessarily shield the entire cavity. A single flat layer of metal can be effective
if the wiring is in close proximity. (see illistration below).
A simple test for this type of noise is to touch a grounded part of the circuit. Normally this includes the 1/4" jack,
cord plug shell (if metal), bridge, and strings. The bridge may be missing a ground connection, so you have to make
sure you are touching a real ground. The 1/4" jack bushing is a reliable ground as long as the amp is properly grounded.
If touching a ground reduces the buzz, then you have have reduced the noise by reducing the noise source. You are now
at zero AC volts, and no longer supplying the noise. To eliminate the buzz when you are not touching ground, the circuit
will require shielding.
If you use the strings or bridge to test, make sure they are grounded. If touching the strings has no effect on the noise,
it could be an indication that the strings are not grounded.
If touching a true ground has no effect, the noise is likely RF. Magnetic noise is typically not a buzz.
These illustrations show the effect of a simple layer of shielding against electrostatic noise:
The first illustration is an electrical field with no shielding. Notice the field remains relatively
strong over distance.
The next two illustrations show how the field falls off in intensity when a grounded shield
is present. Notice that the signal wire is in a very small noise field in both cases.

 

[iiipopes]

One way to tell if it is AC hum or another source of noise: does the hum roughly sound like B 2nd fret on the A string with the tone knob turned down (60 Hz USA-Canada) or G on the E string (50 Hz Europe, Japan, etc.)? If so, it is true line hum. If not, then it is another source of noise.

 

[megafiddle]

Magnetic noise is unique in that there is no practical way to shield against it. You can avoid it some ways like eliminating the source, or using humbucking pickups. But the string signal itself is magnetic in nature, and anything that shields against 60 cycle magnetic fields will also shield against the signal from the string. You could of course place magnetic shielding outside of the strings, but for most of us it would interfere with the very place we play along the length of the strings.

The difference between slowly varying magnetic fields and electromagnetic waves is maybe best seen in an anology.
Suppose you have a large pool of water. At the far end is a small float sitting on the water surface. On the near end you have a very large float sitting on the surface. The water surface is calm. Now you rapidly plunge the large float into the water and then release it. You will create a wave that travels to the other end of the pool and lifts the float as the wave travels beneath it. The wave propagated (travelled) to the opposite end of the pool. The water level only rose locally and momentarily in the proximity of the large float. This rise in water level travelled along the surface of the water and arrived sometime later at the other end of the pool. This is analagous to an electromagnetic wave. The water did not rise uniformily about the pool, but did so in a wave that travelled along the surface.

Now suppose you very gradually submerge the large float at the near end of the pool. There will be a wave, but you will be hard pressed to meausure it. What you will see is a rise in the water level at the far end of the pool. If you now pull the large float out of the water very gradually, you will see the water level at the far end of the pool drop just as gradually. This is analagous to a magnetic field that slowly increasing and slowly decreasing (and changed polarity). This is the type of field that a single coil pickup is immersed in, when it is in the vicinity of a power transformer. It is best understood and treated as a slowly varying magnetic field, not as a super low frequency RF field.

Copper foil shielding does not work against this type of noise. On the grand scale of things, a 60 Hz magnetic field is little different from a permanent magnet. Both permeate thin conductive shielding as if it wasn't there.

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[R&B]

Fascinating. Now, check this out. I acquired a fairly high-end boutique Precision with a hand-wound pickup. From the first time I plugged it in, there was a nasty buzz (not 60 Hz, definitely higher and buzzier). No effect from touching strings, bridge, jack, pickup poles. Twirling while holding the bass caused the buzz to go null and then to intensify with each 90 degree turn. Rolling off tone EQed it out. Power supply is unfiltered but no other basses buzz. The prior owner did not notice such a thing. It may have developed this issue during coast to coast transport. With me so far?

I took it to a respected tech. He thought it sounded very quiet on his bench amp. He measured ground continuity, polarity and phase, said all was OK but he did cause some buzz by touching the outer E pole of the bass side pickup. Took it home, no effect touching the pickup but it buzzed like hell when I turned while holding the bass.

I finally went under the hood. The cavity was shielded with black paint. Wiring was good and all grounds in continuity. The treble side pickup was installed 180 degrees off the proper axis. Put it in position, plugged in, no effect. BZZZZZZ.

So I pulled the pickup and soldered in a Fralin I had lying about. Buzz was killed. Silent. Awesome. I love this bass all of a sudden!

Explanation? (I'm thinking the original pup was acting as a directional antenna for RF, but how, and why?)

 

[khutch]

There are only two types of noise that affect electronic circuits and instruments: thermal and electromagnetic. Thermal noise is ultimately unavoidable but it isn't an issue with the common passive circuits (which is one of the reasons they became the common ones) and generally only those who design active electronics need to deal with it. If you modify your bass with commercial off the shelf active pickups and preamps all that thermal noise work has been done for you. If you spin your own active electronics then you either use proven circuits that someone else has designed or you learn to choose components carefully to keep the noise in check. Thermal noise is broadband: white, pink, or some other "color". It has no pitch associated with it unless you pass it through a very narrow band filter.

Electromagnetic noise is very commonly misunderstood because it is a complex subject. Magnetic and electric fields can exist independently of each other only when they are static. Static fields do not cause any interference at all by themselves because they are at DC, 0 Hz, they do not vary with time, that is what "static" means. So the somewhat and perhaps increasingly common notion in the pop culture of guitar electronics that there is such a thing as electrostatic interference is just peculiar beyond belief. Can you hear zero Hz? Can your speakers reproduce zero Hz? If so then under the right conditions you might have electrostatic or magnetostatic interference. You cannot hear the effects of a static field unless something is changing and changing at a rate that your ears can hear and your speakers can reproduce. So, if you were playing bass in a room that had a strong static magnetic field in it the only way that field could cause an audible interference would be for you to move in such a way that the amount of external magnetic flux linked to your pickup coils varied at a rate you could hear. For example if you mounted your bass on some kind of spindle and spun it so that it rotated at 60 revolutions per second (3600 rpm) you would hear 60Hz. Seriously, how often do you do that? It is just as hard to contrive a situation where a static electric field could become audible. A static electric field can affect the trajectory of electrons flowing in a vacuum tube. So if your bass has a valve preamp and you put it on that spindle and spin it at 3600 rpm you could, in theory, hear a slight 60Hz noise from a static electric field bending the electron paths this way and that since some that would have hit the anode without the field might miss it when the field is oriented just right. There is no such thing as electrostatic interference outside of extremely contrived situations like this. And even then those static fields are part of electromagnetism, the one just does not create the other when the frequency is reduced to zero.

A much better way to describe the situation would be to say that electromagnetic interference can be manifested in three different forms: electric field, magnetic field, and radiated (where the electric and magnetic fields exist in a balance set by the properties of the medium through which they propagate). This reduces the level of complexity to the point where the common man can understand it and make use of it without spreading the misconception that time varying magnetic or electric fields are not electromagnetic fields. Shielding is quite effective against all three manifestations of electromagnetic interference but magnetic energy at low (ie audible) frequencies requires an impossibly thick shield if it is made of an electrically conductive but non-ferromagnetic material like copper, aluminum, or shielding paint. This is an unwelcome complexity to be sure but better to just tell it like it is than to come up with a swimming pool analogy that does not explain why high frequency waves are stopped by a shield while low frequency waves are not. You would see both waves equally well in your swimming pool if you make the pool long enough.... Not everything in this world has a simple explanation.

 

[megafiddle]

The treble side pickup was installed 180 degrees off the proper axis. Put it in position, plugged in, no effect. BZZZZZZ.

Do you mean that you simply rotated the pickup half? You didn't swap the coil connections?

If so, it sounds like it was not wired hum cancelling. Just physically rotating the pickup half
wouldn't change that. It was still not hum cancelling.

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[megafiddle]

There are only two types of noise that affect electronic circuits and instruments: thermal and electromagnetic...

This is from here:

http://en.wikipedia.org/wiki/Electrostatic

Electrostatics is a branch of physics that deals with the phenomena and properties
of stationary or slow-moving electric charges with no acceleration.

The term electrostatic is also used for electrical fields that vary so slowly, that they
can be dealt with as if they were static (same equations etc.).

Where would you draw the line? One cycle per minute? What about one cycle per hour?
Would you really treat one cycle per hour as an electromagnetic wave, whether electric
or magnetic? It's certainaly not static. I maintain that 60 per second is still slow enough
to be classified as electrostatic. The term is in use for that type if interference.

My swimming pool analogy is correct for near field phenomenon. For a long enough pool,
you are correct. But that long pool is an analogy for a wave in free space, many wavelengths
away from the source. And for a 60 Hz EM wave, that would be over 10 million meters.

(edited)
Another example here:

http://www.ramayes.com/aluminum_foil_emi_rfi_shielding.htm

Note that there are different specs for electric field, magnetic field, and EM plane wave.
Specs for magnetic and electric fields are for AC fields, not static.

And some more here:

Invalid Link Removed

• Electric and magnetic field coupling
– In the near field, E and H field coupling are treated separately
– In the far field, coupling is treated as a plane wave coupling

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[R&B]

Wiring looked correct after I rotated the treble side. (Perhaps the leads had been attached wrong in the manufacturing process, I suppose.) The buzz was not typical single coil hum as one would hear with soloing a pickup on a jazz bass. Much nastier and higher frequency.

But the real fix was swapping out the pickup entirely.

 

[megafiddle]

Wiring looked correct after I rotated the treble side. (Perhaps the leads had been attached wrong in the manufacturing process, I suppose.) The buzz was not typical single coil hum as one would hear with soloing a pickup on a jazz bass. Much nastier and higher frequency.

But the real fix was swapping out the pickup entirely.

Did you actually change the wiring, though? I'm not sure I understand exactly
what you changed.

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[R&B]

Did you actually change the wiring, though? I'm not sure I understand exactly
what you changed.

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I did not de-solder the leads from the pickup halves, as I had no point of reference for how to reconnect them any "better" than they were. Once the orientation was correct, the wiring looked like any normal P bass.

But the buzz was unaffected.

So my next move was to excise the offending pickup entirely and drop the Fralin in. I wired and soldered it to the pots exactly as the diagrams show (and the same as the original pickup had been wired). No buzz.

I'm planning to send the bad pickup back to the builder for him to evaluate.

 

[megafiddle]

I did not de-solder the leads from the pickup halves, as I had no point of reference for how to reconnect them any "better" than they were. Once the orientation was correct, the wiring looked like any normal P bass.

But the buzz was unaffected.

So my next move was to excise the offending pickup entirely and drop the Fralin in. I wired and soldered it to the pots exactly as the diagrams show (and the same as the original pickup had been wired). No buzz.

I'm planning to send the bad pickup back to the builder for him to evaluate.

Simply changing the orientation of the one pickup half would not change anything.
You would have to swap the connections on one half to fix it. So it sounds like the
two halves were wired wrong to begin with.

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[R&B]

Simply changing the orientation of the one pickup half would not change anything.
You would have to swap the connections on one half to fix it. So it sounds like the
two halves were wired wrong to begin with.

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Sounds right. Which leads and in which direction would you swap? And does mis-application of the leads adequately explain the buzz?

All academic now, as the immediate problem has been solved. Clearly this was not a typical situation that could be solved by shielding or grounding. Thought I would share and ask for insights. I'll report back on what the builder says when he tests the pup.

 

[megafiddle]

If the buzz disappears when the bass is reorented, it is a good indication of single coil (magnetic) noise.
If one half were wired in reverse from the way it should be wired, you would have in effect, a single coil
pickup, with the two coils simply wired in series aiding. To correct it, you would would disconnect the
connections to only one half, and swap them. Now the two halves would be wired series opposing.

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[R&B]

Aha! Would I see a difference on the ohmmeter?

 

[megafiddle]

No. Resistance has no polarity.

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[R&B]

/thread hijack/

Thanks megafiddle! This is very educational, at least on this end. I may need to do some further experimentation soon.

 

[megafiddle]

Hope it helped.

To actually get the coil polarity, you would have to trace the wire from the terminals
and see which direction the wire was wound, clockwise or counter clockwise. For a
typical P pickup, the wire will travel from the terminal directly to the bobbin and the
wires will not cross between the bobbin (coil) and the terminals. If the coil is unpotted
and untaped, you can see the winding direction and also even see which end of the wire
is at the inner end of the coil, and which end is at at the outer end. The inner and outer
ends don't matter here. Just the direction matters.

If the coils are wound in the same direction, then the like ends of the two coils should be
connected together. So when viewing both coils with the same orientation, the right terminal
on each should be connected to each other, or the left terminal on each should be connected to
each other.

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[khutch]

This is from here:

http://en.wikipedia.org/wiki/Electrostatic

Electrostatics is a branch of physics that deals with the phenomena and properties
of stationary or slow-moving electric charges with no acceleration.
...
Where would you draw the line?

Slow moving? With respect to what? Slow moving is a nonsense definition, it depends entirely on one's perspective so if you have not defined what you mean by it, and this article does not, it means nothing. Now look at the last bit of that definition, the bit you ignored. The charge carriers that support any AC current are constantly accelerating. Finally, the actual charge carriers inside the wires move at a velocity determined by the electric field along the length of the wire and the mobility factor characteristic of the material the wire is made from, independent of frequency. The electrons supporting a 60Hz current are moving as fast as those supporting a 1GHz current and both move no faster than the electrons supporting the DC current in your flashlight's LED, if all are driven by the same voltage difference and if we are talking about the velocity in copper wires of the same area.

Here is another Wikipedia article that calculates the speed of electrons moving in a 1 mm copper wire and supporting a current of 3 A. The answer is rather astonishing really, 1 m/hr!

Where do you draw the line now?

My swimming pool analogy is correct for near field phenomenon.

Yes, exactly. And "near field" is an electromagnetic concept. But the swimming pool analogy cannot hope to illustrate the dualistic electric/magnetic principles of electromagnetic waves because waves in a swimming pool are singular in nature.

Note that there are different specs for electric field, magnetic field, and EM plane wave.
Specs for magnetic and electric fields are for AC fields, not static.

Yes, that is correct, but as we see above a 60 Hz field is not static. I have no problem with calling interference sources electric field or magnetic field because EM near field interferers are one or the other. But electrostatic interference is nonsense. You can find a Link Removed here, and Link Removed. As they explain in [Invalid or Expired Link Removed] these calculators are approximations but they are useful in making comparisons. I have a roll of copper shielding tape on my desk here at work and it measures 1 mil thick. For a magnetic field source 1 m away the Clemson calculator predicts this copper tape will provide 78 dB of suppression if the source is at 1 MHz but only 32 dB at 60 Hz. If we wanted a copper shield to give us 78 dB of suppression at 60 Hz it would have to be 1.76 inches thick. But in the exceedingly unlikely event that we had a far field source at 60 Hz that we were worried about that same 1 mil tape would provide 150 dB of shielding. Near field, far field makes a huge difference. Magnetic versus electric field makes a huge difference in the near field: that 1 mil copper tape gives us 268 dB of shielding for a 60 Hz electric field source 1 m away. The calculator says the copper "tape" would have to be 2300 inches thick to give us 268 dB of shielding at 60 Hz in the 1 m near field of a magnetic source!!

Take those numbers with a grain of salt in terms of the real shielding effectiveness you would get in a practical situation. But they illustrate in relative terms the difference between magnetic near field, electric near field, and far field.

 

[megafiddle]

Slow moving? With respect to what? ...
... Now look at the last bit of that definition, the bit you ignored.

Acceleration is the derivitive of velocity. What happens to the derivitive as the frequency gets lower and lower?
I draw the line when the term becomes so small, that it drops out of the equation.

I used to work with circuits that had a bandwidth that extended from DC to hundreds of megahertz.
We used to call audio frequencies "DC". The circuit capacitance and inductance was so small, it had
no effect on the analysis at lower frequencies. DC analysis worked as well at a few kHz as it did
at zero Hz.

I use the term electrostatic for two reasons. For one, it's simply easier to understand how the noise
is coupled between the noise source and the affected circuit, and how it can be prevented. It is simply
modeled (equivalent circuit) as a capacitor between the source and affected circuit.

And secondly, the term electrostatic is in actual use. Here is one example:

Link Removed

Chapter 2 Electrostatic Interference

2.1 How Electrostatic Interference Works
Consider an amplifier with a single exposed wire connected to its unbalanced input. This wire is in close proximity to another wire which is connected to an ac power source.

The source and the destination both have the same reference - that is, the second connection of both the source and destination is the same.

Even though the two wires do not touch, the signal voltage present on the source wire will have an effect on the exposed amplifier input wire (the destination). The presence of a charge (voltage) on the source wire will either attract or repel electrons in the amplifier input wire, depending on the polarity of the voltage in the source wire. With an alternating voltage in the source wire, there will be a consequential alternating voltage generated in the amplifier input wire.

(edited)

I should add that the term electrostatic can be confusing, and electric field is a better name for it.
I did mention early on, that electrostatic here does not refer to static electricity. It may be better
to just note that the term is in use, which is necessary because some will run into it.

We often use models that are specialized for specific purposes. The electron model of the atom does
not represent physical reality; atoms are not like miniature solar systems with electrons orbiting around
a central "sun". Still, the electron model is extremely useful for explaining and predicting both electrical
and chemical behaviour. So we use it.

Radio waves are modeled as electromagnetic waves, not as streams of photons. But gamma radiation
and light are often modeled as a photon stream even though they all differ only in frequency.

The whole purpose of this is to provide a model that is easy to understand and therefore apply. The
problem of single coil hum and it's prevention (or it's unavoidance) has been known for a long time.
Almost all posts asking for help with this problem receive accurate recommendations for solving the
problem. But electric interference problems often are treated as RF problems, and the suggested
solution is extensive faraday shielding. Certainly it works; but it is often not needed. The problem
may be nothing more than a missing ground connection to existing shielding.

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