least magnetic distortion?

[dcoyle]

This probably a blindingly complicated question, but here goes. Is it widely thought there is more or less distortion inherent any one type of magnetic structure (for a given field strength in the region of the string) ?

For instance, is it thought that a structure such as the p90 type has more distortion than a strat type?

What type has the least distortion?

I believe I once saw on the Web a comparison of magnetic types, describing some feature that I took to be distortion, but more than that is haze. Anyone know what I am referring to?

Thanks

Dan

[madialex]

I wouldn't think the Magnet would have anything at all to do with distortion. More so the windings than anything else...Just MHO

[uvacom]

The magnetic field does introduce some distortion, since the mag field decreases with distance from the polepieces. That means that any movement of the string in the "Y" direction is not detected equally, so there will be some second-order harmonic distortion from the pickup itself.

Also, the magnetic field affects the vibration of the string, which will also introduce distortion, usually called "stratitis". This should also be second-order in nature.

Both of those should be completely dependent on the field strength at the position of the string, so obviously they will be effected by the pickup's topology and the magnet type used, but are not necessarily an inherent function of the magnet or pickup type (i.e. a typical strat pickup might have more inherent distortion than a typical PAF, but that doesn't mean a PAF couldn't be engineered to have more distortion, or vice versa).

Anybody know of any other forms of appreciable distortion in a guitar pickup?

[David King]

Any impedance load on the pickup will affect the string's sustain and pattern so that's another and possibly more important source of "distortion".

[uvacom]

Any impedance load on the pickup will affect the string's sustain and pattern so that's another and possibly more important source of "distortion".

Interesting. So you're saying that loading the pickup affects the string's vibration (and not just the effect on the electrical signal)? What is the mechanism behind this?

[David King]

Think of the pickup as a DC generator with the string driving it. If you short out the generator's leads it will act as a brake on the string's movement. I.e. the string will have to do more work to move through the magnetic field as you're asking it to heat up the shorted coil.
Take a little DC motor sometime and short the leads and try to spin it, same effect.
This is also why it's important not to have loops of wire shorted out in a coil. That will really kill sustain.

[dcoyle]

magnetic distortion

David King,

Could we stipulate, so as to explore magnetic distortion in this thread, that the load is large enough and nonreactive enough as to be constant for the discussion?

Uvacom,

In order to isolate the effects of magnetic structure, lets specify that the field strength, whatever its orientation and topology, is the same at the string area.

There are many sources of distortion in a pick up, due to its inductance, capacitance, magnetic environment, field strength, support structure, etc, but I was hoping to focus on the distortions due to magnetic structure.

When one has all else equal, and please continue to specify what those things are that must be normalized, How do the different metal keepers, magnet types and arrangements change the sound?

Thanks

Dan

[uvacom]

Think of the pickup as a DC generator with the string driving it. If you short out the generator's leads it will act as a brake on the string's movement. I.e. the string will have to do more work to move through the magnetic field as you're asking it to heat up the shorted coil.
Take a little DC motor sometime and short the leads and try to spin it, same effect.
This is also why it's important not to have loops of wire shorted out in a coil. That will really kill sustain.

Okay, I think I understand how this works in an electric motor. When the motor is shorted and you rotate the shaft in one direction, the movement of the coil through the magnetic field induces a current which powers the motor in the opposite direction, that's why it takes more torque to turn the motor in this configuration. If the motor is opened, no current can flow and you don't experience this resistance.

Okay, so we have a shorted coil, and a string moving through the field. That will induce some small current, which intuitively would give rise to a magnetic field in the opposite direction. So what you're saying does make sense to me. The question is what order of magnitude this effect can be in the range of impedance loads into which guitar pickups are typically connected.

[uvacom]

David King,

Could we stipulate, so as to explore magnetic distortion in this thread, that the load is large enough and nonreactive enough as to be constant for the discussion?

Uvacom,

In order to isolate the effects of magnetic structure, lets specify that the field strength, whatever its orientation and topology, is the same at the string area.

There are many sources of distortion in a pick up, due to its inductance, capacitance, magnetic environment, field strength, support structure, etc, but I was hoping to focus on the distortions due to magnetic structure.

When one has all else equal, and please continue to specify what those things are that must be normalized, How do the different metal keepers, magnet types and arrangements change the sound?

Thanks

Dan

Okay, so basically you would like to know which intrinsic magnetic characteristics (i.e. B-H curve, etc.) can induce distortion in an electric guitar pickup.

I found a very interesting discussion of this here. I don't have much to add to what was said there, other than that with my current understanding of guitar pickups there is no property of the magnet which affects distortion without respect to other design considerations.

[dcoyle]

Could we also assume that the magnetic window is only the width of a keeper or magnet, i.e. ignore the effects due to sampling window width?

Uvacom said:

"The magnetic field does introduce some distortion, since the mag field decreases with distance from the polepieces. That means that any movement of the string in the "Y" direction is not detected equally, so there will be some second-order harmonic distortion from the pickup itself."


I wonder about this. It seems to me that the lack of signal due to movement in directions that are not sensitive is not a distortion of the wave form, but the nature of sound percieved by a single sensor, like an eardrum. When we hear the string in the air we experience the same sort of effect of the vibration of the string moving toward us (normal to the ear drum) smothly varying to perpendicular and back, which evolves during the attack. In the air we hear that as a change of loudness, i.e. the same fundamental sine wave, but less excursion, not as the addition of the upper harmonics of second order distortion, which an asymetric wave form produces.

I'm trying to differentiate between the change in amplitude due to orientation and the 2nd order generated when the string moves through regions of different field strength or field orientation DURING ITS EXCURSION and the top and bottom of the wave vary in amplitude.

At any one instant, the behavior of the string is well described in BBSailor's excellent intro to moving coil pickups elsewhere on the board, there are effects over time as the string decays, and in how the abruptness of the motion changes the magnetic field that happen when the string moves. What are these effects and how do the sound?

Thanks

Dan

[dcoyle]

Uvacom,

I'm composing while you're posting. Thanks for the link. That's exactly what I'm curious about.

Thanks

Dan

[David King]

Uvacom, that's a great link. Thanks.

[David Schwab]

Think of the pickup as a DC generator with the string driving it. If you short out the generator's leads it will act as a brake on the string's movement. I.e. the string will have to do more work to move through the magnetic field as you're asking it to heat up the shorted coil.
Take a little DC motor sometime and short the leads and try to spin it, same effect.
This is also why it's important not to have loops of wire shorted out in a coil. That will really kill sustain.

This is what the Creation Audio labs people were saying about the Redeemer, but I think it's hogwash. Changing the load on the pickup does not change the permanent magnet field, and can't effect the strings.

It's very easy to prove that it doesn't happen. Try it.

Shorted loops of wire kill the high end of the pickup, but can't influence the string's motion.

[Mike Sulzer]

Okay, so basically you would like to know which intrinsic magnetic characteristics (i.e. B-H curve, etc.) can induce distortion in an electric guitar pickup.

The output of a guitar pickup is a very small amount of power. It does not move the magnetic material along the B H curve very much, as happens in a motor or a speaker. I suspect that for a pickup there is no significant distortion generated this way.

String damping by power dissipation in the load on the pickup is a linear effect, often not considered distortion. Not sure how big an effect this is, although it could be computed.

The spatial variation of the magnetic field over the pole piece causes distortion as was described above. There are two kinds; the first is due to changes in the vertical field (along the axis of the pole piece). This is asymmetrical of course, and so it makes some even harmonics. The other is due to the fall off of the field in the horizontal direction. Since the field falls off in both directons, this is a frequency doubling effect. I do no think that either of these effects is very big.

The primary "harmonic selector" for a pickup is its location. The primary overall frequency response is set by the pickup-cable resonance. Eddy currents in the steel act like a frequency dependent resistor with an inductor in series, and so can modify the effect of the resonance. But these effects are linear and would not generally be considered distortion.

[David King]

Thanks Mike, that all makes sense to me. I'm just wondering if there is any part of the circuit that can become saturated somehow and then give us a nonlinear response. As usual, I can barely grasp what I'm talking about so forgive me if I make no sense.

David S you'll agree that an active sustainer can be phase flipped to become a string mute. So why not a passive mute effect? Granted most pickups will have less than 10k ohm of loading but I'm betting that at certain frequencies where cable capacitance is part of a tuned LRC circuit we could expect to see noticeable string damping. Again probably easier to check for than to try to calculate.