Impact of neck pickup on electric guitar tone

[bbsailor]

That's an interesting question. In a loaded transformer, you'd have "back EMF" pushing against the original source the magnetic field. I wonder if a guitar pickup produces "back EMF" against the string, and how it might effect the string's movement.

The answer to this question is related to how tightly magnetically coupled the moving element of the transducer is coupled to circuit load. Here is an example. A passive microphone with a moving coil attached to a diaphragm either drives the output load directly or goes through another booster transformer. If the load on the transformer of voice coil is too low it restricts the movement of the coil because it is tightly magnetically coupled. Ribbon microphones tend to exhibit this also because the corrugated ribbon is both the voice coil and the diaphragm moving in a very strong magnetic field. The ribbon mic matching transformer boosts the very low current of the vibrating ribbon to match the input impedance of the preamplifier. A typical 1 to 39 Turns ratio will impose an impedance rise of 39 squared or 1521 times the DCR of the transformer primary and ribbon total resistance as a single loop coil. This puts a 0.1 to 0.2 ohm primary in the 150 to 300 ohm range. If the amplifier input impedance is too low it tend to damp the high frequency sound because of the very close coupling to the amp loading. This is why they make active buffers in the microphone to put a low load on the mic output transformer and not worry about the preamplifier input impedance loading. Then you can go up to a 1 to 50 turns ratio transformer and get a little better signal to noise ratio without worrying about damping an output impedance rise of 50 squared or 2500 that would put the 0.1 to 0.2 ohm primary in the 250 to 500 ohm range. This is much too high for passive preamp impedance loading but an active buffer with a 10K load would not load it too much. Any load on the output of a transformer gets reflected back into the primary (voice coil or ribbon) by the square of the turns ratio.

Guitars, on the other hand, have a very low magnetic coupling to the string, so shorting out the pickup would not reflect any measurable damping on the string vibration. The pickup has such a high DCR winding resistance, unlike the microphone example above, that a shorted or loaded output had very little effect back to the string vibration.

I hope this answers the question?

Joseph J. Rogowski

[Jason Rodgers]

In one of those papers that pops up in the wire break search, they tested 0, 45, and 90 deg plucks.

[John Kolbeck]

I was fooling around recording my string picked up, down, in and out. The waveform looked about the same but different phase. I just realized that a question last week or so about the offset may have the answer in the second harmonic cancelling one peak and reinforcing the other.

The attack is different when plucking sideways because you start out with more horizontal movement initially. Pulling from the top can also cause the string to bang against the fretboard or pickup, which influences the sound quite a bit.

[Richard]

I was fooling around recording my string picked up, down, in and out. The waveform looked about the same but different phase. I just realized that a question last week or so about the offset may have the answer in the second harmonic cancelling one peak and reinforcing the other.

[John Kolbeck]

There's actually a method for making consistent string plucks in such experimental situations which uses a loop of magnet wire (not sure the gauge) pulled to breaking. Just take a short length of wire, loop it under a repeated location on the string, and the consistency of the wire means that it will break at almost exactly the same tension every time, resulting in a consistent pluck.

That sounds like a really good method, I'll have to give it a try. Magnet wire has it's own tolerances that would transfer into the experiment, and I'm not sure how to identify what that tolerance is, as that figure is not always supplied by suppliers of the magnet wire. When I've done such tests, I just delete the outliers, and I bet the method will produce fewer of those.

Also, if anyone does something like this, be sure to pull the string in the direction that it tends to be plucked, and not straight up from the top of the string, as that creates a test condition that is somewhat dissimilar to actual plucking.

[Jason Rodgers]

As a side note, would pulling a magnet off the strings be a repeatable way to test this?

There's actually a method for making consistent string plucks in such experimental situations which uses a loop of magnet wire (not sure the gauge) pulled to breaking. Just take a short length of wire, loop it under a repeated location on the string, and the consistency of the wire means that it will break at almost exactly the same tension every time, resulting in a consistent pluck.

EDIT: google "wire break string pluck."

[John Kolbeck]

The electrical energy from the pickup must come from the kinetic energy of the strings. Does this mean if the pickup is unconnected that it is absorbing less energy?

That's an interesting question. In a loaded transformer, you'd have "back EMF" pushing against the original source the magnetic field. I wonder if a guitar pickup produces "back EMF" against the string, and how it might effect the string's movement.

[Justin Thomas]

I use my bridge pickup 70% of the time, and the neck + bridge in series 25%, and the remaining 2 positions for the last 5%. Yeah, I can get away with one pickup. It's more about construction and amp choice to me. Maybe why I'm so preferential to Bassman heads & big speakers...

Justin

[Mike Sulzer]

The electrical energy from the pickup must come from the kinetic energy of the strings. Does this mean if the pickup is unconnected that it is absorbing less energy? Some must also be lost to hysteresis. If you do go through with the experiment I'd be interested in finding out if my hypothesis is correct although I imagine that for all practical purposes the effect is negligible.

As a side note, would pulling a magnet off the strings be a repeatable way to test this?

I think the power supplied to the electrical circuit is a lot less than the power lost in the neck, body, string, and to the air. I would have to do the calculation again to be sure. One way to do this is first measure the decay time constant off the string, that is, due to all causes. Then from the pickup voltage and the load it operates into, you can compute the power (rate of energy dissipated). Then if you know the mass of the string and the amplitude of its vibration, you can compute its kinetic energy, and the compute the time constant associated with the loss due to driving the electrical circuit. Finally, you can compare the two time constants. I have done this very approximately in the past, and the result was that the time constant from the electrical loss is a lot longer than the measured time constant (decay due to all causes). But this whole process is a pita, and should be done again more carefully if a really good answer is needed.

[tedmich]

Just a player here but I did away with my neck pickup years ago, along with all tone controls and phase/coil tap switches. I have only a single volume knob and a kill switch on the guitar and my tone gets controlled at the amp. I miss running in stereo with two amps but that's it.

[Richard]

The electrical energy from the pickup must come from the kinetic energy of the strings. Does this mean if the pickup is unconnected that it is absorbing less energy? Some must also be lost to hysteresis. If you do go through with the experiment I'd be interested in finding out if my hypothesis is correct although I imagine that for all practical purposes the effect is negligible.

As a side note, would pulling a magnet off the strings be a repeatable way to test this?

[Mike Sulzer]

You often hear that it's a bonus to use weaker magnets because less string pull means less sustain, but do you think it's possible for there to be too little pull to cause "Stratitus", but yet enough pull to have a meaningful consequence on the sustain?

I do not know, but I would not make that claim without careful testing. I would also want to know why the magnetic field causes more energy dissipation in the string when the magnets are not strong enough to cause stratitis.

[John Kolbeck]

If the neck pickup has very (way too) strong magnets, then the "stratitis" that it causes is heard when the bridge pickup is used.

You often hear that it's a bonus to use weaker magnets because less string pull means less sustain, but do you think it's possible for there to be too little pull to cause "Stratitus", but yet enough pull to have a meaningful consequence on the sustain?

[rjb]

That's not a guitar.


That's a guitar.

With apologies to

[jack briggs]

Nah, needs another pickup.

cheers,