Polepiece effects on tone

[Chill]

What characteristics of polepieces have what effects on tone? E.g.:
-longer/ shorter screws
-larger/ smaller diameter polepieces
-larger/ smaller screw head
-slugs vs screws

[hewo]

isn't the whole pickup suppose to determine the overall sound?
the view aperture is created in part by the pole pieces.
variation includes the curvature blade, knifelike thin, or thick like the melodymaker alnico blade without fingerboard radius of curvature.

the more focused the aperture, the more accurate the signal should resemble string vibration, and, i think it can tolerate higher string velocity speeds, meaning freqencies.

in the limit, if we downsize the polepiece to a diameter of zero, the sensing aperture becomes a point on the string. the signal would quantized at that limit, go-no-go, wouldn't it?

[Ruel]

I could be wrong but I think the size of the diameter of the pole pieces and the amount of magnetic material they contain would affect the overall characteristics of the tone of the pickup more than the length of the pole pieces.

[David Schwab]

I think there's so many variables, it's hard to make a definite correlation between each parameter and its effect... you change one aspect and it changes two...

For instance... pole pieces are both the conductor of the magnetic circuit and also the inductive core of the coil. So a fatter pole piece or blade is not only making a larger flux aperture around it, but also increasing the inductance.

Then you have the effect of counter emf, or eddy currents which form in metal parts on the pickups, including the poles. These currents produce their own magnetic fields that oppose the main field. This could cause a loss of highs... and maybe not. I think this is why some pickup makers use real thin blades, like the original Lawrence pickups. Bartolini used to make pickups with laminated steel blades, like that used in transformer cores to reduce eddy currents.

In Bill Bartolini's two patents he discusses the shapes of the pole pieces and how they sense the motion of the string. He says round poles get a plucky tone because they pickup the side to side and up and down motion equally. He had originally used flat, square, or parallelogram poles, and he claims they pick up more of the up and down motion, the way an acoustic guitar sound board works.

I see many attempts to focus the magnetic field in pickups... even Leo Fender added magnetic guides to his pickup designs after he left Fender... and of course Lace does the same thing. My thoughts are that any extra flux that's not sensing the strings is being wasted, so I would think having very high, wide fields don't add much to anything... but I haven't done any testing myself. I stop when I get a pickup design I like!

Then of course I'm sure different metals sound different too.

[hewo]

that was very informative. thanks for the explanations.

some improvements that technology has brought us would be reductions of inductances by materials, neodymium samarium cobalt & oxygen free copper.

i haven't seen any of these materials in pickups yet, maybe they are but unannounced.

the intent is to reduce inductance as well as irreversibilities, parasitic energy losses, e.g., the eddy currents previously discussed.

what i am wondering about is whether or not you physically need to have irreversibilities in order to have any signal at all. e.g., if we do not have impedance, then we can not have a signal generated.

[Chill]

Thanks for the info...I think someone is using neodymium in a pickup, but it's a non-standard design. Neo might also be good for an airbucker type design because it's field is so strong.

Say we have a PAF or a Duncan 59, what would the effect on tone be if we,

Change the polepieces to 1/2" screws instead of the 3/4" screws?

Use two slug coils?

Change the polepieces to wider diameter screws (with boring out the frames, etc...not something I'd do, but for the purposes of discussion)?

Change them to headless screws like set screws?

Change them to screws with heads larger than fillister heads?

[David Schwab]

that was very informative. thanks for the explanations.

some improvements that technology has brought us would be reductions of inductances by materials, neodymium samarium cobalt & oxygen free copper.

i haven't seen any of these materials in pickups yet, maybe they are but unannounced.

Fender has some pickups designed by Bill Lawrence that use samarium cobalt, as do the Q-Tuner pickups.

I've seen a few pickup makers using neo magnets, and they swear by them. I haven't tried them out myself yet.

the intent is to reduce inductance as well as irreversibilities, parasitic energy losses, e.g., the eddy currents previously discussed.

what i am wondering about is whether or not you physically need to have irreversibilities in order to have any signal at all. e.g., if we do not have impedance, then we can not have a signal generated.

You don't need to have a metal core in a pickup at all. Lawrence made air core pickups for acoustic guitars. Then you take Alembic for instance, they wind their pickups directly on ceramic magnets. Their pickups are just two pieces of copper clad printed circuit board with a ceramic magnet sandwiched between. They wind about 1500 tuns of 40 ga wire, and that's it! They are stacked pickups, with the bottom coil having a plastic core.

Of course they are low impedance pickups that require preamplification. Obviously Alembic pickups must have pretty low inductance.

I've seen some patents by DiMarzio where they are adding material to increase inductance... so I guess it's all about the tone you are going for!

As long as you have a coil and a magnet, you should get a signal.. the strength of the signal is another matter of course. Ribbon mics don't have coils, but instead a ribbon of copper or aluminum is hung in the field of a strong magnet. The ribbon has wires on each end. When sound makes the ribbon vibrate, it produces current. This ribbon is not magnetic, but still produces current because of the eddy currents produced.

The Lace Transsensor and Alumitone pickups also have no coil windings, except for a step-up transformer. I think the Villex pickups must work the same way, since he invented the Lace versions.

More ways to skin a cat?

I bet there's a lot of stuff yet undiscovered.

[hewo]

if the vacuum tube preamplifier input impedance is 1 megaohm, then we can settle basically our passive pickup performance specs. this excludes any intra preamplifiers or buffers.

based upon said amplifier's reception of the signal, wouldn't we specify the pickup as being equal to or less than 1 megaohm output impedance?

of course given that over the range of passive tone and volume impedance changes that realistically occur in the guitar, the pickup's impedance generally does color the guitar frequency spectrum. just like a speaker, it exhibits colorations. impedance is not constant over the spectrum.

analogously, the same can be said of inductance.

i think the trick is to at least generate enough frequencies. you can throw away what you already have, but you cannot throw away what you don't have. if you don't have highs, you can't throw them away. neither will your amp ever receive the highs. the same can be said of lows, although throwing them away passively requires tuned coils, inductors, which significantly add to parasitic losses.

this means the pickup designer and maker knows just about how much impedance and inductance that can coexist mutually to provide the spectrum of frequency generation. and part of that design making involves the thesis topic, polepieces. it appears that historically, the most sought after designs have not altered their specs, probably for darn good reasons. every part of the design in the pickup works. can we say it's optimized?

[ken]

A guitar's impedance is set more by the pots than the pickups, because you can
put higher value pots in the guitar, and as the pot value goes 'up', the guitar's tone gets brighter because of better impedance matching.

Ken

[hewo]

i tried switching from a half meg to a full meg, the difference was loudness and greater resolution for lighter volumes using the full meg. but the high pass bridge cap becomes invariantly sensitive in the lighter volume resolution regime. so i switched back to half meg. the sound thickens substantially for any pot value after exceeding a third of full scale.

[Luijo]

The reason why guitars sound brighter with bigger pots is not because of better Z matching. It is because there are less losses to ground with bigger pots than there are with smaller pots. Think of it this way, the signal has a harder job leaking to ground and an easier way to get to the amp when you have a path of very high resistance to ground than when you have a path of less resistance to ground. That's why some manufacturers offer a switch that allows you to bypass volume and tone controls altogether...less losses.
I read somewhere that's the reason why EVH just used a single volumen control in his guitars.
Impedance matching for audio porpuses is best when a very low Z signal (like the one at the output of buffered pedals) feeds a very high Z signal (like the input of an amp). Keep in mind that this not applies when we deal with power amplification, in which case you DO need to have equal impedances at both the source (amplifier) and the load (speaker).

[hewo]

think of the pot's wiper as the input signal fed to the vacuumtube preamp. when the wiper is zeroed the vacuumtube preamp's 1 megaohm input impedance is receiving a short to ground, zero impedance, by the wiper.

as we dial volume upwards, the preamp will be fed the combined impedance resultant of the pot's low side to ground, the pot's high side thru the coil impedance then to ground, including this same analogy for the tone pot if present. the tone pot high side usually is open ended, wiper is at same potential as preamp, and, the low side is high pass cap earthed.

as i earlier stated, for usual log taper, the 1st third range varies invariantly compared to the latter two thirds of range. this permits fine resolution control of soft volumes. i set the amp controls with capacity and merely dial the guitar in for needs.

comparing the full meg versus the half meg pot, if there's no high pass bridge cap for the volume pot, the incoming low frequencies entering the pot's high potential tab have a significantly easier time getting thru the carbon track and out to the wiper. the high frequencies will experince a significant attenuation before reaching the wiper tab. that's why the high pass cap is needed as a detour from high potential tab to the wiper tab. the reason why the low frequencies have the advantage of penetrating the carbon track is explained by visualizing low frequency, in the limit, as DC, not AC.

the tortuosity of carbon track passage is significantly greater for high frequencies as the pot's value is increased, the 1 meg versus 1/2 meg pot.
that is why the high pass cap detour route engages because it easily circumvents the normal tortuous path of carbon track. for higher value pots, the result is an over-sensitized high frequency route, and the resultant sound we hear appears over-brilliant.

by downgrading the pot's value, you are essentially allowing a proportional and favorable blended mixture of low frequency strength and high frequency strength, via two different routes to get to the wiper tab.

[David Schwab]

if the vacuum tube preamplifier input impedance is 1 megaohm, then we can settle basically our passive pickup performance specs. this excludes any intra preamplifiers or buffers.

based upon said amplifier's reception of the signal, wouldn't we specify the pickup as being equal to or less than 1 megaohm output impedance?

The input impedance and the output impedance are two separate things.

Input impedance is analogous to having a resistance across the input to ground, even if there isn't an actual resistor in the circuit. This resistance shunts part of the signal away to ground, thus diminishing the signal's strength.

Output impedance is analogous to having a resistance in series with the output of your device. This also diminishes the signal strength.

A pickup has an output impedance, which can be thought of as the DC resistance. In actual practice the impedance changes with frequency, generally getting higher with higher frequencies. But for now we will say our output impedance is 8,000 Ohms. This is a high impedance device.

So what happens when you take a high-Z pickup and connect it to a low-Z input? You are no longer getting all the signal from the pickup, but are picking up the signal from at the junction of two resistances, the output impedance of the pickup, and the input impedance of the amp. So in essence you have a voltage divider, just like a potentiometer. So for maximum transfer of signal, we want as high a resistance as possible between the signal and the ground (input impedance) and as low a resistance as possible between the signal and the amp (output impedance of the pickup).

So if the output impedance is higher than the input impedance, much of the signal will be shunted to ground. But if the input impedance is much higher than the output impedance, very little signal will be shunted to ground. If our output impedance is getting higher with higher frequencies, we will lose high frequency response. We can see that pickup with high DC resistance tend to lose highs.

Ideally the input impedance should be about 10 times that of the output impedance. So for an 8K pickup, you want at least 80K input impedance... except now we have things like cable capacitance creeping in, and the higher the output impedance, the more effect a small capacitance will have on the signal. And the higher the input impedance the more noise you pick up.

i think the trick is to at least generate enough frequencies. you can throw away what you already have, but you cannot throw away what you don't have. if you don't have highs, you can't throw them away. neither will your amp ever receive the highs. the same can be said of lows, although throwing them away passively requires tuned coils, inductors, which significantly add to parasitic losses.

This is where lower impedance pickups really shine. Or at the very least, by buffering the pickup with a very high input impedance device, like a FET, you retain the actual frequency range of the pickup more than with a totally passive system. This isn't always the tone guitar players are accustomed to however, but it works real well with bass.

[hewo]

agree, the fet or buffer can isolate the lickup from being loaded down. the sound gets too clean and clear for guitar.

but the vacuumtube essentially does the same as the fet, it buffers the incoming guitar signal from the downstream preamplifier stages.

so why the preference for having a vacuumtube instead of a fet for doing the same thing, buffering the pickup from downstream preamp stages that are not high impedance circuits?

or, can fet's be used to achieve the identical result of the vacuumtube? it must have something to do withthe coloration frequency response.

[ken]

FET's and tubes have the same very high input impedance, so they react 'almost' alike. Tubes are 'warmer' than FET's are because of interelectrode
capacitance effects, among other things. Each interelectrode capacitance drains off a tiny bit of treble, and if a tube has many parts, the effects add up.

Ken