Is it possible to accurately describe the tone of magnets and/or pickups?

[Antigua]

Now wouldn't an alnico magnet also have internal eddy currents and therefore a damping action or is the material's conductivity too low for that to be an issue? Could different alnico formulations have different conductivities that might affect those eddy currents?

Here's a comparison plot I made, you can see how AlNiCo and steel compare to air core, in a plastic bobbin Strat pickup:



Note how AlNiCo 5 has a higher Q and yields a higher peak frequency than AlNiCO 2, 3 and 4, which differ must mostly in terms of conductivity, since the Q differs but the peak frequency is the same. Steel obviously has both a higher conductivity and permeability. This is in a "no load" context though, with a volume and tone pot in the mix the differences shrink. The AlNiCo 5 will have a peak resonance that is maybe only 1dB higher than AlNiCo 2/3/4, which in turn has a peak that is only 2dB higher than a steel core pickup. If ceramic were used as a core it can be expected that it would perform similar to air core.

Another take away from this is that the use of lower 250k pots in conjunction with AlNiCo pole pieced pickups, and higher 500k values for steel pole pieced humbuckers and P-90s, suggests that these values serve to put the Q factor of both types of pickups into a range that is neither too high or too low, like a balancing act. There are exceptions to the rule, but those are also less popular in the market place. Low cost Fenders use steel pole pieces instead of AlNiCo, and often pair them with 250k pots, which lowers a Q factor which is already made lower by the steel, so one way to get a steel pole ceramic Strat pickup to perform more like an AlNiCo pickup would be to use higher resistance control pots, although the inductance of those pickups is also greater by about 50%, and that's not something that is as easy to change.

[the great waldo]

Hi Mike
Can you give us simple folk (me) an explanation of your measuring setup please ? (If it's not to much hassle that is !)
Cheers
Andrew

[Mike Sulzer]

Hi Mike
Can you give us simple folk (me) an explanation of your measuring setup please ? (If it's not to much hassle that is !)
Cheers
Andrew

Sure. First, there is some discussion of the most recent use of this method here: http://guitarnuts2.proboards.com/thr...interface-unit.

I will stick to basics.

What do we want from pickup impedance measurements? Well, the resistance and inductance are the most important parameters, but you need the capacitance as well, since it contributes to the resonance under playing conditions even though the cable capacitance is usually more important. These parameters characterize the resonance, and this (including the cable capacitance, of course) are very important in determining the sound. However, that is not the whole story since metal parts influence the impedance in a frequency dependent way. It is hard to evaluate these effects when the impedance is dominated by the resonance caused by the inductance and pickup capacitance.

So if you measure the capacitance accurately, then it is possible to take out its effect on the impedance, and then the "eddy current" effects are easier to evaluate. All this requires computer code, and so the practical way to make such measurements is with a two channel (or more) recording interface, readily available and cheap. (Although I prefer the more expensive models that have digitally settable attenuators.)

Impedance is the ratio of voltage to current; we can measure two voltages referred to ground. So put a small resistor in series with the pickup and connect as shown below. The computer code finds the voltage across the pickup and the current through it. You could sweep the frequency, but I prefer a waveform that generates all frequencies at once, and then do the Fourier analysis.

Then you can use fitting techniques to find the C, take it out, and display the real and imaginary parts (or it could be amplitude and phase if you wanted), comparing them to the values that would be obtained without the eddy currents.

[the great waldo]

Sure. First, there is some discussion of the most recent use of this method here: http://guitarnuts2.proboards.com/thr...interface-unit.

I will stick to basics.

What do we want from pickup impedance measurements? Well, the resistance and inductance are the most important parameters, but you need the capacitance as well, since it contributes to the resonance under playing conditions even though the cable capacitance is usually more important. These parameters characterize the resonance, and this (including the cable capacitance, of course) are very important in determining the sound. However, that is not the whole story since metal parts influence the impedance in a frequency dependent way. It is hard to evaluate these effects when the impedance is dominated by the resonance caused by the inductance and pickup capacitance.

So if you measure the capacitance accurately, then it is possible to take out its effect on the impedance, and then the "eddy current" effects are easier to evaluate. All this requires computer code, and so the practical way to make such measurements is with a two channel (or more) recording interface, readily available and cheap. (Although I prefer the more expensive models that have digitally settable attenuators.)

Impedance is the ratio of voltage to current; we can measure two voltages referred to ground. So put a small resistor in series with the pickup and connect as shown below. The computer code finds the voltage across the pickup and the current through it. You could sweep the frequency, but I prefer a waveform that generates all frequencies at once, and then do the Fourier analysis.

Then you can use fitting techniques to find the C, take it out, and display the real and imaginary parts (or it could be amplitude and phase if you wanted), comparing them to the values that would be obtained without the eddy currents.

[ATTACH=CONFIG]50117[/ATTACH]

Hi Mike
Thanks for the reply. I have a Roland Rubix24 An old Kenwood oscillator and a syscom usb oscilloscope and a couple of LCR meters. What software are you using for the measurements ? I have some knowledge of audio but it's more practical than theoretical.
Cheers

Andrew

[Helmholtz]

Eddy current losses in alnico are low because conductivity is relatively low. This is why strat PUs show a high (unloaded) resonance peak.

I collected some data from magnet literature regarding the specific resistance of some alnico grades. There is some variation (+/-10%) between different sources. Below are average numbers:
A2: 65 (µOhmcm)
A3: 60
A4: 70
A5: 50

Higher specific resistance means lower losses.
Eddy losses also depend on the magnet volume exposed to the AC field, i.e. bigger magnets produce more losses.

The values above are from old (mostly american) magnet literature and correspond to the original/standardized alnico formulations.
I don't know, if they apply to "modern"/ far-east reproduction types as well.

For comparison some steel data:

416 stainless: 58
417 stainless: 69
Carbon steel : 17
Low carbon steel (1010): 12..14
Pure iron : 10

[David King]

Helmholtz, any numbers for leaded steels like 12L14?

[Helmholtz]

Helmholtz, any numbers for leaded steels like 12L14?

Sorry, no.
But I have 1215: 17.4


Edit:

Easy find with Google:
http://www.noblefix.com/pdfs/mechanical_properties.pdf
i.e. values for 1215 and 12L14 are the same.

[Possum]

Its not possible to describe the tonality of any alnico magnets. Why? Because no two alnico manufacturers use the same formulas. So you think alnico 2 is warmer, less trebly? Nope. I have different company A2 magnets that are super bright and really high gauss readings when hot charged. Its the same for all the alnicos, A4 from one company won't sound like A4 from another company. This is why its good to buy from many different manufacturers, you get so many different flavors. When I choose a magnet for a certain design, I try ALL my alnicos in that design and find one that sounds the best in that particular model. It also depends on physical dimensions. The older PAF/P90 magnets were all thick and 2/5" long. Mojo for example, sells all their bucker/P90 magnets in a shorter dimension than anyone else. Alnico is mostly IRON. And iron sucks up treble, so the more massive a magnet is, the less brightish it will be. Some TTop magnets were super thin and short, those things are really shrill. Keep a good stock of various company's products and use where it works.

[Mike Sulzer]

In a humbucker the steel is high loss and inside the coil; therefore it has a much larger effect than the magnet, which is lower loss and outside the coil.

Its not possible to describe the tonality of any alnico magnets. Why? Because no two alnico manufacturers use the same formulas. So you think alnico 2 is warmer, less trebly? Nope. I have different company A2 magnets that are super bright and really high gauss readings when hot charged. Its the same for all the alnicos, A4 from one company won't sound like A4 from another company. This is why its good to buy from many different manufacturers, you get so many different flavors. When I choose a magnet for a certain design, I try ALL my alnicos in that design and find one that sounds the best in that particular model. It also depends on physical dimensions. The older PAF/P90 magnets were all thick and 2/5" long. Mojo for example, sells all their bucker/P90 magnets in a shorter dimension than anyone else. Alnico is mostly IRON. And iron sucks up treble, so the more massive a magnet is, the less brightish it will be. Some TTop magnets were super thin and short, those things are really shrill. Keep a good stock of various company's products and use where it works.

[Possum]

In a humbucker the steel is high loss and inside the coil; therefore it has a much larger effect than the magnet, which is lower loss and outside the coil.

You're forgetting that all the metal parts including the magnet are all directly in contact with eachother. Alnico and "alni" are somewhere around 75% or more IRON. The steels used in pickups are largely iron with some carbon content. In a humbucker or P90 those magnets are large chunks of iron. So the metallurgy of the magnets is a big piece of the magnetic circuit and its formula has a very noticeable effect, electrically on the sound of the coils. Proof of this is if you swap in a ceramic magnet that has no iron content, just hook up your LCR meter and watch those Henries drop like a rock when ceramic magnets are used because ceramic is nonconductive, non-metallic. All the metal parts including the baseplate are electrically connected and losses are from the entire magnetic circuit parts. This is why if you change a single part, a slug, or a screw, to say something with high carbon it changes the sound of the pickup. But as I said in my reply, no two factories make the same kinds of alnico, there isn't a universal recipe being used. So, if you have two radically different alnico magnets, each one is going to make the pickup sound different, and each magnet could be called "alnico 2" but have completely different tone signatures. A really brittle sounding magnet will far outshadow the metallurgy in the ferrous parts. This is where the "fun" is. Change a single part, change the sound. In vintage PAF's there are 4 different alloys (magnet included). Play with those and now you have a design tool ;-)

[Helmholtz]

Proof of this is if you swap in a ceramic magnet that has no iron content, just hook up your LCR meter and watch those Henries drop like a rock when ceramic magnets are used because ceramic is nonconductive, non-metallic.

If you remove the alnico magnet of a humbucker, the inductivity drops around 6..8%. The PU's L difference between different alnico grades charged to similar Gauss values is 1..2%.
If Henries drop for more than 8% when exchanging a ceramic magnet for alnico, I suppose the ceramic was much stronger than the alnico. Strong magnets reduce the µ of the ferrous core parts causing an additional drop of L.

While the iron content of alnicos lies between 50% and 60%, their µ is only 5..8. Low carbon steel is 200...500, lower carbon (and other "impurities') content resulting in higher values.
The µ of ceramic magnets is close to 1.

[Possum]

I would ignore the inductance readings and use AC resistance readings at 1khz. I always found this much more useful than Henries. I seem to remember than putting a base plate on a Tele bridge raised the AC resistance while henries went down, so inductance is less intuitive for pickup designers. In pickups, everything affects everything else, just like in guitar amplifiers where everything affects the whole.

[Helmholtz]

I would ignore the inductance readings and use AC resistance readings at 1khz. I always found this much more useful than Henries. I seem to remember than putting a base plate on a Tele bridge raised the AC resistance while henries went down, so inductance is less intuitive for pickup designers. In pickups, everything affects everything else, just like in guitar amplifiers where everything affects the whole.

Well you talked about Henries, not AC resistance. The differencies in AC resistance caused by different alnicos are even lower than for inductance.

[Mike Sulzer]

The fact that the metal parts are in contact is irrelevant because the overall magnetic circuit is dominated by the huge air gap, and thus it is a wide open circuit without any possibility of interpretation similar to a transformer or other closed magnetic circuit. Your idea that an ally containing a significant amount iron must act like iron is just plain silly. You do not know metallurgy from witchcraft. I also think you are mentioning measurements that you have never made. I know you can fool some guitarists with this kind of talk, but you cannot fool everyone.

You're forgetting that all the metal parts including the magnet are all directly in contact with eachother. Alnico and "alni" are somewhere around 75% or more IRON. The steels used in pickups are largely iron with some carbon content. In a humbucker or P90 those magnets are large chunks of iron. So the metallurgy of the magnets is a big piece of the magnetic circuit and its formula has a very noticeable effect, electrically on the sound of the coils. Proof of this is if you swap in a ceramic magnet that has no iron content, just hook up your LCR meter and watch those Henries drop like a rock when ceramic magnets are used because ceramic is nonconductive, non-metallic. All the metal parts including the baseplate are electrically connected and losses are from the entire magnetic circuit parts. This is why if you change a single part, a slug, or a screw, to say something with high carbon it changes the sound of the pickup. But as I said in my reply, no two factories make the same kinds of alnico, there isn't a universal recipe being used. So, if you have two radically different alnico magnets, each one is going to make the pickup sound different, and each magnet could be called "alnico 2" but have completely different tone signatures. A really brittle sounding magnet will far outshadow the metallurgy in the ferrous parts. This is where the "fun" is. Change a single part, change the sound. In vintage PAF's there are 4 different alloys (magnet included). Play with those and now you have a design tool ;-)

[jack briggs]

The fact that the metal parts are in contact is irrelevant because the overall magnetic circuit is dominated by the huge air gap, and thus it is a wide open circuit without any possibility of interpretation similar to a transformer or other closed magnetic circuit. Your idea that an ally containing a significant amount iron must act like iron is just plain silly. You do not know metallurgy from witchcraft. I also think you are mentioning measurements that you have never made. I know you can fool some guitarists with this kind of talk, but you cannot fool everyone.

there is some truth to what he is saying - you can't fool my ears......