Pickup Inductance Change with Ferrous vs. Non Ferrous Bridge

[tdowns]

After some discussion about ferrous vs. non-ferrous bridges here, I immediately planned to take some measurements.

I indicated in the post above that a ferrous object in the lines of flux of a ferrite core inductor should increase its inductance. Also, in most cases a non-ferrous metal in the flux should slightly lower the inductance slightly because of eddy currents. The real question is how much. Well, I was somewhat surprised how LITTLE the inductance changed with the ferrous vs. non-ferrous bridge.

Inductance meters are not very good for measuring guitar pickups because there is so much DC resistance in the windings. The reactance of a 3.3 henrys at 1kHz is ~20k-ohms, so a 7.4k-ohm DC resistance will cause a big error in the reading. The common inductance meter will however suffice for relative measurements.

I used a standard measurement technique, which is simply a audio signal generator connected to the pickup with a 10k-ohm series resistor. Knowing the voltage of the signal generator AND the voltage across the pickup AND the DC resistance of the pickup, you can calculate the inductance of the pickup. I started by taking measurements at 1kHz on a MIM Tele bridge pickup and calculated the standard 3.3 henries.

The chart below shows the voltage across the pickup at various frequencies with the ferrous bridge, non-ferrous bridge, and bare pickup. At each frequency, the lower voltage means lower inductance. The lower the frequency, the lower the voltage, because it is inductive.



Notice that in each case, the voltage is higher with the ferrous bridge. The higher voltage means it is more inductive. The change is not measurable at low frequencies. Notice the whopping 3.62% change at 1kHz. You will never hear that.

I also have an inductance meter, but it operates at 100kHz at the 20 henry range. The reading is way off as expected. But, for grins I took measurements anyway. The inductance meter readings are as follows:

Bare – 6.22H
MIM brass bridge – 6.21H
Ferrous bridge – 6.25H (a 0.6%) increase.

So my conclusion is the ferrous vs. non-ferrous bridge has negligible effect on the inductance of the pickup. This stands to reason since its overall physical size relative to the flux pattern is small (40-90 mils thick). In the post linked above, Twangbanger discussed some inductance changes he made with ferrous materials, but I believe he was wrapping the inductor with high permeability material. As he stated, that would make a bigger difference. It would have a much greater affect than a thin steel plate in the flux lines (like a Tele bridge).

It is possible the shape of the flux lines, as distorted by the metal, may change the sound. However, it certainly does not appear to be affected by an inductance change.

This is not intended to degrade any product line. It is just some measurements that show the property of "pickup inductance" has negligible change with the bridge metal.

FWIW. :D

[Jack Wells]

Good job Terry ............... I think ............. I used to understand that stuff but that was back in the '60. Didn't use it ............... forgot most of it.

[tdowns]

Thanks jwells. What is your experience with ferrous vs. non-ferrous?


I forgot one measurement. I stuck the pickup to a nearby small machinist vise and it raised the LC meter reading up to 6.79H (a 9% increase from the bare pickup).

[Roli]

HAHAHA, Terry, I bow to your knowledge! Very interesting test again. So the question is: can the human ears (and brain) hear these tiny differences or is it just delusion?

Hmm... or is it the saddles that make the difference in tone (if any)?

[BrianF]

I vote for the saddles / bridge mass and type of metal used that creates the most tonal difference.

[tdowns]

Originally Posted by Roli

HAHAHA, Terry, I bow to your knowledge! Very interesting test again. So the question is: can the human ears (and brain) hear these tiny differences or is it just delusion?

Hmm... or is it the saddles that make the difference in tone (if any)?

To answer your question, 3.62% change in inductance would not be noticable at all.

I surmize the saddles make the bigger difference. If the ferrous vs. non-ferrous bridge changes the tone, it does not appear to be because the metal has changed the inductance of the pickup (according to my experiments). Perhaps it distorts the flux lines and makes it sound different.

I'm not saying there is no sound difference in ferrous vs. non-ferrous. I'm just saying it does not appear to affect inductance.

[alheeley]

Thats interesting stuff, it confirms my 'gut' feeling for this, yet people wander off into the realms of the mystic at times when it comes to magnetic flux and such issues. I'm all for outing the bogus mystics, give me some good hard facts and proper investigations, then my guts can rest easier :P

[fullerplast]

Originally Posted by tdowns

If the ferrous vs. non-ferrous bridge changes the tone, it does not appear to be because the metal has changed the inductance of the pickup (according to my experiments). Perhaps it distorts the flux lines and makes it sound different.

I'm not saying there is no sound difference in ferrous vs. non-ferrous. I'm just saying it does not appear to affect inductance.

This agrees with what I suggested in the previous thread. Thanks for taking the time to conduct this experiment. Well done Terry!

Originally Posted by fullerplast

Inductance doesn't change due to the bridge material; it's a function of the number of turns of wire, size of the coil, and wire size, but not the magnetic field.

The pickup works because of what is called magnetic reluctance, which is the ratio of magnet force to magnetic flux. Magnetic reluctance is in turn affected by what is called permeability. Permeability tells you how much a material is affected by a magnetic field. This is where the bridge plate comes into play. You are slightly changing the way a magnet responds to string movement by altering the magnetic field of the pickup. The strings are vibrating in this field and generate an AC signal in the coil so any change in the field will affect the signal. The field will be affected by the bridge pickup plate, as well as the bridge itself, if it is magnetic. Inductance is not affected by any of this however.

All that said, use your ears. There will be a difference. Which bridge you prefer tonally (or even if you actually hear a difference), is completely subjective.

[Kelsey]

Wow, a scientific approach to a tone question at the TDPRI! Now that's not something you see every day. Nice work!

[stephent2]

Thanks Terry, great work.

[tdowns]

Thanks for the responses.

Originally Posted by fullerplast

Inductance doesn't change due to the bridge material;

That is not what data shows. Inductance DOES change with bridge material, its just not enough to matter.


I think what you said below is key to the whole topic, and a better overall explaination for the differences we hear.

Originally Posted by fullerplast

You are slightly changing the way a magnet responds to string movement by altering the magnetic field of the pickup. The strings are vibrating in this field and generate an AC signal in the coil so any change in the field will affect the signal. The field will be affected by the bridge pickup plate, as well as the bridge itself, if it is magnetic.

...as well as an ashtray if used.

[yegbert]

Originally Posted by tdowns

...as well as an ashtray if used.

...and with a bridge pickup baseplate, and a magnetic neck pickup cover, right?

[fullerplast]

Originally Posted by tdowns

That is not what data shows. Inductance DOES change with bridge material, its just not enough to matter.

For a coil with no core, the magnetic field will have no effect. With a core, as we both mentioned earlier, the field will have an effect. Pickup polepieces are kind of a special case, it's not really a core and it's not really free space either. In addition to all this, you have some capacitive components between the insulated wire. Just looking at your data, you can see that there's more going on. Your voltage is not increasing in a linear fashion with frequency, as it would with pure inductance (X=2*pi*f*L). It's rising to a peak and then actually decreasing with frequency.

Accurately modeling the pickup inductance, capacitance, and pure resistive components, taking into account the pseudo-core properties of the polepieces in the presence of a magnetic field would make somebody a very interesting project! As you seem to have verified, the pickup seems to be acting closer to a coil in free space than a coil with a core. I agree with you that any change in inductance due to the effect of the polepieces as a core is essentially negligible.

[tdowns]

Fullerplast,
I totally agree with all you said above. You can see a resonance peak because whatever winding and parasitic capacitance exists. Some of the non-linearity could be an effect of eddy currents too.

[Big Mike Blues]

Wow. Thanks Terry. That's extremely cool.

Now we can simply argue about saddles and mass. LOLOL

Seriously, great info. Much appreciated.

[dividedsky]

Found this during a search. Certainly worthy of a bump. Thanks!

[Smokestack]

Posts like your, TDowns, make for great reading. I really enjoyed that. :)

[Smokestack]

I just came back to this for a second read. Answered all my questions and negated many assumptions. It is REALLY a surprise to me that the inductance doesn't change much. I'm restoring a 1941 Philco floor model radio and was thinking about the way the output coil sits on the chassis... then I remembered this thread. Once again, THANK YOU FOR YOUR RESEARCH, TDOWNS! Just one more reason to love this forum. :)

[tdowns]

Thanks for the kind post. I guess I was a bit surprised too. But when I think of how sensitive a BFO is in a metal detector, it makes sense.