Blade pole pickups: how does blade thickness affect tone?

[shawnl]

In my exploration of pickup winding, I've been trying to wind a collection of different sized coils with a variety of shapes. Particularly, in the case of blade pole pickups, I started thinking about how the thickness of the blade would influence the tone. I would think that the thickness the blade obviously determines the shape of the coil. So if the blade is thicker, the wider the coil has to spread out which would probably affect the resistance. At the same time however, wouldn't a wider blade sense a larger area of the string making it more sensitive?

So I've been playing around with a design and I'm curious if others have explored this or can stop me from wasting my time if this doesn't work. This pickup will have two 1/16 inch thick blades with a 1/4 inch gap between them. Both blades will be charged by the same magnet with the same polarity. My thoughts are that this will allow me to increase the sensing area but reduce weight by having a hollow core. In turn, this blade arrangement will create more of a rectangular coil which will use more surface area and need less windings. Can anyone predict what sort of tone this will yield?

[David Schwab]

In my experience a thicker blade sounds smoother. This probably has to do with the load on the coil as well as eddy currents, but it might also have to do with the width of the blade to a lesser extent.

As an example take a Bill Lawrence L-500XL and then a DiMarzio X2N. The L-500 is very bright and piercing, while the X2N is fat and bassy.

[Joe Gwinn]

In round numbers, eddy current loading varies as the square of the blade thickness. This is why lamination works.

[Mike Sulzer]

In round numbers, eddy current loading varies as the square of the blade thickness. This is why lamination works.

This is an extremely poor approximation for pickups, or any device that operates over some reasonable frequency range. What you are describing is the low frequency limit, suitable for power transformers. There is a high frequency limit in which eddy current loading does not vary at all with the thickness of the laminations. The problem is the skin effect, and the high frequency limit is reached when the frequency approaches that of total shielding. See, for example, equation 41 in

HYSTERESIS AND EDDY-CURRENT LOSSES OF
A TRANSFORMER LAMINATION VIEWED AS
AN APPLICATION OF THE POYNTING THEOREM
byJohn Barranger
Lewis Research Center Cleveland, Ohio
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON, D. C. NOVEMBER 1965





Hysteresis and eddy-current losses of a transformer lamination ...



The lamination thickness is not in the equation for the high frequency limit. The results I showed in the latest discussion of the effects of eddy currents on pickup impedance show that the frequency range of pickups includes the complicated region in the middle. You should not expect to be able to relate the predictions of the low frequency limit with what you hear when varying the blade thickness or using a laminated blade. It is much more complicated than that.

[Joe Gwinn]

This is an extremely poor approximation for pickups, or any device that operates over some reasonable frequency range. What you are describing is the low frequency limit, suitable for power transformers. There is a high frequency limit in which eddy current loading does not vary at all with the thickness of the laminations. The problem is the skin effect, and the high frequency limit is reached when the frequency approaches that of total shielding. See, for example, equation 41 in

HYSTERESIS AND EDDY-CURRENT LOSSES OF A TRANSFORMER LAMINATION VIEWED AS AN APPLICATION OF THE POYNTING THEOREM
byJohn Barranger
Lewis Research Center Cleveland, Ohio
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON, D. C. NOVEMBER 1965


Hysteresis and eddy-current losses of a transformer lamination ...


The lamination thickness is not in the equation for the high frequency limit. The results I showed in the latest discussion of the effects of eddy currents on pickup impedance show that the frequency range of pickups includes the complicated region in the middle. You should not expect to be able to relate the predictions of the low frequency limit with what you hear when varying the blade thickness or using a laminated blade. It is much more complicated than that.

Look at equation 40, where a is the lamination thickness, and no assumption is made about relative skin depth. Equation 41 assumes that the lamination is far thicker than the skin depth, which isn't generally the case for blade pickups around the fundamental frequencies of guitar strings. Most of the energy is in the lower harmonics. The whole point of laminating is to ensure that the laminations are far thinner than the skin depth, precisely to reduce eddy currents, and while 50/60-Hz power transformer laminations are no thinner than 0.014" or so, wideband audio transformers often use far thinner laminations. (Audio transformer design is a black art.)

But, the general point is valid, that the square-of-thickness is simply a rule of thumb, and one must also do experiments and listen.

[Mike Sulzer]

Look at equation 40, where a is the lamination thickness, and no assumption is made about relative skin depth. Equation 41 assumes that the lamination is far thicker than the skin depth, ...

This is not correct. Equation 38, the complicated one, is for the general case. Both eqs. 39 and 40 are the low frequency limit, but expressed differently.

...which isn't generally the case for blade pickups around the fundamental frequencies of guitar strings. Most of the energy is in the lower harmonics.

The higher harmonics are very important, and huge changes in the sound result from modifying them. Remember the discussion on reducing them when sampling the string as a humbucker does? (http://music-electronics-forum.com/t26893/) A broad dip with a null at the 34th harmonic on the 6 string is a large effect. Theory, measurement, and listening used together show the validity of this. Remember also that certain kinds of bass playing require pickups (and a sound system) that have extended high frequency range, perhaps well past 5KHz.

The whole point of laminating is to ensure that the laminations are far thinner than the skin depth, precisely to reduce eddy currents, and while 50/60-Hz power transformer laminations are no thinner than 0.014" or so, wideband audio transformers often use far thinner laminations. (Audio transformer design is a black art.)

The major point of laminations is to reduce the voltage around conducting loops by making large loops non-conducting, thus reducing the potential that drives the eddy currents. Skin effect certainly can matter, as this paper shows.

But, the general point is valid, that the square-of-thickness is simply a rule of thumb, and one must also do experiments and listen.

I have to disagree with the validity of your point, and state that unless the theory is developed far past what the low frequency limits implies, one can only experiment and listen.

[dor baruch]

the The major difference between the x2n and bill is the wire and magnets the blade are jast for Adjust the funel resolt
so you cant Take into account only the thickness of the blade also the Material from which they are made is a big factor on the sound
the bill l500l use 1 magnet and the x2n or gibson l500l use 3

[David Schwab]

the The major difference between the x2n and bill is the wire and magnets the blade are jast for Adjust the funel resolt
so you cant Take into account only the thickness of the blade also the Material from which they are made is a big factor on the sound
the bill l500l use 1 magnet and the x2n or gibson l500l use 3

I used them as an example because they are wound about the same. The extra magnets would make the pickup brighter, but in this case it's darker.

Also, I have tried this myself, with different blades. I have taken an X2N and replaced the blades with thinner ones, and the pickup sounds much cleaner and brighter.

The L-500L has 4 magnets which pretty much amount to two magnets:

[bbsailor]

Look at equation 40, where a is the lamination thickness, and no assumption is made about relative skin depth. Equation 41 assumes that the lamination is far thicker than the skin depth, which isn't generally the case for blade pickups around the fundamental frequencies of guitar strings. Most of the energy is in the lower harmonics. The whole point of laminating is to ensure that the laminations are far thinner than the skin depth, precisely to reduce eddy currents, and while 50/60-Hz power transformer laminations are no thinner than 0.014" or so, wideband audio transformers often use far thinner laminations. (Audio transformer design is a black art.)

But, the general point is valid, that the square-of-thickness is simply a rule of thumb, and one must also do experiments and listen.

Joe,

See the laminations used in the Edcor RMX1 ribbon microphone transformer. EDCOR - Nickel Lamination . It is made from Super "Q" 80% Nickel with a lamination thickness of 0.014" for a specification of 20 to 20KHz frequency response. Using the Extech 380193 LCR meter, the primary has a 14.9mh inductance and an R of 6.29 ohms at 120 Hz about 0.1 ohm DCR (unloaded secondary). The secondary is 19.6H with an R of 7.765K ohms at 87.8 ohms DCR, also at 120 Hz (unloaded primary). Obviously, the loaded values will be lower. This transformer is designed to be used with a ribbon DCR of less than 0.4 ohms (Edcor Specification) but it will be typically be about 0.1 to 0.2 ohms DCR to get the output in the 150 to 300 ohm range and accomodate a bridging microphone mixer impedance of about 2.5K ohms. So I guess input transformer design is a "black art".

Ric Turner reported that using 7/44 litz wire that he made pickups where could "hear his fingerprints on the strings". So I guess that eddy currents in/or between the actual wire itself may enter the analysis picture in addition to eddy currents in pickup cores to account for additional upper frequency tonal color.

Thanks

Joseph Rogowski

[dor baruch]

WTF is this a original l500l pickup David Schwab all 10 of my original look Quite different
they have only one magnet and epoxy in them
This is aruined pickup that someone gave me but all the l500l i have built this way

[David Schwab]

Wow, I never saw one with such a small magnet! I have seen them with one magnet, but it's a big magnet like this:




I guess Bill changes how he does things from time to time. Anyway, a larger magnet, or more magnets will make the pickup louder and brighter. That is not the case with the X2N. The thick blades give it a darker tone.

BTW it's polyester resin in the pickup, and not epoxy.

[Mike Sulzer]

Ric Turner reported that using 7/44 litz wire that he made pickups where could "hear his fingerprints on the strings". So I guess that eddy currents in/or between the actual wire itself may enter the analysis picture in addition to eddy currents in pickup cores to account for additional upper frequency tonal color.

Thanks

Joseph Rogowski

I guess not. 7/44 is approximately equivalent in area to 36, which has a diameter of .005 inches (by definition). Using an equation from Skin effect - Wikipedia, the free encyclopedia, the diameter of wire suffering a 10% effective increase in resistance due to skin effect at 20 KHz is .056 inches, about 11 times greater. Thus there is only a tiny loss at 20KHz using 36 wire, and so the change in resistance over the range of any electric instrument is very small. Using 7/44 is not a significant change. Some people have found that style is more effective than substance.

[David Schwab]

7/44 is approximately equivalent in area to 36...

Using 36 gauge wire would certainly give you that tone of hearing your finger prints on the strings! That's why a lot of people don't like low Z pickups. They show off sloppy playing because you hear every little nuance due to the extra high frequency information.

[bbsailor]

I guess not. 7/44 is approximately equivalent in area to 36, which has a diameter of .005 inches (by definition). Using an equation from Skin effect - Wikipedia, the free encyclopedia, the diameter of wire suffering a 10% effective increase in resistance due to skin effect at 20 KHz is .056 inches, about 11 times greater. Thus there is only a tiny loss at 20KHz using 36 wire, and so the change in resistance over the range of any electric instrument is very small. Using 7/44 is not a significant change. Some people have found that style is more effective than substance.

Mike,

See the next web link. http://www.litz-wire.com/New%20PDFs/...lculations.pdf The recommendation for an individual strand diameter is based on a formula on the referenced pdf web link as well as on the summary table. AWG 28 is good for signals up to the 60Hz to 1KHz range. Maybe this is why AWG 28 is used on the Jack Casady low impedance bass pickup. AWG 33 is recommended for audio up to the 10KHz to 20KHz range. The only tradeoff in lower frequency ranges is the space occupied by non-conducting material (insulation space and air space) and the consequential slightly higher resistance than a solid wire occupying the same space. AWG 44 is recommended for signals up to the 350KHz to 850KHz range. When the wire is "served" the litz wire will have a thin synthetic thread outer insulation layer to minimize wire-to-wire capacitance in a coil.

My style is to try something, listen and then report on my findings. Knowledge of science just lets me know what to expect and to help explain any unanticipated acoustic outcomes. I have some 40/44 litz that is .018" diameter and can fit about 100 to 110 turns on a standard humbucker bobbin. I plan to use it with a miniature 8 ohm to 20K ohm output transformer used backwards with the 8 ohm side being the input from the litz-wound bobbin coil. This transformer has a 1 to 50 turns ratio, a 1 to 2500 impedance ratio and should get the 100 to 110 turns on the bobbin up near a normal 5000 turn equivalent but with a very different resonance peak and extended high frequency range. I plan on using one transformer on each coil and then treat the output of the transformers in series to maintain the humbucking effect, a parallel connection or use a single coil. With about 3 ohms DCR on each bobbin, I should get near 14K ohms on each transformer output or about 28K (measured at 1Khz) for both transformers in series. A common 250K ohm volume pot should work to minimize loading and tame any upper resonances.

Litz wire should offer some interesting possibilities for making a standard size humbucker with matching transformers (in the control area) to explore some other design alternatives for achieving a more acoustic flavor.

Joseph Rogowski

[Mike Sulzer]

Mike,
AWG 44 is recommended for signals up to the 350KHz to 850KHz range.

Exactly. The 7/44 Litz wire is overkill. The 36 equivalent single consuctor is just fine.

There is another factor here, the inductance of the coil. If the impedance that the pickup looks into is high enough so that the low frequency resistance is small enough to prevent significant loss at low frequencies, then the increase in resistance due to the skin effect is not much of an issue. It only increases with the square root of frequency, while the inductive reactance increases linearly.