Interesting Pickup Design from Fishman

[bbsailor]

Even in a high impedance pickup, the coil capacitance is not the dominant effect (It is the cable.), although it does matter. In a medium impedance pickup, it is even less important. In order to get he traditional boost in the upper middle, it is necessary to either resonate the pickup with additional C or use an active filter in the preamp. But what the hey, if you take the trouble to make a new pickup, you have to sell it. You can claim that the lower C opens things up; who can stop you? But that is not how it works.

Mike,

I did a little back of the envelope analysis and here is what I found out. If each stack of 48 coil layers represents 25mH of inductance (one hundredth of a 2.5H coil) then two 48 stacked humbucker sets represents 50 mH of inductance. Then, to obtain an 80Khz resonant frequency with 50mH you need about 79pf of capacitance or .8229pf capacitance between each of the 96 total layers. Here is where the spacing between the conductive coil layers is near .009". Finding the lowest dielectric constant substrate will go a long way to tuning these pickups or adding bulk switched capacitance to bring the resonant frequency down to the 3Khz to 5Khz range will allow duplication of classic pickup sounds.

Yes Mike, the guitar cable connecting the guitar to the amp imposes about 350 pf of capacitance for a decent 10 foot cable quality and this capacitance typically dwarfs the wire turn-to-turn capacitance. Eliminating the effect of cable capacitance only allows you to extend the high frequency range if so desired.

Once they refine the substrate dielectric, substrate thickness and winding density on each winding layer, they will have a good way to make a whole range of pickups with various amounts of coil stacks to create a highly replicatable and manufacturable design with very close tolerances.

Joseph Rogowski

[charrich56]

Even in a high impedance pickup, the coil capacitance is not the dominant effect (It is the cable.), although it does matter. In a medium impedance pickup, it is even less important. In order to get he traditional boost in the upper middle, it is necessary to either resonate the pickup with additional C or use an active filter in the preamp. But what the hey, if you take the trouble to make a new pickup, you have to sell it. You can claim that the lower C opens things up; who can stop you? But that is not how it works.

Yep. The pickups I am winding right now are about 600-700 turns of #32 with an inductance of about 150 mH, DCR about 50 ohms, and impedance less than 200 ohms at 1 KHz. Winding capacitance is about 170 pf so the self-resonant frequency worst case is a bit over 30 KHz. With an impedance matching transformer, the L is lower in the primary circuit and the self-resonance hits over 60 KHz. Would it make any difference if I engineered the hell out of it, reduced C, and got the self-resonant frequency to 80 KHz? Nope, not a bit. Not even to a hi-fi junkie bat named Nyquist.

[Mike Sulzer]

Joseph, I was thinking that they have significantly less than 2.5 H of total inductance; certainly I could be wrong about that, but they have a preamp. Why make the inductance so high? They would need at least 5,000 turns total, or about 100 per board (fifty on each side). If they did that it could be sold without the electronics, yet this, as offered, requires a preamp, so I am assuming medium impedance and electronics to shape the frequency response and provide the necessary gain.

Also, be careful with the inductance per card calculation. The different boards couple, and so the inductance rises faster than linear as boards are added. I think it would be a lot less than 25 mH per card in the scenario you described. (Also not sure about how much effective permeability their magnet structure has.)

There are things we do not know here, but it is interesting to try to figure it out.

[ScottA]

From the article:
"The interaction between the vibrating string and the magnetic field greatly affects the physical dynamics."

That's not an unreasonable statement, and basically true, although the information content is near zero.

The other huge part of it, as you have been sniffing around lately Mike, is what's going on in the core and how it drives frequency response. It's not straightforward, it's not linear, and I suspect it's not something that can be replicated accurately with simple active electronics. Personally, I think the core of the coil (i.e. the pole piece) is the single most important piece of a passive pickup in defining the tonal response.

I think this an interesting design idea and approach and I applaud Frank and the Fishman team for thinking out of the box.

[charrich56]

Whoops, sorry, meant to say that the last pickup I wound was really only about 35 mH. 150 mH is the inductance of the transformer's primary winding I am using. Still the same point though although the numbers might be different. Outside the audible range is the key concept here.

The concept "speed" in relation to listening to audio or music or (now) musical instruments, has been mangled to death by the high-end hi-fi crowd (and high-end hi-fi marketing crowd) and is semantically null and void at this point.

[Mike Sulzer]

Can you even define what the "physical dynamics" are? How is there an "interaction" between the string and the magnetic field? Interaction is a mutual thing. For example, the magnetic field from the permanent magnet in a speaker affects the motion of the voice coil, and the current in the voice coil produces a magnetic field which can shift the properties of the permanent magnet, especially if is alnico, but not much if it is neo. The motion of the string has a very small, if any, effect on the properties of the permanent magnet in a pickup.

Yes, the core or cores can be important, and this kind of pickup can have them, too. I would use active electronics, or a capacitor across the pickup, to act like the coil inductance and capacitance, as well as the cable capacitance, but not attempt to replicate subtle core effects. I am not convinced that such effects are non linear, however. Different from a normal damped LC resonance, yes. How different is still an issue.

That's not an unreasonable statement, and basically true, although the information content is near zero.

The other huge part of it, as you have been sniffing around lately Mike, is what's going on in the core and how it drives frequency response. It's not straightforward, it's not linear, and I suspect it's not something that can be replicated accurately with simple active electronics. Personally, I think the core of the coil (i.e. the pole piece) is the single most important piece of a passive pickup in defining the tonal response.

I think this an interesting design idea and approach and I applaud Frank and the Fishman team for thinking out of the box.

[charrich56]

Scott,

IMHO you are very qualified to comment on pole piece and magnetic circuit design since you have gone through that process one way or another, developing the Zexcoils. You thought out of the box, too.

BTW, I would love to re-open a dialogue with you about the possibility of a low-impedance, preamped, externally voiced Zexcoil. It may be possible to get there with drastically cheaper electronics than you might think on first pass.

-Charlie

[frankfalbo]

Are you ridiculing me here?

No, I wasn't in the least. I genuinely don't see it that way.

Is this from the sales dept?

No, but now it sounds like you're ridiculing me. I'm just sharing my experiences. I'm not trying to make any of you customers. This is supposed to be a place we come to talk about what we do. I've always said what I could, and told you when I can't say more.

That's pure hyperbole.

I was involved in the development of one, I'm involved in the development of the other, and I'm telling you what I hear and feel as a life-long guitar player with a proven set of ears and adequate playing skills. I believe I have a history of integrity on this and other forums. I now own my company and I get to decide who and what I want to work with. If people want to disagree with or discard any of my statements that's fine. I'm not trying to solve all your problems, just share what I've been a part of.

I don't get "fast", and how can dynamics be improved in an active circuit? With all due respect Frank. From the outside it looks like an active system with a new fangled type of coil. I think the technology of it is fascinating, but I'm not excited.

And you haven't played a set, right? Because they're not available. The new fangled type of coil is the thing. I can appreciate you don't get the use of the term "fast" when describing pickups, but its a term that works for me. Others have "gotten" it. If you wanted to quantify it, talk about differences (perceived or measurable, I don't care) in rise time, round wire vs. the more tape-like traces, skin effect, distributed capacitance, etc. All that matters is everyone who hears/plays the pickups gets it. (so far anyway) Maybe my/our terminology doesn't satisfy other pickup makers but if we're not trying to get music makers to have more fun making music, then what are we doing?

What is the life expectancy of these in our RoHS compliant world?

I have no idea.

[John_H]

...This is supposed to be a place we come to talk about what we do. I've always said what I could, and told you when I can't say more..

Fair enough, I apologize for my attitude there, but some of your description was a bit over the top, and your remark seemed a little condescending.

Maybe my/our terminology doesn't satisfy other pickup makers but if we're not trying to get music makers to have more fun making music, then what are we doing?

True, that's what it's all about. I guess I'm afraid of the day when it's commonplace for everyone to have DSP's built into their self-tuning guitars.

[bbsailor]

Joseph, I was thinking that they have significantly less than 2.5 H of total inductance; certainly I could be wrong about that, but they have a preamp. Why make the inductance so high? They would need at least 5,000 turns total, or about 100 per board (fifty on each side). If they did that it could be sold without the electronics, yet this, as offered, requires a preamp, so I am assuming medium impedance and electronics to shape the frequency response and provide the necessary gain.

Also, be careful with the inductance per card calculation. The different boards couple, and so the inductance rises faster than linear as boards are added. I think it would be a lot less than 25 mH per card in the scenario you described. (Also not sure about how much effective permeability their magnet structure has.)
There are things we do not know here, but it is interesting to try to figure it out.

Mike,

The 2.5 H is for a high impedance coil with 8,000 turns per coil and I assumed that this new Fishman pickup with a preamp (for lack of any technical details) uses one tenth the number of turns or 800 turns and is 25mH per coil. With a maximum of 48 layers that would mean that each coil layer is about 16 turns. Controlling the thickness of each coil layer, number of coil layers and the dielectric constant of the substrate could control the design to make any pickup sound highly replicapable. Buffering isolates the effect of the guitar cable capacitance from the final design. The only issue is reliability with each layer requiring two series contacts or connections that could cause open coil issues. Time will tell!

Joseph Rogowski

[salvarsan]

I ran some numbers and got a 131 ohm 325 wind pickup assuming a standard strat pickup dimension.
It could probably be approximated on a strat bobbin with ~400 winds of AWG 38.5 wire.

For people with too much spare time:

Estimating the winds
The 25 PCB vias (interconnect plate-thru holes) indicate 25 layers.
Flange width is 0.625" with a 2.25" X .1875" bore length by width for magnet(s).
Available PCB width is 0.21875" ~7/32"
Smallest reliable PCB trace is 8 mils (0.008") with an 8 mil space between traces,
i.e., the space is half copper and half space.
(0.21875 / 0.008) = ~27 spaces + traces
or 13 winds per layer with an average length of 5.125" per wind.
This pickup has 13 winds per layer and 25 layers or 325 winds,
and a total length of 1665" or 138.8 feet.

Estimating the resistance
The copper is most likely 1 oz/ sqft or 1.37 mils thick,
so the Copper cross section is 10.96 circular mils
Copper resistivity is 10.371 ohm-circular mils/ft
(resistivity/circular mils) * length = resistance
(10.371 / 10.96) * 138.8ft = 131.3 ohms

Equivalent wire AWG
10.96 circular mils => radius = sqrt(10.96/PI) = 1.867 mils,
and the equivalent diameter is 3.736 mils or .003736"
Bare wire AWG 39 and 38 are .0035" and .0040" inches in diameter
so we can call it a 38.5 AWG winding.

[Mike Sulzer]

Joseph,

I am sorry, I missed your other post and got confused. if i understand the descriptions of the pickups they will offer, each one has to main types of sound, and I think it is implied that they are achieving this with active filters.

Mike,

The 2.5 H is for a high impedance coil with 8,000 turns per coil and I assumed that this new Fishman pickup with a preamp (for lack of any technical details) uses one tenth the number of turns or 800 turns and is 25mH per coil. With a maximum of 48 layers that would mean that each coil layer is about 16 turns. Controlling the thickness of each coil layer, number of coil layers and the dielectric constant of the substrate could control the design to make any pickup sound highly replicapable. Buffering isolates the effect of the guitar cable capacitance from the final design. The only issue is reliability with each layer requiring two series contacts or connections that could cause open coil issues. Time will tell!

Joseph Rogowski

[Mike Sulzer]

I was thinking you can do .004" without much trouble, and do it two sided. This would give more than 1,000 turns. You would need only a very thin insulating layer between boards, and I do not think capacitance between cards is a problem.

[salvarsan]

I was thinking you can do .004" without much trouble, and do it two sided. This would give more than 1,000 turns. You would need only a very thin insulating layer between boards, and I do not think capacitance between cards is a problem.

As a data point, Custom Circuit Boards specs capabilities of 4 mil @ .5oz traces, and 24 layers. This may be an expensive maximum capability. I'm not asking for the cutting edge, merely what would make sense for a near-commodity pickup construction.

[Joe Gwinn]

Once they refine the substrate dielectric, substrate thickness and winding density on each winding layer, they will have a good way to make a whole range of pickups with various amounts of coil stacks to create a highly replicatable and manufacturable design with very close tolerances.

This is well within the state of the art. Think of the motherboard in an ordinary PC, or the circuit board in a smartphone.

And testing is easy - just pass an amp through the coil and listen for noise. If no current, toss coil. If noisy while carrying current, toss coil.