Low Impedance Pickup Research

[fieldwrangler]

As magnetic interference is polarized/directional and electric interference is not, a strong noise dependence on the instrument's orientation indicates dominating magnetic interference.

Sure 'nuff. And, man, have I ever made people happy by coming as close to 100% shielding as humanly possible, or as their budgets allow.

By the way, what do you think is the best way to make a practical directional electrostatic noise source?

Bob Palmieri

[Helmholtz]

By the way, what do you think is the best way to make a practical directional electrostatic noise source?

Maybe I wasn't clear enough in my statement above: Electric (noise) fields are directional (but not polarized like magnetic fields). They typically radiate/spread from the noise source like light from a light source. Spatial field distribution is influenced/changed by conductors, especially grounded ones. This is the principle of "electrostatic" shielding.

An "electrostatic" noise source is a noise voltage on a radiating conductive surface (e.g. a wire).

A uniform noise field can be produced between 2 large metal plates connected to a noise voltage source.

Typical everyday sources of electrostatic noise are dimmers, switched power supllies, CRTs and fluorescent lamps as well as the wiring connected.

By nature of the "electrostatic" coupling (i.e. capacitive coupling) higher frequencies are preferred.

Don't know if this helps. Maybe you can be a little more specific about your intentions.


BTW, electrostatic PU interference can be 100% shielded, but not magnetic interference. As both interference types act differently they need to be attenuated by different means.

[fieldwrangler]

Maybe I wasn't clear enough in my statement above: Electric (noise) fields are directional (but not polarized like magnetic fields). They typically radiate/spread from the noise source like light from a light source. Spatial field distribution is influenced/changed by conductors, especially grounded ones. This is the principle of "electrostatic" shielding.

An "electrostatic" noise source is a noise voltage on a radiating conductive surface (e.g. a wire).

A uniform noise field can be produced between 2 large metal plates connected to a noise voltage source.

Typical everyday sources of electrostatic noise are dimmers, switched power supllies, CRTs and fluorescent lamps as well as the wiring connected.

By nature of the "electrostatic" coupling (i.e. capacitive coupling) higher frequencies are preferred.

Don't know if this helps. Maybe you can be a little more specific about your intentions.


BTW, electrostatic PU interference can be 100% shielded, but not magnetic interference. As both interference types act differently they need to be attenuated by different means.

You were quite clear, as always!
The 100% shielding I referred to is indeed shielding against electric fields.

I was just thinking that I'd like to make a nice focussed electric field source for testing shielding setups. Seems like if one can make directional antennas for RF applications it might be feasible.

[Helmholtz]

I was just thinking that I'd like to make a nice focussed electric field source for testing shielding setups.

That's what I thought. Interesting idea and approach.

Seems like if one can make directional antennas for RF applications it might be feasible.

Electrostatic fields are not travelling electromagnetic fields like radiowaves, so different rules apply.

The best way to produce a confined and quasi-uniform field for testing purposes is to use 2 large metal plates connected to a noise voltage/generator and place the DUT in the middle between the plates. The plates of this large "air-capacitor" could be made from cardboard covered with aluminum foil.

[bea]

Joseph,

exactly that was the impression from my prototype with the thick wire. But nevertheless the 6 mm² loop of my prototype produced an at least reasonably balanced signal. Despite of that i do consider playing with 4x1.5 mm² next time.

practical question: how to insulate these?
Source for the copper will problably normal NYM-Cable where i have a lot of spare material. Remove the thick PVC insulation and then apply something thin. But what?

Insulation does obviously not need to be perfect, just good enough to insulate these tiny currents over most of the distance in order to avoid the skin effect of the large cross section.

@micha: welcome - but why didn't You show the last one in GBB? And what's the design? low output wide bandwith or high output smaller bandwidth?

[Helmholtz]

The best option is to use enamelled copper wire, like https://www.reichelt.de/kupferlackdr...?&trstct=pol_8 . If you can't find thick enough wire at low quantity, just use more strands.

I don't think there is a good DIY way to produce a uniform surface coating.


The Skin effect can't have much effect on the HF response of the PU. As the transformer is not terminated by a low impedance at the secondary, it doesn't operate as current transformer. The primary current (= loop current) is used almost completely as magnetizing current which does not transform to the secondary. At low frequencies the magnetizing current is high and loop resistance matters. But the primary current decreases with increasing frequency and the influence of the primary DCR on output voltage gets much less.

[bbsailor]

Joseph,

exactly that was the impression from my prototype with the thick wire. But nevertheless the 6 mm² loop of my prototype produced an at least reasonably balanced signal. Despite of that i do consider playing with 4x1.5 mm² next time.

practical question: how to insulate these?
Source for the copper will problably normal NYM-Cable where i have a lot of spare material. Remove the thick PVC insulation and then apply something thin. But what?

Insulation does obviously not need to be perfect, just good enough to insulate these tiny currents over most of the distance in order to avoid the skin effect of the large cross section.

@micha: welcome - but why didn't You show the last one in GBB? And what's the design? low output wide bandwith or high output smaller bandwidth?

Bea,

Try using solid core magnet wire with a thin insulation.
Four strands of AWG11 will equal the size of AWG 8 and offer about 52 micro ohms per inch for the 4 parallel AWG11 strands. Make sure you make a low resistance joint on each of the strands. I find that using thin wall copper tubing that closely matches the wire diameter makes a good low resistance joint. Use silver solder.

I hope this helps.

Joseph J. Rogowski

[micha70]

Hi there!

Micha, great work on these, they are really well executed and imaginative designs.

Thanks a lot! As said, still learning.

@micha: welcome - but why didn't You show the last one in GBB? And what's the design? low output wide bandwith or high output smaller bandwidth?

Thank you and sorry for the late reply. I don't follow this forum on a daily basis. The last design is now on the GBB forum as well as this one. https://www.gitarrebassbau.de/downlo...0043&mode=view
I don't know why, but I can't upload files, so I linked to the picture of the GBB forum. Sorry!!!
My basic idea is always to get low output / high bandwith pickups and color the sound with active electronics. I don't know if this will work out, but I'll keep trying. As you might have seen it, there are also some frequency plots of the last two designs take by a friendly forum menber from the GBB forum and they came fairly close to what I was looking for. First the "MW" pickup, second the "D" pickup
https://www.gitarrebassbau.de/downlo...0383&mode=view
https://www.gitarrebassbau.de/downlo...0384&mode=view
Not suprising, since the current sensors are exactly doing what they are designed for.

Micha,

When making low impedance pickups with thick string loop wire, look up on the web the "skin effect" This skin effect says that higher frequencies do not penetrate fully to the core of thicker wire diameters. http://diyaudioprojects.com/Technical/American-Wire-Gauge/
Here are a few wires that I have used.
AWG 4 .2043 inches diameter 5.189mm diameter 650Hz for full current depth penetration
6 .162 4.1148mm 1100Hz
8 .1285 3.2639mm 1650Hz
24 .0201 .51059mm 68Khz

When I used very thick wire for the string loop I noticed that the output level increased due to lower resistance of the string loop with more current being generated but I also noticed that the lower frequency response was being more emphasized. In your experiments, try using multiple parallel strands of thinner wire but with the same area/resistance as thicker wire and listen for the tonal changes due to the skin effect.

You did a nice job on your pickups. Thanks for sharing with us.

Joseph J. Rogowski

Thanks Joseph! Very useful information and exactly right as you can see. Thumbs up!

I did some experiments with multiple strands and found the sound a little to focused on the higher frequencies and too weak on the low end. In the two designs I want to use on my current guitar project, the string loop is always 14mm² times two, but it is reduced due to the available diameter in the current sensors to max. 19mm². So, this 19mm² is the effective cross section. The rest of the string loop it's just "show".
The difference between the "MW" and the "D" pickup is that the "MW" uses a copper tube with 5mm outer dia and 3,2mm inner dia, while the "D" uses a solid 5mm dia bar. This may explain the tonal differences you can see in the frequency plots. I thought that this is more, because of the differences in orientation of the magnetic field. But this is far beyond my knowledge. For the time being, I keep trying building and listening to the result.

Bests,
Micha

[Helmholtz]

The rest of the string loop it's just "show".

Not necessarily. What matters is the total current loop resistance (determined by cross section times length). So if the cross section is lower only for a fraction of the loop length, its contribution to total resistance doesn't necessarily dominate. But even a continuous cross section of 19mm² should be more than enough for excellent bass response.
When you use rivets, their unpredictable contact resistance may significantly increase the total loop resistance.

By principle these "transformer PUs" tend to have a more uneven audio frequency response than a directly wound multi-turn PU of same(!) impedance.

[micha70]

Not necessarily. What matters is the total current loop resistance (determined by cross section times length). So if the cross section is lower only for a fraction of the loop length, its contribution to total resistance doesn't necessarily dominate. But even a continuous cross section of 19mm² should be more than enough for excellent bass response.

Great! Lesson learned.

When you use rivets, their unpredictable contact resistance may significantly increase the total loop resistance.

Absolutely true. I had a hard time finding the right rivets and „process“ to get a fairly consistent results. Meanwhile I use the solid 5mm bar and drill the holes for the rivets rather than using tubing. Before pulling them in, they are pressed in first. Still a risk. I‘ll have to see how tone will change over time, too.

By principle these "transformer PUs" tend to have a more uneven audio frequency response than a directly wound multi-turn PU of same(!) impedance.

OK. Need to dig into that. Don‘t understand all of that.

Bests,
Micha

[Joe Gwinn]

practical question: how to insulate these?
Source for the copper will problably normal NYM-Cable where i have a lot of spare material. Remove the thick PVC insulation and then apply something thin. But what?

Insulation does obviously not need to be perfect, just good enough to insulate these tiny currents over most of the distance in order to avoid the skin effect of the large cross section.

If this is a one-turn coil, the voltages will be tiny, so any kind of good-quality varnish intended for wood will work. Wood? Yes, so the cured varnish film is stretchy enough that it won't crack and fall off the copper. The metal must be very clean (no flux or oils), or adhesion will fail. Cleaning in an ordinary domestic dishwasher could do it, followed by drying the water off, dipping in varnish, and hanging out to dry in a warm clean place.

[bbsailor]

Bea,

Try using solid core magnet wire with a thin insulation.
Four strands of AWG11 will equal the size of AWG 8 and offer about 52 micro ohms per inch for the 4 parallel AWG11 strands. Make sure you make a low resistance joint on each of the strands. I find that using thin wall copper tubing that closely matches the wire diameter makes a good low resistance joint. Use silver solder.

I hope this helps.

Joseph J. Rogowski

I need to correct my post

Two equal length wire strands of the same size will equal a strand that is three sizes larger. Two strands of AWG 11 at about 104 micro ohms per inch will equal a single strand of AWG 8 at about 52 micro ohms per inch.
When I used 4 strands of AWG 11 this resistance should have been cut in half again to be about 26 micro ohms per inch or equal to a single strand of AWG 5 wire. This technique can work with pairing any number of AWG solid wire strands.

Sorry, for my mistake, I just returned form a vacation in Europe.

Here is the way I have measured various wire sizes to get a visual graph of wire size and string loop resistance effects.

I use a Velleman PCSGU250 that is a 2 channel digital scope and a 50 ohm output signal generator. Here is how it is set up.

I put a BNC Tee connector on the graphics generator 50 ohm output and a short BNC coax to channel 2 scope input.

The open end of the BNC Tee connector end is terminated with a red and black screw terminal to attach to the low impedance pickup stimulation coil.

I use a low impedance pickup stimulation coil wire size that is 10 times larger area than the high impedance pickup wire size. My AWG 32 stimulation coil has about 500 turns and I put a 500 ohm resistor in series to minimize the loading effects on the stimulation coil by being directly connected to the 50 generator output source resistance.

The Velleman tester comes with a software package to use the signal generator alone or the oscilloscope alone or a combination of both to see the signal frequency graph driving the stimulation coil and the output of the low impedance pickup directly from the current transformer output feeding the oscilloscope channel 1. The graphs can be labeled and saved. The good part is what you can learn by using a variety of string loop wire sizes, number of thicker or smaller strands, and output level all graphed from low to high frequency low impedance pickup output.

When your ear tells you something is different with this wire string loop or current transformer number of secondary turns being in the range of 500 to 100 turns. The typical mic input is about 2400 ohms to act as a bridging impedance (10 times higher than the pickup output impedance) for rated 150 ohm microphones which have a real output impedance ranging from about 75 ohms to 250 ohms.

The largest wire I can fit in a Triad CSE186L current transformer is one strand of AWG 8. But first the three strands of the CSE186L AWG 16 wire must be removed to expose a single square opening to allow a snug fit of one turn of AWG 8 solid core wire. You need to wrap a turn of insulating tape around the laminated transformer core on each side to prevent direct contact with the exposed transformer laminations that might short out the string loop. Once you make a good string loop joint (low resistance) the resistance of the string loop will depend on its length. A minimal string loop of about 7 inches will span a 2 inch string width, the bend and string joint and the amount of string loop wire going through the current transformer. Making a 7 inch string loop of AWG 8 is 52 micro ohms times 7 or 364 microohms. I then add 10 percent to account for typical leakage inductance in this type of pickup now 364 plus 36 is now about 400 ohms. Since a 500 turn current transformer has an output impedance change of turns ratio squared. Each additional inch of AWG 8 wire loop only adds 52/4 or 13 micro ohms to the output impedance. The 500 turn transformer is now the DC Resistance of the string loop times 250,000 or one quarter million. Since the string loop is measured in micro ohms resistance, the actual output impedance is very close to one quarter the string loop resistance plus 10 percent to come very, very close to the actual output impedance measured by an LCR meter at 120 HZ. If you use a 1000 turn current transformer you now need to use a string loop with 4 times that area because a 1000 turn current transformer is now turns ratio squared or 1000 times 1000 or 1,000,000 (1 million). Since the out is based on turns ratio, a 1000 turn current transformer only gets about twice the output level but the size of the string loop wire is now very, very large and harder to work with. That is why I have focused on a 500 turn current transformer but you can use what you have in stock but understand the consequences and the output impedance limitations bases on the actual input impedance of your amplifier device.

Tinker with different types and numbers of string loop wire and let your ear tell you what sounds best. The theory that I offer only tells you how to target you input device impedance but high impedance pickups have evolved since about 1935 when the first electric guitars allowed the guitarist stand up, solo, and be heard while sitting in the rhythm section. All guitar pickups have resonant peaks due to targeting high impedance amplifiers, many thousand turns of very fine wire to generate enough voltage to drive these amps, 100pf of coil wire capacitance, coax cable capacitance of about 10 pf per foot to feed the high impedance amplifier. The consequence of this is the evolution of the typical "electric guitar sound" is due to the resonant point of most high impedance pickups being in about the 2 to 4 KHZ peak resonance range. Even high impedance magnetic pickups designed for acoustic guitars tend to have some of that typical "electric guitar sound". The low impedance pickups that I describe have a relatively flat response, allowing you to electrically alter the sound to match you desired sound.

Keep on tinkering!

Joseph J. Rogowski

[Nickdino]

Hi, new to the forum. Does anyone have soundclips of their low-z pickups? Anyone selling them?

[Singer15]

I've had great success making lowZ PU's with what I learned on this forum. However, they all require a transformer or an amp with a mic channel. Now I'm wanting to make a low z pu that will not need a mic transformer, and I wonder if something akin to the old Gibson Charlie Christian would work with these specs: 5000 turns x 36ga. The DC resistance calculates at around 1200 Ohms. Inductance calculation says it would have about 3.8H. I would use 6 tiny neod mags 6mm diam x 3mm thick as the poles.Before I go ahead and waste a lot of wire on this idea it would be good to know if there is any hope of it working in a common guitar amp.

[Helmholtz]

I've had great success making lowZ PU's with what I learned on this forum. However, they all require a transformer or an amp with a mic channel. Now I'm wanting to make a low z pu that will not need a mic transformer, and I wonder if something akin to the old Gibson Charlie Christian would work with these specs: 5000 turns x 36ga. The DC resistance calculates at around 1200 Ohms. Inductance calculation says it would have about 3.8H. I would use 6 tiny neod mags 6mm diam x 3mm thick as the poles.Before I go ahead and waste a lot of wire on this idea it would be good to know if there is any hope of it working in a common guitar amp.

How did you determine inductance?

A PU having an inductance of 3.8H definitely is a high impedance PU. In fact its impedance at 1kHz is around 24k. DCR is insignificant for impedance.