A new direction for Low-Z pickups with SMD inductors

[cbrimbor]

I've been following bbsailor's posts on low-impedance pickups for a number of years. His innovative approach to pickup design has inspired many enthusiasts, including myself.

One of the standout features of Joseph's design is the use of a one-loop sensing coil that can fit directly under the strings. This compact design, however, comes at the cost of requiring a bulky current transformer (CT) nearby. Over the years, many have attempted to conceal the CT but struggled due to the extreme sensitivity of the system to the resistance of the primary coil. Eventually, someone suggested replacing the 1:500 transformer with 500 turns of wire as the primary coil.

I had the same thought, but I lacked the tools and skills to wind a custom pickup, so I shelved the project for a while. Recently, while repairing a circuit board, I realized that tiny surface-mount inductors are essentially pre-wound coils around a ferrite core. These components are available in standardized specifications, making them a cheap alternative for experimentation. So I went ahead and ordered some samples from Mouser to test the concept.

----

After some trial and error, I settled on two designs:

1. Single-Coil Configuration

For the single-coil prototype, I used six Bourns SDR1006-472KL inductors (9mm diameter x 6mm height, 4.7mH, 18Ω). The inductors are held together by metal wires soldered to their terminals, and I mounted them on a popsicle stick using sticky tack. Neodymium magnets (5mm diameter x 1mm thickness) attach directly to the ferrite core without adhesives, allowing easy adjustments. To balance the output, I placed two magnets on the middle four strings. The overall height is about 8-9mm. My LCR meter measured approximately 32mH @ 1kHz and 98Ω between the two ends.




For the test setup, I connected the outputs to a TRS output jack using alligator clips. The signal passed through a 10ft/3m TRS-to-XLR cable into my interface. Although the interface has a mic input, I routed the signal through a Shure A85 transformer (as recommended in Joseph's posts) before plugging it into the guitar input on channel 1. This setup allowed a direct comparison with stock humbuckers on channel 2.



With both channels' gain set at 50%, the single-coil prototype's output was about 3dB lower than the humbucker. Despite being a single-coil design, the noise level was comparable to the humbucker. I initially worried that the output would drop significantly if the string vibrated above the gap rather than directly over the inductor. However, sliding the stick back and forth by about 5mm each way during testing showed no significant change in output. Maybe it is because of the ferrite core concentrating magnetic flux through the coil. I also plan to use an Ebow in future experiments for more consistent string vibrations.

2. Hum-cancelling Configuration

The natural progression would be to make a humbucker. Unfortunately I only have 10 of each type of inductors, so I came up with a 5+5 configuration using SDR0805-472KLs (7mm diameter x 5mm height, 4.7mH, 21Ω). The bottom row is flipped in both coil direction and magnet polarity to achieve hum cancellation. My LCR meter measured approximately 50mH @ 1kHz and 210Ω across the two ends. The overall height is about 7-8mm.





For a more modular design akin to Zex-coil pickups, I arranged the circuit in a zigzag pattern instead of a linear configuration, as seen in the photo which shows the back side of a (failed) practice run. I know it is not the best way to do it as the inductors are merely held together by solder.



The test setup mirrored the single-coil prototype. However, the humbucker's output was 5-10dB lower than the single-coil design, likely due to the 1mm height difference. Raising the pickup by an additional stick (1/16"~=1.5mm) significantly boosted the output by 15dB. Interestingly, this increase was more attributable to proximity to the strings than the additional number of inductors.


----

With baseline designs established, the next phase involves plotting the frequency response of these pickups. The ultimate goal for low-Z pickups is to achieve a flat response across the audible range. While awaiting the arrival of my USB oscilloscope and signal generator, I ran simulations in LTspice to check my numbers. Using a 600pF cable capacitance and 4kΩ input impedance from the mic input, the 1006-472KL design shows a stable response, while the 0805-472KL humbucker exhibits a -1.6dB drop at 10kHz and about -4dB at 20kHz. These results are promising, but I’m eager to see how the actual circuits perform.

===
Edit: I'm having troubles with uploading and resizing the images. You may have to click on them to see the full-sized picture.

[bbsailor]

I've been following bbsailor's posts on low-impedance pickups for a number of years. His innovative approach to pickup design has inspired many enthusiasts, including myself.

One of the standout features of Joseph's design is the use of a one-loop sensing coil that can fit directly under the strings. This compact design, however, comes at the cost of requiring a bulky current transformer (CT) nearby. Over the years, many have attempted to conceal the CT but struggled due to the extreme sensitivity of the system to the resistance of the primary coil. Eventually, someone suggested replacing the 1:500 transformer with 500 turns of wire as the primary coil.

I had the same thought, but I lacked the tools and skills to wind a custom pickup, so I shelved the project for a while. Recently, while repairing a circuit board, I realized that tiny surface-mount inductors are essentially pre-wound coils around a ferrite core. These components are available in standardized specifications, making them a cheap alternative for experimentation. So I went ahead and ordered some samples from Mouser to test the concept.

----

After some trial and error, I settled on two designs:

1. Single-Coil Configuration

For the single-coil prototype, I used six Bourns SDR1006-472KL inductors (9mm diameter x 6mm height, 4.7mH, 18Ω). The inductors are held together by metal wires soldered to their terminals, and I mounted them on a popsicle stick using sticky tack. Neodymium magnets (5mm diameter x 1mm thickness) attach directly to the ferrite core without adhesives, allowing easy adjustments. To balance the output, I placed two magnets on the middle four strings. The overall height is about 8-9mm. My LCR meter measured approximately 32mH @ 1kHz and 98Ω between the two ends.




For the test setup, I connected the outputs to a TRS output jack using alligator clips. The signal passed through a 10ft/3m TRS-to-XLR cable into my interface. Although the interface has a mic input, I routed the signal through a Shure A85 transformer (as recommended in Joseph's posts) before plugging it into the guitar input on channel 1. This setup allowed a direct comparison with stock humbuckers on channel 2.



With both channels' gain set at 50%, the single-coil prototype's output was about 3dB lower than the humbucker. Despite being a single-coil design, the noise level was comparable to the humbucker. I initially worried that the output would drop significantly if the string vibrated above the gap rather than directly over the inductor. However, sliding the stick back and forth by about 5mm each way during testing showed no significant change in output. Maybe it is because of the ferrite core concentrating magnetic flux through the coil. I also plan to use an Ebow in future experiments for more consistent string vibrations.

2. Hum-cancelling Configuration

The natural progression would be to make a humbucker. Unfortunately I only have 10 of each type of inductors, so I came up with a 5+5 configuration using SDR0805-472KLs (7mm diameter x 5mm height, 4.7mH, 21Ω). The bottom row is flipped in both coil direction and magnet polarity to achieve hum cancellation. My LCR meter measured approximately 50mH @ 1kHz and 210Ω across the two ends. The overall height is about 7-8mm.





For a more modular design akin to Zex-coil pickups, I arranged the circuit in a zigzag pattern instead of a linear configuration, as seen in the photo which shows the back side of a (failed) practice run. I know it is not the best way to do it as the inductors are merely held together by solder.



The test setup mirrored the single-coil prototype. However, the humbucker's output was 5-10dB lower than the single-coil design, likely due to the 1mm height difference. Raising the pickup by an additional stick (1/16"~=1.5mm) significantly boosted the output by 15dB. Interestingly, this increase was more attributable to proximity to the strings than the additional number of inductors.


----

With baseline designs established, the next phase involves plotting the frequency response of these pickups. The ultimate goal for low-Z pickups is to achieve a flat response across the audible range. While awaiting the arrival of my USB oscilloscope and signal generator, I ran simulations in LTspice to check my numbers. Using a 600pF cable capacitance and 4kΩ input impedance from the mic input, the 1006-472KL design shows a stable response, while the 0805-472KL humbucker exhibits a -1.6dB drop at 10kHz and about -4dB at 20kHz. These results are promising, but I’m eager to see how the actual circuits perform.

===
Edit: I'm having troubles with uploading and resizing the images. You may have to click on them to see the full-sized picture.

cbrimbor,

Thanks for the kind words about my low impedance posts.

Here are some thoughts for you to consider as you test and evolve your individual inductor pickup design.

1. Do you want individual electrical volume control for each string?
Mounting six small trim pots to be turned by a screwdriver near the inductors would allow easy string volume adjustment.
2. Matching these six string outputs ( after the volume controls) to a matching transformer needs some experimentation to find the best match for a sound that has a good low noise output as well as sounds good to your ears.
2a. Try matching one coil that is 4.7mH to a step up transformer to drive the amp input impedance. This will produce an XL of 29.516 ohms at 1000 Hz. A 10 foot guitar cable has about 250 to 300 pf of capacitance. Take measurements at the amplifier end of the coax cable as well as on the guitar end to see how the guitar cable affects the sound when swept across the full audio range.
2b. Try combining the six string inductors in series to see the measured inductance. Then load each string with a miniature pot of about 300 ohms, about 10x the input impedance.
2c. You can also send each transformer output through a 6 conductor shielded cable using an 8 pin connector set, and send each string to a separate channel of a small six input mixer.

Pickup design is based on the following characteristics.

1. Physical appearance.
2. Ease of fit to replace pre installed pickups.
3. Sound to the ear.
4. Noise or hum pickup.
5. String sustain.
6. Ability to be different from fully satisfying 1 to 5 above and introduce a new standard.

Enjoy your new trip down the low impedance pickup lane!

Joseph J. Rogowski

[cbrimbor]

Thanks for the kind words about my low impedance posts.

Of course! Your original post was actually one of the three main inspirations for me to kick off this project. The other two were Dr. Lawing's Zexcoil and Cycfi Research's Nu pickup (plus their research on audio op-amps).

Here are some thoughts for you to consider as you test and evolve your individual inductor pickup design.
Do you want individual electrical volume control for each string?

Yes, balancing the volume between strings is definitely a key focus right now. But down the line, I’m planning to add things like EQ and distortion, too.

At the moment, I’ve been adjusting the physical properties of the inductors—like changing their height, using a different inductor from the same family, or swapping magnets—to balance the output. This method isn’t fully modular, since the pickup only has 2 (or 3, in the case of differential) outputs. But it’s simple to make and understand, and that’s what I’ve been focusing on.

Eventually, though, I want to go fully modular. My plan is for each inductor to have its own circuit (like a volume trim pot) and separate output. So, for a standard 6-string guitar, the pickup would end up having at least 7 wires (6 outputs plus ground). That’s the direction I want to take, though I’m still figuring out the best way to do it. Your suggestions have definitely given me some new ideas!

2. Matching these six string outputs (after the volume controls) to a matching transformer needs some experimentation to find the best match for a sound that has a good low noise output as well as sounds good to your ears.
2a. Try matching one coil that is 4.7mH to a step up transformer to drive the amp input impedance. This will produce an XL of 29.516 ohms at 1000 Hz. A 10 foot guitar cable has about 250 to 300 pf of capacitance. Take measurements at the amplifier end of the coax cable as well as on the guitar end to see how the guitar cable affects the sound when swept across the full audio range.

Right now, I’m just running the output of the pickup through a 10ft XLR cable into a Shure A95 transformer, then into my interface (IK Multimedia Axe IO). I initially tried plugging the XLR directly into the mic input (which has a 4k ohm input impedance), but the gain scale was different from the instrument input. So, I switched to the A95 transformer into the guitar input—it makes it easier to compare the inductor pickup to the stock neck humbucker.

The TRS-to-XLR cable I’m using isn’t the best quality and has quite a bit of capacitance—around 600pF. I measured the inductance and resistance at the XLR end, and the numbers didn’t change too much. The 6-inline SDR1006-472KL still came out at around 30mH at 1kHz and 100 ohms DLR. So, calculating the impedance, 30m * 2 * pi * 1000 ~= 188 ohms, plus 100 ohms, gives me around 288 ohms, which works fine for the A95.

The 5+5 humbucker with SDR0805-472KL came out at about 50mH at 1kHz and 230 ohms, so around 544 ohms total, which is a little high for the A95, but the sound is still acceptable.

The open string tones passed my ear test, but that’s a bit subjective. I’m still waiting for my USB oscilloscope to arrive to do a full frequency sweep. Since I’m on vacation right now, I’ll have to put that off until next year.

2b. Try combining the six string inductors in series to see the measured inductance. Then load each string with a miniature pot of about 300 ohms, about 10x the input impedance.

Right now, the six string inductors are already set up in series. I’ve ordered some trimpots from Mouser, but I’ll probably hold off on this until after the holidays to dive back into it.

2c. You can also send each transformer output through a 6-conductor shielded cable using an 8-pin connector set, and send each string to a separate channel of a small six-input mixer.

This is exactly what I’m envisioning for when I go fully modular. Eventually, the six individual outputs will need to be combined in a mixer, which would likely be an active circuit. Now that we’re talking active components, I’m not sure whether to stick with a transformer or go with something like an op-amp or JFET preamp.

As my college professor always said, "Gain is cheap, but gain without noise is expensive." So, it’ll really come down to signal-to-noise ratio and some other factors. I really appreciate you sharing your expertise with transformers—I’m more familiar with op-amps, but I’ll definitely try both when I get back to this project.

Enjoy your new trip down the low impedance pickup lane!

I’m a bit late to the game, but better late than never! Thanks again for keeping this topic alive all these years. Happy Holidays to you too!

[bbsailor]

Of course! Your original post was actually one of the three main inspirations for me to kick off this project. The other two were Dr. Lawing's Zexcoil and Cycfi Research's Nu pickup (plus their research on audio op-amps).


Yes, balancing the volume between strings is definitely a key focus right now. But down the line, I’m planning to add things like EQ and distortion, too.

At the moment, I’ve been adjusting the physical properties of the inductors—like changing their height, using a different inductor from the same family, or swapping magnets—to balance the output. This method isn’t fully modular, since the pickup only has 2 (or 3, in the case of differential) outputs. But it’s simple to make and understand, and that’s what I’ve been focusing on.

Eventually, though, I want to go fully modular. My plan is for each inductor to have its own circuit (like a volume trim pot) and separate output. So, for a standard 6-string guitar, the pickup would end up having at least 7 wires (6 outputs plus ground). That’s the direction I want to take, though I’m still figuring out the best way to do it. Your suggestions have definitely given me some new ideas!



Right now, I’m just running the output of the pickup through a 10ft XLR cable into a Shure A95 transformer, then into my interface (IK Multimedia Axe IO). I initially tried plugging the XLR directly into the mic input (which has a 4k ohm input impedance), but the gain scale was different from the instrument input. So, I switched to the A95 transformer into the guitar input—it makes it easier to compare the inductor pickup to the stock neck humbucker.

The TRS-to-XLR cable I’m using isn’t the best quality and has quite a bit of capacitance—around 600pF. I measured the inductance and resistance at the XLR end, and the numbers didn’t change too much. The 6-inline SDR1006-472KL still came out at around 30mH at 1kHz and 100 ohms DLR. So, calculating the impedance, 30m * 2 * pi * 1000 ~= 188 ohms, plus 100 ohms, gives me around 288 ohms, which works fine for the A95.

The 5+5 humbucker with SDR0805-472KL came out at about 50mH at 1kHz and 230 ohms, so around 544 ohms total, which is a little high for the A95, but the sound is still acceptable.

The open string tones passed my ear test, but that’s a bit subjective. I’m still waiting for my USB oscilloscope to arrive to do a full frequency sweep. Since I’m on vacation right now, I’ll have to put that off until next year.


Right now, the six string inductors are already set up in series. I’ve ordered some trimpots from Mouser, but I’ll probably hold off on this until after the holidays to dive back into it.


This is exactly what I’m envisioning for when I go fully modular. Eventually, the six individual outputs will need to be combined in a mixer, which would likely be an active circuit. Now that we’re talking active components, I’m not sure whether to stick with a transformer or go with something like an op-amp or JFET preamp.

As my college professor always said, "Gain is cheap, but gain without noise is expensive." So, it’ll really come down to signal-to-noise ratio and some other factors. I really appreciate you sharing your expertise with transformers—I’m more familiar with op-amps, but I’ll definitely try both when I get back to this project.



I’m a bit late to the game, but better late than never! Thanks again for keeping this topic alive all these years. Happy Holidays to you too!

cbrimbor,

Here is a new pickup concept that you can quickly test with just a small audio transformer 8 ohms to 10K or higher, like 20K to 50K ohms on the secondary side.
1. Attach only one test string to the guitar and tune it. Having other strings may short out the test string and make this test perform badly.
2. Run a heavy stranded wire from the string tuning key to near the bridge.
3. Run a heavy stranded wire from the string tail end to near the bridge.
4.Attach these wires to the 8 ohm side of a transformer to step up the voltage. If the transformer secondary is 20 K ohms simply divide 20K by 8 and get 2500. Take the square root of 2500 and get 50. This is the approximate turns ratio.
5. Hand hold a magnet near the test string. Attach the transformer secondary to the amp input and listen to the plunked test string. A longer magnetic field under the string will increase the output.
6. Since the resistance of the string is near one ohm, wire connections between the string ends and the transformer need to be as short as possible and as thick as possible to reduce input loss.

My own research on this new type of pickup design looks to make the nut side of the string the common ground side with the hot end of each string being behind the bridge. The bridge should not have common metal for the strings to rest on but a non metal alternative like nylon or any other non metal bridge slider. String attachment to the tail piece needs a creative way to isolate the strings from each other or any common metal. Using the metal truss rod as a ground return from a metal nut is a creative way to return the ground to the body region near the bridge where the individual string transformers can be located.

This concept is totally new and requires a major leap of faith into the benefits of individual string volume and tonal control.

Give it a try and let me know what you think about this new pickup concept.

Thanks

Joseph J. Rogowski

[cbrimbor]

This concept is totally new and requires a major leap of faith into the benefits of individual string volume and tonal control.

Give it a try and let me know what you think about this new pickup concept.

This concept really reminds me of your ribbon mic-inspired idea that I came across a few years ago. Once again, it’s a brilliant innovation on your part! It’s a bit under-discussed compared to the single loop pickup, but I think it has great potential. Now that I’ve dipped my toes into this area, I think I’ll give this one a try too.

Thanks for sharing your insights, I’m looking forward to experimenting with it after the holidays.