Low Impedance Pickup Research

[bbsailor]

Hi Joseph,

In response to your questions:

1. The problem regarding soundholes is that I have some guitars without them, so this pickup will have to be thin enough to fit between the strings and the top.
2. String width at end of fingerboard = 50mm
3. String ht = 12mm, down to 9mm when using the higher frets.
4. I am using Daddario gypsy strings which have silver plated copper windings, gauges from 10 to 44 thou.
5. Not much left except a roll of 44ga which I have had no luck with!
6. No grounding on acoustic guitars generally. Could be done however, since these all have metal tailpieces.
7. Yes, in fact that is how I will have to get my wire in future, having exhausted the supply in town here.

To add some more experimental results to this, today I wound 2 split coil pickups with 44 ga wire, with mixed results, generally poor. The first pickup had 135 windings per coil, the second had 500. I used plastic for the plates between which I glued the magnets.

The first one was definitely an improvement regarding hum cancelling, compared to the previous single coils, but it was very low output. I used the same 10mm x 3mm ceramic magnets from a dollar store, one set of 3 for the 456 and one set for the 123 strings, reversed windings, and opposite polarity.

The second try was not so good, and I couldn't get a good DC resistance reading, although the pickup actually produced a signal. I though I might have it wired out of phase but even switching the coil ends did nothing to improve the signal.

I suspect that heavier wire is the way to go here, the question is what gauge next? Also, would more powerful magnets improve the signal? These little ceramic magnets seem about as strong as an alnico bar magnet, judging by how well they stick to things.

Cheers,

Don Nathan

Don,

Try this.

Obtain this magnet from K&J Magnetics . https://www.kjmagnetics.com/proddetail.asp?prod=BY042SH
This magnet is 2" long by .25" wide and .125" high. Cut up an old credit card for making the top and bottom plate about 2.5" long by .75" wide and glue to the magnet. If you use AWG 36 single build magnet wire it is about .0056" in diameter. https://mwswire.com/wp-content/uploa...-Wire-Data.pdf. This will allow you to machine wind (tightly wound) 22 winds per layer and with 22 layers make about a 500 turn pickup that would only need the top and bottom plastic plate to be about .5" wide. If you are not making a machine wind by doing it by hand, you will need more area for the coil.

Attach the coil wires to mic wire that is 2 conductor shielded cable. Place a thin stranded bare wire against the metal magnet coating before winding. Use an knife to cut plastic tape .125" wide to place a wind around the magnet binding the thin stranded wire to the metal coating on the magnet to ground it and keep the magnet wire from shorting out against the magnet coating. Run this 2 conductor shielded wire to an XLR connector and to either a mic mixer input with a 2400 ohm real input or a Shure mic matching transformer to boost this signal to be near what a 6000 turn high impedance pickup might produce but without the effects of coax capacitance on shaping the tone.

If there is noise and it stops when you touch the strings, the strings are re-transmitting noise into the pickup. In this case run a ground wire from the metal tailpiece to the ground connection in the pickup to minimize string induced noise.

Your most creative process will be mounting the pickup on your specific guitar. From the low tech method of using double stick tape to hold the pickup to using thin bungee cord warped around the end of the fingerboard to under the heel of the neck will help you mount it with minimal modification to your guitars.

I hope this helps?

Post a photo of your result.

Joseph J. Rogowski

[Diego Padovani]

Hello everyone! This is my first post.

I started reading these publications a few months ago, starting with zero knowledge on the subject. I've been fascinated with them ever since I heard about these types of pickups.
So this is my first prototype.

I used 2 neodymium magnets with 30 x 5 x 5 mm (1.18 x 0.19 x 0.19 in) wrapped with an awg 10 copper wire. (Have few other neo magnetics as options too)
The single loop was connected through the transformer hole, which is a ZHT103, 1000: 1A / 5mA. The pickup was attached to the guitar and connected directly to the UR22 audio interface, which has an input of 4 kΩ (mic) and 20 kΩ (line) and another input with 1 MΩ (Hi-Z).

So, here are my impressions:

- The pickup produced a fairly clean sound, sounding similar to an acoustic guitar, as expected due to producing a flatter response.
- With the pickup connected at input 2 with the Hi-Z on I had to put twice the gain I usually put in a normal pickup to have the same volume.
- I was not able to perceive a noise directly from the pickup, just the noise generated increasing the gain at the interface.
- As I installed the pickup directly on the guitar's pickguard, the magnets were pulling the guitar strings a lot.
- I didn't notice any difference in the output when using 2 transformers, probably I connected them in series (a wire connecting one transformer to the other).

A few questions I would like to share.

1- Is the transformer (through a hole) that I used bad? Do I need a thicker wire to fill the 5mm hole in order to improve its performance? This was the only transformer I got to buy here in Brazil.

2- I am not sure if I made the earth and input connections correctly. I think I forgot to ground the single loop.

3- I intend to use a 5 x 30 mm (0.19 x 1.18 in) thick copper plate to make the single loop. But I do not know exactly how to make a good design to look beautiful (like an Alumitone) while having a good joint connection in the single loop and simple to do. (see the sketch of a prototype)

4- I have some transformers removed from light bulb reactors, maybe it would be easier or better to do with some of them? The two at the back of the photo are identical, taken from one of these reactors. Perhaps it would be possible to make a joint similar to Lace Alumitone by building a ferrite core or other something like that?

5- Maybe I should make the design as simple as possible and then put a cover over it?

I wrote too much, didn't I?

Regardless, I would like to thank everyone for sharing so much information!

[bbsailor]

Hello everyone! This is my first post.

I started reading these publications a few months ago, starting with zero knowledge on the subject. I've been fascinated with them ever since I heard about these types of pickups.
So this is my first prototype.

I used 2 neodymium magnets with 30 x 5 x 5 mm (1.18 x 0.19 x 0.19 in) wrapped with an awg 10 copper wire. (Have few other neo magnetics as options too)
The single loop was connected through the transformer hole, which is a ZHT103, 1000: 1A / 5mA. The pickup was attached to the guitar and connected directly to the UR22 audio interface, which has an input of 4 kΩ (mic) and 20 kΩ (line) and another input with 1 MΩ (Hi-Z).

So, here are my impressions:

- The pickup produced a fairly clean sound, sounding similar to an acoustic guitar, as expected due to producing a flatter response.
- With the pickup connected at input 2 with the Hi-Z on I had to put twice the gain I usually put in a normal pickup to have the same volume.
- I was not able to perceive a noise directly from the pickup, just the noise generated increasing the gain at the interface.
- As I installed the pickup directly on the guitar's pickguard, the magnets were pulling the guitar strings a lot.
- I didn't notice any difference in the output when using 2 transformers, probably I connected them in series (a wire connecting one transformer to the other).

A few questions I would like to share.

1- Is the transformer (through a hole) that I used bad? Do I need a thicker wire to fill the 5mm hole in order to improve its performance? This was the only transformer I got to buy here in Brazil.

2- I am not sure if I made the earth and input connections correctly. I think I forgot to ground the single loop.

3- I intend to use a 5 x 30 mm (0.19 x 1.18 in) thick copper plate to make the single loop. But I do not know exactly how to make a good design to look beautiful (like an Alumitone) while having a good joint connection in the single loop and simple to do. (see the sketch of a prototype)

4- I have some transformers removed from light bulb reactors, maybe it would be easier or better to do with some of them? The two at the back of the photo are identical, taken from one of these reactors. Perhaps it would be possible to make a joint similar to Lace Alumitone by building a ferrite core or other something like that?

5- Maybe I should make the design as simple as possible and then put a cover over it?

I wrote too much, didn't I?

Regardless, I would like to thank everyone for sharing so much information!

[ATTACH=CONFIG]52622[/ATTACH] [ATTACH=CONFIG]52623[/ATTACH]

Diego, and all interested

Here are some things to consider when making these types of pickups.

The output impedance of the current transformer (CT) will be, at minimum, the DCR of the string loop wire times the turns ratio squared. AWG 10 is 89 micro ohms (uohms) per inch. The minimum length of the AWG 10 string loop, span the strings and go through the CT is about 7 inches. 7X89=623 ohms, assuming a very good string loop connection of very low resistance. Assuming you are using a 1000 turn CT, the output impedance will be about 1 million times the DCR of the string loop in micro ohms and that just eliminates the micro. That would indicate that the input impedance of your mixer or preamp should be about 6230 ohms to have less than a 1 db loss due to impedance loading. However, the leakage inductance typically adds about a 10 to 20 percent higher output impedance depending on how much of the CT primary opening area is occupied by copper wire. This now drives the input impedance to about 7.5K ohms.

Typical microphone inputs rated at about 150 ohms have about a 2.4K ohm actual impedance to become a bridging impedance for most low impedance microphones. This should now dictate the maximum output impedance of your pickup which now also dictates the string loop wire gauge size and CT turns ratio. AWG 8 is 52 uohms per inch and using a 7 inch length through a string loop going through a 500 turn CT gives you the following minimum output impedance. 7X52= 364 plus 10% leakage inductance is about 400 ohms. This 400 times 500 squared (CT turns ratio) is 400 uohms X 250,000 or about 100 ohms output impedance.

A good CT to try is the Triad CSE-186L which is rated at 1 to 166 turns ratio. Remove the 3 primary turns of AWG 16 wire to reveal a square opening of about .125 inches square. With a single primary turn, the transformer will now be a 1 to 500 turns ratio. File the plastic in the transformer primary area to accommodate AWG 8 wire. The AWG 8 wire occupies most the the primary area for less leakage inductance. Wrap a thin piece of tape around the metal frame to prevent the raw copper wire from shorting out to the CT frame. Insert about a 2 inch length .25 inch wide of a magnet or individual .25 inch magnets between the hairpin AWG 8 string loop. But first, find a low resistance way to join the AWG copper wire together to create a low resistance joint. This requires the most creative thinking and wire bending resources. Tip: copper tubing helps to make a good low resistance joint. Clean the wire and inside of the copper tube with very fine and paper and secure with silver solder. Any string loop overlap in your joint method would require a little extra string loop length, so plan ahead!!!

Attach 2 conductor shielded cable to each output, pins 2 and 3 on an XLR connector. Run a thin wire from the string loop to a sanded portion of transformer frame and then to the cable shield to minimize noise. The two center pins are not connected so you can use them for the shield connection. To not make any mods to an acoustic guitar here is a good trick. Wrap .125 inch diameter elastic rope (bungee cord) around the fingerboard end over the sound hole and then under the heel of the neck. Connect a stereo 1/8 inch female connector to the pickup using all three connection points and secure to the bungee cord. Then, make a male stereo connector connected to an XLR connector to plug in or disconnect with no permanent mods to your guitar.

For those wanting to quickly make a test pickup, try using it on an acoustic guitar. Here are some design considerations. The string loop may need to be made a little longer to accommodate a 90 degree bend for the CT to fit below the round sound hole opening or a 30 to 45 degree bend to accommodate sliding the CT under to wood top near the fingerboard.

For those who really want to try something new on an acoustic pickup, try this. Use seven 0.25 inch diameter magnets about 1/8 inch thick. Place the magnets so the strings go through the approximate .125 inch space between adjacent magnets so they do not pull too hard on the acoustic strings which only applies to the ferrous string cores. This will emphasize the initial strum second harmonics as in one side ways string cycle the string passes a magnetic field twice before it rotates. This CT pickup design gives experimenters an easy way to test different magnets and magnet layouts. If you do, report back here on your findings.

I hope this helps?

Joseph J. Rogowski

[Possum]

Its not really low impedance if you're converting it to high impedance with a transformer. I don't get the point of that. A real low impedance pickup is large wire with enough turns to satisfy a tube amp. Bigsby made those, Les Paul did too, no transformers.

[bbsailor]

Its not really low impedance if you're converting it to high impedance with a transformer. I don't get the point of that. A real low impedance pickup is large wire with enough turns to satisfy a tube amp. Bigsby made those, Les Paul did too, no transformers.

Possum,

If you look up the Les Paul Recording Guitar schematic you will see that the output of the low impedance pickups are directly feeding a 2.5K ohm volume pot and an internal transformer to allow an output before the transformer primary for connection to a low impedance mic mixer input or after the transformer (the secondary) for connection to a high impedance guitar amp. By using my rules of thumb that a typical volume pot value is about 40 times higher than the DC resistance of the pickup. 2500/40 is 62.5 ohms. Another rule of thumb is that AWG wire gauges that are 10 sizes thicker have one tenth the resistance of the thinner wire. Thus, 6000 turns of AWG 42 at 6000 ohms would be 600 ohms if using AWG 32. But, since the low impedance pickup has about one tenth the number of turns of the high impedance pickup, we now have 60 ohms which is in close alignment with the pot loading rule of thumb.

My use of current transformers to match typical 150 ohm mic impedance ratings can range up to about 250 ohms. That is why the typical low impedance mic input impedance is 2,400 ohms to act as a bridging impedance to minimally load the mic or low impedance pickup output voltage level. By using transformer theory, I can predict the output impedance of a heavy gauge string loop wire (AWG 8) going around a magnet and through a current transformer (CT) primary by doing this: multiply the CT turns ratio squared times the DCR of the string loop to estimate the CT output impedance. AWG 8 is 52 micro ohms per inch. A 7 inch string loop would be 7 X 52 or 364 micro ohms. Now add about 10% for leakage inductance and you have near 400 Ohms. Multiply 400 micro ohms by 250,000 and you now have an output impedance near 100 ohms, perfect for a low impedance mic input for less than 1 db input loading losses. If a 1000 turn CT were used, the output impedance would be about 4 times higher and the mic input loading would be higher.

This thread is all about what I have learned from tinkering with this stuff and sharing it with MEF members.

Thanks

Joseph J. Rogowski

[big_teee]

This is the most watched thread on the Pickup Forum.
With over 100,000+ views!
Because of it's popularity, stuck the thread to the top.

[bbsailor]

This is the most watched thread on the Pickup Forum.
With over 100,000+ views!
Because of it's popularity, stuck the thread to the top.

Big Teee and other forum members,

I have been sharing my private research using current based guitar pickups for a few simple reasons. (1) easy construction (2) less electric guitar typical sound due to resonant points being near where the ear is the most sensitive and (3) targeting a low impedance mic input typically 2400 ohms (actual input impedance) being a good load for microphones in the 150 to about 250 ohm range (4) simple access to over the counter current transformers (CT) to make experimentation very easy.

Here is a little more detail. I initially used a Triad CSE-187L 500 turn CT which works very well but found that the pre-installed AWG 12 primary CT turn tends to restrict the current even when a thick string loop wire is used because the 1.8 inch primary is in series with the thicker string loop using 2 solder joints. When I tried the Triad CSE- 186L current transformer rates at 166 turns, i simply removed the three turns of AWG 16 primary wire that is pre-installed to open a large square area to accommodate an AWG 8 wire going through the transformer opening and continue for a few inches to make a string loop to span about 2.25 inches with some room to bend the transformer 45 degrees to 90 degrees to easily fit an acoustic guitar round hole opening or to slide the pickup under an electric guitar with the transformer bent up. This way you can build a low impedance pickup attach the 2 conductor shielded mic wire and run it directly into a mic XLR input or put a Shure A95U matching transformer directly into the guitar amp input and play the guitar without any major modifications.

Once you have a working CT based pickup, you can easily experiment with magnet, (1) strength, (2) type, (3) covering, (4) size and (5) layout to listen to sound changes as well as string loop wire gauge sizes and stranding to hear the effect on the sound.

Solid AWG 8 at 52 micro ohms per inch makes a good pickup in the 104 to 120 ohm range for a length of 8 inches making the hairpin string loop to fit directly in the CSE-186 primary opening with tape on the transformer lamination, on each side, to prevent the string loop from shorting against the transformer lamination. Working with this thick wire and making a good low resistance joint to finish the string loop can be a challenge. then I discovered this wire bundle:https://www.goldmine-elec-products.c...?number=G22583. This fine stranded wire has less skin effect than the solid wire and is much easier to work with, especially on making an acoustic guitar pickup. I use a copper crimping sleeve by Garden Bender, part number 10-301C for wire gauges 18 to 10. First, I squeeze the round sleeve into an oval and push the two ends of the string loop through the sleeve. Clean the sleeve with contact cleaner to make the lowest resistance joint. Compress the sleeve on the wire and use silver solder on the end to fully secure of the strands to the sleeve. I get a reading of about 180 ohms on my Extech LCR meter using this stranded copper wire.

With my pickup using an epoxy coated magnet 2.25 long by .375 wide and .25 high with a .125 inch wide groove on each side to hold the wire better to the magnet (K&J Magnetics). With the pickup .5 inches below my low E electric guitar string I get about 10mv and about 5mv from the high E string at the same location below the strings. On an acoustic guitar with a thin magnetic string core on the low E string, the output would be closer to the same.

I ordered a 100 epoxy coated .25 inch diameter by 1/8 inch thick Neo magnets from K&J Magnetics part number D42E. I chose epoxy coated to eliminate any eddy currents that would be generated in the metal coating of some of their models. I use ice cream pop sticks to make test pickups.

Stype 1 places 6 magnets to fit directly under the string near the end of the fingerboard on an acoustic guitar. Then the string loop goes around these magnets and the XLR cable hot wires are soldered to the two end pins on the CSE-186L CT. The shield goes to the string loop and to a jumper wire from the string loop to the CSE-186 metal transformer frame. Clean the glue or other stuff off the transformer frame before soldering to ensure a good joint. This will ensure a good low noise pickup.

Style 2 places the strings between adjacent magnets spaces about .125 inches apart. Use 7 magnets so the string is sitting exactly in between the magnets. This allows the magnets to be a little closer to the magnets as it sits in a slightly null zone. This setup will emphasize the 2nd harmonic of the initial pluck or strum as it generates a pulse at each end of the string sideways swing. Then you can slide the pickup sideways to put the string directly over the magnets and hear the effect of this magnet position relative to the strings. Fender did this on their bass guitars to minimize the fundamental tone called the the muddy sound on their early electric bass guitars.

Things to consider when making CT based pickups. The output impedance is very close to the total resistance of the string loop wire times the CT turns ratio squired. Using AWG 8 at 52 micro ohms per inch makes an 8 inch string loop of 416 Micro ohms. Just move the decimal point 6 places left to do the math. .000416 ohms times 500 squared is 250,000 times .000416 or 104 ohms as the theological lowest resistance, minimum output with the highest current and highest output. Since the joint that completes the string loop will add some resistance the output will have a slightly higher resistance with slightly less current and a little less output. Now, look at the area of the transformer primary space that is occupied by wire and you have leakage inductance that also raises the output impedance some. Once you tinker with this stuff enough you will discover these things for yourself and hear how it all affects the sound you hear from your own creations.

I am glad I have stimulated many readers to get me over the 100,000 mark. Thanks to all of you that have read, supported and experimented based on my "Low Impedance Pickup Research" postings.

Thanks

Joseph J. Rogowski

[Diego Padovani]

Many thanks for sharing your knowledge Joseph and all folks! It's really nice that you still reply our questions up today! I've learnt so much by reading the posts on this forum.

Here are some of my progress moths ago.
While I don't put my hands on a better transformer... I'm thinking to do some tests with the one that I got.

I'm using a 5 mm (0.19 in/ ~4 AWG) copper bar to make the primary. It still needs some cuts to do. (picture).
I pretend to do the lower cut to fix its height, then use the cut leg to make the connection between the primary legs, closing the primary circuit. The connection is shaped to fill the gap using just pressure.

So I think it will work, but not sure if it will increase or decrease the resistance, when compared with a solder joint.

Anyway, Ill post the results here.

[bbsailor]

Diego,

Try this low current Kemet Current Transformer. https://content.kemet.com/datasheets/KEM_SE0203_CT.pdf

The most important task is to make a very low resistance connection by fitting the CT in the slot and putting a piece of copper tubing in the CT hole and then placing a screw sideways to clamp the metal on the slot sides against the copper tube to make a good low resistance connection. The copper tube should be just a little longer than the current transformer thickness so you do not crush the CT when you add the screw. Try both the 500 turn CT and the 1000 turn CT and use either a mic mixer XLR connection or use the Shure A95U type transformer. Make sure you clean the spots where the metals connect to get the lowest resistance connection.

Your design is a very innovative way to make a string loop with a current transformer located on the side and under the magnet(s). If you get the CT too close the the magnet(s), you may want to use a piece of ferrous metal to shield the CT from the magnetic field.

Joseph J. Rogowski

[Singer15]

I have built a few Lo Z pickups with the help of forum members. They are about 5mm thick and use a neod magnet 50mm long, and 500 to 750 turns of 36 ga. This will give a very clean sound into my Henricksen Jazz amp, through the XLR or the regular jack (with a mic xformer). The xformer increases the signal but is not strictly necessary. However, once I introduce a volume pot, it only works well thru the xfromer. Otherwise the pot introduces horrible static. I don't know why that is. Maybe someone can explain it? I also wonder why Gibson used resistors in the Les Paul Lo Z guitar circuits?.
I am very pleased with these thin pickups, and they can be simply attached to a guitar top with blue tack. They have no hum and are very even sounding, perfect for acoustic guitar. I plan to install one permanently on an archtop guitar however, with volume and tone controls, but this static is a puzzle.
Don Nathan

[bea]

What do You mean by static?

I am running a 500 turns LoZ pickup with a 2.5-k-Pot (and a C-Switch), into an assymetric cable. No probs. But it was also ok, when i sent the signal through a symmetric cable into my mixing console. So i do net get the problem.

[Singer15]

Using the volume pot creates the static, which is very loud. My cable is not balanced, as I use a single conductor wire with shield. Is it possible this is coming from the 48v phantom power the amp puts out on the xlr? I don't have a schematic and the manufacturer, Henriksen, did not reply to my request for help. I use a 10k pot, and this seems to work ok through the xformer side.

[babysnake]

The correct way to wire this is to make sure the XLR ground goes only to shielding on the guitar, and not any part of the signal path. If the volume control is connected only to the XLR (+) and (-), there should be no noise because the +48V is present on both of them.

Using the volume pot creates the static, which is very loud. My cable is not balanced, as I use a single conductor wire with shield. Is it possible this is coming from the 48v phantom power the amp puts out on the xlr? I don't have a schematic and the manufacturer, Henriksen, did not reply to my request for help. I use a 10k pot, and this seems to work ok through the xformer side.

[Singer15]

On my amp there is no switch for the 48v phantom power, which comes on the #1 pin. Without connecting that pin there is no signal. No problem with xlr on an amp without phantom power. Does anyone have a way to switch the 48v? Henriksen Jazz amp 310.

[babysnake]

On my amp there is no switch for the 48v phantom power, which comes on the #1 pin. Without connecting that pin there is no signal. No problem with xlr on an amp without phantom power. Does anyone have a way to switch the 48v? Henriksen Jazz amp 310.

How did you check for voltage? If you saw voltage between pins 1-3, you should also see voltage on 1-2. Phantom power should appear on pin 2 and 3, pin 1 is common. If your wiring is non-standard, that is the source of your problem. Again, the voltage between 2-3 should be negligible, and won't affect any microphone that doesn't use the common except as an electrostatic shield (such as passive mics). You should do the same with your guitar. Use a balanced connection to the guitar circuit and your problem won't exist.