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.
Joseph J. Rogowski