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Low Impedance Pickup Research

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  • Nick,

    Here are a few things to consider when selecting the right size, shape and coating for your magnet selection on your current transformer based pickup. Consider using a neo type magnet due to its strength, especially important when using a thin height magnet.

    1. Size. The magnet height should be based on how high the strings are from the guitar top when presses at the last fret on the neck. This is usually based on the fingerboard thickness plus the fret height. I have found that getting a magnet that is 0.25 inches thick makes testing on existing guitars very easy. Since string spacing width is about 2 inches wide, select a magnet that is 2.25 inches long to ensure that the two E strings have full magnetic coverage for string vibrations. A magnet width of 0.375 inches is a good starting point but a 0.25 inch wide will work also.

    2. Shape. A rectangular shape with the magnet magnetized through the thickness is a good start for your design. However you can get what is called a stepped in design magnet that contains a semi circular groove along the length of the pickup magnet on each side about 0.125 inch wide will make mounting the string loop wire to the magnet easier using a heat shrink tubing. If you want to fine tune the individual string output due to the height difference, a flat (not stepped) magnet side wall will make shaping the string loop to follow the arch of the fingerboard curve easier.

    3. Magnets come with many selections of coatings. I would select an epoxy coating to avoid shorting out a bare copper wire AWG 8 string loop going through the current transformer. If you choose to use a metal coated magnet, you will be generating eddy currents in the metal coating and affect the pickup tone. If you choose to try it, just place a piece of tape over the side of the magnet to prevent shorting out the string loop to the conductive magnet surface.

    Many magnet supply vendors will allow you to select a magnet type, size and coating in addition to the common sizes they offer on their web sites.

    Another key design consideration is the current transformer turns ratio. Try using CTs with 1000, 1500, 2500 and 5000 turns to get a good high impedance output. Balancing a strong guitar sound signal with noise and tone is a balancing act that your ears can select. However, when trying to fine tune some alternatives, you will need access to some test equipment.

    Before selecting your final magnet size, type and coating, plan on how you will mount the pickup to the guitar. Typically it should fit under the strings with no cutout needed under the strings. The only cut out should be about a cubic inch for the current transformer. Plan on stuffing some foam in the current transformer opening to prevent any movement or vibrations between the string loop wire and the current transformer. To minimize noise, make sure that you ground the string loop.

    Let us know what type, size, coating and shape magnets you are using. There are many magnet vendors on the web. Experiment with different magnets and wire sizes and let your ears point you in the right final design direction.

    Joseph J. Rogowski
    Last edited by bbsailor; 10-28-2023, 04:40 PM.


    • Hi, I have been busy building my new guitar to utilize the low Z pickups.
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      In the neck is the Aluminum single loop with ceramic strat magnet and with the 1:1000 transformer. I added a steel plate under magnet which shifted the resonant peak and improved output. I used hot glue on the magnet and plate and it's keeping the microphonics to a minimum. I had tried some neodymium magnets which were far too strong. The current transformer is installed from the back of the guitar with short 18ga wires

      The bridge pickup is a 3 loop single coil of 18ga wire wrapped around a ceramic magnet and with 2 primary loops thru the transformer to give a 1:500 ratio. To make the pickup I glued two plastic flanges to the magnet, wrapped the wires around the magnet and then fed the wires thru an access hole to the transformer. The pickup is thin enough not to require routing on the top of the body. It is a little microphonic, so potting or glue might be needed.
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      I wanted a very light guitar, so a headless construction made sense for balance. The neck is a beheaded strat copy. The body is from a repurposed pine bed frame with a center strip of oak to give it a little strength. The thickness is 1.35 inch which makes it very light. I was concerned about the strength of the pine and rightly so as during shaping of the body contours it developed a large crack that had to be glued. I then added some areas of hardwood in the neck pocket, and electronics cavity. Routing in the center of the body was kept to a minimum. I also added a steel plate under the tuners. It seems to hold the 9 gauge string tension well. With careful design I was able to mount the standard tuners with straight string pull past the bridge and a sufficient break angle. Tuning access is a bit cramped, but seem to hold pitch well. Besides the cost savings of this headless tuning setup, it has the advantage that no clamping is needed at the nut end.

      The electronics are simple, a 250ka volume pot and two mini switches. One switch for pickup selection and a tone switch adds capacitance to ground. Currently using a 4.7nf to cut off sometimes piercing highs and a 15nf to get in humbucker range.

      The guitar and pickups produce clean and bright sounds and are quiet unless using high gain. Since the output is a little lower than even a Strat, higher gain setting are needed to get overdriven tones. More real world experience is needed to see if these pickups are a success.
      I uploaded a sound clip played into my Fender HRD, recorded into my phone's mic. Bridge pickup then alternating with the neck.


      • Nick

        Nice job in trying something new!

        To quell the microphonic noise, put some packing foam in the current transformer hole where the wire goes through. This will prevent the wire vibrating near a loud amp or vibrations from any noise created with an contact or impact on the guitar body.

        Using thicker and a shorter length of wire around the magnets will enhance the lower guitar frequencies. What test equipment do you have to do electrical measurements?

        What is your current goal for improving the sound of you low impedance pickups?

        I listened to your audio recording and heard some string fret buzzing. Either adjust the neck, raise the string height or go up one gauge in string thickness to make a cleaner sound.

        Whatever gauge wire you choose to go around the magnets and through the current transformer, check the AWG wire table to see the frequency of the skin effect where the output of higher frequencies starts tapering off through less of high frequency energy being converted in the current transformer.

        When designing pickups let your ears be the guide then measure what sounds good and take good notes about your measurements. In my previous posts I covered what variables affect the low impedance pickup sounds.

        The mechanics of a low impedance current driven pickup have some practical physical design issues to consider.
        1. Keep the pickup thickness small so it can slide under the guitar strings with no hole needing to be cut. Look for magnets that are 2.25 inches long, 0.25 inch to 0.375 inch wide and no thicker than about 0.25 inch tall to easily fit under the strings.
        2. Locating the current transformer close to the pickup and magnet to keep the wire as short as practical. Bending the heavy wire loop down 90 degrees locates the current transformer near the pickup and only requires about a one cubic inch hole to fit the current transformer in. The wood in line with the neck and bridge stays uncut and provides a better mass for the strings to vibrate.
        3. Estimate the output AC resistance of the current by following information that I have posted before. Try to make the volume pot value about 10 times higher than the current transformer output AC resistance when the copper or other metal string loop is attached.
        4. Sample current transformers that have different published frequency ranges to determine what your ears say sounds better.
        5. You can decide if you want active electronics mounted in the guitar but you will need to change the battery when it goes dead. You can make a buffer-amplifier mounted in the plug end of the guitar jack by going to the web site where Tillman shows how to construct this device where the batteries or power source is located outside the guitar body.

        Keep up your good and creative work. Please keep us forum members updated about your progress or design issues needing some insight.

        Joseph J. Rogowski


        • Thanks for the feedback Joseph. I haven't quite finished the guitar and I hadn't done a good setup. The nut was too low and I've been trying various ideas for the bridge. I think the output is acceptable passive going to an amp, but playing with my band will tell the tale. I like the jangly sounds I'm getting. I have put a Tillman preamp in another guitar, not much space to do it on this one.


          • Interesting update: In the new guitar I had the 2 pickups in middle switch position wired in parallel which was ok, but tried wiring the transformers in series and I really like it. Gives a nice boost to the mids.


            • Last night I brought the new guitar to band practice for some real world experience. Had to use a clean boost or Eq pedal to get the output up to useable levels with the amp I was using. Once boosted the guitar performed well clean as well as with low or moderate amounts of overdrive. Even with higher gain it was pretty quiet, about on par as a noisy humbucker, but well below that of a typical single.