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Thread: Low Impedance Pickup Research

  1. #316
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    There is a lot to do, even in prototypes (reduce skin effect, maybe also proximity effect).
    Skin and proximity effects have no influence on the bass response of your prototype: Skin and proximity effects essentially vanish at low frequencies.
    The skin depth at 160 Hz is 5.24 mm. I.e. a wire of 10 mm diameter shows no resistance increase by the skin effect at 160Hz and lower.
    The proximity effect requires the proximity of current carrying wires. A sole wire loop does not develop a proximity effect.

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    The only way to replace the transformer with an active circuit would require very manyf devices (for example, junction transistors) in parallel. It would be huge and consume a too much power.

    The way to replace the transformer is to use a coil with about 500 turns.

    If I understand, the proximity effect is like the skin effect, but resulting from the magnetic field produced by current in other turns. I do not think that this is an issue in a 500 turn pickup coil since the normal resistance of the wire would dominate except at very high frequencies.

    Quote Originally Posted by bea View Post
    Josef,

    as soon as You have active electronics in Your instrument anyway, the idea of the transimpedance amplifier will become interesting, especially if we find a cheap traditional (low noise, low power consumption) opamp that will do the job. Which actually means that i'd have a lot t learn until i could make some prototype. (i must have a few TL071 in my box... ) The active design will be more compact than the passive design and allow for traditional shapes or pickups that could be fit to an archtop

    But anyway: at least for the new future i'll concentrate on the passive version. There is a lot to do, even in prototypes (reduce skin effect, maybe also proximity effect).

    Do You think, something like nail polish or some other NC laquer would suffice as an insulation for a stranded wire? I would like to use the wire of electrical installation material, e.g. 1.5 mm^2, diameter 0.67 mm.

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  3. #318
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    Quote Originally Posted by Helmholtz View Post
    Skin and proximity effects have no influence on the bass response
    Indeed. But potentially a bit on the treble response: at 10 kHz the layer thickness is roughly 0.7 mm. Which translates into "at that depth the effective specific resistivity of the wire has almost tripled". Might already be noticeable regarding the wire thickness of 3.6 mm

    Proximity effect - there is more than one. The 2nd type is caused by strong magnetic fields close to a wire. Although i tend to neglect that i would like to understand it better.

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    Quote Originally Posted by Mike Sulzer View Post
    The only way to replace the transformer with an active circuit would require very manyf devices (for example, junction transistors) in parallel. It would be huge and consume a too much power.
    If a transimpedance amplifier is made using an opamp (there are even lists of suitable types on the net) its input impedance will be reduces by the negative feedback.

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    ..at 10 kHz the layer thickness is roughly 0.7 mm.
    This means that there will be no noticeably effect for wire diameters <= 1.4mm. For bigger diameters the resistance will slowly increase proportional to (f)^0.5. But the final effect depends on the ratio of load impedance to the reflected DCR of the loop. A few hundred more Ohms of reflected resistance at the secondary will hardly be noticeable if the CT sees a load of 2.5k. Even if so, the easiest way to compensate is using the treble control of your amp.

    The 2nd type is caused by strong magnetic fields close to a wire.
    But not by DC/permanent fields.

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    Last edited by Helmholtz; 09-30-2018 at 09:07 PM.
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    Maybe. IMO the skin effect might already have an effect on the current induced in the primary loop. Same argument why Josef always pointed out the importance of a large cross section, just for the higher end of the audio spectrum.
    Anyway, it will be a simple experiment to use a few loops of 1.5 mm^2 wire, all through the CT and possibly interconnected (although i don't thing that will be needed because the CT will always see the sum of the currents in all wires through it).
    If it works at least as good, it should have the additional advantage of being easier to solder, and it would also allow for flatter setups.

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    Quote Originally Posted by bea View Post
    If a transimpedance amplifier is made using an opamp (there are even lists of suitable types on the net) its input impedance will be reduces by the negative feedback.
    There is no need to decrease the input impedance. The purpose of paralleling devices is to lower the total noise voltage, and this is not practical for such a low voltage. You must either use a transformer or put enough turns on the pickup coil.

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  8. #323
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    Quote Originally Posted by bea View Post
    Maybe. IMO the skin effect might already have an effect on the current induced in the primary loop. Same argument why Josef always pointed out the importance of a large cross section, just for the higher end of the audio spectrum.
    Anyway, it will be a simple experiment to use a few loops of 1.5 mm^2 wire, all through the CT and possibly interconnected (although i don't thing that will be needed because the CT will always see the sum of the currents in all wires through it).
    If it works at least as good, it should have the additional advantage of being easier to solder, and it would also allow for flatter setups.
    Don't get me wrong. Skin and proximity are high frequency effects. The large cross section of the primary loop, however, is necessary for good bass response. The lower corner frequency of a transformer is given by 2pi*f=Rp/L, where L is the inductance of the primary loop and Rp is given by the parallel resistance of the primary loop and the reflected secondary load resistance. As L is small, Rp has to be very small.

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    Please don't misunderstand me - i do not intend to reduce Rp. Just use a few thinner wires in parallel.

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    Quote Originally Posted by bea View Post
    Please don't misunderstand me - i do not intend to reduce Rp. Just use a few thinner wires in parallel.
    If a single turn pickup made with a single large conductor does not give enough high frequencies, then instead of putting many wires in parallel, put them in series, thus making a multi turn pickup and use a different or no transformer.

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  11. #326
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    Helmholtz, Mike, Bea and other forum members,

    To calculate the inductance of a single transformer turn (typically what I call the string loop) is done by doing this. This string loop and transformer loop is only a 3/4 loop around the transformer core.
    1. Set the Extech 380193 LCR meter to 120 HZ measurement frequency.
    2. With the primary open and the Extech LCR attached to the secondary of the CSE-187L current transformer, I get a 1H reading.
    3. Divide the number in step 2 above by the turns ratio squared. In this case it is 500 squared or 250,000.
    4. This answer or 1/250,000 is 0.000004 or 4 micro Henries per coil turn going through this particular transformer core.

    The CSE-187L uses 1.8 inches of AWG 12 wire at 132.4 micro ohms per inch for a total of 238.32 micro ohms. Using transformer theory I then calculated what the ideal impedance of this single turn loop shorted loop should be. I took a needle nose pliers and bent the AWG 12 winding inward sort of like if you put one of your index fingers over the finger nail of your other index finger. Then I pulled them apart and bent the wires so they could be in line and touch each other sort of like if your two index fingers are now directly touching each other with a slight pressure still keeping these wires touching. Now, I use silver solder to join this joint with a blob of silver solder to make a shorted loop to measure. The measured output impedance of this shorted CSE-187L is 80 ohms after the joint has cooled. Transformer theory teaches us that it should have been close to 60 ohms. Why the difference?

    Leakage inductance represents one of the major losses in current transformer based pickups and here is why. How much of the primary winding area is occupied by current carrying wire? Even if you connect the CSE-187L to very a thick AWG 4 string loop wire by drilling holes into the U- shaped ends to accommodate the AWG 12 primary turn wire diameter. Since these are in series, the most resistance losses are in the AWG 12 primary turn. The CT primary turn only makes a 3/4 turn with 1/4 of the turn open to add to leakage inductance.

    Once you know the shorted primary impedance reading of your chosen CT, you can just add the resistance of the string loop to that impedance to get a very close estimate of the total output impedance of your CT pickup design using the wire size you choose to use.

    I obtained a Triad CSE-186L current transformer and I removed the 3 primary turns of AWG 16 and it revealed a primary turn opening of about 4 mm square which could accept AWG 8 wire to form the string loop as well as the CT primary turn but with only one joint to form the continuous current transformer (CT) primary and string loop with the neo type magnet (.250 inch wide by 2 inch long, .125 thick) in the middle. I obtained an Extech LCR output impedance reading close to 100 ohms. After further investigation and calculations, I came to the conclusion that occupying more primary coil with the CSE-186L area (primary removed) made a significant contribution to these measurements and improved output level.

    Now a balancing act occurs with skin depth and maximum current development with a simple solution. Round wire in a square transformer primary opening and round wire creates four small areas in each corner of the opening in which to run a thin wire to be placed on top of the wire now allow creative pickup designers to run 4 smaller wired on top of the thick string loop by using spiral wrap to secure the wire location. If the B string is too loud just dip the thin wires lower under the B string.

    The evolution of this pickup technology has just begun!!!

    Joseph J. Rogowski

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    Last edited by bbsailor; 10-01-2018 at 12:23 AM.

  12. #327
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    Concerning leakage flux:

    Transformer cores contain nearly all of the flux produced by current through a winding. At the bottom of the frequency the effect of the little bit of leakage flux is irrelevant. The effect becomes important at high frequencies. Suppose the transformer must work from 20 HZ to 20 KHz. Then the effect of the leakage flux (a small inductance in series, say, with the secondary) is 1000 times greater at the top of the range than at the bottom. Leakage flux limits the bandwidth of the transformer.

    So I do not see how leakage explains the measurement.

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  13. #328
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    Transformer cores contain nearly all of the flux produced by current through a winding.
    True, especially for high toroidal cores. But good coupling requires the wire to be closely wound around the core. The string loop instead runs outside the core for most part and I think there must be some small uncoupled inductance, given approximately by the free air inductance of the loop. A single wire has a typical inductance of 10 to 20nH/cm depending on the shape of the loop (the inductance value tends to zero for zero loop area, of course).
    If I assume the uncoupled part of the inductance to be around 100nH, this gets stepped up to a serial inductance of 25mH (or a serial reactance of 19 Ohms@120Hz) at the secondary. It should be possible to (approximately) measure this inductance at the secondary (string loop closed) with the Extech.

    It would be interesting to repeat the measurements at a higher frequency (1kHz) to see if the secondary impedance rises accordingly.

    Edit: Forgot to mention that also the DCR of the secondary coil appears in series with the stepped-up string loop resistance at the output.

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    Last edited by Helmholtz; 10-01-2018 at 06:06 PM.
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  14. #329
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    I think the inductance looking back into the secondary is about 1H on such a system. Also, I believe on the primary side it is possible to think of an inductance looking into the primary of the transformer, and another inductance looking back into the pickup even though it is only one turn. Also you want to make the inductance of the former larger than the latter to avoid attenuating the signal. That is, even though we might call these things current transformers because their normal application is to sense the current through a path without affecting the circuit much, in this application the pickup puts out a very small voltage, and the transformer steps it up to a much higher voltage.

    Quote Originally Posted by Helmholtz View Post
    True, especially for high toroidal cores. But good coupling requires the wire to be closely wound around the core. The string loop instead runs outside the core for most part and I think there must be some small uncoupled inductance, given approximately by the free air inductance of the loop. A single wire has a typical inductance of 10 to 20nH/cm depending on the shape of the loop (the inductance value tends to zero for zero loop area, of course).
    If I assume the uncoupled part of the inductance to be around 100nH, this gets stepped up to a serial inductance of 25mH (or a serial reactance of 19 Ohms@120Hz) at the secondary. It should be possible to (approximately) measure this inductance at the secondary (string loop closed) with the Extech.

    It would be interesting to repeat the measurements at a higher frequency (1kHz) to see if the secondary impedance rises accordingly.

    Edit: Forgot to mention that also the DCR of the secondary coil appears in series with the stepped-up string loop resistance at the output.

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  15. #330
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    I think the inductance looking back into the secondary is about 1H on such a system.
    As bbsailor wrote, 1H is the secondary inductance (measured with open primary loop). It is given by 500 times the AL value of the core. If the primary loop/circuit is closed, the impedance measured at the secondary is given by the secondary open loop inductance in parallel with the stepped-up/reflected primary loop/circuit impedance + the DCR of the secondary coil.

    Measurements taken with my LCR meter on a different "CT" with and without a shorted primary loop in principle confirmed my theoretical considerations (based on transformer equivalent circuit, which is the same for "current" and "voltage" transformers). For instance, the inductance value at the secondary increases with the area of the shorted primary loop.

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  16. #331
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    Quote Originally Posted by Helmholtz View Post
    As bbsailor wrote, 1H is the secondary inductance (measured with open primary loop). It is given by 500 times the AL value of the core. If the primary loop/circuit is closed, the impedance measured at the secondary is given by the secondary open loop inductance in parallel with the stepped-up/reflected primary loop/circuit impedance + the DCR of the secondary coil.

    Measurements taken with my LCR meter on a different "CT" with and without a shorted primary loop in principle confirmed my theoretical considerations (based on transformer equivalent circuit, which is the same for "current" and "voltage" transformers). For instance, the inductance value at the secondary increases with the area of the shorted primary loop.

    Thanks, I see. So you have a "pickup" with an inductance of about 100 nanoH looking into a transformer primary of about 4 microH, and so the inductance of the transformer is large enough so that the voltage of the pickup is not significantly attenuated.

    In the case you mentioned you have about 25 mH looking back into the transformer secondary with the pickup loop connected. So if the pickup is loaded by the actual input impedance of the mic preamp (assume 2500 ohms), then the high frequency roll off of the system is at

    2500/25e-3/2/pi ~ 16 KHz

    But there also must be some leakage inductance that we can think of as in series with the secondary of the transformer, and so this lowers the high frequency response some more.

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    Yes, that's the model, the way I see it. The 100nH outer uncoupled primary loop inductance is just a rough guess, though. Could be off by a factor of 2 or so.

    The 4H primary inductance shunts the reflected load as seen from the primary, and its magnetizing current limits bass transfer. The most effective way to improve bass response is to lower the resistance of the primary loop. The reason is that the increasing magnetizing current at lower frequencies causes a voltage drop across the loop resistance proportional to 1/f and thus reduces the available transformer primary voltage. Lower resistance decreases this effect and shifts the roll-off to lower frequencies.

    Any leakage inductance just adds to the 25mH at the secondary and further lowers the upper corner frequency. A good LCR meter should be able to read the total secondary serial inductance Ls at 1kHz.

    Bass response could maybe be improved a little by a lower load resistance but at the cost of worse high frequency response.


    Are there any frequency response measurements of such LoZ "CT" PUs?

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    Last edited by Helmholtz; 10-06-2018 at 03:45 PM.
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    If you use a Triad CSE-186L current transformer with the primary three turns of AWG-16 wire removed you can insert an AWG 8 solid wire as both the primary transformer turn with an extension to become the string loop at about 53 micro ohms per inch. This method only requires a single joint to make the string loop, unlike two joints required to connect a string loop to connect to a pre-installed CT primary connection..
    We all know that AWG 42 has much less area than AWG 8. AWG 8 has about between 2614 to 2662 less area (depending on whether you use inches or mm) to compare how many turns of AWG 42 would occupy the same area as AWG 8.

    Packing the equivalent of about 2650 turns of very thin wire (which is the same area as AWG 8) compacted in a pickup very close to the string inducing the voltage/current in the string loop is more efficient than trying to induce the same voltage/current in a high impedance pickup with AWG 42 wire distributed about .75 inches from the top wingdings with having a higher induced voltage than the lower induced voltage in the lower winding.

    The efficiency of the Current Transformer (CT) pickup depends on a new set of criteria. These now include:
    1. Transformer turns ratio
    2. Permeability of the CT core
    3. Resistance of the CT primary with the string loop being in series
    4. Diameter of the string loop wire with the skin effect taken into consideration
    5. Creative use of wire and wire sizes to affect the audio effect of CT pickup design variables.

    The difference between the ideal transformer theory and real transformer theory creates a filter defined by the variables discussed this forum thread.

    As more of the MEF members tinker with this stuff and present their results, we will all benefit from a worldwide design evolution.

    Joseph J. Rogowski

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  19. #334
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    Quote Originally Posted by bbsailor View Post
    Packing the equivalent of about 2650 turns of very thin wire (which is the same area as AWG 8) compacted in a pickup very close to the string inducing the voltage/current in the string loop is more efficient than trying to induce the same voltage/current in a high impedance pickup with AWG 42 wire distributed about .75 inches from the top wingdings with having a higher induced voltage than the lower induced voltage in the lower winding.
    2650 turns of #42 occupies a lot less than .75 inches height if the coil has the width of a standard humbucker bobbin, and you can use a wider bobbin if you want, further reducing the height of the coil. But I think a better comparison is to a coil of 500 turns of larger wire, which is easier to work with. This would have a mic. preamp level/impedance similar to the output of the CT.

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  20. #335
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    Quote Originally Posted by Helmholtz View Post
    Yes, that's the model, the way I see it. The 100nH outer uncoupled primary loop inductance is just a rough guess, though. Could be off by a factor of 2 or so.

    The 4H primary inductance shunts the reflected load as seen from the primary, and its magnetizing current limits bass transfer. The most effective way to improve bass response is to lower the resistance of the primary loop. The reason is that the increasing magnetizing current at lower frequencies causes a voltage drop across the loop resistance proportional to 1/f and thus reduces the available transformer primary voltage. Lower resistance decreases this effect and shifts the roll-off to lower frequencies.

    Any leakage inductance just adds to the 25mH at the secondary and further lowers the upper corner frequency. A good LCR meter should be able to read the total secondary serial inductance Ls at 1kHz.

    Bass response could maybe be improved a little by a lower load resistance but at the cost of worse high frequency response.


    Are there any frequency response measurements of such LoZ "CT" PUs?
    You might know that a guy in Germany (GBB forum) is currently doing such pickups - and he is trying to measure the responses.

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  21. #336
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    Quote Originally Posted by bea View Post
    You might know that a guy in Germany (GBB forum) is currently doing such pickups - and he is trying to measure the responses.
    Haven't seen a frequency plot yet.

    Regarding bass response:

    A 1:500 CT showing an open loop secondary inductance of 1H produces a 4H inductance in the single primary loop, as mentioned above. If the wire loop has a resistance of 1mOhm, the lower corner frequency calculates to about 40Hz, using the formula in post #323.

    If the copper wire has a cross section of 10mm, the loop resistance will be around 0.25mOhm. This results in a corner frequency of 10Hz, which isn't bad at all.

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  22. #337
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    Quote Originally Posted by Helmholtz View Post
    resistance will be around 0.25mOhm. This results in a corner frequency of 10Hz, which isn't bad at all.
    Which
    a) is consistent with the impression i got from playing it with bass strings through my headphones and
    b) means that we have plenty enough of tolerance in the low frequency range which will allow us to play with different (slimmer!) cross sections or material with larger specific resistivity (even brass at 6-10 mm^2 ...)

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    Quote Originally Posted by bea View Post
    Which
    a) is consistent with the impression i got from playing it with bass strings through my headphones and
    b) means that we have plenty enough of tolerance in the low frequency range which will allow us to play with different (slimmer!) cross sections or material with larger specific resistivity (even brass at 6-10 mm^2 ...)
    bea and all MEF members trying this,

    I used a CSE-187L with a pre-wound AWG 12 primary wire and about 6.0 inches of AWG 10 making a soldered on string loop in two places and got the following results using the Extech LCR meter:

    216 ohms and about 104mh at 120 Hz: 243 ohms and about 23.8 mH at 1 KHz. Output peak is about 5 mv

    Then I used a CSE-186L current transformer with the pre-installed primary removed using a total of 8 inches of AWG 8 going through the transformer primary and making the string loop with only one solder/crimping sleeve joint making a low Z pickup with these results:

    104 ohms and 35.6 mH at 120 Hz and 111 ohms and 20.6 mH at 1KHz. Output peak is about 10 mv.

    Then I used both pickups on my two string test jig with a Low E and High E strings 25.5 inches long. There was a definite output increase of 2X using the AWG 8 wire loop observed on my oscilloscope.

    A lower string loop impedance means that more current is being developed in the string loop and thus there is more output as the voltage output is a factor of current strength.

    Higher output levels improve the signal to noise ratio. Just try to keep the string loop and wire going through the transformer primary as thick (lowest resistance) as possible and the wire joint as low resistance as possible to minimize the string loop impedance and maximize your output level. Try to use copper and clean any oxide off the wire joint to ensure the lowest resistance connection.

    Joseph J. Rogowski

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    I am trying to make a very thin lo impedance pickup to fit at the end of a fingerboard. The problem is fitting what is required into a form that is 5mm thick and about 40/45 x 80mm. I have attempted a variety of things, using various magnets of 3mm thickness, various plates top and bottom, and various wire gauges. I can’t get the hang of 44 gauge wire, so I gave up on the hi impedance route, and am attempting low impedance.

    My parameters leave no room for an on board CT like the CSE 187 or anything that big, so I have been running through a Shure 85 onto my amp, or directly into an XLR mike input. I determined that I could make a coil with 250 loops of 32 gauge which in theory would have an inductance of 192 ohms at 4khz. I thought this might work directly into the Shure 85 xfromer, but the output was still quite low.

    My form/bobbin is made of two alum plates 40 x 80 x 0.8 mm, with magnets glued between. I tried circular magnets of various types and sizes; neo 12mm x 3mm and plain craft store fridge mags 10mm x 3mm. I made the mistake of lining them up in a daisy chain so they stuck together, but later realized this meant each one was reversed polarity from the adjacent one. Nevertheless I did get some that worked, but poorly. One major difficulty is finding glue that works well. I tried super glue but that failed. JB Kwik works but it too doesn’t hold up to a screwdriver; which is good for opening up failed pickups but not promising for permanent ones.

    There is a lot of theory in this thread, but much of it is beyond my electronic knowledge, so I am looking for some practical tips on how I might best make a simple loZ pickup as described here. I would like it to be 5 x 45 x 80 mm, and able to run into a standard guitar amp through a microphone matching transformer (Shure 85 etc), or possibly into an XLR mike input.

    - Is this form a problem? Would plastic be better? I tried thin steel plates but found the results very microphonic. Aluminum was much less so. Maybe brass? Should the case be grounded?
    - What sort of magnet is necessary? Would a bar be better? It has to be 3mm thick.
    - Would I get better results with more loops of thinner gauge, or would this be defeating the purpose, even staying within lo impedance?

    Thanks so much everyone.

    Don Nathan

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    The aluminum plates will cause extreme eddy current losses resulting in low, muffled output. Good conductors like copper,aluminum or brass are not suitable as PU construction materials.
    Can we see a picture?

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    Quote Originally Posted by Singer15 View Post
    I am trying to make a very thin lo impedance pickup to fit at the end of a fingerboard. The problem is fitting what is required into a form that is 5mm thick and about 40/45 x 80mm. I have attempted a variety of things, using various magnets of 3mm thickness, various plates top and bottom, and various wire gauges. I can’t get the hang of 44 gauge wire, so I gave up on the hi impedance route, and am attempting low impedance.

    My parameters leave no room for an on board CT like the CSE 187 or anything that big, so I have been running through a Shure 85 onto my amp, or directly into an XLR mike input. I determined that I could make a coil with 250 loops of 32 gauge which in theory would have an inductance of 192 ohms at 4khz. I thought this might work directly into the Shure 85 xfromer, but the output was still quite low.

    My form/bobbin is made of two alum plates 40 x 80 x 0.8 mm, with magnets glued between. I tried circular magnets of various types and sizes; neo 12mm x 3mm and plain craft store fridge mags 10mm x 3mm. I made the mistake of lining them up in a daisy chain so they stuck together, but later realized this meant each one was reversed polarity from the adjacent one. Nevertheless I did get some that worked, but poorly. One major difficulty is finding glue that works well. I tried super glue but that failed. JB Kwik works but it too doesn’t hold up to a screwdriver; which is good for opening up failed pickups but not promising for permanent ones.

    There is a lot of theory in this thread, but much of it is beyond my electronic knowledge, so I am looking for some practical tips on how I might best make a simple loZ pickup as described here. I would like it to be 5 x 45 x 80 mm, and able to run into a standard guitar amp through a microphone matching transformer (Shure 85 etc), or possibly into an XLR mike input.

    - Is this form a problem? Would plastic be better? I tried thin steel plates but found the results very microphonic. Aluminum was much less so. Maybe brass? Should the case be grounded?
    - What sort of magnet is necessary? Would a bar be better? It has to be 3mm thick.
    - Would I get better results with more loops of thinner gauge, or would this be defeating the purpose, even staying within lo impedance?

    Thanks so much everyone.

    Don Nathan
    Don,

    Look up Shure A95U matching transformer. It has two primary settings but the default setting of 150 Ohms needs to be changed to the 75ohm setting getting about twice the output voltage. Try this 75ohm setting using 250 turns.

    If if you use AWG 34 to AWG 36, you can put more turns, about 500 to 600 turns, on the pickup to better match the 150 ohm input. The 150 ohm input gets you an output level about 12 times the input level to better approximate the signal level of a single coil pickup with about 6000 turns.

    Post a photo of your guitar so we can all clearly see any further restrictions or opportunities to help you solve your problem.

    Building pickups is a great way to learn!

    Joseph J. Rogowski

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    Last edited by bbsailor; 10-29-2018 at 11:39 PM.

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    Making a 5mm thick pickup

    Quote Originally Posted by bbsailor View Post
    Don,

    Look up Shure A95U matching transformer. It has two primary settings but the default setting of 150 Ohms needs to be changed to the 75ohm setting getting about twice the output voltage. Try this 75ohm setting using 250 turns.

    If if you use AWG 34 to AWG 36, you can put more turns, about 500 to 600 turns, on the pickup to better match the 150 ohm input. The 150 ohm input gets you an output level about 12 times the input level to better approximate the signal level of a single coil pickup with about 6000 turns.

    Post a photo of your guitar so we can all clearly see any further restrictions or opportunities to help you solve your problem.

    Building pickups is a great way to learn!

    Joseph J. Rogowski
    Thanks - more thoughts on that: I bought the Shure A85 thinking it was similar to the A95U, but it isn't! That siad, it certainly get you into the ballpark.
    I've made 2 versions that work reasonably well. Both use aluminum plates with 6 button magnets (from a craft store) of 10mm diameter. This setup fits perfectly under the strings.

    Version 1: 230 winds of 32 ga wire (about 110 ft). Into XLR plug the output is low but usable. Through the A85 it's higher. Sound is clean, except for some hum. E and B strings much louder. To reduce this I placed a small piece of thing steel to partially screen the coil on one side beneath the two high strings. This brought the outputs into better balance.

    Version 2: I first wound this using steel plates, but they were very microphonic and created some odd noises, so I removed the plates and sandwiched the coil between 2 thin sheets of aluminum flashing stock, then taped it all up. It has 430 winds of 32 ga wire and a resistance of 35k. Similar results to version 1, but louder.

    If I switch to 38 gauge, how many windings would it take to increase the output 50%? Is there a dependable way to calculate this? I've been using a website that calculates rectangular coil impedance here: https://www.eeweb.com/tools/rectangle-loop-inductance. I use 4khz to calculate Z. Is this the standard?

    If I wind two coils, using 3 magnets per coil and offset them, would this be a humbucker if wound in opposite directions? I think getting rid of the hum would be desirable!
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  28. #343
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    Don,

    A few more facts about your situation will be helpful.

    1. Please post a photo of your pickup area without a pickup so we can better see the hole and fingerboard end area more clearly.
    2. What is the string width at the end of the fingerboard and right over the round hole where the end strings just pass over the hole?
    3. When you press the end strings on the last fret, what is the distance of the lower part of the string to the guitar top?
    4. What type of strings, acoustic or electric, and string gauge are you using?
    5. What gauge wire do you have in stock?
    6, Are your strings grounded?
    7. Can you access electronic mail order companies?

    Thanks

    Joseph J. Rogowski

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    Quote Originally Posted by bbsailor View Post
    Don,

    A few more facts about your situation will be helpful.

    1. Please post a photo of your pickup area without a pickup so we can better see the hole and fingerboard end area more clearly.
    2. What is the string width at the end of the fingerboard and right over the round hole where the end strings just pass over the hole?
    3. When you press the end strings on the last fret, what is the distance of the lower part of the string to the guitar top?
    4. What type of strings, acoustic or electric, and string gauge are you using?
    5. What gauge wire do you have in stock?
    6, Are your strings grounded?
    7. Can you access electronic mail order companies?

    Thanks

    Joseph J. Rogowski
    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

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  30. #345
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    Don, unfortunately the current transformer pickups are difficult to do that thin. It might be possible under the strings but outside where the CT would need to be in this case, the thickness would be close to 10 mm.

    I would suggest doing sidewinders: a thin horizontal blade of steel, a layer of insulation, a central row of magnets, and two coils windings of relatively thick wire adjacent to the magnets and around the halves of the blade.

    AFAIK, someone has successfully done that and shown it here in the forum. Even thinner than 5 mm.

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    Quote Originally Posted by Singer15 View Post
    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

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    Last edited by bbsailor; 11-03-2018 at 10:07 PM.

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    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!

    Click image for larger version. 

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  33. #348
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    Quote Originally Posted by Diego Padovani View Post
    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!

    Click image for larger version. 

Name:	P_20190221_174311.jpg 
Views:	31 
Size:	1.65 MB 
ID:	52622 Click image for larger version. 

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ID:	52623

    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

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  34. #349
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    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.

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  35. #350
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    Quote Originally Posted by Possum View Post
    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

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