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Thread: Low Impedance Pickup Research

  1. #1
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    Low Impedance Pickup Research

    I want to share some recent low impedance pickup research with the foum members. The photo was taken on top of my research notebook where I placed all the parts that I used in this experiment.

    The Low Z current sensing transformer is CSE187L and has a 1:500 turns ratio.
    The primary side is 250 micro ohms or 1/4000th of an ohm.
    The secondary is measured at 18 ohms DC.
    The secondary impedance is about 400 to 500 ohms when the primary is loaded by a single loop around a magnet as shown in the photo. This impedance is very sensitive to the low Z impedance side as it is reflected back into the secondary.

    I used copper tubing with an ID the same as the wire diameter to make good connections between the low Z primary and the transformer connection. This technique makes changing coils much easier than soldering. The resistance of these connections is very critical that they be very good or low.

    I soldered a two conductor shielded audio cable to a 3-pin mic connector that goes to the 500 ohm to 50000 ohm line or mic matching transformer (Shure 95 series). This provided another 1:10 turns ratio boost. This makes the total turns ratio boost 1:5000 for a single coil pickup. I connected the cable shield to the transformer case. You may also want to try connecting one end of the low-Z primary loop to ground also.

    On my notebook, along with the parts, I skeched out how a humbucking pickup could be made using this same technique. The output would have twice the output but only if you maintain the low impedance of the primary loop windings. Look to use .25" X .125" copper bars that are configured to accomodate your magnet size. Position the magnets N and S as shown. Make the low-Z transformer connection at the two dots as shown on the photo. This will give you the effect of two humbucking coils in series.

    If the magnets are metal or conductive you need to keep the insulation on the wire to keep from shorting out the primary. You can wrap the magnet in electrical tape also to insulate it from the raw copper winding.

    In the photo, the green wire is AWG12 with a 2" X .25" X .125" thick (N/S through the thickness), Neodymium from K&J Magnetics (BY042). The other magnets on the left side of the photo are ceramics I had laying around. Although much weaker than the BY042, they worked nearly as loud as I could can get them closer to the strings without sucking in the string.

    This is not a refined design but is a crude experiment to demonstrate, for my own benefit, how Lace Alumitones work.

    Here is what I learned.

    The matching of the low Z primary single loop is critical to have an optimum energy transfer from the string to the transformers.
    I could hear a level difference between using AWG 12 and the thicker AWG10. Thicker wire is better in this design.
    The Humbucking design should produce a higher output. (My next experiment) It does. I get about 50% more output with the humbucking design using 2 magnets. The resistance of the primary loops needs to be very low (the lower the better) to improve the efficiency of this design.

    I hope this answers questions from those who are curious about low impedance pickups. This technique eliminates coil winding and transfers the design to the mechanics of the pickup assembly.

    If anyone want to make a quick pickup for their acoustic guitar, this pickup technique can be easily mounted at the end of the neck in the sound hole for a cost of $2.50 for the current transformer, $15.00 of the matching transformer (Radio Shack), $6.00 3-pin mic connector, $1.00 short piece of AWG10 solid wire, $5.00 10 ft of 2-conductor mic cable, $2.00 magnet.

    If anyone tries this, I would like to hear about your results.

    Joseph J. Rogowski
    Attached Thumbnails Attached Thumbnails p1040168.jpg  
    Last edited by bbsailor; 01-05-2008 at 11:08 PM.
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  2. #2
    Pickup Maker David Schwab's Avatar
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    Very cool. Thanks for sharing that.
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    the IMmoderator DrStrangelove's Avatar
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    Joseph, yours is an extraordinary bit of work that takes time to ponder so I hope you'll forgive my late response.

    The current mode loop's simplicity lends it to broad application.
    The loop can be prototyped with 1/4" copper tape.
    It can be produced as a printed circuit board.

    The copper could be inexpensively silver plated for minute impedance corrections when matching with a particular current sense transformer.

    The xformr itself could be optimized; toroidal cores tend to have better efficiency and extended high frequency response (-3dB@100kHz? Heh!).

    Buffering the current sense xformr with an active device running on phantom power is another possibility.

    The loop can be on flexible media or arched to follow fingerboard curves on bowed instruments.

    Cello and viola players are still looking for an alternative to piezo sensors and would gladly embrace this for the better sound quality.

    Thanks, Joseph.

    -drh

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    Transformer Design

    The xformr itself could be optimized; toroidal cores tend to have better efficiency and extended high frequency response (-3dB@100kHz? Heh!).

    I have attempted to do this research using commonly available parts to help educate myself about the technical parameters necessary to optimize a current-based vice voltage-based pickup. In the process, I wanted to share this information with the forum members as the minds of many are much better than the mind of one.

    Look at the transbucker sketch and note the transformer core wrapping around one thick conductor of the pickup. See that this is what is also done in the alumitone sketch (single coil shown) and the humbucking form factor (that I recently purchased) works the same way.

    I have also provided a photo of the CSE187L low frequency current sensing transformer. It is commonly available from many electronic supply houses. Do a web search on the part number and check out the pdf specification sheet.

    Here are the key specifications: Primary resistance: 250 micro ohms or 1/4000ohm. Secondary resistance:21 ohms although some of mine are around 18 ohms. Turns ratio: 1:500. Frequency: 50 to 400 Hz although as an input transformer not being saturated, it will go higher into the audio spectrum. Cost: $2.50 to $3.00 depending on source and quantity ordered.

    The single transformer loop primary measures out at the same diameter as AWG 12 copper wire used in houshold electrical cable. However, I found that the optimum current tarnsfer from a guitar string to a single coil loop is highly dependent on the resistance of that single loop. Circuit boards may be too thin to optimize this, but could work for experimentation. The thought to use a toroidal core might work as long as the coupling between the single loop to the toroid is good enough at low frequencies where the largest losses would occur. To match the Lace design, the current sensing transformer should have a secondary in the 2000 ohm plus range with a turns ratio of 1 to several thousand (possibly 5000 or more).

    Since I had a few of the CSE187L at hand, I chose to view the single AWG 12 primary transformer loop as an extension of one loop of the pickup winding that is looped around the magnet. I measured the secondary impedance of the CSE187L at 1000 Hz and is is about 400 to 600 ohms dependent on the pickup loop wire thickness and length that is connected to the transformer primary. Since this is close to the impedance of commercial audio matching transformers (Shure 95 series), I just wired a 3-pin mic connector to the transformer and went though another 500 to 50000 ohm transformer to get some more voltage boost. This is not the most efficient way to go as I now have two transformer losses and optimizations to contend with. This design, however, does give a reasonable output that is very quiet, even near a computer.

    When I increased the single loop wire diameter around the magnet to AWG10, the output improved somewhat. I suspect that using copper bars in the range of .25" to .375" wide by .125" thick would improve it even more. I even made a crude humbucking design using two magnets configured like the sketch on my first post using AWG10 wire soldered together with the transformer connected to the center common loop. This provided about 50% more voltage output compared to the single coil design using the same transformer.

    With this design, the construction of a pickup shifts from winding thousands of turns of wire on a core (voltaged-based design) with the attendent capacitance issues and resonance issues, to the mechanics of current-based design to match the form factor commonly evolved single coil or humbucking sizes to match standardized guitar manufacturing practices.

    Once off-the-shelf parts (transformers, coils, laminated cores) are located, this design opens new possibilities to pickup makers to make new pickup designs that have an extended frequency response that can be electronically shaped to sound like any traditional voltage-based pickup. It is always easier to shape responses that are present than try to create responses that are not, with the consequence of adding noise.

    Drstrangelove and David Schwab fully comprehend why I posted this new design. I hope a stimulating discussion developes and that some folks try using various transformers in a current-based pickup design to educate themselves as well.

    Joseph Rogowski
    Attached Thumbnails Attached Thumbnails transbucker_design.jpg   alumitone_sketch.jpg   cse187lphoto.bmp  
    Last edited by bbsailor; 01-09-2008 at 03:03 AM.

  5. #5
    the IMmoderator DrStrangelove's Avatar
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    Running some numbers, the 10ga. loop is an estimated 5.5 inches long.
    and has a DCR of ~460 micro ohms.

    A .25" wide 5.5" long 1.25mil copper tape loop is 120 MILLI ohms,
    perhaps a bit too much for this application.

    One footnote: I was an early tester for some of the transensors and found their
    humbucking versions were overloaded by 500k pots, unusable without 1M pots
    and even then suffered from an audibly compressed dynamic range.

    Lace's second iteration produced the much-improved California Specials which,
    in addition to handling convention volume/tone pots, have the overall chime of
    Fender single coils albeit with some of the Lace Sensor's lack of detail due to
    comb filtering from it's wider magnetic aperture.

    An active design is worth thinking about, IMO.

    -drh

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    Hey I'm all for this design, I've been meaning to design a decent bass pickup that's thin enough (1/8") to slide around under the strings without needing a routed cavity. Cinemag makes some really nice sounding audio transformers that aren't too expensive. Here's their DI box xformer which might be appropriate as a secondary step-up.
    http://cinemag.biz/direct_box/direct_box.html but they have lots of others, I just don't know what to look for exactly to fit this application. I'm pretty sure they can wind anything you want.

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    Quote Originally Posted by DrStrangelove View Post
    Running some numbers, the 10ga. loop is an estimated 5.5 inches long.
    and has a DCR of ~460 micro ohms.

    A .25" wide 5.5" long 1.25mil copper tape loop is 120 MILLI ohms,
    perhaps a bit too much for this application.

    One footnote: I was an early tester for some of the transensors and found their
    humbucking versions were overloaded by 500k pots, unusable without 1M pots
    and even then suffered from an audibly compressed dynamic range.

    Lace's second iteration produced the much-improved California Specials which,
    in addition to handling convention volume/tone pots, have the overall chime of
    Iender single coils albeit with some of the Lace Sensor's lack of detail due to
    comb filtering from it's wider magnetic aperture.

    An active design is worth thinking about, IMO.

    -drh
    drh

    Funny you should mention the optimum load for Lace low impedance pickups. I contacted Jeff Lace via email and he said that the optimum load for the Alumitone is a 250K Volume pot with either a .02 or .05 uf cap for the tone on a 250K pot. My black Alumitone humbuckers measures out at 3.4 K ohms and 10.5 H, not the 16H that is listed on the Lace web site. It seems my Alumitone humbuckers are closer to the specifications of the Transbucker. I wish Lace would get the technical information about their low impedance pickups closer to reality.

    I plan to feed these Lace Alumitones into an EMG active tone network (treble and bass boost/cut and mid shift boost/cut concentric staked tone network) that has a 200K input impedance. Based on what you say, I may need to build a Tillman active buffer FET right after my six position selector switch to prevent loading them down? These pickups were a little edgy with a 500K volume pot and calmed down some when I reduced the load to 242K by putting a 470K resistor across the selector switch output. Next week I will find out how they sound connected to my active tone network.

    About your active design comment, I have one observation. It is all about signal to noise ratio. We tend to want hotter pickups to get a nice, creamy overdrive sound but must suffer the consequences of the hot pickups also acting like sensitive noise antennas and the consequences of cable capacitance. With the low Z, single loop, current based design, the noise is lower as well as the gain. This lower gain can be activly boosted because the signal to noise ratio is better and the extra active boost does not add any more noise and isolates the pickup coils/transformer from the cable capacitance and amp input circuit loading.

    Thanks for your comments.

    Joseph Rogowski
    Last edited by bbsailor; 01-09-2008 at 02:55 AM.

  8. #8
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by David King View Post
    Hey I'm all for this design, I've been meaning to design a decent bass pickup that's thin enough (1/8") to slide around under the strings without needing a routed cavity.
    And this after you kept telling me no one needs low Z bass pickups...
    It would be possible to describe everything scientifically, but it would make no sense; it would be without meaning, as if you described a Beethoven symphony as a variation of wave pressure. Albert Einstein

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    Cool stuff indeed.

    Random thoughts:
    - a good way to set this up is to put the current-sense stepup transfer right by the string sensing loop to minimize the distance for which heavy wire is needed.
    - square-section or rectangular-section copper bar or a formed rectangular-section aluminum ring would make a good primary loop. Aluminum should work fine and be much cheaper if you used a formed or cast ring so you don't have to make a joint in the primary loop. Aluminum is only slightly less conductive than copper, and large sections are cheap. Could even be cast in place or cast to shape.
    - but because of the difficulty soldering, copper's what the hacker will use
    - if you did the rectangular-section small loop, you could clamp ferrite stepup cores over a section of it at one end of the primary loop, something like a pair of C-cores each with their own half-secondary. A split toroid might work, but this is what C's were made for.
    -if you have to move the high current loop over to the current sense stepup trannie, use a wide, flat conductor pair separated by as little distance as possible; this produces the lowest inductance path to the transformer.
    - if you're going to all this trouble, you want to get to 600 ohms, balanced out of the thing and over to the console. The other end is where to put the stepup. The stepup at the end raises the impedance and voltage up to match the amp it's running into.
    - there are highly linear low noise differential preamps designed for moving coil phono stages that can probably take the place of the second transformer.
    - high-ratio transformers get almost as picky to wind as pickups themselves; it's good to separate the voltage gain into a couple of stages to avoid the high-ratio issues; you'll eat more losses, but have less picky transformers to wind.
    - you'll have to watch what you terminate the stepup chain with. Current transformers get really pickup about the termination impedance if you want wide linear response.
    - if you don't care about wide linear response, you'll be back in the insulation-color-matters issues but on the transformers, not the pickup.

  10. #10
    the IMmoderator DrStrangelove's Avatar
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    Quote Originally Posted by bbsailor View Post
    Funny you should mention the optimum load for Lace low impedance pickups.
    That comment was about the first humbucking Transensors of ~2000.
    They required more than a few detail changes to make them a drop-in replacement for ordinary pickups.

    -drh

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    R.G.

    I had to reflect on your comments for a while, but here goes!!!

    Quote Originally Posted by R.G. View Post
    Cool stuff indeed.

    Random thoughts:
    - a good way to set this up is to put the current-sense stepup transfer right by the string sensing loop to minimize the distance for which heavy wire is needed.

    Yes! This is the way that Lace does with the Alumitones and Transbuckers. My plan is to put the current sensing transformers right under the sensing coils under the magnets. Drsrtangelove suggested that there be magnetic shielding between the sensing coil magnet and the current sensing transformer.

    - square-section or rectangular-section copper bar or a formed rectangular-section aluminum ring would make a good primary loop. Aluminum should work fine and be much cheaper if you used a formed or cast ring so you don't have to make a joint in the primary loop. Aluminum is only slightly less conductive than copper, and large sections are cheap. Could even be cast in place or cast to shape.

    The optimum match between the surrent sensing transformer is based on the resistane value of the primary loop matching the resistance value of teh current sensing primary loop. Beyond that there are diminishing returns.

    - but because of the difficulty soldering, copper's what the hacker will use

    Yes! Copper is solderable while aluminum is not. Aluminum is a good commercial alternative while copper is a great DIY material.

    - if you did the rectangular-section small loop, you could clamp ferrite stepup cores over a section of it at one end of the primary loop, something like a pair of C-cores each with their own half-secondary. A split toroid might work, but this is what C's were made for.

    In my next design, I plan to put one CSE187L on each end of a sensor loop that is around the magnet. This way I can see how the coupling between these transformers can be either between single, series or parallel transformer connections to the 600 ohm microphone matching transformer and monotor the output level and tonal change.

    -if you have to move the high current loop over to the current sense stepup trannie, use a wide, flat conductor pair separated by as little distance as possible; this produces the lowest inductance path to the transformer.

    I plan to put the the first transformer right under the pickup sensor coil

    - if you're going to all this trouble, you want to get to 600 ohms, balanced out of the thing and over to the console. The other end is where to put the stepup. The stepup at the end raises the impedance and voltage up to match the amp it's running into.

    Agree. This minimizes cable capacitance effects on tonal rsponse.

    - there are highly linear low noise differential preamps designed for moving coil phono stages that can probably take the place of the second transformer.

    Yes. I believe that the first stage of impedance conversion should be done inductivly to minimize noise. Later stages could be active.

    - high-ratio transformers get almost as picky to wind as pickups themselves; it's good to separate the voltage gain into a couple of stages to avoid the high-ratio issues; you'll eat more losses, but have less picky transformers to wind.

    This all gets down to signal-to-noise-ratio. With low Z pickups and the potential to provide a balanced line output. This design could take advantage of the extra gain in microphone circuits vice guitar amp input circuits.

    - you'll have to watch what you terminate the stepup chain with. Current transformers get really pickup about the termination impedance if you want wide linear response.

    Agree. But, unlike guitar pickups that have a 12 db per octave response drop off beyond their resonance point, low Z pickups have an extended frequency range making them ideal for electronic EQ and mimicking of traditional single coil and humbucking pickups by just rotating a knob.

    - if you don't care about wide linear response, you'll be back in the insulation-color-matters issues but on the transformers, not the pickup.

    Irrational talk drives scientists crazy.

    I have replied to your observations below each of your comments above.

    Optimum design is based on connecting important dots. What dots are important are not a scientific issues but are marketing and social issues. What sounds good is not a scientific question but a social issue. When scientists are told what sounds good and asked why particular sounds sound good, they can do a scientific analysis and provide answers. Guitar pickups fall into this same categogy. Low impedance pickups connect a particular group of dots better than traditional pickups. The real issue is: how do we connect enough of the important dots to produce a relevant alternative to traditional Hi Z pickups? Should we seek to replace them or simply offer another alternative? These fundamental questions will eventually be answered.

    Thanks or your comments.

    Joseph Rogowski
    Last edited by bbsailor; 01-12-2008 at 07:06 AM.

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    The real issue is: how do we connect enough of the important dots to produce a relevant alternative to traditional Hi Z pickups? Should we seek to replace them or simply offer another alternative? These fundamental questions will eventually be answered.
    As a practical matter, it will be crucial that you produce something that plugs directly into existing guitar amps. Otherwise, uninformed musicians simply won't buy it. That's the social reality you were talking about, I think.

    Otherwise you not only have to innovate pickups, you have to innovate wiring and amps, and educate the public to unthinkingly accept your solution when '59 Les Pauls didn't do that.

    Ugly, but the truth.

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    Quote Originally Posted by David King View Post
    Hey I'm all for this design, I've been meaning to design a decent bass pickup that's thin enough (1/8") to slide around under the strings without needing a routed cavity. Cinemag makes some really nice sounding audio transformers that aren't too expensive. Here's their DI box xformer which might be appropriate as a secondary step-up.
    http://cinemag.biz/direct_box/direct_box.html but they have lots of others, I just don't know what to look for exactly to fit this application. I'm pretty sure they can wind anything you want.

    David,

    I checked out the Cinemag transformers and thay are pretty expensive. My plan was to simply use the Shure A95U 500 ohm to 50K ohm transformer and a Tip-Ring-Sleve .250" stereo jack on the guitar. Then, a standard guitar amp can be used with a balance line 2 conductor microphone cable.

    The output of two CSE187L current sense transformers in series is about 375 ohms. I may be able to switch the wiring in the A95U to the lower input impedance wiring setting and obtain a little higher turns ratio and a potentially higher output. That is my next experiment.

    I am only interested in doing this design with inexpensive off-the-shelf parts so anyone can build it once the design alternatives are fully explored. My progress will be posted here and the results will be in the public domain so others cannot attempt to patent this work without giving full disclosure or enough of a change to make it unique or novel.

    Hopefully, others will follow along and offer ideas as sort of a group design project.

    Joseph Rogowski
    Last edited by bbsailor; 01-16-2008 at 10:04 PM.

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    Senior Member salvarsan's Avatar
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    Quote Originally Posted by R.G. View Post
    As a practical matter, it will be crucial that you produce something that plugs directly into existing guitar amps. Otherwise, uninformed musicians simply won't buy it.
    "Uninformed musicians" is a proportionately smaller group when you look beyond guitar players.

    Bowed instruments, stringed harps, and the like are waiting for something like this.

    -drh

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    bbsailor (and anyone else who can help),

    Sorry to dig up this old thread, but I would like to build pickups of this type myself and I've been thinking about a couple of questions:

    Would it be possible to just have the primary loop under the strings and connect it by wires to a transformer tucked away in the guitar's control cavity? That'd give a cleaner look.

    Is there a particular reason that you're using a microphone adapter? Couldn't you instead have used any 100:1 current sensing transformer in series with the CSE187L?

    Here, I assume that sending the signal through a 100:1 transformer raises the impedance a hundred times. Is this correct? And if not, can anyone tell me the correct way to calculate the change in impedance when sending the signal through a transformer?

    Last question (for now): What if you instead of just one had about ten primary loops and then send the signal through the 500:1 transformer... would that be equivalent to the around 5000 windings of a traditional pickup?

    Please enlighten me. I am about to receive a couple of AS104 transformers this week and would like to know some more about the possibilities they give me.

    /Alex

    Quote Originally Posted by bbsailor View Post
    I want to share some recent low impedance pickup research with the foum members. The photo was taken on top of my research notebook where I placed all the parts that I used in this experiment.

    The Low Z current sensing transformer is CSE187L and has a 1:500 turns ratio.
    The primary side is 250 micro ohms or 1/4000th of an ohm.
    The secondary is measured at 18 ohms DC.
    The secondary impedance is about 400 to 500 ohms when the primary is loaded by a single loop around a magnet as shown in the photo. This impedance is very sensitive to the low Z impedance side as it is reflected back into the secondary.

    I used copper tubing with an ID the same as the wire diameter to make good connections between the low Z primary and the transformer connection. This technique makes changing coils much easier than soldering. The resistance of these connections is very critical that they be very good or low.

    I soldered a two conductor shielded audio cable to a 3-pin mic connector that goes to the 500 ohm to 50000 ohm line or mic matching transformer (Shure 95 series). This provided another 1:10 turns ratio boost. This makes the total turns ratio boost 1:5000 for a single coil pickup. I connected the cable shield to the transformer case. You may also want to try connecting one end of the low-Z primary loop to ground also.

    On my notebook, along with the parts, I skeched out how a humbucking pickup could be made using this same technique. The output would have twice the output but only if you maintain the low impedance of the primary loop windings. Look to use .25" X .125" copper bars that are configured to accomodate your magnet size. Position the magnets N and S as shown. Make the low-Z transformer connection at the two dots as shown on the photo. This will give you the effect of two humbucking coils in series.

    If the magnets are metal or conductive you need to keep the insulation on the wire to keep from shorting out the primary. You can wrap the magnet in electrical tape also to insulate it from the raw copper winding.

    In the photo, the green wire is AWG12 with a 2" X .25" X .125" thick (N/S through the thickness), Neodymium from K&J Magnetics (BY042). The other magnets on the left side of the photo are ceramics I had laying around. Although much weaker than the BY042, they worked nearly as loud as I could can get them closer to the strings without sucking in the string.

    This is not a refined design but is a crude experiment to demonstrate, for my own benefit, how Lace Alumitones work.

    Here is what I learned.

    The matching of the low Z primary single loop is critical to have an optimum energy transfer from the string to the transformers.
    I could hear a level difference between using AWG 12 and the thicker AWG10. Thicker wire is better in this design.
    The Humbucking design should produce a higher output. (My next experiment) It does. I get about 50% more output with the humbucking design using 2 magnets. The resistance of the primary loops needs to be very low (the lower the better) to improve the efficiency of this design.

    I hope this answers questions from those who are curious about low impedance pickups. This technique eliminates coil winding and transfers the design to the mechanics of the pickup assembly.

    If anyone want to make a quick pickup for their acoustic guitar, this pickup technique can be easily mounted at the end of the neck in the sound hole for a cost of $2.50 for the current transformer, $15.00 of the matching transformer (Radio Shack), $6.00 3-pin mic connector, $1.00 short piece of AWG10 solid wire, $5.00 10 ft of 2-conductor mic cable, $2.00 magnet.

    If anyone tries this, I would like to hear about your results.

    Joseph J. Rogowski

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    Quote Originally Posted by alexoest View Post
    bbsailor (and anyone else who can help),

    Sorry to dig up this old thread, but I would like to build pickups of this type myself and I've been thinking about a couple of questions:

    Would it be possible to just have the primary loop under the strings and connect it by wires to a transformer tucked away in the guitar's control cavity? That'd give a cleaner look.

    Is there a particular reason that you're using a microphone adapter? Couldn't you instead have used any 100:1 current sensing transformer in series with the CSE187L?

    Here, I assume that sending the signal through a 100:1 transformer raises the impedance a hundred times. Is this correct? And if not, can anyone tell me the correct way to calculate the change in impedance when sending the signal through a transformer?

    Last question (for now): What if you instead of just one had about ten primary loops and then send the signal through the 500:1 transformer... would that be equivalent to the around 5000 windings of a traditional pickup?

    Please enlighten me. I am about to receive a couple of AS104 transformers this week and would like to know some more about the possibilities they give me.

    /Alex
    bbsailor (and anyone else who can help),

    Sorry to dig up this old thread, but I would like to build pickups of this type myself and I've been thinking about a couple of questions:

    It is very good that others are thinking about this!

    Would it be possible to just have the primary loop under the strings and connect it by wires to a transformer tucked away in the guitar's control cavity? That'd give a cleaner look.

    The short answer is yes but practicaly for only the bridge pickup that is physically closer to the control cavity. So the key question is how do you want to balance the tone of the pickup from the size of the primary loop. Put into scientific language, that means: what is the resistance of the primary loop? You need a primary loop less than about 1 milliohms or 1 one thousandths of an ohm. What is the distance that a continious loop must take to go around the pickup magnetic field (under the strings) and the loop moving inside to the closest place that the current transformer connection can be made? The answer is in your ears! Lower resistance primary loops favors lower frequencies.

    Is there a particular reason that you're using a microphone adapter?

    Yes!!! The output of the CSE 187L just about matches the input impedance of a Shute A95U matching transformer. But two CSE187L current transformers (CT) allow the outputs to be connected in either series, parallel or single CT, resulting to tonal changes. Where there are changes to be heard is totally dependent on the frequencies we are boosting or cutting compared to the organic sound of a high impedance pickup with 6 to 12 thousand turns of very fine wire. The low impedance pickup has a totaly different and wider bandwidth sound that may not please everyone. However, it has potential upper sound harmonics that can be amplified, if desired.


    Couldn't you instead have used any 100:1 current sensing transformer in series with the CSE187L?



    The low resistance of the primaty loop allows a vibrating string to produce anywhere between 1 and 10 milliamps at the plucked frequency. String mass, velocity and vibrating frequency all affect the output level. Changing the load on the CT will affect the tonal balance and the amount of gain needed to be recovered in the matching transformer method or in an active amplifier method.

    Here, I assume that sending the signal through a 100:1 transformer raises the impedance a hundred times. Is this correct?

    No! It changes by the square of the turns ratio. So the CSE187L with a 1 to 500 turns ratio, changes the impedance by 1 to 250,000. Assume that the output load of the CT is 2.500 ohms (like on the Les Paul Low Impedance pickup schematic). Then, the reflected load is 2500 divided by 250,000 or .01 ohms. But you want the source to impedance to be 10 times lower than the load impedance so that means getting a loop resistance in the neighborhood of 1 milliohm or .001 ohms.

    And if not, can anyone tell me the correct way to calculate the change in impedance when sending the signal through a transformer?




    Last question (for now): What if you instead of just one had about ten primary loops and then send the signal through the 500:1 transformer... would that be equivalent to the around 5000 windings of a traditional pickup?

    It is all about how much current is sent through the primary loop. Lower resistance means thicker wire and shorter distances to obtain the right tone to satisfy the ear.

    Please enlighten me. I am about to receive a couple of AS104 transformers this week and would like to know some more about the possibilities they give me.

    See the posts and posed photos by bajaman on this forum.

    /Alex
    Last edited by bbsailor; 05-06-2010 at 06:19 PM.

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    Thank you. That's a very thorough and quick reply.
    I have an additional question to your explanation of the number of turns and the reflected load:

    ---Quote---
    No! It changes by the square of the turns ratio. So the CSE187L with a 1 to 500 turns ratio, changes the impedance by 1 to 250,000. Assume that the output load of the CT is 2.500 ohms (like on the Les Paul Low Impedance pickup schematic). Then, the reflected load is 2500 divided by 250,000 or .01 ohms. But you want the source to impedance to be 10 times lower than the load impedance so that means getting a loop resistance in the neighborhood of 1 milliohm or .001 ohms.
    ---Unquote---

    Does that mean that you must adjust two parameters: To get proper output impedance, you choose a transformer with the right turns ratio _and_ to get a suitable reflected load, you vary the resistance of the loop under the strings? Or is impedance bridging the same thing as ensuring a suitable reflected load?

    But in practical terms, would that mean that to raise the CT's asumed 2.500 ohms output impedance to 250.000 ohms, you'd connect it in series to a 1:10 CT? (since 250.000 divided by 2.500 is 100, and the square root of 100 is 10).

    I saw bajamans pickups in the other thread. Since I'm getting the same CTs (AS104), I expect to make a similar pickup from a piece of flat brass bar. Plus try out the thing I suggested with around ten primary loops of thick copper wire under the strings.

    /Alex

  18. #18
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    Quote Originally Posted by alexoest View Post
    Thank you. That's a very thorough and quick reply.
    I have an additional question to your explanation of the number of turns and the reflected load:

    ---Quote---
    No! It changes by the square of the turns ratio. So the CSE187L with a 1 to 500 turns ratio, changes the impedance by 1 to 250,000. Assume that the output load of the CT is 2.500 ohms (like on the Les Paul Low Impedance pickup schematic). Then, the reflected load is 2500 divided by 250,000 or .01 ohms. But you want the source to impedance to be 10 times lower than the load impedance so that means getting a loop resistance in the neighborhood of 1 milliohm or .001 ohms.
    ---Unquote---

    Does that mean that you must adjust two parameters: To get proper output impedance, you choose a transformer with the right turns ratio _and_ to get a suitable reflected load, you vary the resistance of the loop under the strings? Or is impedance bridging the same thing as ensuring a suitable reflected load?

    But in practical terms, would that mean that to raise the CT's asumed 2.500 ohms output impedance to 250.000 ohms, you'd connect it in series to a 1:10 CT? (since 250.000 divided by 2.500 is 100, and the square root of 100 is 10).

    I saw bajamans pickups in the other thread. Since I'm getting the same CTs (AS104), I expect to make a similar pickup from a piece of flat brass bar. Plus try out the thing I suggested with around ten primary loops of thick copper wire under the strings.

    /Alex
    Alex,

    The Les Paul schematic has a 2500 ohm pot value and that looks like a bridging impedance on the current transformer. The output of two CSE187L CTs can be wired in series, parallel of single to obtain some tonal variation like how two coils on a humbucker pickup can be wired.

    Putting 10 turns under the string loop will increase the resistance and affect the tone. I have tried that by using AWG 16 and AWG 18 but the output did not increase very much over a single heavy AWG 10 wire loop. Put the microphone matching transformer (Shure A95U or similar) at the amplifier end of the guitar cable. The output of the CSE187L current transformers seems to match the input impedance of this transformer which is near 2000 ohms to bridge the output of a typical 150 to 200 ohm microphone to obtain the maximum voltage.

    Try any variation you want as these pickups are pretty quickly put together.
    Use this web link to calculate the resistance of the loop using a variety of metals. Resistivity Calc

    Joseph Rogowski

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    Thanks once again for the help - and for the link.

    The Shure A95 U is pretty expensive here in Europe, so if possible, I'd prefer instead to use another CT to provide the last 100x multiplication of the impedance that you use the microphone adapter for. Would that be possible or does the microphone adapter do something that a CT can't?

    /Alex

    Quote Originally Posted by bbsailor View Post
    Alex,

    The Les Paul schematic has a 2500 ohm pot value and that looks like a bridging impedance on the current transformer. The output of two CSE187L CTs can be wired in series, parallel of single to obtain some tonal variation like how two coils on a humbucker pickup can be wired.

    Putting 10 turns under the string loop will increase the resistance and affect the tone. I have tried that by using AWG 16 and AWG 18 but the output did not increase very much over a single heavy AWG 10 wire loop. Put the microphone matching transformer (Shure A95U or similar) at the amplifier end of the guitar cable. The output of the CSE187L current transformers seems to match the input impedance of this transformer which is near 2000 ohms to bridge the output of a typical 150 to 200 ohm microphone to obtain the maximum voltage.

    Try any variation you want as these pickups are pretty quickly put together.
    Use this web link to calculate the resistance of the loop using a variety of metals. Resistivity Calc

    Joseph Rogowski

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    Quote Originally Posted by alexoest View Post
    Thanks once again for the help - and for the link.

    The Shure A95 U is pretty expensive here in Europe, so if possible, I'd prefer instead to use another CT to provide the last 100x multiplication of the impedance that you use the microphone adapter for. Would that be possible or does the microphone adapter do something that a CT can't?

    /Alex
    Alex,

    You cannot use another CT to boost the output after the first CT. You can use two CTs on the ends of a single low impedance (AWG 10) string loop with a magnet in the center. However the input impedance of a CT is so low that it will not work to boost the secondary of the first CT. It is best to use a microphone matching transformer because of its wide frequency range and the range of the low impedance 150 to 300 ohms input souce impedance and the better coupling. You can use any microphone matching brand that is similar to the Shure A95U series.

    For the lowest noise, ground the metal case of the CSE197 CTs to the main audio ground. Make sure your strings are grounded also.

    Joseph Rogowski

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    Thanks for the explanation. I'll have to get a microphone adapter, then. Apart from that, I'll try to connect the pickup to my sound card mic and line in to see if they suit the impedance of the pickup signal. They should be around 600 ohms and 50 K ohms respectively.

    Haven't got the CTs yet, so I won't be able to try out anything at the moment.

    /Alex

    Quote Originally Posted by bbsailor View Post
    Alex,

    You cannot use another CT to boost the output after the first CT. You can use two CTs on the ends of a single low impedance (AWG 10) string loop with a magnet in the center. However the input impedance of a CT is so low that it will not work to boost the secondary of the first CT. It is best to use a microphone matching transformer because of its wide frequency range and the range of the low impedance 150 to 300 ohms input souce impedance and the better coupling. You can use any microphone matching brand that is similar to the Shure A95U series.

    For the lowest noise, ground the metal case of the CSE197 CTs to the main audio ground. Make sure your strings are grounded also.

    Joseph Rogowski

  22. #22
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    Bump!

    I'm following this thread for a practical purpose.

    I'd like to build a pickup to be placed under my guitar strings that have less then a 3/8 clearance.

    The guitar has a body cavity very near the bridge which occupied by active electronic for piezoelectric pickups.

    Hey Alex how have you progressed in your project?

    Thanks for the science!

  23. #23
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    Randall,

    I haven't progressed much. I have made a pickup consisting of a single loop of thick copper wire through the AS-104 CT. I put it in the microphone input of my computer and the sound was OK. I use guitar effects software on the computer, so a pickup with an impedance suiting the mic-in is very usable to me.

    I am in the process of making a better pickup from a flat bar of brass. I expect it to look something like this:

    [I can't make the picture appear, so here is a link:]

    Picasa Web Albums - agoest

    /Alex
    Last edited by David Schwab; 08-04-2010 at 12:56 PM. Reason: fixed image link

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    Quote Originally Posted by alexoest View Post
    I have made a pickup consisting of a single loop of thick copper wire through the AS-104 CT. I put it in the microphone input of my computer and the sound was OK. I use guitar effects software on the computer, so a pickup with an impedance suiting the mic-in is very usable to me.
    How will you solder the posts to the plates without melting the current transformer? At these low voltages, full metallic bonding is required.

    I am in the process of making a better pickup from a flat bar of brass. I expect it to look something like this:

    [I can't make the picture appear, so here is a link:]
    Picasa Web Albums - agoest
    You can upload images (for which you must "go advanced"), or provide a URL in an "[img]" bracket.

  25. #25
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by alexoest View Post
    [I can't make the picture appear, so here is a link:]
    You have to open the image in a new window to get the link of the image.
    It would be possible to describe everything scientifically, but it would make no sense; it would be without meaning, as if you described a Beethoven symphony as a variation of wave pressure. Albert Einstein

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    Quote Originally Posted by Joe Gwinn View Post
    How will you solder the posts to the plates without melting the current transformer? At these low voltages, full metallic bonding is required.

    You can upload images (for which you must "go advanced"), or provide a URL in an "[img]" bracket.
    I'll use screws rather than rods. Though I considered tapping the copper rods with a hammer to make them expand in the holes to get good contact.

  27. #27
    Senior Member Chris Turner's Avatar
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    I'd also like to build one of these pickups. Very cool stuff!!

    However, is there not a way to get around having to use the Shure A95U (or similar)? Could you not use a transformer with a higher turn ratio? And if that's the case, does anyone know of a good, easy-to-buy-from (preferrably off-the-shelf...) source?
    Last edited by Chris Turner; 08-04-2010 at 10:34 PM.

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    Quote Originally Posted by alexoest View Post
    I'll use screws rather than rods. Though I considered tapping the copper rods with a hammer to make them expand in the holes to get good contact.
    None of these mechanical bonding methods are good enough. Although they may work for a while, in a year or two the contacting surfaces will corrode slightly from atmospheric gases, enough to hinder operation.

    This is why such things as the shorted turn in an induction motor is always welded together or stamped as a ring from sheet copper or aluminum.

    If you look at the alumitone patent drawings, you will see that they have avoided the necessity to make mechanical contacts.

    I suppose that if one gold plates the contact areas of the mating pieces, one can achieve a sufficiently low and stable contact resistance, so long as sufficient clamping pressure can be achieved and maintained over time.

  29. #29
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    Quote Originally Posted by Joe Gwinn View Post
    None of these mechanical bonding methods are good enough. Although they may work for a while, in a year or two the contacting surfaces will corrode slightly from atmospheric gases, enough to hinder operation.

    This is why such things as the shorted turn in an induction motor is always welded together or stamped as a ring from sheet copper or aluminum.

    If you look at the alumitone patent drawings, you will see that they have avoided the necessity to make mechanical contacts.

    I suppose that if one gold plates the contact areas of the mating pieces, one can achieve a sufficiently low and stable contact resistance, so long as sufficient clamping pressure can be achieved and maintained over time.
    Thanks for pointing it out. I wasn't aware that the bonds were that critical.

    What if I seal the bonds with lacqer? Wouldn't that keep the air away?

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    Quote Originally Posted by alexoest View Post
    What if I seal the bonds with lacqer? Wouldn't that keep the air away?
    Not long enough to matter.

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    What about conductive epoxy? Or conductive grease in addition to the mechanical connection?

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    Quote Originally Posted by Rick Turner View Post
    What about conductive epoxy? Or conductive grease in addition to the mechanical connection?
    I have not tried either, but I'm skeptical. Conductive epoxy (grease) is typically silver dust in epoxy (grease), and depends on contact resistance from dust particle to dust particle. Silver will corrode to form a black film. The film is soft and not that good an insulator, but still switches and relays using silver contacts are invariably designed to wipe on make and break, so the rubbing will keep the contacts clean.

  33. #33
    Pickup Maker David Schwab's Avatar
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    One fix would be to bend that side of the top plate over and slot it the way Lace does it.

    Here's one of their "bass bar" Alumitones. They assemble the transformer cores around the frame, so I don't know how that would work with the current transformer.


    img_1640.jpgimg_1638.jpgimg_1637.jpg
    It would be possible to describe everything scientifically, but it would make no sense; it would be without meaning, as if you described a Beethoven symphony as a variation of wave pressure. Albert Einstein

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    Quote Originally Posted by Chris Turner View Post
    I'd also like to build one of these pickups. Very cool stuff!!

    However, is there not a way to get around having to use the Shure A95U (or similar)? Could you not use a transformer with a higher turn ratio? And if that's the case, does anyone know of a good, easy-to-buy-from (preferrably off-the-shelf...) source?

    Anyone?

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    Lace is building their own current transformers. There is no magic here.

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