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Thread: Low Impedance Pickup Research

  1. #71
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    Pickup Analyzer

    LIPR2.10 18. 8. 10

    Possum,

    Sometimes e-mails get lost in spam filters, sorry.

    The price of the new PC-coupled Pickup Analyzer is 850 Euros or
    1100 US-$ (with today's exchange rate) plus air mail freight costs
    (order of 60 $). An English instruction manual will be included. The delivery
    time is about four weeks. It is totally hand-made. I do this in the evenings
    beside my regular job.

    The price does not include the software which was not developed by
    me. It must be purchased separately at:
    audioTester
    (English version available). The price is 39 Euros (51 $). You can
    test it free for one month, after that you have to pay.

    The measured frequency response curves are much more interesting than all
    measurements with the RLC meter. My one also works with 120 Hz and 1000
    Hz. Only the 120 Hz inductance figures are useful. The 1000 Hz figures are
    good for nothing, as well as the AC resistance figures.

    I also tested a Lace Alumitone. The characteristic is not far away from a
    standard humbucker except a slight roll-off the bass end, caused by the
    transformer. Unfortunately it is rather microphonic. When you touch it with the
    pick you hear a loud click in the loudspeaker.

    There is no typical sound of low-impedance pickups. The transfer
    characteristic can be trimmed with external load capacitors and resistors just
    like with normal high-impedance pickups. I developed a new 11 position
    "Decade" switch for the Les Paul Recording with exponential capacitance
    steps. It works much better than the original one.

    ----

    Mike,

    Yes, the dip is caused by the eddy currents. When you wind a pickup with
    lower impedance for test then you can separate this effect from the LC
    resonance effect.

  2. #72
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by MR COFFEE View Post
    David,

    Thanks for the great pics of the Alumitone earlier in the thread. The pic of the top of the pickup look like there is a junction in the magnets part way across the pickup. Is this the case? If so, what do you think that's about?
    They used two shorter magnets instead of one longer magnet. I don't think there is any reason other than not having to order longer magnets. It's probably a size they already had. They act as one long magnet.

    And would you check the poling of the magnets and let us know where the north and south poles lie? I.E., both are poled top N, bottom S, both are poled side-to-side and the N poles go toward the center aluminum rib, and the S poles go toward the edge aluminum ribs, etc.
    One magnet is North up, and the other is South. Assuming you mount the pickup with the wires exiting to the right (transformer on the left), viewed from the top, the magnet closest to the neck is South.
    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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  3. #73
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Helmuth Lemme View Post
    Nobody seems to know how guitar pickups work. Read this and you will be
    informed:

    BuildYourGuitar.com :: The Secrets of Electric Guitar Pickups
    Helmuth welcome to the forum. Your web page and your pickup analyzer has been discussed here a few times in the past. Nice to have you here.
    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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  4. #74
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    Hi David,
    Thanks VERY much for the pics of the alumitone design.

    A couple of questions, if you'd be so kind.

    1) the pic of the top of the alumitone looks like there is a line across the magnets as if two lengths are butted together. Is that the case?

    2) what are the poling of the magnets used? I.E., is the South pole of both magnets toward the center bar, North toward the outside aluminum bars of the frame; North up toward the strings, South toward the pickup cavity\guitar body, or what?

    3) are the magnets those rubberized ceramic strip magnets, hard ceramic or ??

    I've never seen a real Alumitone in person.

    Thanks,

    mr coffee

  5. #75
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by MR COFFEE View Post
    A couple of questions, if you'd be so kind.
    This is the first one I ever saw in person as well.

    They are hard ceramic magnets. They are butted together to make one longer magnet, so think of it as two magnets. One is south up and one is north up.

    I'll post some more photos when I get a chance.
    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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    Alumitone frequency response

    Hi everybody,

    Here are the frequency responses of a Lace Alumitone (humbucker size),
    measured with the Pickup Analyzer from 40 Hz to 10 kHz.

    1. With 470 pF capacitive load and five different resistive loads: 10 M (nearly
    open circuit), 1 M (no pot, tube amp input), 333 k (one 500 k pot and 1 M tube
    amp input), 200 k (two 500 k or one 250 k pot and tube amp input), 111 k (two
    250 k pots and tube amp input). The capacitors on the tone controls are
    nearly a short circuit in the kHz range and can be neglected.

    http://www.gitarrenelektronik.de/ima...nik/alumi1.jpg

    2. With 1 M resistive load and eight different capacitive loads from 47 pF to
    2200 pF.

    http://www.gitarrenelektronik.de/ima...nik/alumi2.jpg

    Small ripples on the curves are caused by interference from the environment
    and have nothing to do with the pickup.

    For experiment, I removed the two plastic magnets and the copper foil on the
    rear side. The resonance peak got about 0.5 dB higher with 10 M load then,
    otherwise there is no fundamental difference.

    The roll-off on the bass side is due to a RL-highpass built of the serial
    resistance of the winding and the input inductance of the transformer.

    The transformer consists of two coils so that a humbucking effect is
    accomplished. If you use one coil only (white wire as hot output) the
    secondary inductance is halfed and the resonance peak appears at a higher
    frequency. It is sensitive to hum then.

  7. #77
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    Interesting. It is pretty much the response you might expect, sort of within the range of a typical humbucker, but without the dip below the resonance due to the eddy current loss. I wonder which aspects if this were intentional and which are just the way it worked out.

  8. #78
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Mike Sulzer View Post
    Interesting. It is pretty much the response you might expect, sort of within the range of a typical humbucker, but without the dip below the resonance due to the eddy current loss. I wonder which aspects if this were intentional and which are just the way it worked out.
    I remember when the pickup first came out they had a flatter response and people didn't seem to like them as much, so they must have decided to wind the transformer hotter.

    Also this design was around a while in the form of the Lace Transsensors which used a copper loop instead of the aluminum frame. So they had time to tweak these and see what people like best. Not surprisingly they voice them similar to hi-z pickups.

    Last edited by David Schwab; 08-23-2010 at 01:29 AM. Reason: typos
    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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    Bringing to mind a question...Why bother? Yeah, I know, cable capacitance and all that, but... Just put in a Stratoblaster and get some real gain happening! Worked for Lowell George...

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    Would winding a continuous length of permalloy ribbon through a couple of small coil form bobbins quasi-toroid style around the single turn loop post work for the low signal level involved?

    I've got some nice tiny-wire coils wound on transistor-radio-sized bobbins. I was thinking I could pull the stacked cores out of the bobbins and use the coils to play with the Alumitone single-turn design idea. I was thinking that putting the transformer outside the frame instead of underneath the frame would get me a really flat pickup with just the loop under the strings at the neck. Might be useful for a custom design.

  11. #81
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    Quote Originally Posted by MR COFFEE View Post
    Would winding a continuous length of permalloy ribbon through a couple of small coil form bobbins quasi-toroid style around the single turn loop post work for the low signal level involved?

    I've got some nice tiny-wire coils wound on transistor-radio-sized bobbins. I was thinking I could pull the stacked cores out of the bobbins and use the coils to play with the Alumitone single-turn design idea. I was thinking that putting the transformer outside the frame instead of underneath the frame would get me a really flat pickup with just the loop under the strings at the neck. Might be useful for a custom design.

    Mr. Coffee,

    Give it a try. What you want to do is put a low resistance (under 1 milliohm) loop under the string with a magnet in the center. Then you need to couple that low resistance loop to the current transformer secondary which will be your transistor radio sized bobbins. The number of trurns on those bobbins will determine the truns ratio and the voltage/impedance of the output. The coupling of the permallow bobbin to the low resistance loop will determine the efficiency of the transformer.

    Feeding a guitar amp, you will need an output in the 100mv to 200mv range but if you feed a mic input you only need about 5mv to 10mv output. Grounding is very important to minimize hum. Ground the low resistance loop. Ground the strings.

    Try ordering a few small CSE187L current transformers and play with using these. They have a 1 to 500 turns ratio. They work very well into an XLR mic input or into a microphone matching transformer with a 1 to 10 turns ratio boost. Putting a piece of thin ferrous metal under the magnet and under the low resistance loop will redirect the underside of the magnetic field and result in about a 25% to 30% increase in output.

    See this link for an enlarged photo of the CSE187L. http://rocky.digikey.com/WebLib/Tria...os/CSE187L.jpg

    Post a photo of what you come up with. Who would have thought that inexpensive current transformers could produce a guitar pickup? Share what you learn from your own experiments and we all learn something.

    Joseph Rogowski

  12. #82
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    All,
    Thanks for the information and encouragement.

    I figure the coils I have are at maybe 500 turns, but I was wondering what I could do for a core I could put around a continuous loop. I have some permalloy that I could cut in long strips and wind through a couple of the coils set up in a humbucking configuration similar to the picture of the Alumitone layout. The coils are a bit shorter and fatter than the Lace design, but I would hope that won't make much difference.

    But with what Helmuth shares about microphonics with the Alumitone he tested, I'd like to know if that represents other people's experience with the Alumitone and Transensor designs. I was mostly excited at the prospect of less noise pickup, not trading noise for microphonics.

    Thanks all. Great Discussion!

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    Why so much microphonics?
    I'd tend to say the whole circuit is mechanically too rigid, plus aluminum alloy makes a high pitched sound when struck, and the mini transformer is secured tightly on one end only leaving it free to vibrate in the field if ever the pickup is struck etc

    Or perhaps the transformer is insufficiently shielded, from outside intererences and from the PU's magnets?

    *are the transformer plates grounded by contacting the aluminum frame or are they grounded via wiring?
    Should the transformer be mechanically secured in some king of potting and not contact rigidly the frame (having the inside gap filled with soft resin or wax between frame & plates for example?)


    *besides redirecting the flux more efficiently under the bar magnets with a ferromagnetic plate as bbsailor proposed, would there be an additional solution to prevent the magnets from inducing current directly in the transformer bobbins?
    Which solutions for shielding the circuit?
    A member stated that there is a copper plate under the magnets: copper is a poorman's choice, it isn't made to block really magnetic fields that well. Unless it isn't copper, on the gutshot photos, it seems to be an alloy more silvery in color than copper....?? (I don't own the pickup so I cannot see for myself)

    LEt's say the aluminum frame was pure aluminum, uncoated, untreated : when the surface corrodes and eventually pits, would that impact the overall quality? The alumitones are apparently anodized, so they're oxidized in a controlled manner, controlled thickness as well.
    Any influence at all on frequency response with degraded & oxidized low-z loop material surface? ( imagine a pure copper low z frame so corroded you could see the copper acetate layer on it, for example?)

  14. #84
    Junior Member TGBoleyn's Avatar
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    Lightbulb

    Joseph,

    Is it possible to use magnetic balls rather than strip or bar magnets in this design? I think that it would have a cool look and wonder if neodym balls would hinder the idea or be worth a try?

  15. #85
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    TGBoleyn,

    Yes, you can try neo-type magnetic balls, however orienting them properly and finding a way to secure them may present you with a challenge. The general advantage of this design is low noise, wide bandwidth (without the classic resonant hump the defines an electric guitar sound) and ease of fabrication. The ease of fabrication is what using neo-type magnetic balls challenges.

    Give it a try and post your results so we can all learn something from your effort!

    Happy Holidays

    Joseph Rogowski

  16. #86
    Junior Member TGBoleyn's Avatar
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    Very encouraging! I figured that I'd try to create some wood covers to insert into the routed humbucker slots in the body (so that the gold, chrome or black chrome ball mags would look like they are in the body itself), and have specially drilled and routed holes to keep the mags in place. As soon as I can get the setup completed, I'll post more info and pics.

    Thanks Joseph!

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    CT pickup with three primary windings

    I've made a ct pickup with three primary windings, not really knowing what to expect. I had a good piece of 3.5 mm copper wire that was just long enough for three windings, so I decided to try and see how it would sound. The sound is decent, but not very exiting. I'm about to try different magnets at different distances to the strings, so I might be able to improve the sound.



    I hope having more primary windings does something positive to the impedance. But I haven't found out what three primary windings instead of one does to the impedance of the signal that comes out of the current transformer. Will it have three times the impedance of the signal from a pickup with only one primary winding. Or nine (3 square) times that high. Or something completely different?

    So the question is: what is the relation between the number of primary windings and the impedance of the final transformed signal?

    If anyone can point to useful formulas or information - or inform me - I'd be happy.

    /Alex

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    Quote Originally Posted by alexoest View Post
    I've made a ct pickup with three primary windings, not really knowing what to expect. I had a good piece of 3.5 mm copper wire that was just long enough for three windings, so I decided to try and see how it would sound. The sound is decent, but not very exiting. I'm about to try different magnets at different distances to the strings, so I might be able to improve the sound.



    I hope having more primary windings does something positive to the impedance. But I haven't found out what three primary windings instead of one does to the impedance of the signal that comes out of the current transformer. Will it have three times the impedance of the signal from a pickup with only one primary winding. Or nine (3 square) times that high. Or something completely different?

    So the question is: what is the relation between the number of primary windings and the impedance of the final transformed signal?

    If anyone can point to useful formulas or information - or inform me - I'd be happy.

    /Alex
    Alex,

    Try one sting loop and a CT at each end of the loop. Then, try listening to the CTs in series and parallel and then again into a mic matching transformer. You should hear some interesting tonal variations doing this experiment.

    It is good to see some daring people try this CT pickup stuff.

    Joseph Rogowski

  19. #89
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    Quote Originally Posted by alexoest View Post
    I've made a current transformer pickup with three primary windings, not really knowing what to expect. I had a good piece of 3.5 mm copper wire that was just long enough for three windings, so I decided to try and see how it would sound. The sound is decent, but not very exiting. I'm about to try different magnets at different distances to the strings, so I might be able to improve the sound. (Image omitted.)
    How is the copper wire bonded into a loop?

    What prevents inter-turn shorts?

  20. #90
    Senior Member mkat's Avatar
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    Quote Originally Posted by bbsailor View Post
    It is good to see some daring people try this CT pickup stuff.
    Joseph, I probably wouldn't have tried this if you hadn't shared your work, so thank you

    I made a couple for bass, both using a single AS-104 current transformer.

    I used 1.6mm copper wire on this first one to check that it worked ok.

    img_2292b.jpg

    The second one was based on bajaman's design. I didn't have 5mm wide aluminium bars for the edge pieces, so had to settle for 10mm. I haven't completed this one yet and just held it over the strings to test, but it works OK. Still need to screw in the base plate I cut from some galvanised iron , which is why the magnets aren't sitting right yet, then cut the screws to size. The base plate is a great idea and does increase output slightly as you mentioned. Anyway, I'm going to use a second CT on this one to utilise the switching options as I do on my standard bass humbuckers.

    img_2354.jpg

    This second effort was quite a bit of work.

    BTW, I wired them into an Alembic Stratoblaster I have in my test bass to test them, didn't bother to check how they matched up.
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    Hi Joe

    I bent the wire into rough shape, filed the two joining areas flat for a larger contact area. Then I slid the CT in place and soldered the loop with a gas blowtorch using ordinary low temperature tin. I placed some damp paper tissue near the CT to absorb heat if it'd travel that far. Having three windings made it easier to get the CT far away from the soldering area than if I'd had only one winding.

    Short circuits are avoided by painting the wire with nail polish (some my wife had left over from the early nineties. I later removed the nail polish except for where the turns are close to one another.

    /Alex
    Last edited by alexoest; 01-24-2011 at 01:19 PM.

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    Hi Joseph

    I will try that option with two CTs at some point. But I'm still to buy the microphone matching transformer.

    I've been postponing it, expecting that I'd get my Linux audio computer up and running and record from the mic in on the sound card instead of using a guitar amp, but that hasn't happened yet.

    /Alex

    Quote Originally Posted by bbsailor View Post
    Alex,

    Try one sting loop and a CT at each end of the loop. Then, try listening to the CTs in series and parallel and then again into a mic matching transformer. You should hear some interesting tonal variations doing this experiment.

    It is good to see some daring people try this CT pickup stuff.

    Joseph Rogowski

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    Nice. The aluminium pickup is going to look really good when it's finished (I assume you'll remove the surplus thead and round the edges a bit)

    How do you like the sound? (and do you use a microphone matching transformer?).

    Plus, have you taken precautions against the oxidation of the contact points of the aluminium pickup, leading to greater resistance in the circuit (it was mentioned earlier in this thread)?

    Btw. +1 on thanking bbsailor for sharing the idea. It's great fun to work with.

    /Alex

    Quote Originally Posted by mkat View Post
    Joseph, I probably wouldn't have tried this if you hadn't shared your work, so thank you

    I made a couple for bass, both using a single AS-104 current transformer.

    I used 1.6mm copper wire on this first one to check that it worked ok.

    Click image for larger version. 

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    The second one was based on bajaman's design. I didn't have 5mm wide aluminium bars for the edge pieces, so had to settle for 10mm. I haven't completed this one yet and just held it over the strings to test, but it works OK. Still need to screw in the base plate I cut from some galvanised iron , which is why the magnets aren't sitting right yet, then cut the screws to size. The base plate is a great idea and does increase output slightly as you mentioned. Anyway, I'm going to use a second CT on this one to utilise the switching options as I do on my standard bass humbuckers.

    Click image for larger version. 

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    This second effort was quite a bit of work.

    BTW, I wired them into an Alembic Stratoblaster I have in my test bass to test them, didn't bother to check how they matched up.

  24. #94
    Senior Member mkat's Avatar
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    Yep, will cut the screws once I account for the base plate and round the edges.

    The tone is ok and like bajaman stated in the other thread on this, positioning is the main thing. I haven't tested this properly, I just held it over the strings while playing the open strings. Also, I'm running this through an Alembic Stratoblaster as stated above instead of a microphone matching transformer, as I don't have an one but do have a Stratoblaster installed in my test bass. I need to work out whether these things match up ok.

    I'm going to route a cavity for this when I get a chance and do some proper testing. I haven't yet done anything to protect against oxidisation, as there is still some work to do with the aluminium (flush sand ends, round edges).
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    Quote Originally Posted by alexoest View Post
    I bent the wire into rough shape, filed the two joining areas flat for a larger contact area. Then I slid the CT in place and soldered the loop with a gas blowtorch using ordinary low temperature tin. I placed some damp paper tissue near the CT to absorb heat if it'd travel that far. Having three windings made it easier to get the CT far away from the soldering area than if I'd had only one winding.

    Short circuits are avoided by painting the wire with nail polish (some my wife had left over from the early nineties. I later removed the nail polish except for where the turns are close to one another.
    Ahh. That works, and explains the appearance in the photo.

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    Quote Originally Posted by alexoest View Post
    Hi Joseph

    I will try that option with two CTs at some point. But I'm still to buy the microphone matching transformer.

    I've been postponing it, expecting that I'd get my Linux audio computer up and running and record from the mic in on the sound card instead of using a guitar amp, but that hasn't happened yet.

    /Alex
    Alex,

    The microphone matching transformer is designed to do two things.
    1. Boost the voltage output about 10 times.
    2. Provide a better impedance match to the high impedance input of a guitar/bass amplifier.

    If you are going directly into a microphone level input between about 1000 to 2000 ohms, then it should work just fine without the transformer.

    For the lowest noise be sure to ground the low impedance string loop. I like using the Triad CSE-187L Current Transformer (CT) because I can make a tight low impedance string loop with a CT on each end. Plus, I can run a ground wire that is directly soldered to the CT metal frame as well a the low impedance string loop. Two of the CSE-187L CTs with parallel outputs match the input impedance of a microphone matching transformer or an XLR microphone input on a mixer pretty well. The low noise of this design allows the extra gain of the microphone mixer input circuit to boost the output to a good working level.


    Joseph Rogowski

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    I'd like to thank and congratulate bbsailor for starting this thread, as well as all who contributed to it. I've been lurking for a couple of weeks, taking this in as time permits. For now, I'm giving the thread a bump while I read over the content thus far before I have anything else to offer.

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    Man Of Steel and all others interested in low impedance pickups,

    Here is an update about some new findings retaled to current transformer low impdance pickups.
    Using two current transformers, one CT is connected to the audio input and the other is used to tune the low impdance loop.

    You can tune the low impedance loop by using two CSE-187L or similar current transformers (CT) with the low resistance leads facing each other and connected by 2.5 inches of AWG 12 solid wire, with a magnet in the center. Measure the output impdance of a single CT with a wire loop the same size and length as the wire in the CT primary. Then measure the impedance of the output CT with another CT forming the other side of the loop. It will be about 10 times higher.

    WHY?
    The theory goes that with a 500 to 1 turns ratio, the impedance ratio will be the square of the turn ratio or 250,000. Any slight change in the primary string loop impedance will be reflected back. A simple test is to use an alligator clip and short out the second CT and listen to the incresed low end response. You can put a 1K pot across the second CT and vary the tone of the loop because the variable rsistance is reflected back onto the primary loop loop impedance which is reflected again back into the output CT. The key is to listen to where the range of the pot value (across the second CT) makes pleasant tonal changes.

    I hope this gives those who tinker with this stuff, something more to play and have fun with.



    Joseph Rogowski
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  29. #99
    Junior Member Man Of Steel's Avatar
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    That's yet another neat idea, bbsailor -- thank you for passing that along, too!

    It raises a couple of questions, like most great ideas:

    1. Is the secondary of the second transformer, which I'll refer to as the tuning transformer, connected to anything, or is it allowed to float?

    2. Could one achieve similar results replacing the tuning transformer with a ferrite slug and a wind or so of the 12 gauge wire, so that the necessary number of joints can be cut in half? I could imagine tuning this by sliding the core in or out.

    Thanks!

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    Quote Originally Posted by Man Of Steel View Post
    That's yet another neat idea, bbsailor -- thank you for passing that along, too!

    It raises a couple of questions, like most great ideas:

    1. Is the secondary of the second transformer, which I'll refer to as the tuning transformer, connected to anything, or is it allowed to float?

    2. Could one achieve similar results replacing the tuning transformer with a ferrite slug and a wind or so of the 12 gauge wire, so that the necessary number of joints can be cut in half? I could imagine tuning this by sliding the core in or out.

    Thanks!
    1. The tuning transformer raises the low impedance string loop. Putting a pot across the tuning transformer does the same thing as changing the size (diameter) of the low impedance string loop. Shorting out the tuning transformer secondary with a variable pot allows audible tuning of the pickup.

    2. Ferrite slug is not variable. However, the tuning transformer offers a chance to alter low frequency response using the low impedance loop while using conventional capacitor values across the audio CT output to tame the high frequencies.

    Also, you can wire an on-on-on type mini switch to select series, parallel or single CT output. In the single CT position the variable pot can be used to tune the pickup using the tuning transformer.

    I hope this helps.

    Joseph Rogowski
    Last edited by bbsailor; 07-12-2011 at 05:38 AM.

  31. #101
    Junior Member Man Of Steel's Avatar
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    Thank you for your reply, Joseph!

    Quote Originally Posted by bbsailor View Post
    1. The tuning transformer raises the low impedance string loop. Putting a pot across the tuning transformer does the same thing as changing the size (diameter) of the low impedance string loop. Shorting out the tuning transformer secondary with a variable pot allows audible tuning of the pickup.
    If I understand correctly, this allows adjustment of the loading of the string loop. Aside from the elements of the tuning transformer's equivalent circuit, a 0-500 Meg pot functions, in essence, as a 0-2 ohm load on the string loop because of the 500:1 turns ratio. Am I understanding this correctly?

    Quote Originally Posted by bbsailor View Post
    2. Ferrite slug is not variable. However, the tuning transformer offers a chance to alter low frequency response using the low impedance loop while using conventional capacitor values across the audio CT output to tame the high frequencies.
    I realize it's not RF, but would not moving a permeable core closer to the center of the loop at the end of a single-transformer current-mode pickup increase the inductance of the pickup, effectively placing inductance in series? Many old Philco radios used ferrite slug tuning for presets.

    "Taming the high frequencies" may be a fundamental issue with the low impedance paradigm. The transformer makes the DC resistance and inductance of the single turn transducer larger by the square of the turns ratio, but the stray capacitance is diminished by the same factor, which may be negligible to begin with. I suspect much of the capacitance comes from the transformer's secondary. The resonant frequency may be too high, and the Q-factor too low, to sound "right." Minimizing the equivalent series resistance of transducer loop and having enough shunt capacitance seem two (of several) key considerations. Fortunately, the latter is easy to fix.

    Quote Originally Posted by bbsailor View Post
    Also, you can wire an on-on-on type mini switch to select series, parallel or single CT output. In the single CT position the variable pot can be used to tune the pickup using the tuning transformer.
    More intriguing possibilities to consider -- thank you!

    Quote Originally Posted by bbsailor View Post
    I hope this helps.

    Joseph Rogowski
    I'm still wrestling with an equivalent circuit of this beast with one transformer. We know that electrical chracteristics of a "conventional" voltage-mode pickup can be modeled as a second-order low-pass filter. For the current-mode pickup, the transformer adds an additional inductor, yielding a third-order filter. This raises the issue of alignment, if this inductance is significant (as well as the shunt stray capacitance). A random alignment is unlikely to perform well (i.e., sound good). Alignments for the "conventional" voltage-mode pickups evolved over the years in the form of winding specifications, magnet/polepiece sizes and shapes, and other factors. I imagine a lot of experimentation, trial, error, and tweaking went into the evolution of these alignments. This paradigm is so far out of my comfort zone (perhaps why it's so interesting?) that much of the conventional wisdom doesn't apply. I'm interested in modeling this current-mode type, plugging in design constraints, and playing around with free parameters to try to align them with less experimentation. Hopefully, come up with alignments that will allow me to actually put one I'll build on a guitar.

    Thank you again!
    Last edited by Man Of Steel; 07-13-2011 at 10:31 PM. Reason: technical correction

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    Quote Originally Posted by Man Of Steel View Post
    Thank you for your reply, Joseph!



    If I understand correctly, this allows adjustment of the loading of the string loop. Aside from the elements of the tuning transformer's equivalent circuit, a 0-500 Meg pot functions, in essence, as a 0-2 ohm load on the string loop because of the 500:1 turns ratio. Am I understanding this correctly?
    Man Of Steel

    Generally you should use a pot value that is betweeen 25 to 50 times the DC resistance of the pickup coil. But since the CSE187L current transformer has a max DCR of 21 ohms and has a closly coupled laminated core, the secondary impedance will be relativly higher than a traditionally wound guitar pickup of the same resistance. I would use a pot value that is between 1K to 2.5K ohms in value and listen to the "sweet spot" value where the change in tone in maximized within the pot range. The minimum DCR of the low impedance string loop with just two CSE187L CTs is 2 times 250 micro ohms plus the resistance of two straight pieces about 2" long to fit the spread of six strings. This puts the total resistance of the string loop to be below 1000 micro ohms or 1 milliohm. Check out Surplus Sales of Nebraska for some rectangular copper wire that is 0.162" on a side and can be drilled to accomodate the "U-Shaped" primary lead with a snug fit to ensure a good, low resistance connection. The lower the total string loop resistance the more efficient the tuning CT will be in changing to tone.

    I hope this points you in the right direction.

    Joseph Rogowski
    Last edited by bbsailor; 07-14-2011 at 06:42 PM.

  33. #103
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    Progress: Modeling a low-current pickup

    Thank you, Joseph.

    An update on modeling: I believe the current-mode pickup may be modeled as a cascade of two-port subnetworks. The model will be slightly simpler if the inductance and series resistance of the transformer primary can be combined with the corresponding quantities of the secondary. It will also be slightly simpler if there are no eddy losses in the single "under string" winding. If both simplifications apply, it will be a fifth-order model. Otherwise, bump up the order of the model by one for each of these assumptions that are not applied.

    This is, of course, before the tone control, cable, etc. are considered.

    I think I can provide an Octave m-file (possibly compatible with Octave's proprietary counterpart, Matlab) that can compute the electrical transfer function as a function of frequency for a given set of parameters. These parameters are:

    1. Under-string inductance;
    2. Under-string resistance;
    3. Under-string capacitance;
    4. Shunt capacitance, series resistance, and series inductance of transformer primary;
    5. Shunt capacitance, series resistance, and series inductance of transformer secondary;
    6. Transformer coupling factor;
    7. Transformer turns ratio;
    8. Volume control resistance and setting;
    9. Tone control capacitance, resistance, and setting;
    10. Load resistance;
      and, if necessary:
    11. Leakage resistance and effective fraction of coil bypassed by eddy current;
    12. Load (cable) capacitance


    Many of the parameters relate to the transformer. The Triad data sheet for the CSE187L transformer provides information useful for its intended application of current-to-voltage conversion, but, other than the turns ratio and DC resistances, does not help here. (The fact that the core starts saturating at 10 to 20 amperes, depending on frequency, is reassuring, but is hardly an issue with a guitar pickup!) Has anybody evaluated the primary and secondary capacitances, primary and secondary inductances, and equivalent core loss resistance of this transformer? It would be a great help if they could share such information with me!

  34. #104
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    Quote Originally Posted by Man Of Steel View Post
    Thank you, Joseph.

    An update on modeling: I believe the current-mode pickup may be modeled as a cascade of two-port subnetworks. The model will be slightly simpler if the inductance and series resistance of the transformer primary can be combined with the corresponding quantities of the secondary. It will also be slightly simpler if there are no eddy losses in the single "under string" winding. If both simplifications apply, it will be a fifth-order model. Otherwise, bump up the order of the model by one for each of these assumptions that are not applied.

    This is, of course, before the tone control, cable, etc. are considered.

    I think I can provide an Octave m-file (possibly compatible with Octave's proprietary counterpart, Matlab) that can compute the electrical transfer function as a function of frequency for a given set of parameters. These parameters are:
    1. Under-string inductance;
    2. Under-string resistance;
    3. Under-string capacitance;
    4. Shunt capacitance, series resistance, and series inductance of transformer primary;
    5. Shunt capacitance, series resistance, and series inductance of transformer secondary;
    6. Transformer coupling factor;
    7. Transformer turns ratio;
    8. Volume control resistance and setting;
    9. Tone control capacitance, resistance, and setting;
    10. Load resistance;
      and, if necessary:
    11. Leakage resistance and effective fraction of coil bypassed by eddy current;
    12. Load (cable) capacitance
    Many of the parameters relate to the transformer. The Triad data sheet for the CSE187L transformer provides information useful for its intended application of current-to-voltage conversion, but, other than the turns ratio and DC resistances, does not help here. (The fact that the core starts saturating at 10 to 20 amperes, depending on frequency, is reassuring, but is hardly an issue with a guitar pickup!) Has anybody evaluated the primary and secondary capacitances, primary and secondary inductances, and equivalent core loss resistance of this transformer? It would be a great help if they could share such information with me!
    Man of Steel,

    You are on the right track with your anaysis, but there are some more details that will help you form a good mental model and ultimately a good math model that will define and graph what is actually happening.

    Remember that a transformer reflects the secondary load back to the primary by the square of the turns ratio (TR). So, in the CSE187L with a 1:500 TR you are reflecting a secondary load impedance back in parallel with the primary string loop impedance which is already pretty low in the sub milliohm range typically measured in hundreds of a microohm. This cannot be directly measured but what you can do is to measure the current transformer (CT) output AC resistance with the Extech LCR meter with the CT having (1) an open primary and (2) having a low imepdance string loop connected. Now, add a second CT on the other end of the same length string loop and note that the second CT will increase the output impedance of the output CT connected to the Extech LCR meter. Assume that the CT output is being sent into a mic input impedance of 2K ohms. Then, a reflected impedance of 2K divided by 250,000 (500 squared) or 0.008 ohms (8 milliohms or 8,000 microohms) will appear in parallel with the string loop impedance. Changing the gauge of the primary string loop will help tune the impedance range of the output CT. If you are going into an XLR input or a remotely mounted microphone matching transformer the microphone matching transformer input load can be optimized so that when the matching tranformer load impedance is reflecte back into the primary string loop, various harmonic ranges can be optimized to satisfy what the ear considers as being pleasing.

    Lets take a more extreme case using a Prem Magnetics SPCT-251 CT with a TR of 1:2000 makes a 4,000,000 string reflected impedance. If you use two 5" lengths of .162 square copper wire, which easily fits into the SPCT-251 open primary space. Drill two .12" holes on each end of the 5" lengths to accomodate a solid piece of copper wire with a CT on each end forming a few hundred microohm primary loop. Look at the output AC resistance of once CT with the other CT open and you will see about a 20K ohm AC resistance both at 1000Hz and almost the same at 120Hz. Now, short out the second CT with a short jumper and see the reflected AC resistance will be about 10X lower or about 2K ohms on the Extech LCR meter. This is due to primary loop impedance being changed by the second CT reflecting a short back into the primary loop by an impedance ratio of 4,000,000, minus the leakage inductance but this shows how the gauge of the wire on the primary loop can change the tonal characteristics of the two-CT-string-loop under the guitar strings.

    Velleman makes a 2-channel scope with a function generator PCSGU250 and software package PC-Lab 2000LT that can run Bode plots by feeding the signal generator into a 500 turn coil of AWG 32 wire on a HB pickup bobbin to stimulate the CT assembly. Try it with a CT open and then again with the CT shorted. Switch the outputs of the two CTs in series and parallel with a simple DPDT switch and listen to the tonal changes. Use an on-on-on swith to make a series/parallel/single coil selection. In the single CT mode, put a variety of pot loads across the second CT as wall as a variety of capacitors and listen to the tonal differences and then note where the most pleasing range of tone changes occurs and then run the bode plot to actually see what is happening. This will be most educational as well as open you mind to what is actually happening in an extremely low impedance string loops with a variety of CTs with different turns ratios. The SPCT-251 will provide you with about 100mv peak output from the 305 ohm secondary coil. Only the ear will tell you what sounds good, then you can graph and print out the Bode plots to capture a picture of the frequency response of what sounds pleasing and then see why it is pleasing.

    Guitar strings tend to emphasize certain harmonics but since typical high impedance pickups with 6 to 10 thousand turns of very thin wire (AWG 42, 43) tend to be very low Q coils and have a low inductance. Enter the low impedance CT pickup domain and you can see new opportunities to voice the sound of the pickup by selecting the right gauge of the low impedance string loop as well as tuning that low impedance string loop with the second CT helping to tune the primary loop impedance. This analysis uses traditional transformer theory but because CTs have very high turns ratios, reflected loads can have a very noticable effect on the final pickup sound and the harmonics that are emphasized by the combination of CTs types, turns ratios, primary loop wire size, second CT loading and personal tonal preferences. Ultimately, you will notice that the extended frequency response can be well out beyond the 5KHz to 8KHz audio region where high impedance pickups do not produce very much upper harmonic content.


    Lace, the makers of the Alumitone pickup, is missing an opportunity to put a second set of coils on the opposite side of the Alumitone frame where the second CT can be modeled to tune the pickups to sound new ways without needing active electronice. Lace can also put the two sets of secondary outputs in series, parallel or single (as explained above). I estimate that the tiny coils under the frame of the Alumitone frame have about 10,000 turns of wire smaller in diamter than typical AWG 42 or 43. Hint, if Don Lace is reading this post, this is a free design idea to expand the Alumitone pickup design to fully capture it's full design potential. This design concept is all based on using the second CT to tune the low impedance string loop formed by the Alumitone alumium frame.


    This is just some more "food for thought" for those curious enough to play with CT pickup designs. Also, this design can make very good sounding magnetic acoustic guitar pickups without the classic resonant hump in the mid audio range that cuts off the upper harmonics that usually helps to define an acoustic guitar sound not adequtely sensed by high impedance magnetic passive pickups.

    Joseph Rogowski
    Last edited by bbsailor; 07-29-2011 at 05:37 PM. Reason: spelling, grammar and fat fingers

  35. #105
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    The Lace Alimatone pickups caught my interest recently when I was researching some info related to pickup replacement on my Bass guitar. After reading some of these threads on constructing a Lo-Z pickup (thanks to contributing members, especially bbsailor) and being an occasional DIYer, I ordered a couple of CSE187L transformers and constructed the pickup pictured below. I physically designed the layout around a magnet I scrounged from a discarded magneto-optical drive (guessing neodymium) looped through 2 Low Z transformers with 8 awg copper wire. Output was fed to a line matching transformer with an XLR cord. Volume wise I thought the output was about the same as my passive pickups, but the tone sounded hollow and very trebly. Any suggestions on how to improve construction to enhance low end?
    dscn2451.jpg

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