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Thread: Low Impedance Pickup Research

  1. #36
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    If you can wind pickups, you can wind tiny transformers.

    Heck, if Gibson could make inductors out of bolts, any of us should be able to do this stuff!

  2. #37
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Rick Turner View Post
    If you can wind pickups, you can wind tiny transformers.
    Anyone have an idea how to wind a transformer like that?

    Heck, if Gibson could make inductors out of bolts, any of us should be able to do this stuff!
    That cracked me up the first time I saw it! A bolt, a few washers and a nut! Do you know if that was Bill Lawrence's doing? I know they used those in the L6-S and Ripper bass. It was a humbucker inductor too.
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  3. #38
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Joe Gwinn View Post
    Lace is building their own current transformers. There is no magic here.
    Exactly. I pointed out the bent side in response to melting the current transformer in the above design.
    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. #39
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    Quote Originally Posted by David Schwab View Post
    Anyone have an idea how to wind a transformer like that?
    Little transformer laminations and bobbins are catalog items. You will have no problem using a pickup winder to fill the little bobbin.

    Another alternative is to note that the fancy bit of punched copper sheet going through the current transformer need not have that shape. A piece of heavy copper wire threaded through the current transformer, formed into shape, and soldered into a ring well away from the current transformer will also work. You may need to make an aluminum (not copper) clamp to hold the wire being soldered and to keep soldering heat from reaching the transformer. Insulate the wire as needed using teflon tubing, which will not melt from soldering.

    Alternatively to the aluminum clamps, if the current transformer has been fully wax impregnated, simply hold the transformer end under water while soldering.

    In all cases, use a BIG iron or a torch to do the soldering. Quicker is better. Soldering should take literally one second, maybe two. It helps if the wire ends have been pre-tinned.

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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?
    I'd like to know too, but I guess they'd have to have a lot of windings. I've tried to figure out how many:

    Instead of increasing the impedance to something that similar to a microphone (I believe around 200 ohms) as we do with the AS104, the transformer would have to increase the impedance to something similar to a traditional pickup (around 200.000 ohms). That's a thousand times as big an impedance, and hence, the transformer would have to have around 32 times as many windings as the AS104 (since the impedance increases by the square of the number of windings, and the square root of 1000 is around 32). So, in order to get an output impedance similar to a traditional pickup, we're looking for a transformer with 32 times as many windings as the AS104. The AS104 has 500 windings so that would mean that we need a transformer with 16.000 windings (32 times 500).

    I might have made one or more mistakes in the above, but unless I'm far off, that's a lot of windings. I don't think such a transformer exists. If I'm wrong, I'd like to know.

    [edit: In this thread, bbsailor mentions current transformers with 5000 windings]

    /Alex
    Last edited by alexoest; 08-10-2010 at 08:49 AM.

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    Quote Originally Posted by Joe Gwinn View Post
    Little transformer laminations and bobbins are catalog items. You will have no problem using a pickup winder to fill the little bobbin.

    ---<snip>---
    Alternatively, if you don't have a pickup winder, you might use a sewing machine thread bobbin - and a sewing machine for winding. I made a between-the-strings pickup that way.

    /Alex

  7. #42
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by alexoest View Post
    I'd like to know too, but I guess they'd have to have a lot of windings. I've tried to figure out how many:

    Instead of increasing the impedance to something that similar to a microphone (I believe around 200 ohms) as we do with the AS104, the transformer would have to increase the impedance to something similar to a traditional pickup (around 200.000 ohms). That's a thousand times as big an impedance, and hence, the transformer would have to have around 32 times as many windings as the AS104 (since the impedance increases by the square of the number of windings, and the square root of 1000 is around 32). So, in order to get an output impedance similar to a traditional pickup, we're looking for a transformer with 32 times as many windings as the AS104. The AS104 has 500 windings so that would mean that we need a transformer with 16.000 windings (32 times 500).

    I might have made one or more mistakes in the above, but unless I'm far off, that's a lot of windings. I don't think such a transformer exists. If I'm wrong, I'd like to know.
    What pickups are 200K?

    The transformer on the Alumitone pickup I posted in the photo reads 5K. I have no idea what gauge wire it is.

    It's not unheard of to wind 10,000 turns on a small bobbin. Look at P bass pickups. It's not a lot of winding on a small bobbin.
    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 David Schwab View Post
    What pickups are 200K?
    In the neighborhood of the resonance with the cable, a pickup can easily be higher than 200K.

    A 3 Henry pickup would be about 40K at 2KHz.

    The impedance approaches the dc resistance only at very low frequencies.

    I am puzzled by this interest in single turn pickups. Why not just wind a regular pickup with fewer turns? The inductive impedance drops roughly with the square of the number of turns as turns are removed, while the voltage openly drops linearly. The "advantages" of low impedance (flat frequency response, low electrostatic hum pickup) are realized without reducing the number of turns very many times. If you need to boost level, you can do it at the amp with a transformer or a preamp with bipolar transistor input or some really low noise FETs.

    The transformer should go at the amp because then it is not loaded by the cable capacitance.

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    Quote Originally Posted by Mike Sulzer View Post
    I am puzzled by this interest in single turn pickups. Why not just wind a regular pickup with fewer turns?
    There might be different reasons, but my own interest was triggered by the fact that with this kind of pickup, you don't have to route a large cavity in the structurally critical area where the neck joins the body. A good example of this advantage can be seen on Rick Toone's Skele guitar.

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    Quote Originally Posted by alexoest View Post
    There might be different reasons, but my own interest was triggered by the fact that with this kind of pickup, you don't have to route a large cavity in the structurally critical area where the neck joins the body. A good example of this advantage can be seen on Rick Toone's Skele guitar.
    That is an interesting project!
    Yes, it is a bit tough to make a very thin pickup by ordinary means; bobbins become extremely flimsy if you try to make them from thin material so that the flat work does not occupy too great a percentage of the available space. An approach I have considered is to make 12 small flat coils so that thin bobbin material is not so much of an issue. The pickup body would be a piece of plastic of the right thickness, drilled out to hold the coils and milled on the back for conductor paths, etc. The pole pieces would be short ferrite beads with thin (1/32") neo magnets on top, of course.

  11. #46
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Mike Sulzer View Post
    In the neighborhood of the resonance with the cable, a pickup can easily be higher than 200K.
    Yes, but in the context of his post, he was talking about winding a transformer that high.

    that's a thousand times as big an impedance, and hence, the transformer would have to have around 32 times as many windings as the AS104
    In context, the Alumitone's transformer reads 5K, not 200K. Not a very big coil. Not a lot of winds.
    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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  12. #47
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    Quote Originally Posted by alexoest View Post
    A good example of this advantage can be seen on Rick Toone's Skele guitar.
    Hey, it's our very own bbsailor!

    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 Mike Sulzer View Post
    I am puzzled by this interest in single turn pickups.
    This type of pickup could also find a nice home at the end of the fingerboard of an EUB. A single loop of wire could easily conform to the short radius.

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    Quote Originally Posted by David Schwab View Post
    In context, the Alumitone's transformer reads 5K, not 200K. Not a very big coil. Not a lot of winds.
    5K is the dc resistance? If so then what one needs to know is the impedance as a function of frequency with the pickup attached. It would probably fall within the range of what you can do with an ordinary pickup

  15. #50
    Pickup Maker David Schwab's Avatar
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    Yes, the DC resistance is 5K. The output level is like that of a regular pickup.
    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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  16. #51
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    Quote Originally Posted by David Schwab View Post
    Yes, the DC resistance is 5K. The output level is like that of a regular pickup.
    What's the inductance, as measured while connected to the pickup turn? That's the core of Mike's question.

  17. #52
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    Quote Originally Posted by Joe Gwinn View Post
    What's the inductance, as measured while connected to the pickup turn? That's the core of Mike's question.
    I have no way of measuring that. However, I checked the Lace site and they do list that info:

    Lace Alumitone Bass Bars
    Sizes: 3.5", 4.0" and 4.5"
    Position: Neck, Bridge
    Resistance: 5.0K
    Peak Frequency: 3800
    Inductance: 2.7 henries

    Lace Music Products | Since 1979 | Electric 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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    And why not put one or more additional turns on the very low Z pickup? Frequency response and resonance will probably not drop into the audible zone, and you'd get more current to work with, right? That would mean a lower turns current transformer. Heck, make the primary coil out of silver to keep the DCR as low as possible.

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    Don't forget to trademark "Silvertone". If it's still available.

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    Quote Originally Posted by David Schwab View Post
    I have no way of measuring that. However, I checked the Lace site and they do list that info:

    Lace Alumitone Bass Bars
    Resistance: 5.0K
    Peak Frequency: 3800
    Inductance: 2.7 henries

    Lace Music Products | Since 1979 | Electric Pickups
    I would guess that the Peak Frequency (3800 Hz) is the resonant frequency; this is far lower than expected, and explains why it's sold for Bass.

    The inductance is far larger than expected from ordinary current transformers, unless they have a whole lot of turns. The transformer will also cause the inductance of the one-turn primary to appear, suitably transformed.

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    Brilliant, Joe! I'll look into it.

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    SMC Music Oh, well...

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    Quote Originally Posted by Rick Turner View Post
    And why not put one or more additional turns on the very low Z pickup? Frequency response and resonance will probably not drop into the audible zone, and you'd get more current to work with, right? That would mean a lower turns current transformer. Heck, make the primary coil out of silver to keep the DCR as low as possible.
    Actually, the fact that you have a one turn pickup with an inductance similar to that of a normal pickup (see specs David quoted above) suggests that you do not gain much of anything by using a transformer and fiddling with the ratios.

    And you do not.

    Suppose you double the number of turns on the pickup. You get twice the voltage; you use half the turns ratio. The pickup coil has four times the inductance (if you hold the geometry the same). But the impedance transformation ratio of the transformer is the square of the turns ratio so you end up with the same inductance looking back into the secondary. Even the resistance looks the same because you use smaller wire when you increase the number of turns.

    So where is the improvement? In fact, transformers are not perfect, so you actually lose something.

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    Quote Originally Posted by Mike Sulzer View Post
    Actually, the fact that you have a one turn pickup with an inductance similar to that of a normal pickup (see specs David quoted above) suggests that you do not gain much of anything by using a transformer and fiddling with the ratios.

    And you do not.

    Suppose you double the number of turns on the pickup. You get twice the voltage; you use half the turns ratio. The pickup coil has four times the inductance (if you hold the geometry the same). But the impedance transformation ratio of the transformer is the square of the turns ratio so you end up with the same inductance looking back into the secondary. Even the resistance looks the same because you use smaller wire when you increase the number of turns.

    So where is the improvement? In fact, transformers are not perfect, so you actually lose something.
    Mike and all

    Sorry for my delay in responding to this thread but I just returned from a 10 day vacation in Europe and a cruise up the Rhine River from Amsterdam to Switzerland. I had no cell phone, access to the internet or desire to stay connected as I immersed myself in European culture, architecture and history. But for now, it's back to reality and comments about pickup technology.

    Joe Gwinn is quite right about about keeping the the primary loop resistance low and maintaining the integrity of the joint when threading the loop through a toroid current transformer. To make the loop joint, I have been using thin copper tubing, tinning the wire making and silver soldering the joint. To minimize heat thansfer to the toroid you can just use two large alligator clips as heat sinks. The CSE187L is an E-I laminated core type with a single loop of AWG 12 that requires two solder connections.

    Mike is correct about transformers not being perfect. So where is the advantage to current based transformers? This discussion should be going in the direction of improved signal to noise ratio. High impedance pickups make good induced noise antennas with shielding attempts to minimize the noise but with some eddy current effects impacting the sound. Current-based pickups have a lower noise pickup, espicially when you ground the E-I laminated core frame to the low Z loop and send the signal out of the guitar in a balanced microphone way. Even in the unbalance mode, the noise is still lower. The amount of wire on a current transformer (CT) is less thus the distributed capacitance is less. With lower noise the output of a CT can drive the higher gain of an XLR microphone input preamp and then into a guitar amp for a wider range sound. Add some frequency shifting resonant peak EQ circuits (EMG-BQC) and you can tune a CT pickup to sound like most any traditional high impedance pickup.

    The Alumitone interleaved "U-shapes" cores wrap around the metal frame so as to not need any secure connections of the low Z loop. It is inductivly coupled and uses two coils that have very fine wire and that are connected in parallel from what I can decuce from the Lace published resistance specifications. The coils be connected in either series or parallel for a tonal shift change. To get the output comparable with high impedance pickups, I suspect the number of turns on each small coil on the "U-shaped" laminated core is about 10,000 turns. Maybe Mike can do some reverse engineering on the physical core sizes with Dave's coil size input and offer some possibe turns ratios if the coils are in parallel or series. I believe the wire gauge is AWG 44 but a close examination of one of the Lace under- shell coils should put that to rest pretty quickly. The under-string loop resistance and turns ratio affects the tonal coloring or voicing of the pickup to favor the lower or higher frequencies.

    I have offered up my research to kick off this topic, and a few folks are starting to expand on my initial posts and are making nice current-based pickups. Rick Toone lives near me and saw some of my earlier posts on this forum and invited me to his shop. He needed a very thin pickup that did not need the traditional hole behind the fingerboard. With a CT-based pickup you can arrange the "U-shaped" laminated cores on each end of the pickup so the only recess that is needed is about 1 sq inch on each side (to accommodate the coils) about 3 inches apart to not interrupt the acoustic string energy transfer from the neck to the bridge.

    Summary: Low noise, less wire, wider frequency response and thinner physical size all offering guitar makers some new alternatives.

    Thanks for your interest and I hope to see more comments and refinements in these new types of pickup.

    Look at Bajaman posts for his clever CT design.

    Thanks

    Joseph Rogowski
    Last edited by bbsailor; 08-15-2010 at 02:29 PM.

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    Joe Gwinn,


    On the same web page Lace published the following information. Lace Aluma J-Bass
    • Position: Neck, Bridge
    • Resistance: 5.0K
    • Peak Frequency: 3800
    • Inductance: 60 henries
    This 60 Henries seems too high but can 60 Henries be possibly the correct value. It seems more realistic that it is more like 6 Henries. What do you think?

    Thanks

    Joseph Rogowski
    Last edited by bbsailor; 08-15-2010 at 03:15 PM.

  26. #61
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Mike Sulzer View Post
    Actually, the fact that you have a one turn pickup with an inductance similar to that of a normal pickup (see specs David quoted above) suggests that you do not gain much of anything by using a transformer and fiddling with the ratios.
    I think much of that inductance comes from the transformer coils (I believe they use two for hum canceling). For the "hotter" voiced Alumitones, they wind the transformer hotter. They don't change the primary coil at all.

    One complaint when they first came out was they were to thin sounding, so then they increased the winds on the transformer.

    Similarly the old Les Paul Signature guitar used low Z pickups with a transformer with different taps to get different tones.
    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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  27. #62
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by bbsailor View Post
    This 60 Henries seems too high but can 60 Henries be possibly the correct value. It seems more realistic that it is more like 6 Henries. What do you think?
    I think they meant 6.0 H.
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    Quote Originally Posted by bbsailor View Post
    Summary: Low noise, less wire, wider frequency response and thinner physical size all offering guitar makers some new alternatives.
    Joseph,

    That sounds like a wonderful vacation. I hope you have returned refreshed and ready to go!

    Yes, those are the potential advantages. But how well do they apply to a single turn pickup, and do you have to go to a single turn to realize them?

    1. Comparing a one turn pickup to one with 10,000 turns in terms of hum pickup from electric fields, the single turn pickup is better by 80 db (signal to hum ratio improvement). You do not need so much reduction, and so a pickup with maybe a few hundred turns would be good enough.

    2. Realizing the full benefit of wide bandwidth and high output requires a transformer at the amp (like the original Gibson recording pro). A single turn pickup requires a transformer in the guitar since the impedance is too low to send over a cable with connectors, etc. Therefore, you would need two transformers to realize the full frequency range when using a single turn pickup, one to go to an intermediate impedance, such as a couple hundred ohms, and a second at the amp to produce the normal voltage level.

    3. A low impedance pickup still needs to reduce hum from magnetic fields with a canceling scheme, like a humbucker.

    4. A single turn pickup is certainly a natural for a thin design. Other designs are possible, but harder.
    Last edited by David Schwab; 08-16-2010 at 02:46 PM. Reason: fixed quote tag

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    Quote Originally Posted by David Schwab View Post
    I think much of that inductance comes from the transformer coils (I believe they use two for hum canceling).
    It should not. Audio transformers are designed to have very good coupling between the windings. If the coupling is perfect, there is no series inductance in the output from the transformer no matter how high the inductance of the coils is.

    Imperfect coupling (leakage flux) results in such inductance, known as a leakage inductance. Remember a discussion of pickup impedance, where the effect of eddy currents in the cores was treated as imperfect coupling to a transformer secondary.

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    Quote Originally Posted by bbsailor View Post
    Joe Gwinn,


    On the same web page Lace published the following information. Lace Aluma J-Bass
    • Position: Neck, Bridge
    • Resistance: 5.0K
    • Peak Frequency: 3800
    • Inductance: 60 henries

    This 60 Henries seems too high but can 60 Henries be possibly the correct value. It seems more realistic that it is more like 6 Henries. What do you think?

    Thanks

    Joseph Rogowski
    I wonder if that resistance is just the resistance of the secondary coils or if it includes the resistance of the pickup loops, as tremendously increased by the transformer turns ratio?

  31. #66
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    Quote Originally Posted by bbsailor View Post
    Joe Gwinn,


    On the same web page Lace published the following information. Lace Aluma J-Bass
    • Position: Neck, Bridge
    • Resistance: 5.0K
    • Peak Frequency: 3800
    • Inductance: 60 henries
    This 60 Henries seems too high but can 60 Henries be possibly the correct value? It seems more realistic that it is more like 6 Henries. What do you think?
    I don't believe 60 Henrys for a minute. Even 6.0 seems pretty high, although I suppose it isn't impossible. While transformers will change the inductance or capacitance presented to one side to a different value seen from the other side, conservation of energy still holds.

    I bet they used a LCR meter to measure it, and got bit -- most LCR meters simply don't work on low Q inductors such as pickups, yielding wild answers. The Extech being one of the few exceptions.

    I got 40 H and 60 H results when I used a B+K LCR meter on a pickup, which flummoxed me for a while. This is what led me to build the Maxwell-Wein bridge, which in turn allowed me to show that the Extech got the right answer. Unlike the B+K.

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    Magnet structure question for David S.

    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?

    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.

    The interest and research into very low impedance pickups by you, Joe, and other luminaries around here is fascinating. Keep it up!

    Thanks,
    mr coffee

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    Nobody seems to know how guitar pickups work. Read this and you will be
    informed:

    BuildYourGuitar.com :: The Secrets of Electric Guitar Pickups

    With my Pickup Analyzer I can measure the frequency response of any pickup
    under real operating conditions.

    The DC resistance of a pickup or a transformer is an irrelevant magnitude.
    Most manufacturers tell a lot of nonsense. The inductance is essential.
    Because of the winding capacitance in parallel and eddy currents in an iron
    core the reading depends on the measuring frequency. It must be measured
    at lowest frequencies as possible, e. g. in the order of 100 Hz. Measuring at 1
    kHz or more will lead to false results. If a pickup manufacturer publishes
    inductance figures (only a few do) you cannot rely on these.

    I experimented a lot with low impedance pickups, especially with the Gibsons
    used in the Les Paul Recording (1971). The transformer is of very poor
    quality. It has high losses by eddy currents and loose coupling. In the "Les
    Paul Signature" of 1973 (a golden semiacoustic, looks like an upper half of an
    ES335 combined with a lower half of a Les Paul) Gibson used a high-quality
    studio transformer with different taps on the primary side, selected by a rotary
    switch. So one can simulate three different pickup inductances. I measured all
    the frequency response curves.

    The very first invention of a low-impedance pickup probably was made by the
    British guitarbuilder Jim Burns. See US patent no. 3,249,677. He never had
    success with it.

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    3,951
    Helmuth, welcome, some of us own your book. I emailed you once about your pickup analyzer and how much it costs and if it has English instructions, never got a reply. Would like to know more about how it functions in detail.

    Alot of us us the Extech LCR meter, which has test frequencies of 120hz and 1khz. It also read AC resistance. I find that it is useful to have the 2 frequencies so one can see what is happening in the bass frequencies a little bit. While inductance is useful, personally I find AC resistance MORE useful and usually look at that first. If you add more iron into a pickup design usually you see the inductance go DOWN so its hard to compare one pickup against another by how high the inductance reading is, but AC resistance always goes up the more wire you add or more iron, or putting covers on etc. I don't quite see how only using a 100HZ test signal would be useful, its seem a very limited look at things.

    I have to say I don't see the appeal of these one turn pickups either, they don't sound particularly good to me and to me they are just too bright. A good Filtertron sounds way more musical and pleasing than those Alumatones, but I guess they sell them, I just wonder if they remain on player's guitars or after a year get replaced by something more traditional. They DO look cool, I will give them that!
    http://www.SDpickups.com
    Stephens Design Pickups

  35. #70
    Supporting Member
    Join Date
    Sep 2007
    Posts
    2,827
    Helmuth,

    I was interested to see that your measurements predict a dip in the frequency response (below the resonance) of a pickup with significant eddy current losses. This would be due to the effects of the cores, etc. I suspect that a lot of work that Possum has done on his PAF sound-a-likes is in getting the eddy currents right by choosing the proper metal in order to reproduce this dip accurately.

    Mike

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