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Thread: A six coil pickup and on board electronics for individual string distortion.

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    A six coil pickup and on board electronics for individual string distortion.

    The purpose of this project was to make an electric guitar pickup system with distortion on individual strings, that is, with low intermod distortion on multiple strings. A constraint is that the guitar must work with a standard cable and use standard amplifiers. This is accomplished by making a pickup with six individual coils and on board electronics for generating the distortion, combining the outputs, and filtering the combined signal.

    A coil like the ones in the pickup is shown here: http://www.naic.edu/~sulzer/coilPrototype.jpg. The coil is longer than it appears due to the shortening from the viewing angle. The core is made from two ferrite beads, each .2 by .437 inches. It has 5000 turns of #43 wire. The inductance of the coil is about .5 Henries. The material is so-called #73 with a relative initial permeability of 2500. (I might have mistakenly said this was 5000 in an earlier reference to this project.) The magnet is 1/8 inch in diameter and 1/32 inch thick neodymium. The field strength provided by this magnet backed by this core, 3/32 above the magnet, is larger than that of a typical humbucker, but less than a typical ajnico single coil.

    These coils can be used in a variety of ways. For example, six connected in alternating polarity in series makes a "single coil type" bridge pickup with high hum cancelation. The resonance is at about 5KHz with a normal guitar cable. The ferrite is lower loss than alnico, and so the pickup must be loaded with a lower total resistance than a standard single coil. The pickup delivers more than 1.5 volts p-to-p with a six string chord.

    As an individual string pu, loaded as explained later, the output is about 0.2 volts p-to-p. An earlier version of this project used a seventh coil to provide hum cancelation for each of the six coils. This is unnecessary; reversing the polarity does the job, even though the effective gains of the non-linear circuits can be very different. (All that really matters is that there be good cancelation when there is no, or very little signal present.)

    Also in an earlier version, a cancelation network was used to get the signal leakage between adjacent coils down from the inherent 20 db to 30 db. This is also not necessary.

    The six coils are simply glued into a strat pu cover. This is shown here: http://www.naic.edu/~sulzer/6coilPUonG.jpg. This system is only useful for a bridge pickup (string bending). It should be possible to make a 12 coil system for a neck pu. (But that is another project.)

    In later posts, I will discuss the pickup some more and describe the electronics. The circuit diagram is shown here: http://www.naic.edu/~sulzer/sixCoilPre.png

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    Pickup Maker David Schwab's Avatar
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    Great project Mike!

    I've been wanting to make a hex fuzz ever since I played an ARP Avatar guitar synth back in the 70's.

    A buddy of mine wants me to make him a pickup like this for using with six Pro Co RAT fuzzes.

    That's a nice looking guitar, what is it?

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    Very cool project. I would be interested in hearing it. I had worked on a similar project and really enjoyed the sound.

    That looks like a Black Korina body guitar. Nice tone wood! I have one too.

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    Thanks to you both! For this project I bought an unfinished body and neck from Warmoth. (I am a slow wood worker; I would still be carving!) The body is their VIP model, black Korina, yes, chambered body with a black Korina top as well. The neck is mahog. This body style has a large control cavity; so there is plenty of room for fancy electronics.

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    Pickup Maker David Schwab's Avatar
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    I have a white limba (korina) neck through fretless bass. It's a nice sounding wood. The black variety is nice looking wood. Apparently it's all from the same tree.

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    i'd be tempted to have a outboard box for the distortion controls. having a control per string could be difficult to fit on the guitar, or are you planning to have the distortion all preset without a difference between the individual strings, but keeping the distortion from crossing over.

    i'm interested in this project!

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    Quote Originally Posted by black_labb View Post
    i'd be tempted to have a outboard box for the distortion controls. having a control per string could be difficult to fit on the guitar, or are you planning to have the distortion all preset without a difference between the individual strings, but keeping the distortion from crossing over.

    i'm interested in this project!
    The second: no individual distortion controls. Strings 1 and 2 have a different gain from strings 3-6, but this is built into the electronics. The gain difference was chosen to get the sound I wanted. It is both a gain and frequency response difference.

    (That schematic I posted has one error that I know of; there is a 33K resistor in series with the distortion control pot. Other than that, what I am playing is what is on the schematic as far as I know. I expect that there are a few more errors!)

    The electronics is not so simple; I am going to try to get the first part of the explanation done today.

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    The electronics module in the guitar is shown here: http://www.naic.edu/~sulzer/sixCoilEmod.jpg. The selector switch is not on the main aluminum sheet so that it can be unscrewed and taken out the back while the rest stays in place. This allows resistors mounted on the switch to be changed easily for tweaking the design.

    In general, frequency compensation is needed both before and after distortion generating stages. It seemed to me at the beginning of this this project, that it should only be necessary to compensate afterwards when generating the distortion on each string individually. The reason, it would seem, is that with a single string one can only generate harmonics (not cross modulation products), and it should be possible to set the best ratio of harmonics with compensation only afterwards. My ears have told me something different.

    There might be several explanations, but the one that appeals to me, and seems to agree with what I hear, is that there really is cross modulation, but not what you might expect. All strings are anharmonic; light gauge guitar strings are more so that most. The harmonics generated by the distortion from the fundamental are locked to multiples of that frequency. Since the strings are anharmonic, the higher harmonics in particular are significantly shifted from those frequencies. Thus harmonics generated by distortion from the fundamental and first few harmonics (which are not much shifted from the fundamental) are not at the same frequency as the higher natural harmonics. This produces ugly beat frequencies, or at least that is how I hear it.

    The solution is simple: filter out the high frequencies, before distortion. But do not take out too much, or the sound is not "alive". Since this must be done for each string individually, there is a premium on doing it in a simple way. Therefore this circuit uses the pickup inductance as the frequency selective element, together with a resistor. The op amps are used in the inverting mode so that the resistor that loads the inductor is one that would be required in the circuit in any case.

    The circuit has also be set up so that strings 1 and 2 have more gain than the others at low frequencies, but also the gain starts rolling off at a lower frequency. This is to compensate (at least as I hear it) for a thinner sound in those two strings.

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    ...

    Velvet Hammer made such a pickup called the "Stereo." But it took half the poles in an even/odd selection out to 2 outputs. I've never seen one, Velvet Hammer is back in business but not sure if they are making those particular ones.

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    Quote Originally Posted by Possum View Post
    Velvet Hammer made such a pickup called the "Stereo." But it took half the poles in an even/odd selection out to 2 outputs. I've never seen one, Velvet Hammer is back in business but not sure if they are making those particular ones.
    Then they likely had no hum cancellation.

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    Quote Originally Posted by Possum View Post
    Velvet Hammer made such a pickup called the "Stereo." But it took half the poles in an even/odd selection out to 2 outputs. I've never seen one, Velvet Hammer is back in business but not sure if they are making those particular ones.
    Then they likely had no hum cancellation.

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    Here are some requirements for the distortion circuit:
    1. The level of distortion must be voltage controlled. This means that a large number of distortion circuits can be controlled together, either by a pot. or by a switch. The actual circuit uses both: five levels from clean to very distorted are selected on a switch. A pot allows finer levels of adjustment, and also allows compensation for changes in the battery voltage with use or age. This avoids the need for a regulated supply. One biasing circuit feeds all six distortion circuits.

    2. The circuit must have a gradual onset of distortion rather than abrupt clipping, at least in the intermediate positions of the switch. One could design for symmetrical distortion or not, or perhaps making it selectable. This circuit is symmetrical in order to keep it simple. It is thus roughly analogous to the distortion of a push-pull output stage rather than that of a single-ended triode.

    3. Since we need six distorters in a small space, it must use few parts. The goal was one op amp, two diodes, and a few resistors and capacitors.

    The circuit (http://www.naic.edu/~sulzer/sixCoilPre.png) is a variation of the standard diode clipping circuit. The diodes can be forward biased using voltages selectable by switch and pot. When the forward bias at its maximum, the dynamic resistance of the diodes is low, and so the gain of the op amp circuit is low. The ac signal from the guitar string is low compared to the dc bias current, and so the circuit is very close to linear. As the bias current is reduced, using the switch and or pot., the ac signal increases due to the increasing gain from the increasing diode dynamic resistance. The ac signal increases relative to the decreased dc current, and so the circuit becomes non-linear. How non-linear depends on the bias current. When this current is reduced close to zero, the circuit behaves very much like a standard clipper. However, the biasing resistors have a component in series with the diodes, and so the circuit never clips completely flat.

    The waveforms for the first four switch positions are shown here: http://www.naic.edu/~sulzer/sixCoilWaveforms.png. The input is a 200 mv p-to-p sine wave.

    The eight op amps used in the circuit are two LMV654 chips. These are a new generation of low voltage, low current chips with good characteristics for audio use. They come only in the tiny TSSOP14 case; I had them soldered onto adapter cards (very expensive). They are very linear right up to very near the power supply rails, and so the five volt maximum limit is not a problem for use in a guitar. I use three AA alkaline cells as a power supply; these cells are inexpensive, and they are the cockroach of the battery world, available in quantity wherever there are people. A set should last nearly 1000 hours.

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    A variation on your design

    Mike,

    Your hex pickup is very impressive. I will suggest another way to feed the individual string channels.

    Consider that each string to be the dynamic element such as in a ribbon microphone. You can do a quick experiment by measuring the impedance of each individual string. They vary from a fraction or an ohm to over 1 ohm depending on its diameter for solid strings and the core size for wound strings.
    Just use a miniature ourput transformer across a string with a 4 or 8 ohm side and a 20K to 100K side for a high turns ratio. Alligator clips will work for a quick experiment on one string. Generally, you want the transformer input impedance to be 10 times the individual string impedances for the maximum voltage transfer. By using an input transformer with a cente tap, you can optimize the balance between wound and solid strings a little better. You can even use a 70V speaker line matching transformer to try this out.

    The laws of physics dictate that the amount of voltage induced into the string is higher when the length of the magnetic field under the string is increased. I use a .5" wide X .125" thick X 7" long rubberized/flexible magnets under the strings from the neck to the bridge. This ensures that a full range of harmonics are being induced into the string.

    With this type of pickup, each string is fully isolated from each other. On one of my early prototypes, that I made about 25 years ago, I took an old Applause guitar, removed the fingerboard and placed a copper strip over the truss rod and extended the end into the guitar body in the heel of the neck as a ground return. I used a brass nut and soldered the copper strip to make the connection behind the brass nut.

    Each string had the common ground at the nut end of the string and the six hot connections behind the bridge. The Applause guitar had the strring ball ends secured behind the bridge rather than inside the body. I just put a copper rivet in the bridge for each string to go through and soldered a wire to each rivet. I routed a hot wire from each rivet to the low impedance side of an 8 ohm to 50K ohm transformer. I enclosed all six transformers in coper foil and mounted it on the heel of the neck. By alternating the phase of each alternate string I was able to minimize hum similar to a humbucking pickup. I routed a common ground from each transformer and one hot connnection from each transformer to an eight pin microphone connector (using only 7 pins, one for ground and one hot for each string). I made breakout cable with six 1/4" plugs that I fed into a six channel Radio Shack Mixer. I had individual string level, EQ, and L-R panning control. At that time I mounted a large ceramic magnet on a wood block that I suspended in the sound hole. But, longer magnets, putting more string length in a magnetic field, work better as it produces a higher output and produces a greater amount of higher harmonics. Just put the long magnet closer to the string near the bridge where the string motion is less. I believe you will get an invividual string output very close to your reported 0.2V P-P output.

    So what is the point to all of this? This design produced a wide bandwidth pickup that sounded more acoustic than electric with a greater amount of higher harmonics, as I did not loose all the upper harmonics beyond the resonant point as would happen in high impedance pickups. In your case, I believe you mentioned that each string coil has a 5KHz resonant point.

    Since you are going to the effort to add active EQ and distortion in your design, I thought that you might want to see what happens when you have more upper harmonics on each string to play with. You can even produce many common electric guitar sounds (both single coil and humbuching) by active EQ shaping.

    Since it only takes two alligator clip wires and a junk box transformer, I thought you might want to quickly try this. I believe this method has some potential that might contribute to the next generation of guitar pickups.

    If you try it, let us know what happens?

    Joseph Rogowski

    PS. Here is an additional thought. I even had success by using a 3000 turn, low frequency torroid current transformer by wrapping a different amount of low impedance turns through the torroid ring from each string. This is a simple way to make combined output with a single transformer.

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    Last edited by bbsailor; 06-23-2009 at 05:10 PM.

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    Maybe it was already mentioned, but there are two distinct kinds of hex pickups. One is the kind like what was used in the Ripley guitar, or the Bartolini type, where the intent is to provide for a more interesting stereo image. here, the coils don't have to sense ONLY one string, but simply have to sense that string more than the others.

    The other type of hex pickup is that used for guitar synthesis, where the purpose of the individual coils is to make sure that the sensing circuitry receives no information from "irrelevant" strings, such that it can generate a trigger, gate, and proportional control voltage for only the note being played on that string at that time.

    The second kind imposes more stringent constraints than the first.

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    Joseph,

    That is a great idea, using the string as the conductor, that is, one side of the loop around which one integrates to determine the induced voltage. I like the hum canceling, too. The loop for hum pickup is bounded on the top by the string and then follows the return path for the current. Thus it is quite large, but most of it goes away completely with your hum cancellation because the return path is common to all strings. (That is, B dot da cancels when the six loops are considered with the sign inversions.)

    It would be interesting to try a sequence of small neo magnets along the string. One could probably get the limiting field (that is, limited by "string pull") without too many.

    The 5 KHz resonant frequency for my coils is for using them in series to make a "single coil" pickup with a cable of about 500 pf. The inductance for such a pickup is about 3 H, very much what one would expect. When these coils are used individually right into a preamp (much lower capacitance than a cable), the resonance is much higher than 20 KHz; so one can have flat response in the audio range. However, what I wanted to do was to cut high frequencies by loading the coil.

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    Quote Originally Posted by Mark Hammer View Post
    Maybe it was already mentioned, but there are two distinct kinds of hex pickups. One is the kind like what was used in the Ripley guitar, or the Bartolini type, where the intent is to provide for a more interesting stereo image. here, the coils don't have to sense ONLY one string, but simply have to sense that string more than the others.

    The other type of hex pickup is that used for guitar synthesis, where the purpose of the individual coils is to make sure that the sensing circuitry receives no information from "irrelevant" strings, such that it can generate a trigger, gate, and proportional control voltage for only the note being played on that string at that time.

    The second kind imposes more stringent constraints than the first.

    Maybe it would be useful to think of this pickup as a third type, where the stringency of the constraints is between the other two.

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    I remember reading in the 80's about some guy by the name of Steve Ripley or something like that who came up with a similar concept, individually distorted strings which sounded like a synth, or a real double lead when playing chords.

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    Quote Originally Posted by Zhangliqun View Post
    I remember reading in the 80's about some guy by the name of Steve Ripley or something like that who came up with a similar concept, individually distorted strings which sounded like a synth, or a real double lead when playing chords.
    This certainly is not a new idea; it has been discussed on this forum at least once. I bet it has been done several times. The difference today is that it is easier to build high quality low power complex electronics.

    My goal is not a sound like a synth, but rather to reduce the IM enough to allow a greater variety of chords to be played.

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    The summer and the filter

    Summing the outputs of the six amplifiers is easy. This inverting op amp circuit also has variable gain implemented in the feedback loop to balance the levels in the five switch positions.

    The filter is a Sallen-Key second order low pass section with some additions. The resonance of these coils is way above the audio range and damped out. Therefore, if one wants the traditional electric guitar sound, it has to be added back in at the right frequency. A resonance just below 5 KHz gives the brightest sound. The circuit is designed to have a Q somewhat higher than necessary; then a pot and capacitor are added across the capacitor to ground, just like a guitar tone circuit. A non-mathematical way to think of this circuit is that the capacitor from the amp input to ground in the Sallen-key circuit is analogous to the pickup and cable capacitance in a guitar. The resistors and feedback capacitor replace the pickup inductance. So a guitar-type tone circuit should work in this circuit, and it does. But in this case we make the effective value of the impedance variable by switching in different amounts of feedback. This is useful to reduce the high frequencies when switching to the positions with more distortion.

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    Pickup Maker David Schwab's Avatar
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    The Ripley guitar was stereo. It used a hexaphonic Bartolini pickup with each string going to a built in mixer channel, so you could pan the strings. I don't think it had distortion though. Steve Ripley is a member of the band The Tractors.

    ARP had a guitar synth with a hex fuzz back in the 70's. It was a really cheesy sounding fuzz, but allowed you to play chords without the usual intermodulation distortion. The fuzz made up for the lack of more than two note polyphony from the synth, but allowed you to run the fuzz through the filter.

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    ....

    I don't know if they were hum cancelling or not, they easily could have been, one set N up the other S up and thats all it would take. turns out VH isn't selling those, probably no one bought them, stereo guitars aren't exactly big sellers...

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    Pickup Maker David Schwab's Avatar
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    I think those old Bart Hi-A pickups were six single coils, alternating N-S. The Bart single coils are pretty quiet though.

    Yeah, EVH had one of them, and a bass version, when Kramer was making them.

    It might be interesting in the studio, but that's about it.

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    Quote Originally Posted by Possum View Post
    I don't know if they were hum cancelling or not, they easily could have been, one set N up the other S up and thats all it would take. turns out VH isn't selling those, probably no one bought them, stereo guitars aren't exactly big sellers...
    If the set of three coils for the left channel and that for the right channel had opposite electrical polarity with respect to hum pickup, and if they were played through different amplifiers as one expects for "stereo", the hum could cancel for a listener in just the right place. But in general, no.

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    Very interesting thread. Any updates?

    On a related note, have you considered using the Roland GK series pickups? These are normally wired to a 13pin DIN connector and drive guitar synth type platforms that require Hex signal. However, fundemantally, they are made of 6 miniature humbuckers. You could potentially rework the cable connector and gain access to the signal from the individual humbuckers.

    Take a look at the following site & thread for a close look. Hope this helps.

    Inside a Roland Ready Strat GK-2A

    Regards

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    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by MCK View Post
    Take a look at the following site & thread for a close look. Hope this helps.

    Inside a Roland Ready Strat GK-2A

    Regards
    Nice link! Man those are small coils!

    Yeah, I guess buying one of those is the easiest way to do it. But where's the fun in that?

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    Quote Originally Posted by MCK View Post
    Very interesting thread. Any updates?
    Yes, I have made another pickup with slightly smaller coils and steel between them to improve the isolation, and a few other details. I will be writing this up in the next few weeks.
    Quote Originally Posted by MCK View Post
    On a related note, have you considered using the Roland GK series pickups? These are normally wired to a 13pin DIN connector and drive guitar synth type platforms that require Hex signal. However, fundemantally, they are made of 6 miniature humbuckers. You could potentially rework the cable connector and gain access to the signal from the individual humbuckers.

    Take a look at the following site & thread for a close look. Hope this helps.


    Regards
    Interesting, thanks. No, never thought about using them. Wow, they are small as David said.

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    Quote Originally Posted by Mike Sulzer View Post
    ...I will be writing this up in the next few weeks...
    Writing it up? ...for who, is it a school project or something?
    (BTW; no offense intended)

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  28. #28
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    Greetings,

    Interesting project and discussion. Some neat ideas, thanks for sharing this..

    I have been toying with something similar. My objective is to maintain the beloved SC character/voicing with good noise reduction.

    A picture of my prototype: http://www.johanforrer.net/Guitars/E...talPickup1.jpg

    My prototype follows along the lines of a P-Bass split pickup with each side using opposing magnetic polarity and coils wired out of phase in order to get the noise reduction benefit. To implement the offset poles, the coils are relatively small, only occupy about half the width of a SC. About 500 Ohm of very fine wire per coil. Each coil uses a 1/8" Neodymium rod. The assembly fits into a SC housing.

    I was curious what the prototype sounded like compared to a standard SC and made a little experimental recording. For this experiment I fabricated a jig to hold the pickup in place above the strings of my Strat --- the distance from the poles to the strings as like for the regular neck pickup of the guitar.

    In the sample clip below, you'll first hear the standard SC, followed by the experimental prototype. Each sound segment consists of a quiet period to sample the noise followed by a strum on the open strings to sample the harmonic content. Hopefully, that will give you some idea of the noise cancellation and PU voice properties.

    Here's the sound sample: http://www.johanforrer.net/Guitars/E...ntalPickup.mp3

    Still work (a lot) in progress.

    Regards.

    JB.

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    It is very interesting that you used three individual coils for each half rather than winding around all three polepieces. I think the result is somewhat lower resistance. The indutance is probably lower as well since the total area enclosed by the windings is smaller than if you wound around all three. (Neo has a permeability near that of a vacuum, and so the pole peices contribute little more than empty space. Therefore it is the total area that counts.)

    Also the fact that the the coils are small means that there are not so many turns, and this also lowers the inductance. I would expect that the resonant frequency with a standard guitar cable would be quite high.

    This agrees with what I hear from your clip. The standard SC has a bright sound from the resonance, which typically might be at about 5KHz with a cable. Your pickup sounds very neutral; this probably means that the resonance is so high that it does not affect the sound.

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    Hello Mike,

    Sounds like a feasible explanation of the physics. You are right, I measured the unloaded, resonance frequency and it is somewhere in the order of 44KHz. The Strat pickup was just over 5KHz. Only time and experimentation with EQ will tell whether the sound of this pickup will work for me.

    Thanks for your feedback.

    JB.

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    Quote Originally Posted by jbforrer View Post
    Hello Mike,

    Sounds like a feasible explanation of the physics. You are right, I measured the unloaded, resonance frequency and it is somewhere in the order of 44KHz. The Strat pickup was just over 5KHz. Only time and experimentation with EQ will tell whether the sound of this pickup will work for me.

    Thanks for your feedback.

    JB.
    Start by putting a few thousand pf across the pickup to bring down the resonant frequency. When you get it to about 5 KHz, you can adjust the resistive load across it to get the Q that sounds best.

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    The idea of this six-coil pickup with six individual distortion units has been going through my head for decades. I was simply called stupid by people I told it to. Nice to see someone actually began to build it! Main question now: has the project been finished, does it work and how does it sound? Is there a complete plan somewhere (or a commercial product that I can buy)?

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    Quote Originally Posted by harvey_tbee View Post
    The idea of this six-coil pickup with six individual distortion units has been going through my head for decades. I was simply called stupid by people I told it to. Nice to see someone actually began to build it! Main question now: has the project been finished, does it work and how does it sound? Is there a complete plan somewhere (or a commercial product that I can buy)?
    The project is continuing; it is now in a two-pickup guitar with a single knob for controlling the whole range of distortion and FET switches for changing pickups, both of which I will describe on this forum eventually. The project is not complete until I find a way to control the nature of the distortion as well as its intensity. This would be something like a symmetry control; it is not easy to control this on six devices together over a wide range of distortion intensity.

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    Mike, this would be interesting using parallel processing on each string. Split the signal and EQ one "track" to taste for best distortion, then use the other split clean and mix it back in after the distortion for more clarity.

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    Mike!

    Nice work! This is very ambitious! Have you heard the distortion of your preamp/distortion circuit? The waveforms are interesting!

    I really like how you are putting a neo mag on the top of a high perm slug. Is there a reason to put it on top, not on the bottom?

    I used to be obsessed with the hex coil... Or Rather the Sex coil as I was winding for a seven string. I wrote about it on Ampage in the nineties under the user name "Eth". But I didn't own any test equipment at the time and my main stumbling block was using Alnico cores. The permeability was so low, they never sounded the way I wanted. I only realized that later.

    Your core looks good, and I would like to try it! I connected all my coils in series and used diff amps between each node. At the time I was strictly against IC's (religious reasons). So the diff amps were discrete. Adjusting the bias current would control gain and thus distortion(VC idea). The benefit of having the coils all in series, you can plug a mono cable into it and jam passively in mono, for instance if your batteries die.

    At the time I was stoked on the sex coil, Ken Gilbert pointed out that IM distortion is very important to tone. I found that he was right. The sound of two strings modulating each other is amazing, though I agree, the next level of complex chords, in saturation, is yet to have been heard. I encourage you to go there!

    Keep up the good work, and thanks for sharing!
    Eth

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