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Thread: Circuits involve active electronics and a dummy coil.

  1. #36
    Supporting Member Steve A.'s Avatar
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    Quote Originally Posted by NateS View Post
    I'm considering this for a two pickup RW/RP guitar, which necessitates invertering the coil for one of the pickups...
    Nate:
    IMO there is a simpler way to accomplish this without having to use electronic circuitry to invert the dummy coil- check out the wiring diagram for the Gibson Blueshawk:

    bhcircuit.jpg

    bh_wiring_update.jpg

    The first picture is the official Gibson wiring diagram while the second one was what the curator of the BluesHawk website drew up himself with some corrections made after he was finally able to obtain the official schematic. (Gibson ought to post his drawing since it is much easier to understand!) He goes into a lot more detail in his very excellent single-purpose website, which is undoubtedly everything you wanted to know about one of Gibson's most interesting guitars!

    www.blueshawk.info/resources

    I believe that the dummy coil used in the Blueshawk is basically a P-90 coil without the magnets or any kind of metal load. The BH selector switch has just 3 positions, all of which are humcancelling, so you don't get to A/B the effect of the dummy coil (which from my experiments adds some compression and loses some of the brightness.) It seems like most people think that it is the RW/RP properties that create the humcancelling effect but I figured out on my own strictly by accident that it was the [B]reverse winding[/B] that was responsible for the humcancelling effect and that the reverse polarity that keeps the pair in phase. I guess that is common knowledge now among guitar techs but I was shocked when I discovered that since the Brosnac book (1st edition) was my bible and he did not consider the possibility of a humcancelling pair that was out-of-phase (see the chart on Page 35 1st ed. which has a 4 x 4 grid showing all 16 combinations of strat pickups).

    In any case my interpretation of the drawing is that the two pickups are NW/RP with the windings of the neck pickup inverted in the middle switch position. The neck pickup is South up and the bridge pickup is North up, but all three positions of the selector switch are humcancelling (the dummy coil is bypassed in the middle position which is a series linkage.) It would be easy to add a switch (or a pole of a 4P/5T or 4P/6T switch) to bypass the dummy coil in the neck only or bridge only positions when humcancelling is not needed. (I think that Gibson left that option out to keep people from A/B-ing the effect of the dummy coil and discovering that it sounded better without it!) One more thing to note in the drawing- for the middle position of the switch the signal to the controls is shunted through a treble bleed RC network consisting of a 100k resistor and a 0.0047uF cap to brighten up the sound of the series linkage. You should be able to add a switch (or switch poles) to be able to get a parallel linkage in the middle position.

    Good luck!

    Steve Ahola


    EDIT I had drawn up a wiring diagram inspired by the BluesHawk which had a RW/RP pair of P-90's wiring in series with a dummy coil in the middle. The middle position would have the two P-90's in series- I forget if I was able to get them in parallel as well. I'll have to see what I can come up with again. (My LP Jr Special is just begging for a dummy coil between the two P-90's under the pick guard.)
    Last edited by Steve A.; 02-07-2012 at 05:26 AM.

  2. #37
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    That schematic makes the active circuit look simple. I'm glad to finally see that schematic though.

    There are two choices: 1) make the dummy coil an insignificant load 2) make the the dummy coil an insignificant load. Active electronics is actually the least technical way to do this. IMO

  3. #38
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    The Cap is actually a good idea. The bias network in series with the winding resistance creates a LR high pass... A few dB... But none the less, I like that you add it

    The 2.7M are also wise! Good thinking. The 2.7M's are in parallel with your volume. Thermal noise will be shunt by the volume control. I'll check that link next time I jump on a computer. Thanks for sharing. I like your "hair brained" ideas

  4. #39
    Senior Member uneumann's Avatar
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    I'm new here, but recently did some work with a dummy coil on a Tele ... seems this is not a new idea to you all - but it was to me.
    I have the whole story posted here with pictures, if you're interested.

    https://sites.google.com/site/string...for-sc-pickups

  5. #40
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    Great article! I admire people who can explain things as well as this. I don't find your name anywhere on your site

    one typo:
    "....One is electromagnetic and this noise source is addressed by shielding. Again, there are lots of websites and articles about shielding. Ideally, a metal shield surrounds the guitar electronics, including the PUs, and is tied to the guitar cable ground...."

    A conductive shield, such as foil, "shields" from the electric field, not the electromagnetic field.

    By reading your article, I am quite certain that you know this and it is only typographic

    Excellent job, I think you should sign your name to it!
    Cheers,
    Ethan

  6. #41
    Senior Member uneumann's Avatar
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    Quote Originally Posted by CRU JONES View Post
    Great article! I admire people who can explain things as well as this. I don't find your name anywhere on your site;)

    one typo:
    "....One is electromagnetic and this noise source is addressed by shielding. Again, there are lots of websites and articles about shielding. Ideally, a metal shield surrounds the guitar electronics, including the PUs, and is tied to the guitar cable ground...."

    A conductive shield, such as foil, "shields" from the electric field, not the electromagnetic field.

    By reading your article, I am quite certain that you know this and it is only typographic:)

    Excellent job, I think you should sign your name to it!
    Cheers,
    Ethan
    Thanks Ethan - good catch - EM is not my strength so I used the terminology loosely. I've made the fix - and actually caught another in the process. Thanks again.

    My site has my gmail address (uneumann@gmail.com) if anyone wants to reach me. As for signing, it seems like email is a sig these days. ;-)

    -Ulrich Neumann

  7. #42
    Senior Member uneumann's Avatar
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    I've now made a hum cancelling coil for a strat-like guitar (my G&L S-500). It works great and it's pretty easy to do.
    Details are at https://sites.google.com/site/string...for-sc-pickups

  8. #43
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    Att. Mike Sulzer.
    Old thread but looking for answers :-)

    I know my way around tubes but not much about transistors/fets etc.
    Anyway I breadboard your dummy coil buffer circuit, testing several J201 i'm getting at the output (top of 27k resister) between 0.55v to 0.85v (Vdd is 8.6v).
    Is that ok?
    This means current drain is like 30nA...
    I'm not getting any voltage reading on the gate... i guess its in nano-volts... shouldn't the gate be biased to some level like mid Vdd?

    * post edit: guess i haven't built anything in a while... gate is at 0V while source is at 0.55-0.85V so its fine for mV noise signal :-)
    I was just thrown off by googling jfet followers schematics and seeing them all being biased by a divider to mid Vdd.

    Btw, if i'll use a gain stage (common source, which inverts the signal) before the buffer, with the dummy at same wind direction as the pickups, does it mean I'll also get some level of electrical noise cancellation?
    Last edited by Amit; 10-15-2015 at 12:34 PM.

  9. #44
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    I prefer the source follower in this application because of the low impedance looking back into it. (If you use the circuit for two pickups this provides isolation.) But the circuit with gain should work as you say. Also be careful with noise (hiss). If you use fewer turns on the dummy and amplify up, you might notice hiss.

    I like the low current because battery life is long, and it works, but there is nothing wrong with changing the circuit for higher current with traditional biasing if you like.

    Quote Originally Posted by Amit View Post
    Att. Mike Sulzer.
    Old thread but looking for answers :-)

    I know my way around tubes but not much about transistors/fets etc.
    Anyway I breadboard your dummy coil buffer circuit, testing several J201 i'm getting at the output (top of 27k resister) between 0.55v to 0.85v (Vdd is 8.6v).
    Is that ok?
    This means current drain is like 30nA...
    I'm not getting any voltage reading on the gate... i guess its in nano-volts... shouldn't the gate be biased to some level like mid Vdd?

    * post edit: guess i haven't built anything in a while... gate is at 0V while source is at 0.55-0.85V so its fine for mV noise signal :-)
    I was just thrown off by googling jfet followers schematics and seeing them all being biased by a divider to mid Vdd.

    Btw, if i'll use a gain stage (common source, which inverts the signal) before the buffer, with the dummy at same wind direction as the pickups, does it mean I'll also get some level of electrical noise cancellation?

  10. #45
    Supporting Member Steve A.'s Avatar
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    Quote Originally Posted by uneumann View Post
    I've now made a hum cancelling coil for a strat-like guitar (my G&L S-500). It works great and it's pretty easy to do.
    Details are at https://sites.google.com/site/string...for-sc-pickups
    I just checked out uneumann's link and found it to be interesting although it deals strictly with passive dummy coils...

    I've attached a PDF capture of the page for future reference...

    saf.dc.pdf

    Steve A.

  11. #46
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    Yup just a follower is obviously the best way if you use matching pickup and dummy.
    In my case I'm trying to tame a Tele, so bridge pickup and neck pickup are way different.

    As a middle ground between the pickups for the dummy I'll use a Strat single, hence why I look to add a bit of boost, to match the bridge noise level. neck obviously will need just passive pot adjustment.
    I thought about using local feedback inverting amp with a single jfet like in tubes:
    Designing Single-Stage Inverting Feedback Amplifiers
    But I can't find schematics of such an amp using jfets on the web... jfets are not able to do that?


    Thanks for the replay after all those years

  12. #47
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    Sure, you can do that with FETs. But remember what we are trying to do. The idea is to put the voltage from the dummy in series with the pickup coil without the dummy's inductance adding to that of the pickup coil. Therefore we measure the dummy's voltage: a very high impedance is used for this so that you do not load the dummy and thus alter the relative levels of the different hum harmonics. The follower lowers the impedance while "following" the voltage so you can insert it in series with the "ground" lead of the pickup with minimal effect on the sound of the pickup from the adjustment pot..

    Quote Originally Posted by Amit View Post
    Yup just a follower is obviously the best way if you use matching pickup and dummy.
    In my case I'm trying to tame a Tele, so bridge pickup and neck pickup are way different.

    As a middle ground between the pickups for the dummy I'll use a Strat single, hence why I look to add a bit of boost, to match the bridge noise level. neck obviously will need just passive pot adjustment.
    I thought about using local feedback inverting amp with a single jfet like in tubes:
    Designing Single-Stage Inverting Feedback Amplifiers
    But I can't find schematics of such an amp using jfets on the web... jfets are not able to do that?


    Thanks for the replay after all those years

  13. #48
    Junior Member AlDiMeowla's Avatar
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    Hello and sorry for necrobumping the thread. I am very intrigued by this FET-dummy coil buffering, that Mike was talking about. But it seems the schematic has disappeared. Would anyone of you happen to have a copy of it to post again? Thanks

  14. #49
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    buffereddummy.png
    Quote Originally Posted by AlDiMeowla View Post
    Hello and sorry for necrobumping the thread. I am very intrigued by this FET-dummy coil buffering, that Mike was talking about. But it seems the schematic has disappeared. Would anyone of you happen to have a copy of it to post again? Thanks


    I am having trouble getting it to attach.

  15. #50
    Junior Member AlDiMeowla's Avatar
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    Maybe PM me, or upload and link to an external imagehosting site?

  16. #51
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    Quote Originally Posted by AlDiMeowla View Post
    Maybe PM me, or upload and link to an external imagehosting site?
    Ah! I am also interested in this. Funny how I find myself here 3 days after your request.
    If you would be so kind, Mike Sulzer, I would really appreciate that!
    eschertron likes this.

  17. #52
    Supporting Member Steve A.'s Avatar
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    Quote Originally Posted by AlDiMeowla View Post
    Quote Originally Posted by Mike Sulzer View Post
    Quote Originally Posted by AlDiMeowla View Post
    Hello and sorry for necrobumping the thread. I am very intrigued by this FET-dummy coil buffering, that Mike was talking about. But it seems the schematic has disappeared. Would anyone of you happen to have a copy of it to post again? Thanks
    I am having trouble getting it to attach.
    Maybe PM me, or upload and link to an external imagehosting site?
    Al: Perhaps if you include the Post # [this post is #52] the original schematic can be retrieved. Or not.

    Mike: Can you try attaching the schematic again? I just checked and was able to attach an image file here. Or PM/email it to me and I can try to reattach it to the original post (I'd need to know Post # to do that.)

    Al: If you have run across a good imagehosting site still around let us know. Photobucket used to be great but the last time I checked I could not link directly to 100's of images I had uploaded there (links went to an HTML page that included the image- along with tons of ads! )

    %=%=%=%=%=%=%

    Most people seem to be using cloud host sites like DropBox these days. DropBox gives you 2GB space for free*** but the code generated to share files includes "dl=0" as the last entry which disables downloading.

    Change the 0 to 1 (or delete the entry) for a direct link to the file.

    There are other cloud host sites but DropBox has been around forever and is well-integrated on most platforms, programs and mobile apps. I use my stable of Android tablets for 99% of my computing these days and DropBox is indispensable for sharing files between my various mobile devices and my PC (you'll want to install DropBox on a host computer which mirrors the contents in the cloud.)

    Steve A. "Super Moderator" (I mention that only because I have moderator powers for all of the forums here and do not want to step on the toes of the actual moderator of each forum.)

    *** There is free bonus space available. My Samsung Tab 4 7.0 tablet came with a one-time bonus of an additional 48GB space there but that expired after 2 years. Or you can pay $9.99/mo for 1TB space (ouch!) Here's a link explaining how to earn more space for free:

    https://www.dropbox.com/help/space/get-more-space

    Getting family, friends or co-workers to join DropBox gives you a 500MB bonus apiece (up to 16GB.)
    Last edited by Steve A.; 05-31-2017 at 06:19 PM.

  18. #53
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    I made a new pdf file. That seems to work.

    bufferedDummy.pdf

  19. #54
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    Quote Originally Posted by Mike Sulzer View Post
    I made a new pdf file. That seems to work.

    bufferedDummy.pdf
    Thank you very much, Mike.

  20. #55
    Junior Member AlDiMeowla's Avatar
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    Quote Originally Posted by Mike Sulzer View Post
    I made a new pdf file. That seems to work.

    bufferedDummy.pdf
    Many thanks Mike!
    That circuit is brilliant in its simplicity. I like the individual pickup trimpots. Looks like it could be powered with a very small battery. Any recommendations on a specific J-FET?

  21. #56
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    Quote Originally Posted by AlDiMeowla View Post
    Many thanks Mike!
    That circuit is brilliant in its simplicity. I like the individual pickup trimpots. Looks like it could be powered with a very small battery. Any recommendations on a specific J-FET?


    Just did this with a K117. I have a guitar with 2xP90s but only use the bridge, so I pulled the magnets off the neck. I had to modify the circuit to provide some gain, because the bridge is overwound wrt the neck. I used a 6.8k on the drain and 2.2k on the source - maybe a bit high res but wanted to keep current draw low as just using a CR2032. Flipped + and - pickup wires and took output from the source thru 10uF. It works brilliantly. No discernable effect on tone, and just pop the battery out to see what it would be like with all the noise!

    Thanks again Mike. I wish I had done this years ago!

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    Quote Originally Posted by Mike Sulzer View Post
    I made a new pdf file. That seems to work.

    bufferedDummy.pdf
    Hi - I'm trying this out, no luck so far - but I took the PDF and created a more detailed view of how this works for an actual guitar circuit.
    dummy-coil.png

    Mike, if you are still reading this thread, could you confirm that this is correct?

    I included a 'defeat' switch to eliminate the circuit - may not be necessary.

  23. #58
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    Quote Originally Posted by cheezit View Post
    Hi - I'm trying this out, no luck so far - but I took the PDF and created a more detailed view of how this works for an actual guitar circuit.
    Click image for larger version. 

Name:	dummy coil.png 
Views:	42 
Size:	60.5 KB 
ID:	47650

    Mike, if you are still reading this thread, could you confirm that this is correct?

    I included a 'defeat' switch to eliminate the circuit - may not be necessary.
    Hi cheezit,
    That is how I interpreted it.
    For my application, the circuit wasn't working with unity gain. I was using a dummy in the neck position to cancel out hum from an overwound bridge. So I modified it slightly as per my post.

    I have tried it on 3 guitars now:

    1) Neck P-90 cancelling out hum from bridge P-90: Worked great. So much noise reduction.
    2) Strat pickup under metal pickguard cancelling out hum from Jazzmaster pickup: Works so-so. It is a marked improvement though.
    3) Strat pickup under pickguard cancelling out hum from other strat pickups: Also works fairly well, removes enough hum for me, but not as quiet as positions 2 and 4 with RWRP.

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    Quote Originally Posted by uneumann View Post
    Thanks Ethan - good catch - EM is not my strength so I used the terminology loosely. I've made the fix - and actually caught another in the process. Thanks again.

    My site has my gmail address (uneumann@gmail.com) if anyone wants to reach me. As for signing, it seems like email is a sig these days. ;-)

    -Ulrich Neumann

    (To be precise, the term electromagnetic fields covers both : electric and magnetic fields. Conductive shields reduce electric fields/interference but can as well damp magnetic fields via eddy currents at high frequency, depending on conductivity of the shield. They are ineffective for low frequency magnetic fields though.)

    Seems you have reinvented the patented Ilitech system:
    HUM CANCELING SYSTEMS ? ILITCH ELECTRONICS

    This is probably the best passive humcancelling system known.

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    Quote Originally Posted by harry View Post
    Hi cheezit,
    That is how I interpreted it.
    For my application, the circuit wasn't working with unity gain. I was using a dummy in the neck position to cancel out hum from an overwound bridge. So I modified it slightly as per my post.

    I have tried it on 3 guitars now:

    1) Neck P-90 cancelling out hum from bridge P-90: Worked great. So much noise reduction.
    2) Strat pickup under metal pickguard cancelling out hum from Jazzmaster pickup: Works so-so. It is a marked improvement though.
    3) Strat pickup under pickguard cancelling out hum from other strat pickups: Also works fairly well, removes enough hum for me, but not as quiet as positions 2 and 4 with RWRP.
    I'm doing this with P90s - I had an Epi LP with dual P90s, sold it to a friend because I couldn't take the hum, and he sold it back to me ... eight years later. So here I am with hum again.

    Did you happen to note any voltages from your versions? I'm getting back to mine and trying to trace where things have gone wrong.

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    Quote Originally Posted by harry View Post
    Hi cheezit,
    That is how I interpreted it.
    For my application, the circuit wasn't working with unity gain. I was using a dummy in the neck position to cancel out hum from an overwound bridge. So I modified it slightly as per my post.

    I have tried it on 3 guitars now:

    1) Neck P-90 cancelling out hum from bridge P-90: Worked great. So much noise reduction.
    2) Strat pickup under metal pickguard cancelling out hum from Jazzmaster pickup: Works so-so. It is a marked improvement though.
    3) Strat pickup under pickguard cancelling out hum from other strat pickups: Also works fairly well, removes enough hum for me, but not as quiet as positions 2 and 4 with RWRP.

    It sometimes gets overlooked that magnetic (interference) fields vary with position and especially orientation. The dummy coil needs to have its magnetic axis oriented parallel to the PUs' and should be placed as close to the PUs as possible.

    When you remove the ferromagnetic core parts from a PU coil, it will produce less hum than the original PU. Thus it is advisable to use an overwound dummy coil or a coil with larger area than the PUs (e.g. P-90 dummy coil in Strat) in conjunction with the unity gain buffer circuit.

    Your modified Jfet source amplifier circuit is critical with such low battery voltage and will work properly only with specially selected Fets having a very low pinch-off voltage. Also, to use it as an amplifier means to take the output from the drain. But the internal impedance at the drain is considerably higher than at the source. Thus you sacrifice one of the benefits of the simple source follower, namely inserting only little series resistance with the PUs.

    Placing the duimmy coil under a metal pickguard not only reduces the HC signal but also changes its frequency content, as the conductive metal preferably absorbs the higher frequency components of the magnetic interference fields. But for perfect HC you want the dummy coil's signal to replicate the PUs' interference content as close as possible.
    Last edited by Helmholtz; 04-01-2018 at 02:59 PM.

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    Quote Originally Posted by Helmholtz View Post
    Your modified Jfet source amplifier circuit is critical with such low battery voltage and will work properly only with specially selected Fets having a very low pinch-off voltage. Also, to use it as an amplifier means to take the output from the drain. But the internal impedance at the drain is considerably higher than at the source. Thus you sacrifice one of the benefits of the simple source follower, namely inserting only little series resistance with the PUs.
    BTW, in my searching around I consider this approach to be very similar in spirit to https://patents.google.com/patent/US5569872, which is the Dudley Gimpel patent for Ernie Ball. Take a look at Figure 5, which shows the same approach: a buffered noise-sensing coil inserted on the ground side of the pickup.

    The circuits are much more complex, though, and I wondered why; your points about the sensitivity of the circuit to the characteristics of the JFET and the amplification problem may be it.

    If I'm right, it's the same reason DIY fuzz pedals can be made out of 8 components but Boss equivalents have dozens and dozens; they have to design for consistency and low cost, and can't mess around with parts that are operating at (or beyond) the edge of their design parameters.

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    Update here...I had an interesting experience with this yesterday. I have a dummy coil i'd harvested from an old P90, and I had it wired in to a small PCB board to try this circuit. Wasn't working, so I detached it and put a connector inline that allowed me to insert the dummy coil, unbuffered, on the ground side of the actual pickup, tucked away behind the volume pots.

    So now I have a P90 with a P90 dummy coil in series, no buffering. Didn't know what to expect....I was assuming it would be mud. But the guitar itself is a little bright, and the existing P90s (GFS Vintage) are not high-output, at least not for P90s. The result was:

    1) Some reduction of hum, but highly directional. If I stand in the right spot it's almost silent, if i turn 90 degrees it's almost as bad as if the dummy coil weren't there. This is likely due to it being down in the control cavity, and parallel to but not on the same plane as the pickup. Might be able to move it a bit.

    2) Significant darkening of tone, but highs are still there. Brings the bridge EQ closer in similarity to the neck, which doesn't have this dummy coil.

    3) Here's the surprise - big increase in low-end punch through high gain. This thing is now way "chunkier" and crunchier than before and just feels really great to play. It's not more output overall; it's that the lows are louder and more dynamic in a way that differs from what an EQ pedal might do.

    The low-end behavior is (I'm guessing) related to the increased inductance of the additional coil, which seems to have lowered the resonant frequency of the pickup in a pleasing way.

  29. #64
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    Quote Originally Posted by cheezit View Post
    Update here...I had an interesting experience with this yesterday. I have a dummy coil i'd harvested from an old P90, and I had it wired in to a small PCB board to try this circuit. Wasn't working, so I detached it and put a connector inline that allowed me to insert the dummy coil, unbuffered, on the ground side of the actual pickup, tucked away behind the volume pots.

    So now I have a P90 with a P90 dummy coil in series, no buffering. Didn't know what to expect....I was assuming it would be mud. But the guitar itself is a little bright, and the existing P90s (GFS Vintage) are not high-output, at least not for P90s. The result was:

    1) Some reduction of hum, but highly directional. If I stand in the right spot it's almost silent, if i turn 90 degrees it's almost as bad as if the dummy coil weren't there. This is likely due to it being down in the control cavity, and parallel to but not on the same plane as the pickup. Might be able to move it a bit.

    2) Significant darkening of tone, but highs are still there. Brings the bridge EQ closer in similarity to the neck, which doesn't have this dummy coil.

    3) Here's the surprise - big increase in low-end punch through high gain. This thing is now way "chunkier" and crunchier than before and just feels really great to play. It's not more output overall; it's that the lows are louder and more dynamic in a way that differs from what an EQ pedal might do.

    The low-end behavior is (I'm guessing) related to the increased inductance of the additional coil, which seems to have lowered the resonant frequency of the pickup in a pleasing way.
    No real surprise, here.

    You may achieve the same lowering of the PU's resonant frequency by wiring a capacitor of around 470pF (exact value depends on your PUs and guitar cable) in parallel with the PU - no noise reduction though.

    The buffer avoids lowering of the resonant frequency.

    Perfect noise cancelling via dummy coil would ideally require the dummy coil to be identical to the PU coil and being in the same place - or surround the PU as with low impedance frame shaped dummy coils. Results of real systems may not be completely satisfying if the noise source(s) are located in close proximity, as in these cases the magnetic interference fields vary strongly with position and orientation.

    There is a problem with placing the dummy coil in the control cavity: The casings of the pots are typically made of ferromagnetic steel and thus change the noise fields close-by. This may prevent the dummy coil's signal to be identical to the PU's noise and consequently cancellation will suffer.

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    Update: Mike Sulzer's Jfet buffer does not really have low output impedance. While typical textbook examples of the source follower give values of some hundred Ohms, a closer look at this circuit reveals that its output impedance increases noticeably with lower drain currents. In our case with Rs=27K, the output impedance will be around 3K (!) or even higher. (In can be measured directly with an LCR meter).

    lnserting a series resistance of several k Ohms will damp the PU's resonance and might be audible.

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