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Thread: EMG active pickups preamp circiut schematic.

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    EMG active pickups preamp circiut schematic.

    I found these links on another forum. It claims to the secret EMG pickup preamp circuit.




    What do you have say about this.
    I have heard that opamp in EMG pickups is something like EMG01. But circuit say Lm4250. Do you think its fake.
    And what these loops on output from ground means.

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    Senior Member Stan H's Avatar
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    Quote Originally Posted by ubaid88 View Post
    And what these loops on output from ground means.
    That means it's a shielded cable.
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    It matches the concept that David Schwab has posted in the past, but with the component values shown the gain will not be equal for the two coils, which is suspicious.

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    Quote Originally Posted by Scott S. View Post
    It matches the concept that David Schwab has posted in the past, but with the component values shown the gain will not be equal for the two coils, which is suspicious.
    Yes something like this I have also read in David Schwab post that output will not be equal from the coils.

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    The circuit diagram does not match the circuit of the pictured circuit board.

    Are the two pickup coils identical?

    If yes, the circuit diagram isn't quite right, but is probably close. I would expect a balanced differential amplifier merged with a DC voltage divider to set the DC operating point (~ground) at about one half the battery voltage, allowing operation off a single 9 volt battery.

    If no, the circuit still doesn't quite make sense because the gain of the plus channel will depend too much on the AC resistance of that coil. One would expect a series resistor like the 30 Kohm resistor in the minus path.

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    Old Timer RedHouse's Avatar
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    I wonder if the schem is correct about it being a LM4250

    Checking the data sheet it seems that low power is the only thing a LM4250 has going on, it's a general purpose OpAmp.
    (not a low noise job)

    It's a shame whoever took that pic of the PCB didn't flip it over and give us a look at the other side, might be more SMT over there.

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    Pickup Maker David Schwab's Avatar
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    I know where this schematic came from, and it's the same source that sent me pictures of the inside of an EMG 81 (see below). I got the photos over two years ago, and schematic this past July.

    Yes, this is supposed to be accurate. I haven't traced it from the photos I have though, but the same people did both (see below). It took them a while to trace the PCB from inside the pickup. The only thing they didn't know was the exact op amp, since it had no number on it, besides a private EMG part number.

    The other circuit from the booster switch board is different. That photo is not where this schematic came from. Here's the board attached.

    I'd guess the unbalanced coils are part of the tone modeling thing they do. One of their older pickups, the EMG-58 was too noisy (hummed too much), so they redesigned it as the EMG-85 which is quieter. They also have very fine brass screening acting as a Faraday cage for shielding.

    The source that did the schematic said the coils were matched, but I think that's hard to say since they had to destroy the pickups to find out.

    But the specs were listed as follows:

    EMG81

    Magnet: Ceramic (cuting) 56x3x13mm
    Wire: 0,06mm (PE)
    Core: 54x3x12,5mm (silicon steel?), solid steel.
    Coil: 4,18KOhm (one coil), wax potted, aprox. 5500-6000 turns, h=7,5mm
    Bobbin: 64x13x9mm (or with "tube legs" 12,2mm)


    Coils conection:

    -opamp--------^^^^^^^^-----ground----^^^^^^^^^^---------opamp+


    IC unknown, marked as EMG001

    Electric shematic will be able in a few days.
    Attached Thumbnails Attached Thumbnails emg_81_014.jpg   emg_81_013.jpg   emg_81_010.jpg  
    Last edited by David Schwab; 09-08-2008 at 06:53 AM.
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    Senior Member salvarsan's Avatar
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    National Semiconductor's datasheet examples suggest
    that EMG started with component values from NS'
    gain-of-5 differential amp schematic, and set the current
    bias to ~1 microamp to optimize the noise power figure.

    A 10k output buffer resistor is also prominent throughtout
    the datasheet specifications.

    That lack of a 30k resistor on the non-inverting pin
    looks like an error instead of a design decision.

    LM4250 datasheet:
    http://www.national.com/ds/LM/LM4250.pdf

    -drh

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    This circuit is certainly not balanced: inv, gain = 5, non-inv gain = 6. So, yes, maybe a resistor in series with the non-inv input has been left out. It would have to have a value of 13.7K to lower 6 to 5. (The lower leg of the voltage divider is the two 137K resistors in parallel.)

    The inv. input input impedance is 30K; this is kind of low for a high impedance coil. And the input impedance the other coil sees is 68.5K. This does not make sense.

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    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Mike Sulzer View Post
    This circuit is certainly not balanced: inv, gain = 5, non-inv gain = 6. So, yes, maybe a resistor in series with the non-inv input has been left out. It would have to have a value of 13.7K to lower 6 to 5. (The lower leg of the voltage divider is the two 137K resistors in parallel.)
    I don't know if there is a part left out, but it's common for people to wind unbalanced pickups... maybe they wind balanced coils and unbalance them electronically?

    The inv. input input impedance is 30K; this is kind of low for a high impedance coil. And the input impedance the other coil sees is 68.5K. This does not make sense.
    From the EMG site (italics added)

    http://www.emginc.com/company.asp

    Tone Modeling

    One of the most important aspects governing the tone of a pickup is the resonant frequency. EMG Pickups use "Impedance Modeling" to manipulate the two coils. This innovation allows us to shape a mix of the reactive slope and resonance from each of the two coils. The idea is to achieve a complex mixture of each coils phase and frequency response resulting in a richer tone from the pickup. This means the sound is vibrantly alive with more harmonics than from conventional passive pickups. EMG Pickups like the EMG-S, EMG-SA, and the EMG-60 use this technique to its fullest, while the EMG-81 uses modeling in only a small way. Modeling might work well for a single coil pickup, but not for a design such as the EMG-ACS Acoustic Sound Hole Pickup. As each pickup design is approached differently, it all depends on the final result we're searching for.
    Also remember that the two coils are not wired in series, so loading it down like that will make it darker sounding.
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    A 6/5 gain ratio at low frequencies will not do a good job of canceling the hum from magnetic fields. EMGs do cancel hum well, and so I think something is missing. Perhaps the coils are different as someone mentioned.

    It is true that actively adding the outputs of two different resonant circuits can produce responses that cannot be achieved with two identical resonant circuits. If the two coils are the same, they would have the same resonant frequency, but different Qs. If the coils are different, both parameters could be different.

    The differences that can be achieved with such an active circuit are greater than can be achieved by putting the two different coils in series. If the cable capacitance dominates over the inter-winding capacitance, as it can, then the two inductors pretty much just add in series.

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    Pickup Maker David Schwab's Avatar
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    I think every EMG made has the coils in parallel. That's one thing I don't like about the EMG-P.

    I have an old Bartolini Hi-A pickup with one coil being intentionally 1K different from the other, and it's dead quiet. I had it installed in one of my '74 Ric basses, and along with total copper foil shielding of the bass, I was able to remove the string ground with no noise whatsoever.

    I make some bass pickups with similarly large offsets with no hum problems.

    EMG's have the entire pickup in a Faraday cage made from very fine brass screen. As I mentioned, some EMG models that had very mismatched coil circuits and did indeed hum. The EMG-58 was one of those. We used to use them in the American Showster AS-57 tailfin guitars, and they were the best sounding EMG humbucker, but they were a tad noisy. They replaced them with the 85, which is quieter, but lacks the harmonics of its predecessor.

    Either way, take a look at the photo I posted of the circuit board inside the 81. That's where that schematic came from. The IC and one surface mount part is missing, but you could probably trace the circuit assuming they didn't use a double sided board. I've been meaning to do it but haven't had the time.

    Here's an EMG SA, which is also very mismatched:
    Attached Thumbnails Attached Thumbnails emg_sa3.jpg  
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    Sure, you can use different numbers of turns on the two coils. Cancellation of magnetic hum using two coils requires that the two coils have the same overall sensitivity. Since the sensitivity depends on both the number of turns and the magnetic core, you can get good cancellation with different number of turns if the cores are different in a complementary way. Simply making the number of turns different with two identical cores decreases the degree of cancellation. This is how it is in theory and how it works in practice. It might still appear to be good enough most of the time, but in an environment with really high stray magnetic fields, you need really good cancellation.

    Shielding with a conducting material is for electric fields. Both kinds of hum are a problem, and each has a different solution.

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    Supporting Member Dave Kerr's Avatar
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    Quote Originally Posted by David Schwab View Post
    EMG's have the entire pickup in a Faraday cage made from very fine brass screen. ...
    Here's an EMG SA, which is also very mismatched:
    I'm curious about how the cage is wired - is that (and any other metal) connected to a drain wire or braided shield that's sent to ground, separate from the beginning and end of the coils?

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    So is anyone who wants to make clone of them.

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    Quote Originally Posted by ubaid88 View Post
    So is anyone who wants to make clone of them.
    No way. This is not a circuit I would use when you have both coils available separately. You can put an op amp, non-inverting, on each coil and add the outputs through resistors. If the coils are the same, you can make the amplifier inputs truly identical, adjusting the input resistance and capacitance for the sound you want. If you want to use different coils, you have the most flexibility in adjusting the input impedance and gain to get the sound and good hum cancellation.

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    Quote Originally Posted by Mike Sulzer View Post
    No way. This is not a circuit I would use when you have both coils available separately. You can put an op amp, non-inverting, on each coil and add the outputs through resistors. If the coils are the same, you can make the amplifier inputs truly identical, adjusting the input resistance and capacitance for the sound you want. If you want to use different coils, you have the most flexibility in adjusting the input impedance and gain to get the sound and good hum cancellation.
    I just need clean sound. I heard one of the Emgs it was as clean as acoustic guitar with high gain.

  18. #18
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by ubaid88 View Post
    I just need clean sound. I heard one of the Emgs it was as clean as acoustic guitar with high gain.
    EMG's are not hi-fi clean. They sound like regular pickups with a buffer.

    If you want clean wind a low impedance coil and use a preamp to boost the level up. Just wind a regular humbucker with about 1000 turns on each coil. That will be as clean as an acoustic guitar.
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    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Dave Kerr View Post
    I'm curious about how the cage is wired - is that (and any other metal) connected to a drain wire or braided shield that's sent to ground, separate from the beginning and end of the coils?
    As far as I could tell is was grounded at the same spot as the coils were, which is the common ground.

    It totally enclosed the pickup.
    Attached Thumbnails Attached Thumbnails emg_sa5.jpg   emg_sa7.jpg  
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    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by ubaid88 View Post
    So is anyone who wants to make clone of them.
    Not me. I was just curious how they had their coils arranged. I like my pickups better.
    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
    I think every EMG made has the coils in parallel. That's one thing I don't like about the EMG-P.
    In the circuit diagram, the coils are not in parallel. They are each connected to their own input of a differential amplifier, and so are electrically isolated from one another. Unlike a direct-wired electrical parallel connection, the coils do not load one another.

    I have an old Bartolini Hi-A pickup with one coil being intentionally 1K different from the other, and it's dead quiet. I had it installed in one of my '74 Ric basses, and along with total copper foil shielding of the bass, I was able to remove the string ground with no noise whatsoever.

    I make some bass pickups with similarly large offsets with no hum problems.
    Perfect matching is not required to obtain adequate hum cancellation. Here is a simple numerical example:

    Consider each coil as two voltage sources in series, the music source (Vm) and the hum source (Vh). The coils also differ in that they don't have the same number of turns and thus induced voltages, the ratio being called k, where k=1 if the coils are identical. All of k will be allocated to coil 2. The coils differ in that the algebraic signs of Vm and Vh are the same in coil 1, and opposite in coil 2, so:

    Vc1=Vh+Vm but Vc2=k(Vh-Vm), the difference being the sign and the factor k.

    A differential amplifier amplifies the difference between the two inputs, so (assuming a gain of one for now) we get

    Vout= Vc1-Vc2= Vh+Vm-k(Vh-Vm)= Vh(1-k)+(1+k)Vm

    If k=1, we get double the music voltage and complete cancellation of the hum voltage. But k is never exactly unity, so what is the effect of mismatch?

    The cancellation ratio is (1-k)/(1+k). How large can k be to achieve 20 dB cancellation? For 20 dB, the cancellation ratio is one tenth, so 0.1=(1-k)/(1+k), and k= 0.8181, and 1-k=0.182. This means that if the coils are geometrically similar and differ in turns count by 18% or less, 20 dB or better cancellation will be achieved.

    For 30 dB cancellation, 1-k= 6%.

    Either way, take a look at the photo I posted of the circuit board inside the 81. That's where that schematic came from. The IC and one surface mount part is missing, but you could probably trace the circuit assuming they didn't use a double sided board. I've been meaning to do it but haven't had the time.
    The usual with surface mount cards is to use two-sided boards, perhaps with components on both sides, unless the circuit is so simple that it's cheaper to implement crossovers with wire bridges and drilled holes. So, I'd photograph both sides of the circuit board, and be sure to identify one corner with a magic marker such that it's visible in both photos.

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    Senior Member Stan H's Avatar
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    Quote Originally Posted by David Schwab View Post
    If you want clean wind a low impedance coil and use a preamp to boost the level up. Just wind a regular humbucker with about 1000 turns on each coil. That will be as clean as an acoustic guitar.
    Hi David, what size wire would you use for such a pickup, would 42 work or would you recommend something larger?
    Last edited by Stan H; 09-10-2008 at 02:46 PM. Reason: missssspelling.....
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    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Joe Gwinn View Post
    In the circuit diagram, the coils are not in parallel. They are each connected to their own input of a differential amplifier, and so are electrically isolated from one another. Unlike a direct-wired electrical parallel connection, the coils do not load one another.
    I knew that! That's what happens if you haven't thought through what you are going to type ...

    The EMG-P is in parallel, but not the humbuckers. But I meant to say they aren't in series as you would expect.

    The usual with surface mount cards is to use two-sided boards, perhaps with components on both sides, unless the circuit is so simple that it's cheaper to implement crossovers with wire bridges and drilled holes. So, I'd photograph both sides of the circuit board, and be sure to identify one corner with a magic marker such that it's visible in both photos.
    Now that I think about it, the other side of the board is the back of the pickup, so there are no parts...
    Attached Thumbnails Attached Thumbnails emg_81_009.jpg   emg_81_010.jpg  
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    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Stan H View Post
    Hi David, what size wire would you use for such a pickup, would 42 work or would you recommend something larger?

    I did a bunch of bass pickups using 42 and 43. Generally you want heavier wire for a low Z pickup, but 42 worked fine.

    I used a simple JFET preamp.
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    Senior Member Stan H's Avatar
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    Thanks for the info, David!
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    Quote Originally Posted by Joe Gwinn View Post

    Perfect matching is not required to obtain adequate hum cancellation.

    Yes, I agree with your numbers. Another way of saying it is that you can approach 6 db better than implied by 1-k. That is, 1-k = .1 gives 25.58 db rather than 20 db, and 1-k = .01 gives 45.98 db rather than 40 db, while k-1=.31622 gives 14.53 db rather than 10 db, but only 4.53 db better than implied by 1-k.

    But how much is good enough? Do you plan for the average environment, or the really bad one that might not be too common, but is still out there?

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    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Mike Sulzer View Post
    But how much is good enough? Do you plan for the average environment, or the really bad one that might not be too common, but is still out there?
    It's very common to mismatch PAF coils, and look at all the people who play Strats and Teles!

    So how good is enough? Some people don't seem to mind the hum at all.
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    Quote Originally Posted by David Schwab View Post
    It's very common to mismatch PAF coils, and look at all the people who play Strats and Teles!

    So how good is enough? Some people don't seem to mind the hum at all.

    And look at how much effort there is to make strat pickups that sound like the original and have no hum.

  29. #29
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Mike Sulzer View Post
    And look at how much effort there is to make strat pickups that sound like the original and have no hum.
    Oh I agree. I don't use single coils. You can get away with it in a club, but I do a lot of recording, so I need quiet pickups.
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    Quote Originally Posted by Mike Sulzer View Post
    Yes, I agree with your numbers. Another way of saying it is that you can approach 6 db better than implied by 1-k. That is, 1-k = 0.1 gives 25.58 db rather than 20 db, and 1-k = 0.01 gives 45.98 db rather than 40 db, while k-1=0.31622 gives 14.53 db rather than 10 db, but only 4.53 db better than implied by 1-k.
    Yes. The music voltage Vm is doubled when the coils are exactly matched, and doubling voltage causes a 20 Log10[2]= 6 dB increase. As the coils become increasingly mismatched, the 6 dB will decline, but the effect isn't that strong.

    But how much is good enough? Do you plan for the average environment, or the really bad one that might not be too common, but is still out there?
    As a rule of thumb, 10 dB is quite noticable, 20 dB is pretty good (and would probably make the hum inaudible in a noisy venue), 30 dB is almost perfect, and 40 dB isn't any better than 30 dB in practice.

    When people intentionally mismatch coils, the difference in turns count seems to be of order 10%, which would allow for very good cancellation: 20 Log10[(1-0.90)/(1+0.90)]= -25.6 dB.


    This assumes that the electrostatic shielding is adequate. Perfectly matched coils are still susceptible to electrostatic pickup if not shielded.

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    Quote Originally Posted by Joe Gwinn View Post

    As a rule of thumb,
    I like your rules of thumb. Just like to point out that getting say 20db reliably time after time and under all condition requires aiming for somewhat better.


    Quote Originally Posted by Joe Gwinn View Post
    Perfectly matched coils are still susceptible to electrostatic pickup if not shielded.
    Thank you for writing that: one of my pet peeves. In fact, writing that and arguing that it is correct contributed to me getting kicked off another forum. It is interesting that most humbucker users do not use a cover. But the base plate, even though it covers just the bottom, does have a significant shielding effect. So it could be worse.

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    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Mike Sulzer View Post
    Thank you for writing that: one of my pet peeves. In fact, writing that and arguing that it is correct contributed to me getting kicked off another forum.
    Yeah, but Mike, you would also argue about things, like when you said to me that stacking a bunch of magnets doesn't increase the strength, when clearly it does.

    And Deb has a hair trigger for kicking people off lately...

    It is interesting that most humbucker users do not use a cover. But the base plate, even though it covers just the bottom, does have a significant shielding effect. So it could be worse.
    Because they don't sound the same with metal covers, which is why people started taking the covers off. You can get better noise reduction without changing the tone by using shielded plastic covers.

    Traditional humbucker parts are really quite archaic. Covers are popular these days because of the way they look.
    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 like your rules of thumb. Just like to point out that getting say 20db reliably time after time and under all condition requires aiming for somewhat better.
    The rule of thumb was about the acoustic consquence of various levels of hum cancellation. This is inherently sloppy, as people are involved.

    Achieving these levels of cancellation is another matter entirely. Although matching turns counts to within 10% doesn't seem that hard to me.

    Thank you for writing that: one of my pet peeves. In fact, writing that and arguing that it is correct contributed to me getting kicked off another forum.
    Was it something you said?

    What was their counterargument?

    It is interesting that most humbucker users do not use a cover. But the base plate, even though it covers just the bottom, does have a significant shielding effect. So it could be worse.
    It's a tradeoff to be sure. Most covers do muffle the sound.

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    Quote Originally Posted by David Schwab View Post
    Yeah, but Mike, you would also argue about things, like when you said to me that stacking a bunch of magnets doesn't increase the strength, when clearly it does.

    And Deb has a hair trigger for kicking people off lately...
    David, I think we both know that stacking disc magnets, that is, flat things magnetized along the short dimension, increases the strength, and as your reference says, makes a magnet of the same strength as a single magnet of that geometry.

    But making very long rod magnets out of shorter rod magnets has little effect. (magnetized along the long dimension) Do you remember what we were discussing?

    It took Deb several years to kick me off, but yeah, the final event really was hair trigger, and kind of surprised me.

  35. #35
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Mike Sulzer View Post
    Do you remember what we were discussing?
    Yeah, I had about 10 neo bar magnets stuck together, and was able to pick up a chromed nickel silver Gibson EB-0 pickup cover. One or two of the same magnets wouldn't do it, and a big stack of ceramics wouldn't do it either.

    I think the discussion was about the effect of chrome plating on the tone of a pickup.

    This photo should refresh your memory.
    Attached Thumbnails Attached Thumbnails stack-o-mags.jpg  
    It would be possible to describe everything scientifically, but it would make no sense; it would be without meaning, as if you described a Beethoven symphony as a variation of wave pressure. Albert Einstein

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