Efficient way to read guitar pickup impedance?

Discussion in 'General Electronics Chat' started by blah2222, Dec 10, 2012.

  1. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Hi,

    I was wondering if anyone has tried to characterize their guitar's pickup impedance out of curiosity.

    Through a patch cord (I know more cable resistance and capacitance...) and a jack I fed in signals in the range of 50Hz up to 50kHz in series with the pickup and a resistor.

    I then measured the voltage across the resistor to get a current value and then used that to compute the magnitude of pickup impedance.

    I ended up getting the characteristic shape of the response. Resistive -> increasing impedance due to inductance -> peak -> parallel capacitance driving impedance to zero with increasing frequency.

    I am just wondering if there is a more efficient or effective way of finding this measurement.
     
  2. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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  3. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    haha if I were rich...
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    Why do you want to know the fairly low impedance of the guitar magnetic pickup?
    Instead you should know the recommended input impedance of its preamp which is at least 1M ohms. Then the inductance of the pickup can resonate with the capacitance of its cable to produce a large peak in the frequency response like this:
     
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  5. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    How would one go about measuring this response for themselves? You would need to stimulate the pickup with a known source somehow.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    The inductance, parallel capacitance, series and parallel resistances combine to produce the peak in the frequency response of a magnetic guitar pickup.
     
  7. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    I see. I get what causes the response, but if I wanted to actually measure the impedance at different frequencies using a function generator and plotting the resulting curve, what would be the best way to do it?
     
  8. nigelwright7557

    Senior Member

    May 10, 2008
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    Why not just pluck the different strings and see what the output looks like at each note ?
     
  9. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    The decay of the signal is very weak and quick being hard to get an accurate measurement. Also, I have no idea what the source amplitude is.
     
  10. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Just seems to make sense that I could inject AC signals via a function generator and measure current to get pickup impedance...
     
  11. tubeguy

    Well-Known Member

    Nov 3, 2012
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    I've done exactly the same thing to verify the impedance and correct operation of a 70v transformer with impedance taps.
    Worked fine for me.

    Did you take into consideration that the voltages are 90 degrees out of phase in your calculations ?
     
  12. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    I noticed phase shifts but I was only really looking at the magnitude of impedance.

    Thanks for the reply!
     
  13. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    The pickup is only part of the system. The rest is the mechanical part ie the magnets affecting the string and the string if moving or not.

    With a speaker or microphone those parts are always included in the test, but with a guitar pickup the size, type and distance of the strings and how they are moving are all unknown and cannot easily be tested with an electrical test.
     
  14. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    That's true. I think I saw a device that has an audio pressure emitter to simulate the sound of the strings and measures the impedance here, but I would like to build this myself.

    I guess could point a speaker with a known output signal strength and measure the impedance of the pickup that way, and play around with the scope.

    Thanks for the reply!
     
  15. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    That depends if you are talking about a magnetic pickup from an electric guitar or a piezo pickup from an acoustic one.
     
  16. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    I am specifically measuring coiled pickups. With that being said, piezo would be another project.
     
  17. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    In that link he uses a coil that is mangetically coupled to the pickup, no sound involved in there. I don´t think that is a very repeatable or accurate way of measuring the pickup characteristic. The question is, how do you want to calibrate the measuring coil?
     
  18. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Yeah, that sounds right, not sound. I'm not sure if I really want to go that route as it seems pretty finicky. I just really want a rough idea of the characteristic response and roughly where the resonant frequency lies.

    Using my AC signal generator I have obtained the same looking characteristic curve but I'm not sure if the resonant frequency is correct.
     
  19. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Hi all,

    I took another stab at this and finally got a half decent model which matched very well with my lab tests to measure pickup impedance.

    Besides getting the impedance magnitudes of each pickup, I really wanted to see if I could hone in on what the inductances of the coils are. I had a rough idea of what inductances I'd be looking for so I played around with the inductance in the model until it matched my results.

    For both pickups I used two cases while measuring impedance across frequency:

    1) Volume control pot = 100% and Tone control pot = 0%
    2) Volume control pot = 100% and Tone control pot = 0%

    To actually test the impedances, I used a function generator (source resistance of ~1K) to generate frequencies 10Hz - 100KHz at log10 intervals.

    I placed a 10K series resistor between the function generator and hot lead of the 1/4" jack (connected to the guitar via patch cord) with the ground lead of the jack connected to ground.

    I set a 1Vpp signal and measured the voltage between the hot and ground leads of the jack. The current through the system is just the source signal minus the jack voltage difference, divided by the 10K resistor. Pickup impedance (with cable impedance) was calculated by dividing the measured voltage across the jack leads divided by the calculated current.

    The plots attached are the four cases (two per pickup). The left charts are Excel plots of measured data and the right are the model charts in MATLAB. You can hover over the images to get the descriptions of the charts.

    A bit tedious but kind of neat to know for building effects/amps.

    Hope this was useful or interesting,
    JP
     
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