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Im confused about Sustaining PU's?

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  • Im confused about Sustaining PU's?

    Okay, i know there are lots of DIY'ers out there and a few kits similar to the sustainiac setup.  But im not interested inany of the kits ive seen online. And im a lil bit confused about the principle involved.

     

    Soooooooo, to me, the idea is to vibrate the strings to keep the sound comming.

     

    Why wouldnt a simple circuit and home made electro magnet work without all the complicared stuff added in?

     

    Example:

     

    (remember i am not really educated in circuitry....

     

    9v Battery / on-off switch / capaciter / nail / copper wire.

     

    Wind the wire round the nail and mount it inbetween the neck PU and the fretboard under the strings. Then make a circuit from those two wires of the coil to a switch and add a capacitor somewhere to Pulse the electro magnet from power from the 9v.

     

    The EM turns on and pulls the strings downward a touch and then releases them causing the strings to vibrate. Of course there isnt any harmonics added, or gain added, etc...

     

    So edumacate me guys and explain why this isnt being done. It seems very simple in theory.

     

    By the way,

     

    its been ages since ive been here. Years ago i was a nightly regular all around this place. Just got 3G internet at home after all this time. Im still a pro, work fulltime in church for pay as the house guitarist. And do some gigs here and there for extra cash with blues and other genres. So if ANY of youse guys remember me then HEllo!

     

     

    Yea though i walk through the "Valley-Of-Shadows", i shall not fear. For i have 2 spare sets of "Ernie Balls" in my guitar case! Muahahahaha!

  • #2

    Assuming that your EM worked as planned and vibrated the strings one big problem I see is varying the frequency. You would need some sort of feedback loop like when you put your guitar in front of your amp. 

    Comment


    • #3

      StratKat wrote:

      Wind the wire round the nail and mount it inbetween the neck PU and the fretboard under the strings. Then make a circuit from those two wires of the coil to a switch and add a capacitor somewhere to Pulse the electro magnet from power from the 9v.

      The EM turns on and pulls the strings downward a touch and then releases them causing the strings to vibrate. Of course there isnt any harmonics added, or gain added, etc...

      So edumacate me guys and explain why this isnt being done. It seems very simple in theory.



      Hi Stratcat.

      It isn't being done that way because if you built it, you will find it doesn't work.

      The original E-bow from decades ago and the similar things like Kramer's sustainer are basically oscillator circuits (which inculde an electro-mechanical element). Oscillation requires amplification with feedback. Amplification adds the energy to the system which sustains the oscillation.

      Although there is an energy source in your idea, the 9V battery, what is missing is the amplifier.

      In your circuit, the battery basically does nothing: it sends DC through the circuit consisting of the wire, electromagnet and pickup. On top of this DC, you have the AC signal from the pickup, which is not amplified in any way. The weak AC signal from the pickup cannot energize an electromagnet into moving the string significantly enough to sustain oscillations.

      The battery is being discharged thorugh the circuit, but its energy is not being put to the proper use (is not being modulated by the signal in any way to create a bigger signal), and so what you have is effectively a perpetual motion machine.

      If you include a series capacitor in the circuit, then you won't get the DC flow from the battery, because capacitors block DC. It will preserve the battery charge, but otherwise make no difference. Capacitors do not generate pulses, they only charge and discharge to the voltage level applied to them, and there pass through signals in a frequency-selective way.

      Oscillation requires the "Barkhausen condition" to exist: that the "loop gain" of the system is equal to 1 at some frequency where the total phase shift around the loop is a multiple of 360 degrees. Basically, positive feedback with a gain of 1. In practice, oscillators (including these sustainers) use a gain of more than 1 so they start up quickly, and then the gain will somehow naturally limit itself to 1 (in this case, the string refuses to vibrate any louder because there is no room). 

      "Loop gain" is the total gain around the feedback loop: the amplifier's gain and the feedback fraction. E.g. with a gain of 20, and 10% feedback, the loop gain is 2: 20 times 0.1.   A gain of 1 at the proper phase shift regenerates the oscillation.

      Since you have no amplifier in the system, your "amplifier" gain is 1.  The feedback system consists of the electromagnet, wire, guitar string and pickup.  To have sustain, that feedback system would have to provide 100% feedback to make up for the lack of an amplifier so that the loop gain can be 1. For instance a 1V RMS sinusoidal wave sent by the wire to the electromagnet would have to vibrate the string so much that 1V RMS sinusoidal emerges from the pickup!  100% feedback isn't going to happen (anywhere near!) due to various losses in the system, and due to poor coupling from string to pickup, and electromagnet to string.

      Music DIY mailing list: http://www.kylheku.com/diy
      ADA MP-1 mailing list: http://www.kylheku.com/mp1

      Comment


      • StratKat
        StratKat commented
        Editing a comment

        Thanks guys, that taught me a lot. 

         

        I was thinking that if you play a note the string vibrates distubing the mag field of the pu causing a signal.

         

        So by just vibrating any string in that field somehow you would keep that string alive, continuing to make a signal.

         

        I was trying to find the way to do that without adding to the existing circuitry of the pu's. More of a stand alone thing.













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