Pimping my Piezo Pickup classical guitar

Discussion in 'The Projects Forum' started by Roob, Mar 24, 2015.

  1. Roob

    Thread Starter Member

    Mar 21, 2015
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    Hi,

    I have fitted a piezo pickup under the saddle of my classical nylon strung guitar.

    My plan is to build the following : a 3 channel parametric eq followed by a fuzz box. All will be mounted into the guitar and powered by a 9V battery.

    As a beginner I have many questions and in order to start off on firm foundations I guess it would be sensible to understand output and input impedances. So, as I have assumed that I would need a basic preamp for my piezo pickup before feeding the signal into other places I built the attached single transistor amp.

    The output impedance of the piezo is simple I guess, I measured the resistance between the 2 electrodes to be 2MOhms or more. The output impedance from the amp is not so clear to me as there are currents and voltages and frequency dependent impedances due to the capacitor etc...

    Question 1: what is the output impedance of the circuit in the attached file
    Question 2: Do I need the attached preamp if I then have a parametric eq which must contain amps
    Question 3: Does anyone have a good schematic for a parametric eq, preferable using transistors
    Question 4: is this post breaking the etiquette rules of length and number of questions?

    thank you in advance
     
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Odd transistor choice and odd transistor symbol.

    Have you googled Guitar Piezo Pre-amp? Quite a few to choose from, no point reinventing the wheel.

    [​IMG]

    It's a good idea to put a pre-amp before the eq stage because it probably won't be designed to be driven directly from a guitar pick-up and may only have unity gain and too low an input impedance.

    Probably best to go for an op-amp based parametric equalizer as it's likely to be easier to build.
     
    Last edited: Mar 24, 2015
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  3. Dodgydave

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  4. Roob

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    Mar 21, 2015
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    Thanks for the reply. I will normalise my transistor symbol.

    I used the circuit that you show here but it gave a voltage gain of only about 2 so just tried something else to hand and it gave me a gain of 25...?
    What is it that is odd about the transistor I used? It says it's general purpose and amplifier...
     
  5. Roob

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    Mar 21, 2015
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  6. Alec_t

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    The 2N3055 is a power transistor, for handling heavy currents. It has a poor frequency reponse for audio use.
     
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  7. Roob

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    Mar 21, 2015
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    ok, its out. thanks.
     
  8. KL7AJ

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    Nov 4, 2008
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  9. Dodgydave

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  10. Roob

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    Mar 21, 2015
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  11. KL7AJ

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  12. KL7AJ

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    You may not want a lot of voltage gain. The main point of the onboard preamp is for equalization and impedance matching.
     
  13. Dodgydave

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    Jun 22, 2012
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    a good op amp for high impedance is a Ca3240,
    its best to use stages, so use Jfet transistors or op amps.
     
  14. Roob

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    Mar 21, 2015
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  15. Roob

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    Mar 21, 2015
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    so that brings me to my other question, namely what is the output impedance of this for example:

    is it simply 51K?
     
    Last edited: Mar 24, 2015
  16. Roob

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    Mar 21, 2015
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    Should the load impedance always be higher than the output impedance of the source?
     
  17. KL7AJ

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    Yes. in just about all modern audio circuitry. The load impedance should be 10 x or greater than the source impedance.
     
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  18. Veracohr

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    Jan 3, 2011
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    It would be something under 6.8k. R3||R4||Rout of the transistor.
     
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  19. blocco a spirale

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    Jun 18, 2008
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    The output impedance is around 6k as R4 is effectively in parallel with R3. If you need a lower output impedance, just add an emitter-follower stage.

    Oops, Veracohr got there before meo_O
     
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