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Two signals to the same guitar's endpin jack

Discussion in 'Audio' started by bluebb, Apr 23, 2019.

  1. bluebb

    bluebb

    49
    1
    Feb 7, 2019
    Hello,
    I have an acoustic guitar which have an undersaddle piezo pickup which needs a preamp which is built in in the body of the guitar.
    I want to add to these factory equipment a bridge position piezo transducer which doesn't need the preamp because have a discrete high output,i add after this transducer a log pot of 500k,and i connected the output together with the output of the preamp to the same endpin jack of the guitar.
    All working fine.

    My questions are,do i have to add a series resistor?
    Is the preamp short circuited?

    the output impedance of the preamp is 3.5k,of the piezo without preamp at the order of Mohm...
    Thanks for the answer in advance!
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,306
    1,889
    Nov 17, 2011
    In that configuration the output of the preamp will drive power into the bridge position transducer. And the bridge position transducer will present a variable load (depending on the pot's position) to the preamp which will in turn attenuate the amplified signal from the built-in transducer.

    Imho a better solution would use a switch to select either preamp output or bridge transducer.
    Another method would attenuate the bridge transducer's output to match the signal of the built-in transducer, then sum the outputs of the built-in transducer and the attenuated bridge transducer (using resistors), then feed that sum of signals into the preamp.
     
    bluebb likes this.
  3. bluebb

    bluebb

    49
    1
    Feb 7, 2019
    thx,but i need both signals simultanely summed in the endpin jack.
    The factory piezo needs the preamp and this that i added doesn't.
    All works fine.
    What i must do now?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,306
    1,889
    Nov 17, 2011
    That is what my last proposal will provide: The sum of both transducers at the output.
     
  5. bluebb

    bluebb

    49
    1
    Feb 7, 2019
    Between the piezo (which doesn't need the preamp), and the endpin jack there is the log pot of 500k..
    To avoid that somethink fails i have to add something?
    Can you tell me in other words?i don't understand well.
    The difference in volume is not significant,and all works well.
    I add this piezo because the 2 thinner strings of the guitar have less volume than others..and in this way i compensate this...
    This piezo doesn't match with the built in preamp,it is studied to work without the preamp,too high output.And this is why i put it alone...
    How to sum these 2 signals?
    Many many thanks
     
    Last edited: Apr 24, 2019
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    9,306
    1,889
    Nov 17, 2011
    Have a look at this block diagram:
    upload_2019-4-24_14-25-4.png
     
    bluebb likes this.
  7. bluebb

    bluebb

    49
    1
    Feb 7, 2019
    I tried, but it doesn't work, there is no sound.
    I put 2 pots of 500k nconnected as simple resistors, I tried from 100k up to 1M, and after the 500k log connected as a potentiometer,and it doesn't work.

    It only works with the the resistor in series,from 0 to 1M but the volume does not change, it remains the same.
    When I add the log pot connected as a potentiometer there is no more sound.

    If I wanted to connect to the endpin jack the bridge piezo output and the preamp output how should I do?
    If the sound suits me I can simply connect the 2 hots to the endpin jack?
    Or do I risk something?
    If I put a resistor of 4.7k in series with the bridge piezo(without preamp) to prevent the preamp from short-circuiting?
    What do you say?
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    9,306
    1,889
    Nov 17, 2011
    Please provide a schematic. It is hard to understand your circuit from a verbal description only.
     
    bluebb likes this.
  9. bluebb

    bluebb

    49
    1
    Feb 7, 2019
    thanks!
     

    Attached Files:

  10. bluebb

    bluebb

    49
    1
    Feb 7, 2019
    as you well know the input impedance of the preamp is 10-20M,and the output impedance of a piezo pickup is Megaohms too,maybe 1-2M about....
    The output of the piezo "see" the parallel between the pot of 500k and input impedance of the preamp,and this lower the impedance which isn't enough high.
    This is what i think..
    Thx
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

    9,306
    1,889
    Nov 17, 2011
    1. Your potentiometer is connected incorrectly This is what it should look like:
    upload_2019-4-25_10-4-41.png

    2. you should have a series resistor between the original piezo and the amplifier input. Otherwise the fishman piezo will see the bridge-piezo's signal without attenuation. That's usually not good practice, but frankly I don't know what would jappen with to the transducer as I'm not familiar with this kind of pickup.
     
  12. bluebb

    bluebb

    49
    1
    Feb 7, 2019
    i used the 2 pots as a series resistor(without the lug 3).
    for the third pot i tried to invert the lug 1 with the lug 2,but doesn't work.
    I saw guitars which used the lug1 or the lug2 as a input...
    I will put bridge transducer to the endpin jack without the preamp,together the output of the preamp.
    To prevent shortcircuiting of the preamp i will put a series resistor of 4,7k to the bridge pickup
    Thsnks
     
  13. darren adcock

    darren adcock

    461
    34
    Sep 26, 2016
    Hi there, I've just drawn up Harald's block diagram in post #6. In a similar format to your hand drawing, if that makes things more clear for you? I don't know what jack socket you have. Some shots of your wiring attempts will help in diagnosing where you have made mistakes.

    [​IMG]
     
  14. bluebb

    bluebb

    49
    1
    Feb 7, 2019
    i connected exactly like this,but doesn't work...
    Something doesn't match with the pots...
    It's working only with the pots after the preamp
     
  15. bluebb

    bluebb

    49
    1
    Feb 7, 2019
    with the shaft down
    with the shaft flipped the lugs 1 and 2 can be output or input as you want..
    I tried this 1000 times...
    Harold told that the center lug (2) can be only output..
    I would like to know:
    I have an acoustic guitar which have an undersaddle piezo pickup which needs a preamp which is built in in the body of the guitar.
    I want to add to these factory equipment a bridge position piezo transducer which doesn't need the preamp because have a discrete high output,i add after this transducer a log pot of 500k,and i connected the output together with the output of the preamp to the same endpin jack of the guitar.
    All working fine.

    My questions are,do i have to add a series resistor?
    Is the preamp short circuited?

    the output impedance of the preamp is 3.5k,of the piezo without preamp at the order of Mohm...
    Thank!!
     
  16. darren adcock

    darren adcock

    461
    34
    Sep 26, 2016
    Info on the bridge position transducer will help, part number? If the pre-amp was short circuited you'd get no sound, but let's see the bridge position transducer info, it is quite an expensive fishman pre amp you have there so good to be sure. And if I am understanding correctly when you connect both the output of the pre amp to the output jack socket and also connect the bridge position transducer to the output you get sound from both?
     
    bluebb likes this.
  17. bluebb

    bluebb

    49
    1
    Feb 7, 2019
    Many thanks for the reply.
    Sorry I didn't wrote correctly the question, I wanted to ask if the piezo K&K can cause the short circuit of the preamp.

    Both are connected to the endpin jack.For the K&K bridge piezo there is no need to use a preamp because the output is enough high.
    I thought that connecting a resistor in series with the K&K would protect the preamp from possible short circuits.

    I've connected like this, and everything works fine, I'm satisfied with the results. I'm just afraid the preamp might fail.
    P_20190426_172347_optimized.jpg

    i have the volume for the fishman piezo on the preamp,the volume for the K&K on one pot of 500k and the general volume on the second pot of 500k.
    The 4.7k resistor in series on the transducer which doesn,t have the preamp is (I hope) the protection for a possible short circuit of the preamp.
    What you say?
    I need this?
    Is enough 4.7k?
    I have to sum the signals in a different way?

    If I connect the K&K transducer to the preamp input there is sound, but isn,t good sound because the characteristics of the piezo doesn't match those of the preamp.
    The fishman piezo obviously works perfecty with his (designed for it) preamp.
    If I put a pot between the K&K piezo and the preamp there is no more sound.
    That why i put it alone to the endpin jack as the factory says.

    This is the Fishman piezo called "Matrix":
    12183292_800.jpg

    This is the preamp Fishman:
    temp2.0x0.jpg

    This is the K&K piezo called "k&k pure mini"(but i use only one of these three):
    preuzmi.jpeg
    And this is the fishman 1/4" endpin jack which works as a switch when you insert a mono 6.3mm jack which connect the ring to the sleeve.
    img56651005.jpg

    Thanks!!
     
  18. darren adcock

    darren adcock

    461
    34
    Sep 26, 2016
    If all works fine and you are happy with then go for it. The series resistor you have is acting as a summing resistor, you need to put one from the output of the pre amp also, I don't have the math knowledge to help you work out which is best for each output. I can't see how you can short circuit the pre amp by conecting both signal ouputs, others with more knowledge might need to clarify my statement here. This resource is good for explaining the pitfalls of passive mixing and the pro's of active mixing. http://sound.whsites.net/articles/audio-mixing.htm
     
    bluebb likes this.
  19. bluebb

    bluebb

    49
    1
    Feb 7, 2019
    My fear is that the preamp output also feeding the K&K piezo can burn it and the circuit closes and the impedance that sees the preamp is zero and then the preamp is also burned.
    That's why I would put the resistor only in series at K&K.
    The K&K is passive and cannot do damage,instead the fishman is active.
    You are the experts, not me.
    I thought I would have a greater support here.
    Thank you
     
  20. darren adcock

    darren adcock

    461
    34
    Sep 26, 2016
    I don't understand, "The K&K is passive and cannot do damage, instead the fishman is active." if you knew this already, why do you need "greater support"?
    I am certainly not the most experienced/knowledgeable, person here by a long shot, or an expert, but thought i could help you, if my help is not enough then fair enough, but it's not the best idea to say you've not had enough support here, within mind that this support is what you desire.
     
    bluebb likes this.
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