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2-channel passive mixer-independent volume control for each pickup

Discussion in 'Project Construction Technologies' started by bluebb, Feb 7, 2019.

  1. bluebb

    bluebb

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    Feb 7, 2019
    P_20190206_171025_1.jpg P_20190206_201326.jpg P_20190206_201728.jpg P_20190206_201334.jpg P_20190206_171025_1.jpg P_20190206_201326.jpg P_20190206_201728.jpg P_20190206_201334.jpg
    Hello,

    I'm new here,I need your help, I built this simple 2-channel passive mixer for blending 2 piezos,i used 2 pots of 500 Kohms.
    I am attaching photos,please check the diagram,the mixer has 2 inputs, one for the undersaddle piezo pickup and one for bridge plate sensor piezo pickup it should make a blend,the mixer's output enters to the fishman 501T preamp.
    The problem is that no signal comes from the two pickups,but,if I touch the hot of one of the two inputs with the ground you hear this buzzing, it comes out from the output of the acoustic guitar.
    But the sound of the strings taken from the piezo is not heard.
    I do not understand where I'm wrong !!!

    Thank you very much in advance to those who answer me!
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Add 1: Having your images upright would save us a lot of neck bending.
    Add 2: You connected the pickup to the center tap ot the potentiometes and the end of the potentiometers to the amplifier. Swap these connections as show:
    [​IMG]
     

    Attached Files:

    bluebb likes this.
  3. bluebb

    bluebb

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    Feb 7, 2019
    Thanks for the quick answer but this is not the reason:

    https://components101.com/potentiometer
    I tried before i soldering with two batteries of 1.5v in the way you told and at the output i had 0.75v with one pot completely open and 1.5v with both open.
    In the way i connect i had 1.5 with one open or both open,always 1.5v.
     
  4. Alec_t

    Alec_t

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    Jul 7, 2015
    So where do these voltages come from, if not from the pickups, bearing in mind that this is supposedly a passive mixer?
     
    bluebb likes this.
  5. bluebb

    bluebb

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    Feb 7, 2019
    Before welding I did some tests using two 1.5 volts batteries instead of the two piezo pickups as a current generator.

    With the batteries the circuit worked, I used the central pin as an input.
    I would like some of you to tell me why the voltage produced by the piezo pickups does not reach the output of the simple mixer.
    The voltage of the batteries arrived.
    Thanks for collaboration!
     
  6. Alec_t

    Alec_t

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    Jul 7, 2015
    Piezo pickups have a source impedance which is very high at low frequencies but reduces as frequency increases. Their output voltage is small even when unloaded, but is made even smaller by the loading which the pot resistances place on them.
    Piezo pickups should normally be working into a 10 Megohm input impedance of a preamp such as the one you mention, not into the relatively low impedance of 500k or less (depending on the settings of the pots). You really need a high input impedance buffer amplifier between the pickups and the mixer.
     
    bluebb likes this.
  7. bluebb

    bluebb

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    Feb 7, 2019
    when the pot is at maximum there are zero ohms,so the pickup only see the input impedence of the preamp which is very high.
    I don't understand why this mixer doesn't work,it has the same circuit of the gibson les paul,but there are magnetic pickups,not piezo.
     
  8. Bluejets

    Bluejets

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    Oct 5, 2014
    I'm guessing it's the pickups then....o_O

    Once built an electronic stethoscope for finding location of engine noise.
    Plan called for magnetic earpiece as a pickup mic but all I had was crystal. ( wouldn't work).
    Ordered magnetic mic like was specified and still working 30 years later.
     
    bluebb likes this.
  9. Audioguru

    Audioguru

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    Sep 24, 2016
    When the pot is wired correctly as a signal voltage divider with its center pin feeding the preamp, then the pickup is always connected to the 500k resistance of the pot. A magnetic geetar pickup will work this way but a 3M pot resistance works better. A piezo pickup needs 10M. But your idea of shorting the pots together is no good. You need an amplified mixer circuit.

    Here is an article that says a magnetic pickup picks up string vibrations on an electric guitar that has no housing but a piezo pickup picks up the housing vibrations of an acoustic guitar:
    https://www.seymourduncan.com/blog/the-tone-garage/piezo-vs-magnetic-pickups
     
    bluebb likes this.
  10. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    No, it "sees" also the impedance of the potentiometer in parallel with the amplifier input.
    Magnetic pickups imho have a much lower impedance. That explains it.
    Use a preamplifier as suggested by Alec_T or use magentic pickups.

    You can verify this simply by using the original piezo pickup directly plugged into the amplifier, without your mixer.
     
    bluebb likes this.
  11. bluebb

    bluebb

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    Feb 7, 2019
    Ok, I give up, thanks to everyone who answered me!
    But I have another question.
    I decided at this point to buy a preamp for the bridge piezo,the original for it,the K&K pure amp,and for the undersaddle pickup I already have in the guitar the preamp fishman.
    I will connect bot outputs from the preamps to the endpin jack.
    But between the preamps and the endpin jack i would like to put a volume pot as a general volume.
    Or buy a blend pot.
    But the problem is,I tried to do a test, I connected the 500k linear pot and the signal that enters the mixer is always the same.
    Why?
    I need a different pot?
    and I want to buy a blend pot,of how much ohms should I buy it?
    I thought working with the signal coming out of the preamps was easier.
    Thank you!
     
  12. Alec_t

    Alec_t

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    Jul 7, 2015
    Post a sketch of exactly how you connected it.
     
    bluebb likes this.
  13. bluebb

    bluebb

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    Feb 7, 2019
    When it is turned upside down at the lug in the left the preamp output,at the lug in the middle output to pa.
    The lug at the right to gnd.
    Thx
     

    Attached Files:

  14. BobK

    BobK

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    Jan 5, 2010
    If is not working because you wired it as a variable resistor, not as a potentiometer.

    Wire it as a potiometer, and it will have an effect on the voltage. However, it will reduce the output severely at any setting because of the high impedance of the piezo.

    Bob
     
    bluebb likes this.
  15. bluebb

    bluebb

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    Feb 7, 2019
    sorry,how i have to wiring it?
    Thanks
     
  16. Audioguru

    Audioguru

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    I disagree. The pot is connected backwards so when it is turned clockwise the volume goes down and when it is turned counter-clockwise the volume gores up. But a volume control should be logarithmic like our hearing, not linear.

    Since the preamp feeds your mixer then why doesn't your mixer have an input volume control?

    I do not understand what you are doing here: "I will connect bot outputs from the preamps to the endpin jack.
    But between the preamps and the endpin jack i would like to put a volume pot as a general volume.
    Or buy a blend pot." Why doesn't your mixer have a blend pot? Your above volume control does not blend the two preamps.
     
    bluebb likes this.
  17. bluebb

    bluebb

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    Feb 7, 2019
    i have a guitar washburn ea40sce,the guitar have an undersaddle pickup with the preamp "fishman 501T",i ordered the piezo sensors for the bridge if the guitar(another type of transducers),those sensors have its preamp(K&K pre preamp)but i still waiting the package from USA.
    Today i made a test,but the output from the pot didn't change,at max and at min the same.
    Why?
    My question is,how i have to wiring it?and which value i need?
    I made a test today with one linear 500k pot,i have to decide,i will maybe put one pot as a general volume which receive the two outout from the two preamps,or i will buy one blend pot.
    thx,i hope that my english is sufficient to be understood.
     
  18. Audioguru

    Audioguru

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    Sep 24, 2016
    A volume control is a logarithmic pot. A linear pot is used for adjusting a voltage or as a balance control.
    The value of a volume control depends on the input impedance of the mixer or amplifier it feeds.

    How can you "short circuit" outputs of preamps together? Instead you should mix or blend them together so they do not short circuit each other.

    Of course a volume control will work. When it is turned clockwise then its slider connects to the input. When it is turned down then its slider gets a reduced level because of its voltage divider action. When it is counter-clockwise then its slider connects to ground producing no signal.
     
    bluebb likes this.
  19. bluebb

    bluebb

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    Feb 7, 2019
    ok,just a volume pot to which i will connect the two outputs from the preamps,and the output of the pot to the endpin jack.
    Is it possible?
    I have to buy a 500k log pot?
    The impedence of the input of the amplifier will be different,depend on the situation,different PA,or guitar amplifiers.
    I don't want to complicate anymore,just a generale volume pot,not blend pot.
    Can u tell me please where i made the mistake.
    Thx
     
  20. bluebb

    bluebb

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    Feb 7, 2019
    "short circuit" outputs of preamps together?
    I don't understand,what i have to do?
    Isn't enough connect the two preamp's output in parallel to the pot and the output of the pot to the endpin jack?

    Nevermind for the blend pot,i just want to put a generale volume between the preamps and the endpin jack.

    Please tell which pot put and how to connect it.
    Thx
     
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